3629Hyperbaric and Diving Medicine CHAPTER 463

asthma is considered to increase risk because it could predispose to air

trapping and pulmonary barotrauma (see below); and ischemic heart

disease increases risk because it could prevent a diver from exercising

sufficiently to get out of a difficult situation such as being caught in a

current. It can be a complex matter to recognize the relevant interactions between diving and medical conditions and to determine their

impact on suitability for diving. There may follow an equally complex

discussion about whether such interactions impart a disqualifying

risk, and this may be influenced by the individual candidate’s level of

risk acceptance and the extent to which others involved (such as dive

partners) might be affected. Guidelines are occasionally published

on assessment of diving candidates with risk factors for important

comorbidities like cardiovascular disease or who have suffered topical

problems such as COVID-19 infection (see “Further Reading” list).

Physicians interested in regularly conducting such evaluations should

obtain relevant training. Short courses providing relevant training are

offered by specialist groups in most countries.

BAROTRAUMA

Barotrauma is essentially tissue injury arising as a result of ambient

pressure changes. Middle-ear barotrauma (MEBT) in diving is similar

to the problem that may occur during descent from altitude in an

airplane, but difficulties with equalizing pressure in the middle ear

are exaggerated underwater by both the rapidity and magnitude of

pressure change as a diver descends or ascends. Failure to periodically insufflate the middle-ear spaces via the eustachian tubes during

descent results in increasing pain. As the Pamb increases, the tympanic

membrane (TM) may be bruised or even ruptured as it is pushed

inward. Negative pressure in the middle ear results in engorgement

of blood vessels in the surrounding mucous membranes and leads to

effusion or bleeding, which can be associated with a conductive hearing

loss. MEBT is much less common during ascent because expanding

gas in the middle-ear space tends to open the eustachian tube automatically. Barotrauma may also affect the respiratory sinuses, although

the sinus ostia are usually widely patent and allow automatic pressure

equalization without the need for specific maneuvers. If equalization

fails, pain usually results in termination of the dive. Difficulty with

equalizing ears or sinuses may respond to oral or nasal decongestants.

Much less commonly, divers may suffer inner ear barotrauma

(IEBT). Several explanations have been proposed, of which the most

favored holds that forceful attempts to insufflate the middle-ear space

by Valsalva maneuvers during descent result in transmission of pressure to the perilymph via the cochlear aqueduct and outward rupture

of the round window, which is already under tension because of negative middle-ear pressure. The clinician should be alerted to possible

IEBT after diving by a sensorineural hearing loss or true vertigo (which

is often accompanied by nausea, vomiting, nystagmus, and ataxia).

These manifestations can also occur in vestibulocochlear DCS (see

below) but should never be attributed to MEBT. Immediate review by

an expert diving physician is recommended, and urgent referral to an

otologist will often follow.

The lungs are also vulnerable to barotrauma but are at most risk

during ascent. If expanding gas becomes trapped in the lungs as Pamb

falls, this may rupture alveoli and associated vascular tissue. Gas trapping may occur if divers intentionally or involuntarily hold their breath

during ascent or if there are bullae. The extent to which asthma predisposes to pulmonary barotrauma is debated, but the presence of active

bronchoconstriction must increase risk. For this reason, asthmatics

who regularly require bronchodilator medications or whose airways

are sensitive to exercise or cold air are usually discouraged from diving. While possible consequences of pulmonary barotrauma include

pneumothorax and mediastinal emphysema, the most feared is the

introduction of gas into the pulmonary veins leading to cerebral arterial gas embolism (CAGE). Manifestations of CAGE include loss of

consciousness, confusion, hemiplegia, visual disturbances, and speech

difficulties appearing immediately or within minutes after surfacing.

The management is the same as for DCS described below. The natural

history of CAGE often includes substantial or complete resolution of

symptoms early after the event. This is probably the clinical correlate

of bubble involution and redistribution with consequent restoration

of flow. Patients exhibiting such remissions should still be reviewed

at specialist diving medical centers because secondary deterioration

or re-embolization can occur. Unsurprisingly, these events can be

misdiagnosed as typical strokes or transient ischemic attacks (TIAs)

(Chap. 427) when patients are seen by clinicians unfamiliar with

diving medicine. All patients presenting with neurologic symptoms after

diving should have their symptoms discussed with a specialist in diving

medicine and be considered for recompression therapy.

DECOMPRESSION SICKNESS

DCS is caused by the formation of bubbles from dissolved inert gas

(usually nitrogen) during or after ascent (decompression) from a

compressed gas dive. Bubble formation is also possible following

decompression for extravehicular activity during space flight and with

ascent to altitude in unpressurized aircraft. DCS in the latter scenarios

is probably rare in comparison with diving, where the incidence is

~1:10,000 recreational dives.

Breathing at elevated Pamb results in increased uptake of inert gas

into blood and then into tissues. The rate at which tissue inert gas

equilibrates with the inspired inert gas pressure is proportional to

tissue blood flow and the blood-tissue partition coefficient for the

gas. Similar factors dictate the kinetics of inert gas washout during

ascent. If the rate of gas washout from tissues does not match the rate

of decline in Pamb, then the sum of dissolved gas pressures in the tissue

will exceed Pamb, a condition referred to as “supersaturation.” This is the

prerequisite for bubbles to form during decompression, although other

less well-understood factors are also involved. Deeper and longer dives

result in greater inert gas absorption and greater likelihood of tissue

supersaturation during ascent. Divers control their ascent for a given

depth and time exposure using algorithms that often include periods

where ascent is halted for a prescribed period at different depths to

allow time for gas washout (“decompression stops”). Although a breach

of these protocols increases the risk of DCS, adherence does not guarantee that it will be prevented. DCS should be considered in any diver

manifesting postdive symptoms not readily explained by an alternative

mechanism.

Bubbles may form within tissues themselves, where they cause

symptoms by mechanical distraction of pain-sensitive or functionally

important structures. They also appear in the venous circulation,

almost certainly forming in capillary beds as blood passes through

supersaturated tissues. Some venous bubbles are tolerated without

symptoms and are filtered from the circulation in the pulmonary capillaries. However, in sufficiently large numbers, these bubbles are capable

of inciting inflammatory and coagulation cascades, damaging endothelium, activating formed elements of blood such as platelets, and causing

symptomatic pulmonary vascular obstruction. Moreover, if there is a

right-to-left shunt through a patent foramen ovale (PFO) or an intrapulmonary shunt, then venous bubbles may enter the arterial circulation

(25% of adults have a probe-patent PFO). The risk of cerebral, spinal

cord, inner ear, and skin manifestations appears higher in the presence

of significant shunts, suggesting that these “arterialized” venous bubbles

can cause harm, perhaps by disrupting flow in the microcirculation of

target organs. Circulating microparticles, which are elevated in number

and size after diving, are currently under investigation as indicators of

decompression stress and as injurious agents in their own right. How

they arise and their exact role in DCS remain unclear.

Table 463-3 lists manifestations of DCS grouped according to

organ system. The majority of cases present with mild symptoms,

including musculoskeletal pain, fatigue, and minor neurologic manifestations such as patchy paresthesias. Serious presentations are much

less common. Pulmonary and cardiovascular manifestations can be

life-threatening, and spinal cord involvement frequently results in permanent disability. Latency is variable. Serious DCS usually manifests

within minutes of surfacing, but mild symptoms may not appear for

several hours. Symptoms arising >24 h after diving are very unlikely to

be DCS. The presentation may be confusing and nonspecific, and there

are no useful diagnostic investigations. Diagnosis is based on integration of findings from examination of the dive profile, the nature and

3630 PART 15 Disorders Associated with Environmental Exposures

TABLE 463-3 Manifestations of Decompression Sickness

ORGAN SYSTEM MANIFESTATIONS

Musculoskeletal Limb pain

Neurologic

Cerebral Confusion

Visual disturbances

Speech disturbances

Spinal Muscular weakness

Paralysis

Upper motor neuron signs

Bladder and sphincter dysfunction

Dermatomal sensory disturbances

Abdominal pain

Girdle pain

Vestibulocochlear Hearing loss

Vertigo and ataxia

Nausea and vomiting

Peripheral Patchy nondermatomal sensory disturbance

Pulmonary Cough

Dyspnea

Cardiovascular Hemoconcentration

Coagulopathy

Hypotension

Cutaneous Rash, itch

Lymphatic Soft tissue edema, often relatively localized

Constitutional Fatigue and malaise

■ HYPOTHERMIA

Accidental hypothermia occurs when there is an unintentional drop

in the body’s core temperature below 35°C (95°F). At this temperature,

many of the compensatory physiologic mechanisms that conserve heat

begin to fail. Primary accidental hypothermia is a result of the direct

exposure of a previously healthy individual to the cold. The mortality

rate is much higher for patients who develop secondary hypothermia as

a complication of a serious systemic disorder or injury.

464 Hypothermia and

Peripheral Cold Injuries

Daniel F. Danzl

temporal relationship of symptoms, and the clinical examination. Some

DCS presentations may be difficult to separate from CAGE following

pulmonary barotrauma, but from a clinical perspective, the distinction

is unimportant because the first aid and definitive management of both

conditions are the same.

TREATMENT

Diving Medicine

First aid for either DCS or CAGE includes horizontal positioning

(especially if there are cerebral manifestations), intravenous fluids

if available, and sustained 100% oxygen administration. The latter

accelerates inert gas washout from tissues and promotes resolution of

bubbles. Definitive treatment of DCS or CAGE with recompression

and hyperbaric oxygen is justified in most instances, although some

mild or marginal DCS cases may be managed with first aid measures alone—an option that may be invoked by experienced diving

physicians under various circumstances, but especially if evacuation

for recompression is hazardous or extremely difficult. Long-distance

evacuations are usually undertaken using a helicopter flying at low

altitude or a fixed-wing air ambulance pressurized to 1 ATA.

Recompression reduces bubble volume in accordance with

Boyle’s law and increases the inert gas partial pressure difference

between a bubble and surrounding tissue. At the same time,

oxygen administration markedly increases the inert gas partial

pressure difference between alveoli and tissue. The net effect is to

significantly increase the rate of inert gas diffusion from bubble

to tissue and tissue to blood, thus accelerating bubble resolution.

Hyperbaric oxygen also helps oxygenate compromised tissues and

may ameliorate some of the proinflammatory effects of bubbles.

Various recompression protocols have been advocated, but there

are no data that define the optimum approach. Recompression typically begins with oxygen administered at 2.8 ATA, the maximum

pressure at which the risk of oxygen toxicity remains acceptable in

a hyperbaric chamber. There follows a stepwise decompression over

variable periods adjusted to symptom response. The most widely

used algorithm is the U.S. Navy Table 6, whose shortest format lasts

4 h and 45 min. Typically, shorter “follow-up” recompressions are

repeated daily while symptoms persist and appear responsive to

treatment. Adjuncts to recompression include intravenous fluids

and other supportive care as necessary. Occasionally, very sick

divers require intubation, ventilation, and high-level intensive care.

The presentation of sick divers to physicians or hospitals without

diving medicine expertise creates a risk of misinterpretation of

nonspecific manifestations and of consequent mistakes in diagnosis and management. Physicians finding themselves in this situation are strongly advised to expeditiously contact the 24-h diving

emergency advisory service provided by the Divers Alert Network

(DAN). This can be accessed at +1-919-684-9111, and there are

subsidiary or related services in virtually all jurisdictions globally.

■ FURTHER READING

Bennett MH et al: Hyperbaric oxygen therapy for late radiation tissue

injury. Cochrane Database Syst Rev 4:CD005005, 2016.

Edmonds C et al: Diving and Subaquatic Medicine, 5th ed. Boca Raton,

FL, Taylor and Francis, 2015.

Francis A, Baynosa R: Ischaemia-reperfusion injury and hyperbaric

oxygen pathways: A review of cellular mechanisms. Diving Hyperb

Med 47:110, 2017.

Gorenstein SA et al: Hyperbaric oxygen therapy for COVID-19

patients with respiratory distress: Treated cases versus propensity

-matched controls. Undersea Hyperb Med 47:405, 2020.

Jepson N et al: South Pacific Underwater Medicine Society guidelines

for cardiovascular risk assessment of divers. Diving Hyperb Med

50:273, 2020.

Kranke P et al: Hyperbaric oxygen therapy for chronic wounds.

Cochrane Database Syst Rev 6:CD004123, 2015.

Mitchell SJ et al: Pre-hospital management of decompression illness:

Expert review of key principles and controversies. Diving Hyperb

Med 48:45, 2018.

Moon RE (ed): Hyperbaric Oxygen Therapy Indications, 14th ed. North

Palm Beach, FL, Best Publishing Company, 2018.

Oley MH et al: Effects of hyperbaric oxygen therapy on vascular endothelial growth factor protein and mRNA in crush injury patients: A

randomized controlled trial study. Int J Surg Open 29:33, 2021.

Sadler C et al: Diving after SARS-CoV-2 (COVID-19) infection: Fitness to dive assessment and medical guidance. Diving Hyperb Med

50:278, 2020.

Vann RD et al: Decompression illness. Lancet 377:153, 2011.

Weaver LK et al: Hyperbaric oxygen for acute carbon monoxide poisoning. N Engl J Med 347:1057, 2002.

Whelan HT, Kindwall EP (eds): Hyperbaric Medicine Practice,

4th ed. Palm Beach, FL, Best Publishing Company, 2017.

3631Hypothermia and Peripheral Cold Injuries CHAPTER 464

disrupts the autonomic pathways that lead to shivering and will prevent

cold-induced reflex vasoconstrictive responses.

Hypothermia associated with sepsis is a poor prognostic sign.

Hepatic failure causes decreased glycogen storage and gluconeogenesis as well as a diminished shivering response. In acute myocardial

infarction associated with low cardiac output, hypothermia may be

reversed after adequate resuscitation. With extensive burns, psoriasis,

erythrodermas, and other skin diseases, increased peripheral-blood

flow leads to excessive heat loss.

■ THERMOREGULATION

Heat loss occurs through five mechanisms: radiation (55–65% of heat

loss), conduction (10–15% of heat loss, increased in cold water), convection (increased in the wind), respiration, and evaporation; both of

the latter two mechanisms are affected by the ambient temperature and

the relative humidity.

