Heatstroke Clinical Presentation

Heatstroke is defined typically as hyperthermia exceeding 41°C and anhidrosis associated with an altered sensorium. However, when a patient is