The preoptic anterior hypothalamus normally orchestrates thermoregulation (Chap. 18). The immediate defense of thermoneutrality

is via the autonomic nervous system, whereas delayed control is mediated by the endocrine system. Autonomic nervous system responses

include the release of norepinephrine, increased muscle tone, and shivering, leading to thermogenesis and an increase in the basal metabolic

rate. Cutaneous cold thermoreception causes direct reflex vasoconstriction to conserve heat. Prolonged exposure to cold also stimulates

the thyroid axis, leading to an increased metabolic rate.

■ CLINICAL PRESENTATION

In most cases of hypothermia, the history of exposure to environmental factors (e.g., prolonged exposure to the outdoors without adequate

clothing) makes the diagnosis straightforward. In urban settings,

however, the presentation is often more subtle, and other disease processes, toxin exposures, or psychiatric diagnoses should be considered.

Predicting the core temperature based on the clinical presentation is

very difficult.

After initial stimulation by hypothermia, there is progressive

depression of all organ systems. The timing of the appearance of these

clinical manifestations varies widely (Table 464-2). Without knowing the core temperature, it can be difficult to interpret other vital

signs. For example, tachycardia disproportionate to the core temperature suggests secondary hypothermia resulting from hypoglycemia,

hypovolemia, or a toxin overdose. Because carbon dioxide production

declines progressively, the respiratory rate should be low; persistent

hyperventilation suggests a central nervous system (CNS) lesion or

an organic acidosis. A markedly depressed level of consciousness in a

patient with mild hypothermia suggests an overdose or CNS dysfunction due to infection or trauma.

Physical examination findings will also be altered by hypothermia.

For instance, the assumption that areflexia is solely attributable to

hypothermia can obscure the diagnosis of a spinal cord lesion. Patients

with hypothermia may be confused or combative; these symptoms

abate more rapidly with rewarming than with chemical or physical

restraint. A classic example of maladaptive behavior in patients with

hypothermia is paradoxical undressing, which involves the inappropriate removal of clothing in response to a cold stress. The cold-induced

ileus and abdominal rectus spasm can mimic or mask the presentation

of an acute abdomen (Chap. 15).

When a patient in hypothermic cardiac arrest is first discovered,

cardiopulmonary resuscitation (CPR) is indicated unless (1) a do-notresuscitate status is verified, (2) obviously lethal injuries are identified,

or (3) the depression of a frozen chest wall is not possible. Continuous

CPR is normally recommended, and interruptions should be avoided if

possible. In the field, when the core temperature is <28°C, intermittent

CPR may also be effective.

As the resuscitation proceeds, the prognosis is grave if there is evidence of widespread cell lysis, as reflected by potassium levels >10–12

mmol/L (10–12 meq/L). Other findings that may preclude continuing

resuscitation include a core temperature <10–12°C (<50–54°F), a

pH <6.5, and evidence of intravascular thrombosis with a fibrinogen

value <0.5 g/L (<50 mg/dL). The decision to terminate resuscitation

TABLE 464-1 Risk Factors for Hypothermia

Age extremes

Elderly

Neonates

Environmental exposure

Occupational

Sports-related

Inadequate clothing

Immersion

Toxicologic and pharmacologic

Ethanol

Anesthetics

Antipsychotics

Antidepressants

Anxiolytics

Benzodiazepines

Neuromuscular blockers

Insufficient fuel

Malnutrition

Marasmus

Kwashiorkor

Endocrine-related

Diabetes mellitus

Hypoglycemia

Hypothyroidism

Adrenal insufficiency

Hypopituitarism

Neurologic

Cerebrovascular accident

Hypothalamic disorders

Parkinson’s disease

Spinal cord injury

Multisystemic

Trauma

Sepsis

Shock

Hepatic or renal failure

Carcinomatosis

 Burns and exfoliative dermatologic

disorders

Immobility or debilitation

■ CAUSES

Primary accidental hypothermia is geographically and seasonally pervasive. Although most cases occur in the winter months and in colder

climates, this condition is surprisingly common in warmer regions as

well. Multiple variables render individuals at the extremes of age—both

the elderly and neonates—particularly vulnerable to hypothermia

(Table 464-1). The elderly have diminished thermal perception and are

more susceptible to immobility, malnutrition, and systemic illnesses

that interfere with heat generation or conservation. Dementia, psychiatric illness, and socioeconomic factors often compound these problems. Neonates have high rates of heat loss because of their increased

surface-to-mass ratio and their lack of effective shivering and adaptive

behavioral responses. At all ages, malnutrition can contribute to heat

loss because of diminished subcutaneous fat and as a result of depleted

energy stores used for thermogenesis.

Individuals whose occupations or hobbies entail extensive exposure

to cold weather are at increased risk for hypothermia. Military history

is replete with hypothermic tragedies. Hunters, sailors, skiers, and

climbers also are at great risk of exposure, whether it involves injury,

changes in weather, or lack of preparedness.

Ethanol causes vasodilation (which increases heat loss), reduces

thermogenesis and gluconeogenesis, and may impair judgment or lead

to obtundation. Some antipsychotics, antidepressants, anxiolytics, benzodiazepines, and other medications reduce centrally mediated vasoconstriction. Many hypothermic patients are admitted to intensive care

because of drug overdose. Anesthetics can block shivering responses;

these effects are compounded when patients are not insulated adequately in the operating or recovery units.

Several types of endocrine dysfunction cause hypothermia.

Hypothyroidism—particularly when extreme, as in myxedema coma—

reduces the metabolic rate and impairs thermogenesis and behavioral

responses. Adrenal insufficiency and hypopituitarism also increase

susceptibility to hypothermia. Hypoglycemia, most commonly caused

by insulin or oral hypoglycemic agents, is associated with hypothermia,

in part because of neuroglycopenic effects on hypothalamic function.

Increased osmolality and metabolic derangements associated with

uremia, diabetic ketoacidosis, and lactic acidosis can lead to altered

hypothalamic thermoregulation.

Neurologic injury from trauma, cerebrovascular accident, subarachnoid hemorrhage, and a hypothalamic lesion increases susceptibility to

hypothermia. Agenesis of the corpus callosum (Shapiro’s syndrome) is

one cause of episodic hypothermia. In this syndrome, profuse perspiration is followed by a rapid fall in temperature. Acute spinal cord injury

3632 PART 15 Disorders Associated with Environmental Exposures

before rewarming the patient past 33°C (91°F) should be predicated

on the type and severity of the precipitants of hypothermia. Survival

has occurred with a cardiac arrest time over 7 h. There is an ongoing

search for validated prognostic indicators for recovery from hypothermia. The Swiss grading system considers core body temperature

and the clinical findings. Other scoring systems also consider age,

albumin, and lactate levels. A history of asphyxia, as in an avalanche,

with secondary cooling is the most important negative predictor of

survival.

■ DIAGNOSIS AND STABILIZATION

Hypothermia is confirmed by measurement of the core temperature,

preferably at two sites. Rectal probes should be placed to a depth of

15 cm and not adjacent to cold feces. A simultaneous esophageal probe

can be placed 24 cm below the larynx; it may read falsely high during

heated inhalation therapy. Relying solely on infrared tympanic thermography is not advisable.

After a diagnosis of hypothermia is established, cardiac monitoring

should be instituted, along with attempts to limit further heat loss. If

the patient is in ventricular fibrillation, it is unclear at what core temperature ventricular defibrillation (2 J/kg) should first be attempted.

One biphasic attempt below 30°C is warranted. Further defibrillation

attempts should usually be deferred until some rewarming (1°–2°C)

is achieved and ventricular fibrillation is coarser. Although cardiac

pacing for hypothermic bradydysrhythmias is rarely indicated, the

transthoracic technique is preferable. The J or Osborn wave at the

junction of the QRS complex and ST segment suggests the diagnosis.

Obvious J waves are routinely misdiagnosed by automated readings as

injury current.

Supplemental oxygenation is always warranted, since tissue oxygenation is affected adversely by the leftward shift of the oxyhemoglobin

dissociation curve. Pulse oximetry is often unreliable in patients with

vasoconstriction. If protective airway reflexes are absent, gentle endotracheal intubation should be performed. Adequate preoxygenation

will prevent ventricular arrhythmias.

Insertion of a gastric tube prevents dilation secondary to decreased

bowel motility. Indwelling bladder catheters facilitate monitoring

of cold-induced diuresis and can provide an ancillary approach for

temperature monitoring. Dehydration is encountered commonly with

chronic hypothermia, and most patients benefit from an intravenous

or intraosseous crystalloid bolus. Normal saline is preferable to lactated

Ringer’s solution, as the liver in hypothermic patients inefficiently

metabolizes lactate. The placement of a pulmonary artery catheter can

cause perforation of the less compliant pulmonary artery. Insertion of

a central venous catheter deeply into the cold right atrium should be

avoided since this procedure, similar to transvenous pacing, can precipitate refractory arrhythmias.

Arterial blood gases should not be corrected for temperature

(Chap. 55). An uncorrected pH of 7.42 and a Pco2

 of 40 mmHg reflect

appropriate alveolar ventilation and acid-base balance at any core temperature. Acid-base imbalances should be corrected gradually, since

the bicarbonate buffering system is inefficient. A common error is

overzealous hyperventilation in the setting of depressed CO2

 production. When the Pco2

 decreases by 10 mmHg at 28°C (82°F), it doubles

the pH increase of 0.08 that occurs at 37°C (99°F).

The severity of anemia may be underestimated because the hematocrit increases 2% for each 1°C drop in temperature. White blood cell

sequestration and bone marrow suppression are common, potentially

masking an infection. Although hypokalemia is more common in

chronic hypothermia, hyperkalemia also occurs; the expected electrocardiographic changes are often obscured by hypothermia. Patients

with renal insufficiency, metabolic acidosis, or rhabdomyolysis are at

greatest risk for electrolyte disturbances.

Coagulopathies are common because cold inhibits the enzymatic

reactions required for activation of the intrinsic cascade. In addition,

thromboxane B2

 production by platelets is temperature dependent,

and platelet function is impaired. The administration of platelets and

fresh-frozen plasma is therefore not effective. Coagulation studies can

be deceptively normal and contrast with the observed in vivo coagulopathy. This contradiction occurs because all coagulation tests are routinely performed at 37°C (99°F), and the enzymes are thus rewarmed.

■ REWARMING STRATEGIES

The key initial decision is whether to rewarm the patient passively or

actively. Passive external rewarming simply involves covering and insulating the patient in a warm environment. With the head also covered,

the rate of rewarming is usually 0.5°–2°C (1.10°–4.4°F) per hour. This

technique is ideal for previously healthy patients who develop acute,

mild primary accidental hypothermia. The patient must have sufficient

glycogen to support endogenous thermogenesis.

The application of heat directly to the extremities of patients with

chronic severe hypothermia should be avoided because it can induce

peripheral vasodilation and precipitate core temperature “afterdrop,” a

response characterized by a continual decline in the core temperature

TABLE 464-2 Physiologic Changes Associated with Accidental Hypothermia

SEVERITY BODY TEMPERATURE

CENTRAL NERVOUS

SYSTEM CARDIOVASCULAR RESPIRATORY

RENAL AND

ENDOCRINE NEUROMUSCULAR

Mild 35°C (95°F)–32.2°C

(90°F)

Linear depression of

cerebral metabolism;

amnesia; apathy;

dysarthria; impaired

judgment; maladaptive

behavior

Tachycardia,

then progressive

bradycardia; cardiac

cycle prolongation;

vasoconstriction;

increase in cardiac output

and blood pressure

Tachypnea, then

progressive decrease

in respiratory minute

volume; declining

oxygen consumption;

bronchorrhea;

bronchospasm

Diuresis; increase

in catecholamines,

adrenal steroids,

triiodothyronine, and

thyroxine; increase

in metabolism with

shivering

Increased

preshivering muscle

tone, then fatiguing

Moderate <32.2°C (90°F)–28°C

(82.4°F)

EEG abnormalities;

progressive depression

of level of consciousness;

pupillary dilation;

paradoxical undressing;

hallucinations

Progressive decrease in

pulse and cardiac output;

increased atrial and

ventricular arrhythmias;

suggestive (J-wave) ECG

changes

Hypoventilation: 50%

decrease in carbon

dioxide production

per 8°C (17.6°F)

drop in temperature;

absence of protective

airway reflexes

50% increase in renal

blood flow; renal

autoregulation intact;

impaired insulin

action

Hyporeflexia;

diminishing

shivering-induced

thermogenesis; rigidity

Severe <28°C (<82.4°F) Loss of cerebrovascular

autoregulation; decline

in cerebral blood flow;

coma; loss of ocular

reflexes; progressive

decrease in EEG

abnormalities

Progressive decrease

in blood pressure, heart

rate, and cardiac output;

reentrant dysrhythmias;

maximal risk of ventricular

fibrillation; asystole

Pulmonic congestion

and edema; 75%

decrease in oxygen

consumption; apnea

Decrease in renal

blood flow that

parallels decrease

in cardiac output;

extreme oliguria;

poikilothermia; 80%

decrease in basal

metabolism

No motion; decreased

nerve-conduction

velocity; peripheral

areflexia; no corneal

or oculocephalic

reflexes

Abbreviations: ECG, electrocardiogram; EEG, electroencephalogram.

Source: From DF Danzl, RS Pozos: Accidental hypothermia. N Engl J Med 331:1756, 1994. Copyright © 1994 Massachusetts Medical Society. Reprinted with permission from

Massachusetts Medical Society.

3633Hypothermia and Peripheral Cold Injuries CHAPTER 464

after removal of the patient from the cold. Truncal heat application

reduces the risk of afterdrop.

Active rewarming is necessary under the following circumstances:

core temperature <32°C (<90°F) (poikilothermia), cardiovascular instability, age extremes, CNS dysfunction, hormone insufficiency, and

suspicion of secondary hypothermia. Active external rewarming is best

accomplished with forced-air heating blankets. Other options include

devices that circulate water through external heat exchange pads,

radiant heat sources, and hot packs. Monitoring a patient with hypothermia in a heated tub is extremely difficult. Electric blankets should

be avoided because vasoconstricted skin is easily burned.

There are numerous widely available options for active core

rewarming. Airway rewarming with heated humidified oxygen (40°–

45°C [104°–113°F]) via mask or endotracheal tube is a convenient

option. Although airway rewarming provides less heat than do some

other forms of active core rewarming, it eliminates respiratory heat

loss and adds 1°–2°C (2.2°–4.4°F) to the overall rewarming rate. Crystalloids should be heated to 40°–42°C (104°–108°F), but the quantity of

heat provided is significant only during massive volume resuscitation.

The most efficient method for heating and delivering fluid or blood is

with a countercurrent in-line heat exchanger. Heated irrigation of the

gastrointestinal tract or bladder transfers minimal heat because of the

limited available surface area. These methods should be reserved for

patients in cardiac arrest and then used in combination with all available active rewarming techniques.

Closed thoracic lavage is far more efficient in severely hypothermic

patients with cardiac arrest. The hemithoraxes are irrigated through

two inserted large-bore thoracostomy tubes. Thoracostomy tubes

should not be placed in the left chest of a spontaneously perfusing

patient for purposes of rewarming. Peritoneal lavage with the dialysate

at 40°–45°C (104°–113°F) efficiently transfers heat when delivered

through two catheters with outflow suction. Like peritoneal dialysis,

standard hemodialysis is especially useful for patients with electrolyte

abnormalities, rhabdomyolysis, or toxin ingestion. Another option

involves the use of endovascular temperature control catheters.

Extracorporeal blood rewarming options (Table 464-3) should be

considered in severely hypothermic patients, especially those with

primary accidental hypothermia. Extracorporeal life support, including

bypass, should be considered in nonperfusing patients without documented contraindications to resuscitation. Circulatory support may be

the only effective option in patients with completely frozen extremities

or those with significant tissue destruction coupled with rhabdomyolysis. There is no evidence that extremely rapid rewarming improves

survival in perfusing patients.

TREATMENT

Hypothermia

When a patient is hypothermic, target organs and the cardiovascular system respond minimally to most medications. Generally,

medications are withheld below 30°C (86°F). In contrast to antiarrhythmics, low-dose vasopressor medications may improve the

intra-arrest rates of return of spontaneous circulation. Because of

increased binding of drugs to proteins as well as impaired metabolism and excretion, either a lower dose or a longer interval between

doses should be used to avoid toxicity. As an example, the administration of repeated doses of digoxin or insulin would be ineffective

while the patient is hypothermic, but the residual drugs would be

potentially toxic during rewarming.

Achieving a mean arterial pressure of at least 60 mmHg should

be an early objective. If the hypotension is disproportionate for

temperature and does not respond to crystalloid/colloid infusion

and rewarming, low-dose dopamine support (2–5 μg/kg per min)

should be considered. Perfusion of the vasoconstricted cardiovascular system also may improve with low-dose IV nitroglycerin.

Atrial arrhythmias should be monitored initially without intervention, as the ventricular response should be slow and, unless

preexistent, most will convert spontaneously during rewarming.

The role of prophylaxis and treatment of ventricular arrhythmias

is complex. Preexisting ventricular ectopy may be suppressed by

hypothermia and reappear during rewarming. None of the class I

agents is proven to be safe and efficacious.

Initiating empirical therapy for adrenal insufficiency usually is

not warranted unless the history suggests steroid dependence or

hypoadrenalism or efforts to rewarm with standard therapy fail. The

administration of parenteral levothyroxine to euthyroid patients

with hypothermia, however, is potentially hazardous. Because laboratory results can be delayed and confounded by the presence of

the sick euthyroid syndrome (Chap. 382), historic clues or physical

findings suggestive of hypothyroidism should be sought. When

myxedema is the cause of hypothermia, the relaxation phase of the

Achilles reflex is prolonged more than is the contraction phase.

Hypothermia obscures most of the symptoms and signs of infection, notably fever and leukocytosis. Shaking rigors from infection

may be mistaken for shivering. Except in mild cases, extensive

cultures and repeated physical examinations are essential. Unless

an infectious source is identified, empirical antibiotic prophylaxis is

most warranted in the elderly, neonates, and immunocompromised

patients.

FROSTBITE

Peripheral cold injuries include both freezing and nonfreezing injuries

to tissue. Tissue freezes quickly when in contact with thermal conductors such as metal and volatile solutions. Other predisposing factors

include constrictive clothing or boots, immobility, and vasoconstrictive

medications. Frostbite occurs when the tissue temperature drops below

0°C (32°F). Ice-crystal formation subsequently distorts and destroys

the cellular architecture. Once the vascular endothelium is damaged,

stasis progresses rapidly to microvascular thrombosis. After the tissue

thaws, there is progressive dermal ischemia. The microvasculature

begins to collapse, arteriovenous shunting increases tissue pressures,

and edema forms. Finally, thrombosis, ischemia, and superficial necrosis appear. The development of mummification and demarcation may

take weeks to months.

TABLE 464-3 Options for Extracorporeal Blood Rewarming

EXTRACORPOREAL

REWARMING TECHNIQUE CONSIDERATIONS

Continuous venovenous (CVV)

rewarming

Circuit: CV catheter to CV, dual-lumen CV, or

peripheral catheter

No oxygenator/circulatory support

Flow rates 150–400 mL/min

ROR 2°–3°C (4.4°–6.6°F)/h

Hemodialysis Circuit: single- or dual-vessel cannulation

Stabilizes electrolyte or toxicologic

abnormalities

Exchange cycle volumes 200–500 mL/min

ROR 2°–3°C (4.4°–6.6°F)/h

Continuous arteriovenous

rewarming (CAVR)

Circuit: percutaneous 8.5-Fr femoral catheters

Requires systolic blood pressure of 60 mmHg

No perfusionist/pump/anticoagulation

Flow rates 225–375 mL/min

ROR 3°–4°C (6.6°–8.8°F)/h

Cardiopulmonary bypass (CPB) Circuit: full circulatory support with pump and

oxygenator

Perfusate-temperature gradient 5°–10°C

(11°–22° F)

Flow rates 2–7 L/min (average 3–4 L/min)

ROR up to 9.5°C (20.9°F)/h

Venoarterial extracorporeal

membrane oxygenation

(VA-ECMO)

Decreased risk of post-rewarming

cardiorespiratory failure

Improved neurologic outcome

Abbreviations: CV, central venous; ROR, rate of rewarming.

3634 PART 15 Disorders Associated with Environmental Exposures

CLINICAL PRESENTATION

The initial presentation of frostbite can be deceptively benign. The

symptoms always include a sensory deficiency affecting light touch,

pain, or temperature perception. The acral areas and distal extremities

are the most common insensate areas. Some patients describe a clumsy

or “chunk of wood” sensation in the extremity.

Deep frostbitten tissue can appear waxy, mottled, yellow, or

violaceous-white. Favorable presenting signs include some warmth

or sensation with normal color. The injury is often superficial if the

subcutaneous tissue is pliable or if the dermis can be rolled over bony

prominences.

Clinically, frostbite is superficial or deep. Superficial frostbite does

not entail tissue loss but rather causes only anesthesia and erythema.

The appearance of vesiculation surrounded by edema and erythema

implies deeper involvement (Fig. 464-1). Hemorrhagic vesicles reflect

a serious injury to the microvasculature and indicate severe frostbite.

Damages in subcuticular, muscular, or osseous tissues may result in

amputation. An alternative classification establishes grades based on

the location of presenting cyanosis; that is grade 1, absence of cyanosis;

grade 2, cyanosis on the distal phalanx; grade 3, cyanosis up to the

metacarpophalangeal (MP) joint; and grade 4 cyanosis proximal to the

MP joint.

The two most common nonfreezing peripheral cold injuries are

chilblain (pernio) and immersion (trench) foot. Chilblain results from

neuronal and endothelial damage induced by repetitive exposure to

damp cold above the freezing point. Young females, particularly those

with a history of Raynaud’s phenomenon, are at greatest risk. Persistent

vasospasticity and vasculitis can cause erythema, mild edema, and pruritus. Eventually plaques, blue nodules, and ulcerations develop. These

TABLE 464-4 Treatment for Frostbite

BEFORE THAWING DURING THAWING AFTER THAWING

Remove from

environment.

Consider parenteral

analgesia and ketorolac.

Gently dry and protect part;

elevate; place pledgets

between toes, if macerated.

Prevent partial

thawing and

refreezing.

Administer ibuprofen

(400 mg PO).

If clear vesicles are intact,

aspirate sterilely; if broken,

debride and dress with

antibiotic or sterile aloe vera

ointment.

Stabilize core

temperature and treat

hypothermia.

Immerse part in

37°–40°C (99°–104°F)

(thermometer-monitored)

circulating water

containing an antiseptic

soap until distal flush

(10–45 min).

Leave hemorrhagic vesicles

intact to prevent desiccation

and infection.

Protect frozen

part—no friction or

massage.

Encourage patient to

gently move part.

Continue ibuprofen

(400–600 mg PO [12 mg/kg

per day] q8 to 12h).

Address medical or

surgical conditions.

If pain is refractory,

reduce water

temperature to 35°–37°C

(95°–99°F) and administer

parenteral narcotics.

Consider tetanus and

streptococcal prophylaxis;

elevate part.

Administer hydrotherapy at

37°C (99°F).

Consider dextran or

phenoxybenzamine or, in

severe cases, thrombolysis

rt-PA (IV or intraarterial).

FIGURE 464-1 Frostbite with vesiculation, surrounded by edema and erythema. Abbreviation: rt-PA, recombinant tissue plasminogen activator.

lesions typically involve the dorsa of the hands and feet. In contrast,

immersion foot results from repetitive exposure to wet cold above the

freezing point. The feet initially appear cyanotic, cold, and edematous.

The subsequent development of bullae is often indistinguishable from

frostbite. This vesiculation rapidly progresses to ulceration and liquefaction gangrene. Patients with milder cases report hyperhidrosis, cold

sensitivity, and painful ambulation for many years.

TREATMENT

Peripheral Cold Injuries

When frostbite accompanies hypothermia, hydration may improve

vascular stasis. Frozen tissue should be thawed rapidly and completely by immersion in circulating water at 37°–40°C (99°–104°F)

for 30–60 min and not by using hot air. Rapid rewarming often

produces an initial hyperemia. The early formation of large clear

distal blebs is more favorable than that of smaller proximal dark

hemorrhagic blebs. A common error is the premature termination

of thawing, since the reestablishment of perfusion is intensely painful. Parenteral narcotics will be necessary with deep frostbite. If

cyanosis persists after rewarming, the tissue compartment pressures

should be monitored carefully.

Many antithrombotic and vasodilatory treatment regimens have

been evaluated. The prostacyclin analogue iloprost given within

48 h after rewarming is an option. There is no conclusive evidence

that sympathectomy, steroids, calcium channel blockers, or hyperbaric oxygen salvages tissue.

Patients who have deep frostbite injuries with the potential for

significant morbidity should be considered for intravenous or

intraarterial thrombolytic therapy. Angiography or pyrophosphate

scanning may help evaluate the injury and monitor the progress

of tissue plasminogen activator therapy (rt-PA). Heparin is recommended as adjunctive therapy. Intraarterial thrombolysis may

reduce the need for digital and more proximal amputations when

administered within 24 h of severe injuries. A treatment protocol

for frostbite is summarized in Table 464-4.

Unless infection develops, any decision regarding debridement or amputation should generally be deferred. Angiography or

3635Heat-Related Illnesses CHAPTER 465

technetium-99 bone scan may assist in the determination of surgical margins. Magnetic resonance angiography may also demonstrate the line of demarcation earlier than does clinical demarcation.

The most common symptomatic sequelae reflect neuronal injury

and persistently abnormal sympathetic tone, including paresthesia, thermal misperception, and hyperhidrosis. Delayed findings

include nail deformities, cutaneous carcinomas, and epiphyseal

damage in children.

Management of the chilblain syndrome is usually supportive.

With refractory perniosis, alternatives include nifedipine, steroids,

and limaprost, a prostaglandin E1

 analogue.

■ FURTHER READING

Cauchy E et al: A controlled trial of a prostacyclin and rt-PA in the

treatment of severe frostbite. N Engl J Med 364:189, 2011.

McIntosh SE et al: Wilderness Medical Society practice guidelines for

the prevention and treatment of frostbite. Wilderness Environ Med

25(4 suppl):S43, 2014.

Ohbe H et al: Extracorporeal membrane oxygenation improves outcomes of accidental hypothermia without vital signs: A nationwide

observational study. Resuscitation 144:27, 2019.

Okada Y et al: The development and validation of a “5A” severity scale

for predicting in-hospital mortality after accidental hypothermia

from J-point registry data. J Intensive Care 7:27, 2019.

Zafren K: Out-of-hospital evaluation and treatment of accidental

hypothermia. Emerg Med Clin North Am 35:261, 2017.

Heat-related illnesses include a spectrum of disorders ranging from

heat syncope, muscle cramps, and heat exhaustion to medical emergencies such as heatstroke. The core body temperature is normally

maintained within a very narrow range. Although significant levels of

hypothermia are tolerated (Chap. 464), multiorgan dysfunction occurs

rapidly at temperatures >41°–43°C. In contrast to heatstroke, the far

more common sign of fever reflects intact thermoregulation.

■ THERMOREGULATION

Humans are capable of significant heat generation. Strenuous exercise

can increase heat generation twentyfold. The heat load from metabolic

heat production and environmental heat absorption is balanced by a

variety of heat dissipation mechanisms. These central integrative dissipation pathways are orchestrated by the central thermostat, which is

located in the preoptic nucleus of the anterior hypothalamus. Efferent

signals sent via the autonomic nervous system trigger cutaneous vasodilation and diaphoresis to facilitate heat loss.

Normally, the body dissipates heat into the environment via four

mechanisms. The evaporation of skin moisture is the single most efficient mechanism of heat loss but becomes progressively ineffective as

the relative humidity rises to >70%. The radiation of infrared electromagnetic energy directly into the surrounding environment occurs

continuously. (Conversely, radiation is a major source of heat gain in hot

climates.) Conduction—the direct transfer of heat to a cooler object—

and convection—the loss of heat to air currents—become ineffective

when the environmental temperature exceeds the skin temperature.

Factors that interfere with the evaporation of diaphoresis significantly

increase the risk of heat illness. Examples include dripping of sweat off

the skin, constrictive or occlusive clothing, dehydration, and excessive

humidity. While air is an effective insulator, the thermal conductivity

of water is 25 times greater than that of air at the same temperature.

465 Heat-Related Illnesses

Daniel F. Danzl

The wet-bulb globe temperature is a commonly used index to assess

the environmental heat load. This calculation considers the ambient

air temperature, the relative humidity, and the degree of radiant heat.

The regulation of this heat load is complex and involves the central nervous system (CNS), thermosensors, and thermoregulatory

effectors. The central thermostat activates the effectors that produce

peripheral vasodilation and sweating. The skin surface is in effect the

radiator and the principal location of heat loss, since skin blood flow

can increase 25–30 times over the basal rate. This dramatic increase in

skin blood flow, coupled with the maintenance of peripheral vasodilation, efficiently radiates heat. At the same time, there is a compensatory

vasoconstriction of the splanchnic and renal beds.

Acclimatization to heat reflects a constellation of physiologic adaptations that permit the body to lose heat more efficiently. This process

often requires one to several weeks of exposure and work in a hot

environment. During acclimatization, the thermoregulatory set point

is altered, and this alteration affects the onset, volume, and content

of diaphoresis. The threshold for the initiation of sweating is lowered,

and the amount of sweat increases, with a lowered salt concentration.

Sweating rates can be 1–2 L/h in acclimated individuals during heat

stress. Plasma volume expansion also occurs and improves cutaneous

vascular flow. The heart rate lowers, with a higher stroke volume.

After the individual leaves the hot environment, improved tolerance

to heat stress dissipates rapidly, the plasma volume decreases, and deacclimatization occurs within weeks.

■ PREDISPOSING FACTORS AND

DIFFERENTIAL DIAGNOSIS

When there is an excessive heat load, unacclimated individuals can

develop a variety of heat-related illnesses. Heat waves exacerbate the

mortality rate, particularly among the elderly and among persons

lacking adequate nutrition and access to air-conditioned environments.

Secondary vascular events, including cerebrovascular accidents and

myocardial infarctions, occur at least 10 times more often in conditions

of extreme heat.

Exertional heat illness continues to occur when laborers, military

personnel, or athletes exercise strenuously in the heat. In addition to

the very young and very old, preadolescents and teenagers are at risk

since they may use poor judgment when vigorously exercising in high

humidity and heat. Other risk factors include obesity, poor conditioning with lack of acclimatization, and mild dehydration.

Cardiovascular inefficiency is a common feature of heat illness. Any

physiologic or pharmacologic impediment to cutaneous perfusion

impairs heat loss. Many patients are unaware of the heat risk associated with their medications. Anticholinergic agents impair sweating

and blunt the normal cardiovascular response to heat. Phenothiazines

and heterocyclic antidepressants also have anticholinergic properties

that interfere with the function of the preoptic nucleus of the anterior

hypothalamus due to central depletion of dopamine.

Calcium channel blockers, beta blockers, and various stimulants

also inhibit sweating by reducing peripheral blood flow. To maintain

the mean arterial blood pressure, increased cardiac output must be

capable of compensating for progressive dehydration. A variety of

stimulants and substances of abuse also increase muscle activity and

heat production.

Careful consideration of the differential diagnosis is important

in the evaluation of a patient for a potential heat-related illness. The

clinical setting may suggest other etiologies, such as malignant hyperthermia after general anesthesia. Neuroleptic malignant syndrome can

be triggered by certain antipsychotic medications, including selective

serotonin reuptake inhibitors. A variety of infectious and endocrine

disorders as well as toxicologic or CNS etiologies may mimic heatstroke (Table 465-1).

■ MINOR HEAT-EMERGENCY SYNDROMES

Heat edema is characterized by mild swelling of the hands, feet, and

ankles during the first few days of significant heat exposure. The principal mechanism involves cutaneous vasodilation and pooling of interstitial fluid in response to heat stress. Heat also increases the secretion

3636 PART 15 Disorders Associated with Environmental Exposures

or lotion provides some relief. In adults, localized areas may benefit

from 1% salicylic acid TID, with caution taken to avoid salicylate

intoxication. Clothing with breathable fabric should be clean and loose

fitting, and activities or environments that induce diaphoresis should

be avoided.

Heat syncope (exercise-associated collapse) can follow endurance

exercise or occur in the elderly. Other common clinical scenarios

include prolonged standing while stationary in the heat and sudden

standing after prolonged exposure to heat. Heat stress routinely causes

relative volume depletion, decreased vasomotor tone, and peripheral

vasodilation. The cumulative effect of this decrease in venous return

is postural hypotension, especially in nonacclimated elderly individuals. Many of those affected also have comorbidities. Therefore, other

cardiovascular, neurologic, and metabolic causes of syncope should

be considered. After removal from the heat source, most patients will

recover promptly with cooling and rehydration.

Hyperventilation tetany occurs in some individuals when exposure

to heat stimulates hyperventilation, producing respiratory alkalosis,

paresthesia, and carpopedal spasm. Unlike heat cramps, heat tetany

causes very little muscle-compartment pain. Treatment includes providing reassurance, moving the patient out of the heat, and addressing

the hyperventilation.

■ HEAT CRAMPS

Heat cramps (exercise-associated muscle cramps) are intermittent,

painful, and involuntary spasmodic contractions of skeletal muscles.

They typically occur in an unacclimated individual who is at rest after

vigorous exertion in a humid, hot environment. In contrast, cramps

that occur in athletes during exercise last longer, are relieved by stretching and massage, and resolve spontaneously.

Of note, not all muscle cramps are related to exercise, and the differential diagnosis includes many other disorders. A variety of medications, myopathies, endocrine disorders, and sickle cell trait are other

possible causes.

The typical patient with heat cramps is usually profusely diaphoretic

and has been replacing fluid losses with copious water or other hypotonic fluids. Roofers, firefighters, military personnel, athletes, steel

workers, and field workers are commonly affected. Other predisposing

factors include insufficient sodium intake before intense activity in the

heat and lack of heat acclimatization, resulting in sweat with a high salt

concentration.

The precise pathogenesis of heat cramps appears to involve a relative deficiency of sodium, potassium, and fluid at the intracellular

level. Coupled with copious hypotonic fluid ingestion, large amounts

of sodium in the diaphoresis cause hyponatremia and hypochloremia,

resulting in muscle cramps due to calcium-dependent muscle relaxation. Total-body depletion of potassium may be observed during the

period of heat acclimatization. Rhabdomyolysis is very rare with routine exercise-associated muscle cramps.

Heat cramps that are not accompanied by significant dehydration

can be treated with commercially available electrolyte solutions.

Although the flavored electrolyte solutions are far more palatable, two

650-mg salt tablets dissolved in 1 quart of water produce a 0.1% saline

solution. Individuals should avoid the ingestion of undissolved salt

tablets, which are a gastric irritant and may induce vomiting.

■ HEAT EXHAUSTION

The physiologic hallmarks of heat exhaustion—in contrast to

heatstroke—are the maintenance of thermoregulatory control and

CNS function. The core temperature is usually elevated but is generally

<40.5°C (<105°F). The two physiologic precipitants are water depletion

and sodium depletion, which often occur in combination. Laborers,

athletes, and elderly individuals exerting themselves in hot environments, without adequate fluid intake, tend to develop water-depletion

heat exhaustion. Persons working in the heat frequently consume only

two-thirds of their net water loss and are voluntarily dehydrated. In

contrast, salt-depletion heat exhaustion occurs more slowly in unacclimated persons who have been consuming large quantities of hypotonic

solutions.

TABLE 465-1 Heat-Related Illness: Predisposing Factors and

Differential Diagnosis

ILLNESS PREDISPOSING FACTORS

Cardiovascular inefficiency Age extremes

Beta/calcium channel blockade

Congestive heart failure

Dehydration

Diuresis

Obesity

Poor physical fitness

Central nervous system illness Cerebellar injury

Cerebral hemorrhage

Hypothalamic cerebrovascular accident

Psychiatric disorders

Status epilepticus

Impaired heat loss Antihistamines

Heterocyclic antidepressants

Occlusive clothing

Skin abnormalities

Endocrine and immune-related

illness

Diabetic ketoacidosis

Multiple-organ dysfunction syndrome

Pheochromocytoma

Systemic inflammatory response syndrome

Thyroid storm

Excessive heat load Environmental conditions

Exertion

Fever

Hypermetabolic state

Lack of acclimatization

Infectious illness Cerebral abscess

Encephalitis

Malaria

Meningitis

Sepsis syndrome

Tetanus

Typhoid

Toxicologic illness Amphetamines

Anticholinergic toxidrome

Cocaine

Dietary supplements

Hallucinogens

Malignant hyperthermia

Neuroleptic malignant syndrome

Salicylates

Serotonin syndrome

Strychnine

Sympathomimetics

Withdrawal syndromes (ethanol, hypnotics)

of antidiuretic hormone and aldosterone. Systemic causes of edema,

including cirrhosis, nephrotic syndrome, and congestive heart failure,

can usually be excluded by the history and physical examination. Heat

edema generally resolves without treatment in several days. Simple

leg elevation or compression stockings will usually suffice. Diuretics

are not effective and, in fact, predispose to volume depletion and the

development of more serious heat-related illnesses.

Prickly heat (miliaria rubra, lichen tropicus) is a maculopapular,

pruritic, erythematous rash that commonly occurs in clothed areas.

Blockage of the sweat pores by debris from macerated stratum corneum causes inflammation in the sweat ducts. As the ducts dilate, they

rupture and produce superficial vesicles. The predominant symptom is

pruritus. In addition to antihistamines, chlorhexidine in a light cream

3637Heat-Related Illnesses CHAPTER 465

Heat exhaustion is usually a diagnosis of exclusion because of the

multitude of nonspecific symptoms. If any signs of heatstroke are

present, rapid cooling and crystalloid resuscitation should be initiated

immediately during stabilization and evaluation. Mild neurologic

and gastrointestinal influenza-like symptoms are common. These

symptoms may include headache, vertigo, ataxia, impaired judgment,

malaise, dizziness, nausea, and muscle cramps. Orthostatic hypotension and sinus tachycardia develop frequently. More significant CNS

impairment suggests heatstroke or other infectious, neurologic, or

toxicologic diagnoses.

Hemoconcentration does not always develop, and rapid infusion of

isotonic IV fluids should be guided by frequent electrolyte determinations and perfusion requirements. Most cases of heat exhaustion reflect

mixed sodium and water depletion. Sodium-depletion heat exhaustion is characterized by hyponatremia and hypochloremia. Hepatic

aminotransferases are mildly elevated in both types of heat exhaustion.

Urinary sodium and chloride concentrations are usually low.

Some patients with heat exhaustion develop heatstroke after removal

from the heat-stress environment. Aggressive cooling of nonresponders is indicated until their core temperature is 39°C (102.2°F). Except

in mild cases, free water deficits should be replaced slowly over 24–48 h

to avoid a decrease of serum osmolality by >2 mOsm/h.

The disposition of younger, previously healthy heat-exhaustion

patients who have no major laboratory abnormalities may include

hospital observation and discharge after IV rehydration. Older patients

with comorbidities (including cardiovascular disease) or predisposing

factors often require inpatient fluid and electrolyte replacement, monitoring, and reassessment.

■ HEATSTROKE

The clinical manifestations of heatstroke reflect a total loss of thermoregulatory function. Typical vital-sign abnormalities include tachypnea, various tachycardias, hypotension, and a widened pulse

pressure. Although there is no single specific diagnostic test, the historical and physical triad of exposure to a heat stress, CNS dysfunction,

and a core temperature >40.5°C helps establish the preliminary diagnosis. Some patients with impending heatstroke will initially appear lucid.

The definitive diagnosis should be reserved until the other potential

causes of hyperthermia are excluded. Many of the usual laboratory

abnormalities seen with heatstroke overlap with other conditions. If

the patient’s mental status does not improve with cooling, toxicologic

screening may be indicated, and cranial CT and spinal fluid analysis

can be considered.

The premonitory clinical characteristics may be nonspecific and

include weakness, dizziness, disorientation, ataxia, and gastrointestinal

or psychiatric symptoms. These prodromal symptoms often resemble

heat exhaustion. The sudden onset of heatstroke occurs when the maintenance of adequate perfusion requires peripheral vasoconstriction to

stabilize the mean arterial blood pressure. As a result, the cutaneous

radiation of heat ceases. At this juncture, the core temperature rises

dramatically. Since many patients with heatstroke also meet the criteria

for systemic inflammatory response syndrome (SIRS) and have a broad

differential diagnosis, rapid cooling is essential during the extensive

diagnostic evaluation. Heat-induced SIRS reflects the responses of both

the innate and the adaptive immune systems (Table 465-1).

There are two forms of heatstroke with significantly different manifestations (Table 465-2). Classic (epidemic) heatstroke (CHS) usually

occurs during long periods of high ambient temperature and humidity,

as during summer heat waves. Patients with CHS commonly have

chronic diseases that predispose to heat-related illness, and they may

have limited access to oral fluids. Heat dissipation mechanisms are

overwhelmed by both endogenous heat production and exogenous heat

stress. Patients with CHS are often compliant with prescribed medications that can impair tolerance to a heat stress. In many of these dehydrated CHS patients, sweating has ceased and the skin is hot and dry.

If cooling is delayed, severe hepatic dysfunction, renal failure, disseminated intravascular coagulation, and fulminant multisystem organ

failure may occur. Hepatocytes are very heat sensitive. On presentation, the serum level of aspartate aminotransferase (AST) is routinely

elevated. Eventually, levels of both AST and alanine aminotransferase

(ALT) often increase to >100 times the normal values. Coagulation studies commonly demonstrate decreased platelets, fibrinogen,

and prothrombin. Most patients with CHS require cautious crystalloid resuscitation, electrolyte monitoring, and—in certain refractory

cases—consideration of central venous pressure (CVP) measurements.

Hypernatremia is secondary to dehydration in CHS. Many patients

exhibit significant stress leukocytosis, even in the absence of infection.

Patients with exertional heatstroke (EHS), in contrast to those with

CHS, are often young and previously healthy, and their diagnosis is

usually more obvious from the history. Athletes, laborers, and military recruits are common victims. Unlike those with CHS, many EHS

patients present profusely diaphoretic despite significant dehydration.

As a result of muscular exertion, rhabdomyolysis and acute renal failure are more common in EHS. Studies to detect rhabdomyolysis and

its complications, including hypocalcemia and hyperphosphatemia,

should be considered. Hyponatremia, hypoglycemia, and coagulopathies are frequent findings. Elevated creatine kinase and lactate dehydrogenase levels also suggest EHS. Oliguria is a common finding. Renal

failure can result from direct thermal injury, untreated rhabdomyolysis,

or volume depletion. Common urinalysis findings include microscopic

hematuria, myoglobinuria, and granular or red cell casts.

With both CHS and EHS, heat-related increases in cardiac biomarker

levels may be present and reversible. Heatstroke often causes thermal

cardiomyopathy. As a result, the CVP may be elevated despite significant dehydration. In addition, the patient often presents with potentially deceptive noncardiogenic pulmonary edema and basilar rales

despite being significantly hypovolemic. The electrocardiogram commonly displays a variety of tachyarrhythmias, nonspecific ST-T wave

changes, and heat-related ischemia or infarction. Rapid cooling—not

the initial administration of antiarrhythmic medications—is essential.

Above 42°C (107.6°F), heat can rapidly produce direct cellular

injury. Thermosensitive enzymes become nonfunctional, and eventually, there is irreversible uncoupling of oxidative phosphorylation. The

production of heat-shock proteins increases, and cytokines mediate

a systemic inflammatory response. The vascular endothelium is also

damaged, and this injury activates the coagulation cascade. Significant

shunting away from the splanchnic circulation produces gastrointestinal ischemia. Endotoxins further impair normal thermoregulation. As

a result, if cooling is delayed, severe hepatic dysfunction, permanent

renal failure, disseminated intravascular coagulation, and fulminant

multisystem organ failure may occur.

■ COOLING STRATEGIES

Before cooling is initiated, endotracheal intubation and continuous core-temperature monitoring should be considered. Peripheral

methods to measure temperature are not reliable. Hypoglycemia is

TABLE 465-2 Typical Manifestations of Heatstroke

CLASSIC EXERTIONAL

Older patient Younger patient

Predisposing health factors/

medications

Healthy condition

Epidemiology (heat waves) Sporadic cases

Sedentary Exercising

Anhidrosis (possible) Diaphoresis (common)

Central nervous system dysfunction Myocardial/hepatic injury

Oliguria Acute renal failure

Coagulopathy (mild) Disseminated intravascular

coagulation

Mild lactic acidosis Marked lactic acidosis

Mild creatine kinase elevation Rhabdomyolysis

Normoglycemia/calcemia Hypoglycemia/calcemia

Normokalemia Hyperkalemia

Normonatremia Hyponatremia

3638 PART 15 Disorders Associated with Environmental Exposures

a frequent finding and can be addressed by glucose infusion. Since

peripheral vasoconstriction delays heat dissipation, repeated administration of discrete boluses of isotonic crystalloid for hypotension is

preferable to the administration of α-adrenergic agonists.

Evaporative cooling is frequently the most practical and effective

technique. Rapid cooling is essential in both CHS and EHS, and an

immediate improvement in vital signs and mental status may prove

valuable for diagnostic purposes. Cool water (15°C [60°F]) is sprayed

on the exposed skin while fans direct continuous airflow over the

moistened skin. Cold packs applied to the neck, axillae, and groin are

useful cooling adjuncts. If cardiac electrodes will not adhere, they can

be applied to the patient’s back.

Immersion cooling in ice-cold water is an alternative option in EHS

but can induce peripheral vasoconstriction and shivering. The initial

increase in temperature from peripheral vasoconstriction will rapidly

be overcome by the large conductive thermal transfer into cold water.

This technique presents significant monitoring and resuscitation challenges in many clinical settings. The safety of immersion cooling is

best established for young, previously healthy patients with EHS (but

not for those with CHS). To avoid hypothermic afterdrop (continued

cooling after immersion), active cooling should be terminated at

~38°–39°C (100.4°F–102.2°F).

Cooling with commercially available cooling blankets should not be

the sole technique used, since the rate of cooling is far too slow. Other

methods are less efficacious and rarely indicated, such as IV infusion

of cold fluids and cold irrigation of the bladder or gastrointestinal

tract. Cold thoracic and peritoneal lavage are efficient maneuvers but

are invasive and rarely necessary. Endovascular cooling also provides

effective cooling.

■ RESUSCITATION

Aspiration commonly occurs in heatstroke, and endotracheal intubation is usually necessary. Depolarizing agents should be avoided. The

metabolic demands are high, and supplemental oxygenation is essential

due to hypoxemia induced by thermal stress and pulmonary dysfunction. The oxyhemoglobin dissociation curve is shifted to the right.

Pneumonitis, pulmonary infarction, hemorrhage, edema, and acute

respiratory distress syndrome occur frequently in heatstroke patients.

Seizures are common and can occur during therapeutic cooling. Cold

induced tonic-clonic muscular rigidity mimics seizure activity.

The circulatory fluid requirements, particularly in CHS, may be

deceptively modest. Aggressive cooling and modest volume repletion

usually elevate the CVP to 12–14 mmHg. The reading, however, may

be deceptive. Many patients present with a thermally induced hyperdynamic circulation accompanied by a high cardiac index, low peripheral

vascular resistance, and an elevated CVP caused by right-sided heart

failure. In contrast, most patients with EHS require far more zealous

isotonic crystalloid resuscitation.

The hypotension that is initially common among patients with heatstroke results from both dehydration and high-output cardiac failure

caused by peripheral vasodilation. Inotropes causing α-adrenergic

stimulation (e.g., norepinephrine) can impede cooling by causing significant vasoconstriction. Vasoactive catecholamines such as dopamine or dobutamine may be necessary if the cardiac output remains

depressed despite an elevated CVP, particularly in patients with a

hyperdynamic circulation.

A wide variety of tachyarrhythmias are routinely observed on

presentation and usually resolve spontaneously during cooling. The

administration of atrial or ventricular antiarrhythmic medications is

rarely indicated during cooling. Anticholinergic medications (including atropine) inhibit sweating and should be avoided. With a cardiac

rhythm that sustains perfusion, electrical cardioversion of the hyperthermic myocardium should be deferred until the myocardium is

cooled. Significant shivering, discomfort, or extreme agitation is preferably mitigated with short-acting benzodiazepines, which are ideal

due to their renal clearance. On the other hand, chlorpromazine may

lower the seizure threshold, has anticholinergic properties, and can

exacerbate the hypotension or cause neuroleptic malignant syndrome.

With hepatic dysfunction, barbiturates should be avoided and seizures

treated with benzodiazepines.

Coagulopathies more commonly occur after the first day of illness. After cooling, the patient should be monitored for disseminated

intravascular coagulation, and replacement therapy with fresh-frozen

plasma and platelets should be considered.

There is no therapeutic role for antipyretics in the control of environmentally induced hyperthermia; these drugs block the actions of

pyrogens at hypothalamic receptor sites. Salicylates can further uncouple oxidative phosphorylation in heatstroke and exacerbate coagulopathies. Acetaminophen may further stress hepatic function. The safety

and efficacy of dantrolene are not established. Although aminocaproic

acid impedes fibrinolysis, it may cause rhabdomyolysis and is not recommended in heatstroke.

■ DISPOSITION

Most patients with minor heat-emergency syndromes (including heat

edema, heat syncope, and heat cramps) require only stabilization and

treatment with outpatient follow-up. Although there are no decision

rules to guide disposition choices in heat exhaustion, many of these

patients have multiple predisposing factors and comorbidities that will

require prolonged observation or hospital admission.

Essentially all patients with actual heatstroke require admission to a

monitored setting, and most require intensive care. There are reports

of very high survival rates of patients following prehospital immersion

cooling without intensive care. Most or all of these patients appear to

have had heat exhaustion. Many actual heatstroke patients also require

prolonged tracheal intubation, invasive hemodynamic monitoring,

and support for various degrees of multiorgan dysfunction syndrome.

The prognosis worsens if the initial core temperature exceeds 42°C

(107.6°F) or if there was a prolonged period during which the core

temperature exceeded this level. Other features of a negative prognosis

include acute renal failure, massively elevated liver enzymes, and significant hyperkalemia. As expected, the number of dysfunctional organ

systems also correlates directly with mortality risk.

■ FURTHER READING

Balmain BN et al: Aging and thermoregulatory control: The clinical

implications of exercising under heat stress in older individuals.

BioMed Res Int 2018:8306154, 2018.

Casa DJ et al: National Athletic Trainers’ Association position statement: Exertional heat illnesses. J Athl Train 50:986, 2015.

Hosokawa Y et al: Inconsistency in the standard of care-toward

evidence-based management of exertional heat stroke. Front Physiol

10:108, 2019.

Lawton EM et al: Review article: Environmental heatstroke and

long-term clinical neurological outcomes: A literature review of case

reports and case series 2000-2016. Emerg Med Australas 31:163,

2019.

Leon LR et al: Pathophysiology of heat-related illnesses, in Auerbach’s

Wilderness Medicine, 7th ed. PS Auerbach et al (eds): Philadelphia,

Elsevier, 2017, pp. 249–267.

Lipman GS et al: Wilderness Medical Society practice guidelines for

the prevention and treatment of heat-related illness. Wilderness

Environ Med 24:351, 2013.

Platt M et al: Heat illness, in Rosen’s Emergency Medicine: Concepts

and Clinical Practice, 9th ed. Walls RM et al (eds). Philadelphia,

Elsevier, 2018, pp. 1755–1764.

Genes, the Environment, and Disease PART 16

Principles of Human

Genetics

J. Larry Jameson, Peter Kopp

466

IMPACT OF GENETICS AND GENOMICS

ON MEDICAL PRACTICE

Human genetics refers to the study of individual genes, their role

and function in disease, and their mode of inheritance. Genomics

refers to an organism’s entire genetic information, the genome, and

the function and interaction of DNA within the genome, as well as

with environmental or nongenetic factors, such as a person’s lifestyle.

With the characterization of the human genome, genomics not only

complements traditional genetics in our efforts to elucidate the etiology and pathogenesis of disease, but it now plays a prominent and

continuously expanding role in diagnostics, prevention, and therapy

(Chap. 467). These transformative developments, originally emerging from the Human Genome Project, have been variably designated

genomic medicine, personalized medicine, or precision medicine. Precision medicine aims at customizing medical decisions to an individual

patient. For example, a patient’s genetic characteristics (genotype)

can be used to optimize drug therapy and predict efficacy, adverse

events, and drug dosing of selected medications (pharmacogenomics)

(Chap. 68). The characterization of the mutational profile of a malignancy allows the identification of driver mutations or overexpressed

signaling molecules, which then facilitates the selection of targeted

therapies. Genome-wide polygenic risk scores (PRS) for common

complex diseases are also beginning to emerge and potentially impact

disease prevention.

Genetics has traditionally been viewed through the window of

relatively rare single-gene diseases. These disorders account for

~10% of pediatric admissions and childhood mortality. Historically,

genetics has focused predominantly on chromosomal and metabolic

disorders, reflecting the long-standing availability of techniques to

diagnose these conditions. For example, conditions such as trisomy

21 (Down’s syndrome) or monosomy X (Turner’s syndrome) can be

diagnosed using cytogenetics. Likewise, many metabolic disorders

(e.g., phenylketonuria, familial hypercholesterolemia) are diagnosed

using biochemical analyses. The advances in DNA and RNA diagnostics have extended the field of genetics to include virtually all medical

specialties and have led to the elucidation of the pathogenesis of the

majority of monogenic disorders. In addition, it is apparent that virtually every medical condition has a genetic component. As is often

evident from a patient’s family history, many common disorders such

as hypertension, heart disease, asthma, diabetes mellitus, and mental

illnesses are significantly influenced by the genetic background. These

polygenic or multifactorial (complex) disorders involve the contributions of many different genes, as well as environmental factors that can

modify disease risk. Genome-wide association studies (GWAS) have

elucidated numerous disease-associated loci and are providing novel

insights into the allelic architecture of complex traits. These studies

have been facilitated by the availability of comprehensive catalogues

of human single nucleotide polymorphism (SNP) haplotypes (HapMap, International Genome Sample Resource/1000 Genomes Project).

Next-generation DNA sequencing (NGS) technologies have evolved

rapidly, and the cost of sequencing whole exomes (the exons within

the genome; whole exome sequencing [WES]) or genomes (whole

genome sequencing [WGS]) has plummeted. Comprehensive unbiased

sequence analyses are now frequently used to characterize individuals

with complex undiagnosed conditions or to determine the mutational

profile of advanced malignancies in order to select targeted therapies.

The routine assembly of diploid genomes, which can reveal a comprehensive spectrum of human genetic variation, will be possible in the

near future and should provide further insights into heritability and

disease mechanisms.

Cancer has a genetic basis because it results from acquired somatic

mutations in genes controlling growth, apoptosis, and cellular differentiation (Chap. 71). In addition, the development of many cancers

is associated with a hereditary predisposition. Characterization of the

genome (and epigenome) in various malignancies has led to fundamental new insights into cancer biology and reveals that the genomic

profile of mutations is in many cases more important in determining

the appropriate therapy than the organ in which the tumor originates.

The Cancer Genome Atlas (TCGA) initiative of the National Cancer

Institute and the National Human Genome Research Institute has

already characterized the genomic landscape of >30 malignancies.

TCGA consists of comprehensive analyses of genomic and proteomic

alterations and is providing fundamental new insights into the molecular pathogenesis of cancer. These data, together with comprehensive

catalogues of somatic mutations identified in human cancer, have

direct clinical ramifications that impact cancer taxonomy, as well as the

development and choice of targeted therapies.

Genetic and genomic approaches have proven invaluable for the

detection of infectious pathogens and are used clinically to identify

agents that are difficult to culture such as mycobacteria, viruses, and

parasites, or to track infectious agents locally or globally. In many

cases, molecular genetics has improved the feasibility and accuracy of

diagnostic testing and is beginning to open new avenues for therapy,

including gene and cellular therapies (Chap. 470). Molecular genetics

has also provided the opportunity to characterize the microbiome,

a new field that characterizes the population dynamics of bacteria,

viruses, and parasites that coexist with humans and other animals

(Chap. 471). Emerging data indicate that the microbiome has significant effects on normal physiology as well as various disease states, and

the field is now focusing on defining the mechanisms underlying these

interactions.

Molecular biology has significantly changed the treatment of human

disease. Peptide hormones, growth factors, cytokines, and vaccines

can be produced in large amounts using recombinant DNA and RNA

technology (e.g., mRNA vaccines against SARS-CoV-2). Targeted

modifications of recombinant peptides provide improved therapeutic

tools, as illustrated by genetically modified insulin analogues with

more favorable kinetics.

The astounding rate at which new genetic and genomic information

is being generated has led to major challenges for physicians, health

care providers, and basic investigators. Although many functional

aspects of the genome remain unknown, there are many clinical

situations where sufficient evidence exists for the use of genetic and

genomic information to optimize patient care and treatment. Much

genetic information resides in databases that provide easy access to

the expanding information about the human genome, genetic disease,

and genetic testing (Table 466-1). For example, several thousand

monogenic disorders are summarized in a large, continuously evolving

compendium, referred to as the Online Mendelian Inheritance in Man

(OMIM) catalogue (Table 466-1). The constant refinement of bioinformatics and new developments in big data analytics, together with the

widespread adoption of electronic health records (EHRs), are simplifying the access, analysis, and integration of this daunting amount of new

information. Importantly, genomic data can be integrated readily into

EHRs and thus impact clinical practice.

■ THE HUMAN GENOME

Structure of the Human Genome The Human Genome Project

was initiated in the mid-1980s as an ambitious effort to characterize the

entire human genome and culminated in the completion of the DNA

3640 PART 16 Genes, the Environment, and Disease

TABLE 466-1 Selected Databases Relevant for Genomics and Genetic Disorders

SITE URL COMMENT

National Center for Biotechnology

Information (NCBI)

http://www.ncbi.nlm.nih.gov/ Broad access to biomedical and genomic information, literature (PubMed),

sequence databases, software for analyses of nucleotides and proteins

Extensive links to other databases, genome resources, and tutorials

National Human Genome Research

Institute

http://www.genome.gov/ An institute of the National Institutes of Health focused on genomic and genetic

research; links providing information about the human genome sequence,

genomes of other organisms, and genomic research

Catalog of Published Genome-Wide

Association Studies

https://www.ebi.ac.uk/gwas/ Published high-resolution genome-wide association studies (GWAS)

Ensembl Genome browser http://www.ensembl.org Maps and sequence information of eukaryotic genomes

Online Mendelian Inheritance in Man http://www.ncbi.nlm.nih.gov/omim Online compendium of Mendelian disorders and human genes causing genetic

disorders

American College of Medical Genetics and

Genomics

http://www.acmg.net/ Extensive links to other databases relevant for the diagnosis, treatment, and

prevention of genetic disease

American Society of Human Genetics http://www.ashg.org Information about advances in genetic research, professional and public

education, and social and scientific policies

The Cancer Genome Atlas https://cancergenome.nih.gov/ Comprehensive, multidimensional characterization of the genomic and proteomic

landscape of malignancies with high public health impact

COSMIC Catalogue of Somatic Mutations

in Cancer

https://cancer.sanger.ac.uk/cosmic Comprehensive catalogue of somatic mutations in human cancer

Genetic Testing Registry https://www.ncbi.nlm.nih.gov/gtr/ International directory of genetic testing laboratories and prenatal diagnosis

clinics; reviews and educational materials

Genomes Online Database (GOLD) http://www.genomesonline.org/ Information on published and unpublished genomes

HUGO Gene Nomenclature http://www.genenames.org/ Gene names and symbols

GENECODE https://www.gencodegenes.org/ High-quality reference gene annotation and experimental validation for human and

mouse genomes

MITOMAP, a human mitochondrial genome

database

http://www.mitomap.org/ A compendium of polymorphisms and mutations of the human mitochondrial DNA

The International Genome Sample

Resource (IGSR)

http://www.internationalgenome.org Public catalogue of human variation and genotype data from numerous ethnic

groups

Human Genome Variation Society https://www.hgvs.org/ Collection and documentation of genomic variations including population

distribution and phenotypic associations

ENCODE http://www.genome.gov/10005107 Encyclopedia of DNA Elements; catalogue of all functional elements in the human

genome

Dolan DNA Learning Center, Cold Spring

Harbor Laboratories

http://www.dnalc.org/ Educational material about selected genetic disorders, DNA, eugenics, and

genetic origin

The Online Metabolic and Molecular

Bases of Inherited Disease (OMMBID)

http://ommbid.mhmedical.com Online version of the comprehensive text on the metabolic and molecular bases of

inherited disease

Online Mendelian Inheritance in Animals

(OMIA)

https://www.omia.org/home/ Online compendium of Mendelian disorders in animals

The Jackson Laboratory http://www.jax.org/ Information about murine models and the mouse genome

Mouse genome informatics http://www.informatics.jax.org Mouse genome informatics, potential mouse models of human disease, information

on phenotypic similarity between mouse models and human patients

Note: Databases are evolving constantly. Pertinent information may be found by using links listed in the few selected databases.

sequence for the last of the human chromosomes in 2006. The scope of

a whole genome sequence analysis can be illustrated by the following

analogy. Human DNA consists of ~3 billion base pairs (bp) of DNA

per haploid genome, which is nearly 1000-fold greater than that of the

Escherichia coli genome. If the human DNA sequence were printed

out, it would correspond to about 120 volumes of Harrison’s Principles

of Internal Medicine.

In addition to the human genome, the genomes of thousands of

organisms have been sequenced completely or partially (Genomes

Online Database [GOLD]; Table 466-1). They include, among others,

eukaryotes such as the mouse (Mus musculus), Saccharomyces cerevisiae, Caenorhabditis elegans, and Drosophila melanogaster; bacteria

(e.g., E. coli); and archaea, viruses, organelles (mitochondria, chloroplasts), and plants (e.g., Arabidopsis thaliana). Genomic information

of infectious agents has significant impact for the characterization

of infectious outbreaks and epidemics. Other ramifications arising

from the availability of genomic data include, among others, (1) the

comparison of entire genomes (comparative genomics); (2) the study

of large-scale expression of RNAs (functional genomics), proteins

(proteomics), or protein families (e.g., the kinome, the complete set of

protein kinases) to detect differences between various tissues in health

and disease; (3) the characterization of the variation among individuals by establishing catalogues of sequence variations and SNPs; and

(4) the identification of genes that play critical roles in the development

of polygenic and multifactorial disorders.

CHROMOSOMES The human genome is divided into 23 different chromosomes, including 22 autosomes (numbered 1–22) and the X and Y sex

chromosomes (Fig. 466-1). Adult cells are diploid, meaning they contain

two homologous sets of 22 autosomes and a pair of sex chromosomes.

Females have two X chromosomes (XX), whereas males have one X

and one Y chromosome (XY). As a consequence of meiosis, germ cells

(sperm or oocytes) are haploid and contain one set of 22 autosomes and

one of the sex chromosomes. At the time of fertilization, the diploid

genome is reconstituted by pairing of the homologous chromosomes

from the mother and father. With each cell division (mitosis), chromosomes are replicated, paired, segregated, and divided into two daughter

cells.

Principles of Human Genetics

3641CHAPTER 466

STRUCTURE OF DNA DNA is a double-stranded helix composed of

four different bases: adenine (A), thymidine (T), guanine (G), and

cytosine (C). Adenine is paired to thymidine, and guanine is paired

to cytosine, by hydrogen bond interactions that span the double helix

(Fig. 466-1). DNA has several remarkable features that make it ideal

for the transmission of genetic information. It is relatively stable, and

the double-stranded nature of DNA and its feature of strict base-pair

complementarity permit faithful replication during cell division. Complementarity also allows the transmission of genetic information from

DNA → RNA → protein (Fig. 466-2). mRNA is encoded by the socalled sense or coding strand of the DNA double helix and is translated

into proteins by ribosomes.

The presence of four different bases provides surprising genetic

diversity. In the protein-coding regions of genes, the DNA bases are

arranged into codons, a triplet of bases that specifies a particular

amino acid. It is possible to arrange the four bases into 64 different

triplet codons (43

). Each codon specifies 1 of the 20 different amino

acids, or a regulatory signal such as initiation and stop of translation.

Because there are more codons than amino acids, the genetic code is

degenerate; that is, most amino acids can be specified by several different codons. By arranging the codons in different combinations and

in various lengths, it is possible to generate the tremendous diversity of

primary protein structure.

DNA length is normally measured in units of 1000 bp (kilobases, kb)

or 1,000,000 bp (megabases, Mb). In the human genome, only ~1% of

DNA accounts for protein-coding sequences. The noncoding DNA has

multiple functional and structural roles including (1) sequences that

form introns; (2) regulatory elements (promoters, enhancers, silencers,

insulators); (3) sequences that generate RNAs that do not code for proteins; (4) centromeres and telomeres; (5) regions defining chromatin

structure and histone modifications; (6) various forms of repetitive

sequences of variable length; and (7) pseudogenes and regions without

currently discernible functional or structural roles (Fig. 466-1).

GENES A gene is a functional unit that is regulated by transcription

(see below) and encodes an RNA product, which is most commonly,

but not always, translated into a protein that exerts activity within

or outside the cell (Fig. 466-3). Historically, genes were identified

because they conferred specific traits that are transmitted from one

generation to the next. Now, they are frequently characterized based

on expression in various tissues (transcriptome). The size of genes is

quite broad; some genes are only a few hundred base pairs, whereas

others are extraordinarily large (2 Mb). The number of genes greatly

underestimates the complexity of genetic expression, because single

genes can generate multiple spliced messenger RNA (mRNA) products

(isoforms), which are translated into proteins that are subject to complex posttranslational modification such as phosphorylation. Exons

refer to the portion of genes that are eventually spliced together to form

mRNA. Introns refer to the spacing regions between the exons that

are spliced out of precursor RNAs during RNA processing. The gene

locus also includes regions that are necessary to control its expression

(Fig. 466-2). Current estimates predict roughly 20,000 protein-coding

genes in the human genome with an average of about four different

coding transcripts per gene. Remarkably, the exome only constitutes

1.14% of the genome. Of note, the number of transcripts is close to

200,000 and includes thousands of noncoding transcripts (RNAs of

various length such as microRNAs [miRNA] and long noncoding

RNAs [lncRNA]). These noncoding RNAs are involved in numerous

cellular processes such as transcriptional and posttranscriptional

regulation of gene expression, chromatin remodeling, and protein

trafficking, among others. Not surprisingly, aberrant expression and/or

mutations in these RNAs play a pathogenic role in numerous diseases.

SINGLE-NUCLEOTIDE POLYMORPHISMS Each individual has roughly

5 million sequence variants that differentiate one person from another.

Some of these variants have no impact on health, whereas others may

increase or lower the risk for developing a specific disease. Remarkably,

however, the primary DNA sequence of humans has ~99.9% similarity

compared to that of any other human. An SNP is a variation of a single base pair in the DNA. The identification of the ~10 million SNPs

estimated to occur in the human genome has generated a catalogue of

common genetic variants that occur in human beings from distinct

ethnic backgrounds (Fig. 466-3). SNPs are the most common type of

sequence variation and account for ~90% of all sequence variation.

They occur on average every 100–300 bases and are the major source

of genetic heterogeneity. SNPs that are in close proximity are inherited together (e.g., they are linked) and are referred to as haplotypes

(Fig. 466-4). Haplotype maps describe the nature and location of these

SNP haplotypes and how they are distributed among individuals within

and among populations, information that has been facilitating GWAS

designed to elucidate the complex interactions among multiple genes

and lifestyle factors in multifactorial disorders (see below). Moreover,

haplotype analyses are useful to assess variations in responses to medications (pharmacogenomics) and environmental factors, as well as the

prediction of disease predisposition.

COPY NUMBER VARIATIONS Copy number variations (CNVs) are relatively large genomic regions (1 kb to several Mb) that have been duplicated or deleted on certain chromosomes and hence alter the diploid

status of the DNA (Fig. 466-5). It has been estimated that 5–10% of the

genome can display CNVs. When comparing the genomes of two individuals, ~0.4–0.8% of their genomes differ in terms of CNVs scattered

throughout the genome. Of note, de novo CNVs have been observed

between monozygotic twins, who otherwise have identical genomes.

Some CNVs have no functional consequences, whereas others have

been associated with susceptibility or resistance to disease, and CNVs

also occur in cancer cells.

Histone

H1

Nucleosome

fiber

Metaphase

chromosome

Solenoid

q, long arm

p, short arm

Centromere

Supercoiled

chromatin

Telomere

Nucleosome core

Histone H2A, H2B, H4

Double-strand DNA

without histones

Adenine

H

H

H

O

O

P OO

O

O–

P OO

O

O–

O

H

H

H

H H

H

H

H

H

H H3C

H

Thymine

Cytosine Guanine

C C

C C

C

C

C

C

C C

C

C

C C C

C

N

N

T

A

C

G

G

C

A

T

N

N

N N

N

N N

N

N

C

C

C

N

N

N

N

FIGURE 466-1 Structure of chromatin and chromosomes. Chromatin is composed of

double-strand DNA that is wrapped around histone and nonhistone proteins forming

nucleosomes. The nucleosomes are further organized into solenoid structures.

Chromosomes assume their characteristic structure, with short (p) and long (q)

arms at the metaphase stage of the cell cycle.

3642 PART 16 Genes, the Environment, and Disease

CRE

Enhancer Silencer

RE CAAT TATA 1 2 3

1 2 3

Nuclear

receptor

Transcription

factor

RNA polymerase II

Nuclear

receptor

CBP TAF

GTF CREB CREB TBP

HAT

CoA

Steroids Ca Growth 2+ Cytokines

factors

Hormones

Light

UV-light

Mechanical stress

Regulation of Gene Expression

Transcription

mRNA Processing

Translation

Posttranslational Processing

Cytoplasm

Nucleus

DNA

hRNA

–COOH

5′ -Cap –Poly-A Tail

mRNA

Protein

1 2 3

NH2–

FIGURE 466-2 Flow of genetic information. Multiple extracellular signals activate intracellular signal cascades that result in altered regulation of gene expression through

the interaction of transcription factors with regulatory regions of genes. RNA polymerase transcribes DNA into RNA that is processed to mRNA by excision of intronic

sequences. The mRNA is translated into a polypeptide chain to form the mature protein after undergoing posttranslational processing. CBP, CREB-binding protein; CoA,

co-activator; COOH, carboxyterminus; CRE, cyclic AMP responsive element; CREB, cyclic AMP response element–binding protein; GTF, general transcription factors; HAT,

histone acetyl transferase; NH2, aminoterminus; RE, response element; TAF, TBP-associated factors; TATA, TATA box; TBP, TATA-binding protein.

Replication of DNA and Mitosis Genetic information in DNA

is transmitted to daughter cells under two different circumstances:

(1) somatic cells divide by mitosis, allowing the diploid (2n) genome

to replicate itself completely in conjunction with cell division; and (2)

germ cells (sperm and ova) undergo meiosis, a process that enables

the reduction of the diploid (2n) set of chromosomes to the haploid

state (1n).

Prior to mitosis, cells exit the resting, or G0

 state, and enter the cell

cycle. After traversing a critical checkpoint in G1

, cells undergo DNA

synthesis (S phase), during which the DNA in each chromosome is replicated, yielding two pairs of sister chromatids (2n → 4n). The process

of DNA synthesis requires stringent fidelity in order to avoid transmitting errors to subsequent generations of cells. Genetic abnormalities

of DNA mismatch/repair include xeroderma pigmentosum, Bloom’s

syndrome, ataxia telangiectasia, and hereditary nonpolyposis colon

cancer (HNPCC), among others. Many of these disorders strongly

predispose to neoplasia because of the rapid acquisition of additional

mutations (Chap. 71). After completion of DNA synthesis, cells enter

G2

 and progress through a second checkpoint before entering mitosis.

At this stage, the chromosomes condense and are aligned along the

equatorial plate at metaphase. The two identical sister chromatids, held

together at the centromere, divide and migrate to opposite poles of the

cell. After formation of a nuclear membrane around the two separated

sets of chromatids, the cell divides and two daughter cells are formed,

thus restoring the diploid (2n) state.

Assortment and Segregation of Genes During Meiosis Meiosis occurs only in germ cells of the gonads. It shares certain features

with mitosis but involves two distinct steps of cell division that reduce

the chromosome number to the haploid state. In addition, there is

active recombination that generates genetic diversity. During the first

cell division, two sister chromatids (2n → 4n) are formed for each chromosome pair and there is an exchange of DNA between homologous

paternal and maternal chromosomes. This process involves the formation of chiasmata, structures that correspond to the DNA segments that

cross over between the maternal and paternal homologues (Fig. 466-6).

Usually there is at least one crossover on each chromosomal arm;

recombination occurs more frequently in female meiosis than in male

meiosis. Subsequently, the chromosomes segregate randomly. Because

there are 23 chromosomes, there exist 223 (>8 million) possible combinations of chromosomes. Together with the genetic exchanges that

occur during recombination, chromosomal segregation generates tremendous diversity, and each gamete is genetically unique. The process

of recombination and the independent segregation of chromosomes

provide the foundation for performing linkage analyses, whereby one

attempts to correlate the inheritance of certain chromosomal regions

(or linked genes) with the presence of a disease or genetic trait (see

below).

After the first meiotic division, which results in two daughter cells

(2n), the two chromatids of each chromosome separate during a second meiotic division to yield four gametes with a haploid state (1n).

When the egg is fertilized by sperm, the two haploid sets are combined,

thereby restoring the diploid state (2n) in the zygote.

■ REGULATION OF GENE EXPRESSION

Regulation by Transcription Factors The expression of genes

is regulated by DNA-binding proteins that activate or repress transcription. The number of DNA sequences and transcription factors

that regulate transcription is much greater than originally anticipated.

Most genes contain at least 15–20 discrete regulatory elements within

300 bp of the transcription start site. This densely packed promoter

region often contains binding sites for ubiquitous transcription factors. However, factors involved in cell-specific expression may also

bind to these sequences. Key regulatory elements may also reside

at a large distance from the proximal promoter. The globin and the

Principles of Human Genetics

3643CHAPTER 466

Chromosome 7

p22.3

p22.1

p21.3

p21.1

p15.3

p15.1

p14.3

p14.1

p13

p12.3

p12.1

p1

q11.21

q11.22

q11.23

q21.11

p21.13

q21.3

q22.1

q22.3

q31.1

q31.2

q31.31

q31.33

q32.1

q33

q34

q35

q36.1

q36.3

1.2

Known Genes

(1260)

SNPs

(612,977)

CFTR Gene

116.90 Mb 116.94 Mb 116.98 Mb 117.02 Mb 117.06 Mb

200 Kb

20 Kb

SNPs

Intronic

Coding region, synonymous Coding region, nonsynonymous

Splice site

Coding region, frameshift

FIGURE 466-3 Chromosome 7 is shown with the density of single nucleotide polymorphisms (SNPs) and genes above. A 200-kb region in 7q31.2 containing the CFTR gene

is shown below. The CFTR gene contains 27 exons. Close to 2000 mutations in this gene have been found in patients with cystic fibrosis. A 20-kb region encompassing exons

4–9 is shown further amplified to illustrate the SNPs in this region.

FIGURE 466-4 The origin of haplotypes is due to repeated recombination events

occurring in multiple generations. Over time, this leads to distinct haplotypes. These

haplotype blocks can often be characterized by genotyping selected Tag single

nucleotide polymorphisms (SNPs), an approach that facilitates performing genomewide association studies (GWAS).

immunoglobulin genes, for example, contain locus control regions that

are several kilobases away from the structural sequences of the gene.

Specific groups of transcription factors that bind to these promoter

and enhancer sequences provide a combinatorial code for regulating

transcription. In this manner, relatively ubiquitous factors interact

with more restricted factors to allow each gene to be expressed and

regulated in a unique manner that is dependent on developmental

state, cell type, and numerous extracellular stimuli. Regulatory factors

also bind within the gene itself, particularly in the intronic regions.

The transcription factors that bind to DNA actually represent only the

first level of regulatory control. Other proteins—co-activators and corepressors—interact with the DNA-binding transcription factors to generate large regulatory complexes. These complexes are subject to control

by numerous cell-signaling pathways and enzymes, leading to phosphorylation, acetylation, sumoylation, and ubiquitination. Ultimately,

the recruited transcription factors interact with, and stabilize, components of the basal transcription complex that assembles at the site of the

TATA box and initiator region. This basal transcription factor complex

consists of >30 different proteins. Gene transcription occurs when

RNA polymerase begins to synthesize RNA from the DNA template.

A large number of identified genetic diseases involve transcription

factors (Table 466-2).

The field of functional genomics is based on the concept that understanding alterations of gene expression under various physiologic and

pathologic conditions provides insight into the underlying functional

3644 PART 16 Genes, the Environment, and Disease

1

2

Normal

Deleted

Area

Duplicated

Area

0

–1

–2

log2 (ratio)

Chromosome 8

FIGURE 466-5 Copy number variations (CNV) encompass relatively large regions of the genome that

have been duplicated or deleted. Chromosome 8 is shown with a CNV detected by genomic hybridization.

An increase in the signal strength indicates a duplication, whereas a decrease reflects a deletion of the

covered chromosomal regions.

Homologous

chromosomes

A

B

C

D

a

b

c

d

a

b

c

d

a

b

c

d

Chromatids

A

B

C

D

A

B

C

D

a

b

c

d

No crossover

A

B

C

D

a

b

c

d

A

B

C

D

a

b

C

d

Double crossover

A

B

C

D

a

b

c

d

A

B

c

D

a

b

C

D

Crossover

A

B

C

D

a

b

c

d

A

B

c

d

a

b

c

d

No recombination

in gametes

A

B

C

D

a

b

c

d

A

B

C

D

a

b

C

D

Recombination

in gametes

A

B

C

D

a

b

c

d

A

B

c

d

a

b

C

d

Recombination

in gametes

A

B

C

D

a

b

c

d

A

B

c

D

FIGURE 466-6 Crossing-over and genetic recombination. During chiasma

formation, either of the two sister chromatids on one chromosome pairs with

one of the chromatids of the homologous chromosome. Genetic recombination

occurs through crossing-over and results in recombinant and nonrecombinant

chromosome segments in the gametes. Together with the random segregation of

the maternal and paternal chromosomes, recombination contributes to genetic

diversity and forms the basis of the concept of linkage.

role of the gene. The ENCODE (Encyclopedia of DNA Elements) project aims to compile and annotate all functional sequences in the human

genome. By revealing specific gene expression profiles, this knowledge

can be of diagnostic and therapeutic relevance. The large-scale study

of expression profiles is referred to as transcriptomics because the

complement of mRNAs transcribed by the cellular genome is called

the transcriptome.

Most studies of gene expression have focused on the regulatory

DNA elements of genes that control transcription. However, it should

be emphasized that gene expression requires a series of steps, including

mRNA processing, protein translation, and posttranslational modifications, all of which are actively regulated (Fig. 466-2).

Epigenetic Regulation of Gene Expression (see Chap. 483)

Epigenetics describes mechanisms and phenotypic changes that are not

a result of variation in the primary DNA nucleotide sequence but are

caused by secondary modifications of DNA or histones. These modifications include heritable changes such as X-inactivation and imprinting, but they can also result from dynamic posttranslational protein

modifications in response to environmental influences such as diet,

age, or drugs. The epigenetic modifications result in altered expression

of individual genes or chromosomal loci encompassing multiple genes.

The term epigenome describes the constellation of covalent modifications of DNA and histones that impact chromatin structure, as well

as noncoding transcripts that modulate the transcriptional activity of

DNA. Although the primary DNA sequence is usually identical in all

cells of an organism, tissue-specific changes in the epigenome contribute to determining the transcriptional signature of a cell (transcriptome) and hence the protein expression profile (proteome).

Mechanistically, DNA and histone modifications can result in the

activation or silencing of gene expression (Fig. 466-7). DNA methylation involves the addition of a methyl group to cytosine residues.

This is usually restricted to cytosines of CpG dinucleotides, which

are abundant throughout the genome. Methylation of these dinucleotides is thought to represent a defense mechanism that minimizes the

expression of sequences that have been incorporated into the genome

such as retroviral sequences. CpG dinucleotides also exist in so-called

CpG islands, stretches of DNA characterized by a high

CG content, which are found in the majority of human

gene promoters. CpG islands in promoter regions are

typically unmethylated, and the lack of methylation

facilitates transcription.

Histone methylation involves the addition of a

methyl group to lysine residues in histone proteins

(Fig. 466-7). Depending on the specific lysine residue

being methylated, this alters chromatin configuration,

making it either more open or tightly packed. Acetylation of histone proteins is another well-characterized

mechanism that results in an open chromatin configuration, which favors active transcription. Acetylation

is generally more dynamic than methylation, and

many transcriptional activation complexes have histone acetylase activity, whereas repressor complexes

often contain deacetylases and remove acetyl groups

from histones. Other histone modifications include,

among others, phosphorylation and sumoylation.

Furthermore, noncoding RNAs and RNA regulatory networks that bind to DNA have a significant

impact on transcriptional activity.

Physiologically, epigenetic mechanisms play an

important role in several instances. For example,

X-inactivation refers to the relative silencing of one of

the two X chromosome copies present in females. The

inactivation process is a form of dosage compensation

such that females (XX) do not generally express twice

as many X-chromosomal gene products as males

(XY). In a given cell, the choice of which chromosome

is inactivated occurs randomly in humans. But once

the maternal or paternal X chromosome is inactivated,

Principles of Human Genetics

3645CHAPTER 466

it will remain inactive, and this information is transmitted with each

cell division. The X-inactive specific transcript (Xist) gene encodes a

large noncoding RNA that mediates the silencing of the X chromosome

from which it is transcribed by coating it with Xist RNA. The inactive

X chromosome is highly methylated and has low levels of histone acetylation. While the majority of X-chromosomal genes are silenced by

X-inactivation, ~15% escape inactivation and are expressed.

Epigenetic gene inactivation also occurs on selected chromosomal

regions of autosomes, a phenomenon referred to as genomic imprinting.

Through this mechanism, a small subset of genes is only expressed in

a monoallelic fashion. Imprinting is heritable and leads to the preferential expression of one of the parental alleles, which deviates from the

usual biallelic expression seen for the majority of genes. Remarkably,

imprinting can be limited to a subset of tissues. Imprinting is mediated through DNA methylation of one of the alleles. The epigenetic

marks on imprinted genes are maintained throughout life, but during

zygote formation, they are activated or inactivated in a sex-specific

manner (imprint reset) (Fig. 466-8), which allows a differential

expression pattern in the fertilized egg and the subsequent mitotic

divisions. Appropriate expression of imprinted genes is important for

normal development and cellular functions. Imprinting defects and

uniparental disomy, which is the inheritance of two chromosomes or

chromosomal regions from the same parent, are the cause of several

developmental disorders such as Beckwith-Wiedemann syndrome,

Silver-Russell syndrome, Angelman’s syndrome, and Prader-Willi

syndrome (see below). Monoallelic loss-of-function mutations in the

GNAS1 gene lead to Albright’s hereditary osteodystrophy (AHO).

Paternal transmission of GNAS1 mutations leads to an isolated AHO

phenotype (pseudopseudohypoparathyroidism), whereas maternal

transmission leads to AHO in combination with hormone resistance

to parathyroid hormone, thyrotropin, and gonadotropins (pseudohypoparathyroidism type IA). These phenotypic differences are explained

by tissue-specific imprinting of the GNAS1 gene, which is expressed

primarily from the maternal allele in the thyroid, gonadotropes, and

the proximal renal tubule. In most other tissues, the GNAS1 gene is

expressed biallelically. In patients with isolated renal resistance to

parathyroid hormone (pseudohypoparathyroidism type IB), defective

imprinting of the GNAS1 gene results in decreased Gs

α expression in

the proximal renal tubules. Rett’s syndrome is an X-linked dominant

disorder resulting in developmental regression and stereotypic hand

movements in affected girls. It is caused by mutations in the MECP2

gene, which encodes a methyl-binding protein. The ensuing aberrant

methylation results in abnormal gene expression in neurons, which are

otherwise normally developed.

Remarkably, epigenetic differences also occur among monozygotic

twins. Although twins are epigenetically indistinguishable during the

early years of life, older monozygotic twins exhibit differences in the

overall content and genomic distribution of DNA methylation and

histone acetylation, which would be expected to alter gene expression

in various tissues.

In cancer, the epigenome is characterized by simultaneous losses

and gains of DNA methylation in different genomic regions, as well

as repressive histone modifications. Hyper- and hypomethylation are

associated with mutations in genes that control DNA methylation.

Hypomethylation is thought to remove normal control mechanisms

that prevent expression of repressed DNA regions. It is also associated

with genomic instability. Hypermethylation, in contrast, results in

the silencing of CpG islands in promoter regions of genes, including

tumor-suppressor genes. Epigenetic alterations are considered to be

more easily reversible compared to genetic changes; modification of

the epigenome with demethylating agents and histone deacetylases is

being used in the treatment of various malignancies.

■ TRANSMISSION OF GENETIC DISEASE

Origins and Types of Mutations The term mutation or variant

is used to designate the process of generating genetic variations as well

as the outcome of these alterations. A mutation can be defined as any

change in the primary nucleotide sequence of DNA regardless of its

functional consequences, although it often has a negative connotation.

The more neutral term variation is now increasingly used to describe

sequence changes and is recommended by several professional organizations and guidelines instead of mutation. Some variations may be

lethal, others are less deleterious, and some may confer an evolutionary

advantage. Variations can occur in the germline (sperm or oocytes);

these can be transmitted to progeny. Alternatively, variations can occur

during embryogenesis or in somatic tissues. Variations that occur

during development lead to mosaicism, a situation in which tissues are

composed of cells with different genetic constitutions. If the germline

is mosaic, a mutation can be transmitted to some progeny but not

others, which sometimes leads to confusion in assessing the pattern

of inheritance. Somatic mutations that do not affect cell survival can

sometimes be detected because of variable phenotypic effects in tissues (e.g., pigmented lesions in McCune-Albright syndrome). Other

somatic mutations are associated with neoplasia because they confer

a growth advantage to cells. Epigenetic events may also influence gene

expression or facilitate genetic damage. With the exception of triplet

TABLE 466-2 Selected Examples of Diseases Caused by Mutations and

Rearrangements in Transcription Factors

TRANSCRIPTION

FACTOR CLASS EXAMPLE ASSOCIATED DISORDER

Nuclear receptors Androgen

receptor

Complete or partial androgen

insensitivity (recessive missense

mutations)

Spinobulbar muscular atrophy (CAG

repeat expansion)

Zinc finger proteins WT1 WAGR syndrome: Wilms’

tumor, aniridia, genitourinary

malformations, mental retardation

Basic helix-loop-helix MITF Waardenburg’s syndrome type 2A

Homeobox IPF1 Maturity onset of diabetes mellitus

type 4 (heterozygous mutation/

haploinsufficiency)

Pancreatic agenesis (homozygous

mutation)

Leucine zipper Retina leucine

zipper (NRL)

Autosomal dominant retinitis

pigmentosa

High mobility group

(HMG) proteins

SRY Sex reversal

Forkhead HNF4α, HNF1α,

HNF1β

Maturity onset of diabetes mellitus

types 1, 3, 5

Paired box PAX3 Waardenburg’s syndrome types 1

and 3

T-box TBX5 Holt-Oram syndrome (thumb

anomalies, atrial or ventricular

septum defects, phocomelia)

Cell cycle control

proteins

P53 Li-Fraumeni syndrome, other

cancers

Co-activators CREB binding

protein (CREBBP)

Rubinstein-Taybi syndrome

General transcription

factors

TATA-binding

protein (TBP)

Spinocerebellar ataxia 17 (CAG

expansion)

Transcription

elongation factor

VHL von Hippel–Lindau syndrome

(renal cell carcinoma,

pheochromocytoma, pancreatic

tumors, hemangioblastomas)

Autosomal dominant inheritance,

somatic inactivation of second allele

(Knudson two-hit model)

Runt RUNX1 Familial thrombocytopenia with

propensity to acute myelogenous

leukemia

Chimeric proteins due

to translocations

PML-RAR Acute promyelocytic leukemia

t(15;17)(q22;q11.2-q12) translocation

Abbreviations: CREB, cAMP responsive element–binding protein; HNF, hepatocyte

nuclear factor; PML, promyelocytic leukemia; RAR, retinoic acid receptor; SRY,

sex-determining region Y; VHL, von Hippel–Lindau.

3646 PART 16 Genes, the Environment, and Disease

Methylated DNA

Methylation

Histone Acetylation

Unmethylated DNA

Transcription

NH2

O

N

N

NH2

O

N

N

CH3

Histone Modifications

Cytosine Methylation

Acetylation

Phosphorylation

Methylation

NH2

FIGURE 466-7 Epigenetic modifications of DNA and histones. Methylation of cytosine residues is associated with

gene silencing. Methylation of certain genomic regions is inherited (imprinting), and it is involved in the silencing of

one of the two X chromosomes in females (X-inactivation). Alterations in methylation can also be acquired, e.g., in

cancer cells. Covalent posttranslational modifications of histones play an important role in altering DNA accessibility

and chromatin structure and hence in regulating transcription. Histones can be reversibly modified in their aminoterminal tails, which protrude from the nucleosome core particle, by acetylation of lysine, phosphorylation of serine,

methylation of lysine and arginine residues, and sumoylation. Acetylation of histones by histone acetylases (HATs),

e.g., leads to unwinding of chromatin and accessibility to transcription factors. Conversely, deacetylation by histone

deacetylases (HDACs) results in a compact chromatin structure and silencing of transcription.

nucleotide repeats, which can expand (see below), variations are usually stable.

Mutations are structurally diverse—they can involve the entire

genome, as in triploidy (one extra set of chromosomes), or gross

numerical or structural alterations in chromosomes or individual

genes. Large deletions may affect a portion of a gene or an entire gene,

or, if several genes are involved, they may lead to a contiguous gene syndrome. Unequal crossing-over between homologous genes can result

in fusion gene mutations, as illustrated by color blindness. Variations

involving single nucleotides are referred to as point mutations. Substitutions are called transitions if a purine is replaced by another purine base

(A ↔ G) or if a pyrimidine is replaced by another pyrimidine (C ↔ T).

Changes from a purine to a pyrimidine, or vice versa, are referred to as

transversions. If the DNA sequence change occurs in a coding region

and alters an amino acid, it is called a missense mutation. Depending

on the functional consequences of such a missense mutation, amino

acid substitutions in different regions of the protein can lead to distinct

phenotypes.

Variations can occur in all domains of a gene (Fig. 466-9). A point

mutation occurring within the coding region leads to an amino acid

substitution if the codon is altered (Fig. 466-10). Point mutations that

introduce a premature stop codon result in a truncated or missing

protein. Large deletions may affect a portion of a gene or an entire

gene, whereas small deletions and insertions alter the reading frame

if they do not represent a multiple of three bases. These “frameshift”

mutations, also designated as amphigoric amino acid changes, lead to

an entirely altered carboxy terminus. Mutations in intronic sequences

or in exon junctions may destroy or create splice donor or splice acceptor sites. Variations may also be found in the regulatory sequences of

genes, resulting in reduced or enhanced gene transcription.

Certain DNA sequences are particularly susceptible to mutagenesis.

Successive pyrimidine residues (e.g., T-T or C-C) are subject to the

formation of ultraviolet light–induced photoadducts. If these pyrimidine dimers are not repaired by the nucleotide excision repair pathway,

mutations will be introduced after DNA synthesis. The dinucleotide

C-G, or CpG, is also a hot spot for a specific type of mutation. In this

case, methylation of the cytosine is associated with an enhanced rate

of deamination to uracil, which is then replaced with thymine. This

C → T transition (or G → A on the opposite

strand) accounts for at least one-third of

point mutations associated with polymorphisms and mutations. In addition to the

fact that certain types of mutations (C →

T or G → A) are relatively common, the

nature of the genetic code also results in

overrepresentation of certain amino acid

substitutions.

Polymorphisms are sequence variations

that have a frequency of at least 1%. Usually,

they do not result in a perceptible phenotype,

but because allele frequency and functional

consequences are often not known, the term

variation is now increasingly recommended

for the description of these sequence changes.

Often, they consist of single base-pair substitutions that do not alter the protein coding

sequence because of the degenerate nature

of the genetic code (synonymous polymorphism), although it is possible that some

might alter mRNA stability, translation, or

the amino acid sequence (nonsynonymous

polymorphism) (Fig. 466-10). The detection

of sequence variants poses a practical problem because it is often unclear whether it creates a change with functional consequences

or a benign variation. In this situation, the

sequence alteration is also described as variant of unknown significance (VUS).

MUTATION RATES Mutations represent an important cause of genetic

diversity as well as disease. Mutation rates are difficult to determine

in humans because many mutations are silent and because testing is

often not adequate to detect the phenotypic consequences. Mutation

rates vary in different genes but are estimated to occur at a rate of

~10−10/bp per cell division. Germline mutation rates (as opposed

to somatic mutations) are relevant in the transmission of genetic

disease. Because the population of oocytes is established very early

in development, only ~20 cell divisions are required for completed

oogenesis, whereas spermatogenesis involves ~30 divisions by the time

of puberty and 20 cell divisions each year thereafter. Consequently,

the probability of acquiring new point mutations is much greater in

the male germline than the female germline, in which rates of aneuploidy are increased. Thus, the incidence of new point mutations in

spermatogonia increases with paternal age (e.g., achondrodysplasia,

Marfan’s syndrome, neurofibromatosis). It is estimated that about 1

in 10 sperm carries a new deleterious mutation. The rates for new

mutations are calculated most readily for autosomal dominant and

X-linked disorders and are ~10−5−10−6/locus per generation. Because

most monogenic diseases are relatively rare, new mutations account

for a significant fraction of cases. This is important in the context of

genetic counseling, because a new mutation can be transmitted to the

affected individual but does not necessarily imply that the parents are

at risk to transmit the disease to other children. An exception to this is

when the new mutation occurs early in germline development, leading

to gonadal mosaicism.

UNEQUAL CROSSING-OVER Normally, DNA recombination in germ

cells occurs with remarkable fidelity to maintain the precise junction

sites for the exchanged DNA sequences (Fig. 466-6). However, mispairing of homologous sequences leads to unequal crossover, with gene

duplication on one of the chromosomes and gene deletion on the other

chromosome. A significant fraction of growth hormone (GH) gene deletions, for example, involve unequal crossing-over (Chap. 379). The GH

gene is a member of a large gene cluster that includes a GH variant gene

as well as several structurally related chorionic somatomammotropin

genes and pseudogenes (highly homologous but functionally inactive

relatives of a normal gene). Because such gene clusters contain multiple

homologous DNA sequences arranged in tandem, they are particularly

Principles of Human Genetics

3647CHAPTER

Active

466

unmethylated

mat pat

Inactive

methylated

Maternal somatic cell

Inactive

methylated

pat mat

Active

unmethylated

Paternal somatic cell

Inactive

methylated

pat mat

Active

unmethylated

Zygote

Active

unmethylated

mat pat

Active

unmethylated

Maternal germline Paternal germline

Inactive

methylated

pat mat

Inactive

methylated

Germline development:

Imprint reset

FIGURE 466-8 A few genomic regions are imprinted in a parent-specific fashion. The unmethylated chromosomal

regions are actively expressed, whereas the methylated regions are silenced. In the germline, the imprint is reset

in a parent-specific fashion: both chromosomes are unmethylated in the maternal (mat) germline and methylated

in the paternal (pat) germline. In the zygote, the resulting imprinting pattern is identical with the pattern in the

somatic cells of the parents.

prone to undergo recombination and, consequently, gene duplication

or deletion. On the other hand, duplication of the PMP22 gene because

of unequal crossing-over results in increased gene dosage and type IA

Charcot-Marie-Tooth disease. Unequal crossing-over resulting in deletion of PMP22 causes a distinct neuropathy called hereditary neuropathy

with liability to pressure palsies (HNPP) (Chap. 446).

Glucocorticoid-remediable aldosteronism (GRA) is caused by a

gene fusion or rearrangement involving the genes that encode aldosterone synthase (CYP11B2) and steroid 11β-hydroxylase (CYP11B1),

normally arranged in tandem on chromosome 8q. These two genes

are 95% identical, predisposing to gene duplication and deletion by

unequal crossing-over. The rearranged gene product contains the

regulatory regions of 11β-hydroxylase fused to the coding sequence of

aldosterone synthetase. Consequently, the latter enzyme is expressed in

the adrenocorticotropic hormone (ACTH)–dependent zona fasciculata

of the adrenal gland, resulting in overproduction of mineralocorticoids

and hypertension (Chap. 386).

Gene conversion refers to a nonreciprocal exchange of homologous

genetic information. It has been used to explain how an internal

portion of a gene is replaced by a homologous segment copied from another allele or

locus; these genetic alterations may range

from a few nucleotides to a few thousand

nucleotides. As a result of gene conversion,

it is possible for short DNA segments of two

chromosomes to be identical, even though

these sequences are distinct in the parents. A

practical consequence of this phenomenon is

that nucleotide substitutions can occur during gene conversion between related genes,

often altering the function of the gene. In

disease states, gene conversion often involves

intergenic exchange of DNA between a gene

and a related pseudogene. For example, the

21-hydroxylase gene (CYP21A2) is adjacent

to a nonfunctional pseudogene (CYP21A1P).

Many of the nucleotide substitutions that are

found in the CYP21A2 gene in patients with

congenital adrenal hyperplasia correspond to

sequences that are present in the CYP21A1P

pseudogene, suggesting gene conversion as

one cause of mutagenesis. In addition, mitotic

gene conversion has been suggested as a

mechanism to explain revertant mosaicism

in which an inherited mutation is “corrected”

in certain cells. For example, patients with

autosomal recessive generalized atrophic

benign epidermolysis bullosa have acquired

reverse mutations in one of the two mutated

COL17A1 alleles, leading to clinically unaffected patches of skin.

INSERTIONS AND DELETIONS Although

many instances of insertions and deletions

occur as a consequence of unequal crossingover, there is also evidence for internal

duplication, inversion, or deletion of DNA

sequences. The fact that certain deletions or

insertions appear to occur repeatedly as independent events indicates that specific regions

within the DNA sequence predispose to these

errors. For example, certain regions of the

DMD gene, which encodes dystrophin, appear

to be hot spots for deletions and result in muscular dystrophy (Chap. 449). Some regions

within the human genome are rearrangement

hot spots and lead to CNVs.

ERRORS IN DNA REPAIR Because mutations

caused by defects in DNA repair accumulate

as somatic cells divide, these types of mutations are particularly important in the context of neoplastic disorders. Several genetic disorders

involving DNA repair enzymes underscore their importance. Patients

with xeroderma pigmentosum have defects in DNA damage recognition or in the nucleotide excision and repair pathway (Chap. 76).

Exposed skin is dry and pigmented and is extraordinarily sensitive to

the mutagenic effects of ultraviolet irradiation. More than 10 different

genes have been shown to cause the different forms of xeroderma

pigmentosum.

Ataxia-telangiectasia is a multisystem disorder that includes progressive neurodegenerative cerebellar ataxia, immunologic defects, telangiectatic lesions, lymphomas and leukemias, and hypersensitivity to

ionizing radiation (Chap. 439). The discovery of the ataxia-telangiectasia

mutated (ATM) gene revealed that it is homologous to genes involved

in DNA repair and control of cell cycle checkpoints. Mutations in

the ATM gene give rise to defects in meiosis as well as increasing

susceptibility to damage from ionizing radiation. Fanconi’s anemia

is also associated with an increased risk of multiple acquired genetic

abnormalities. It is characterized by diverse congenital anomalies and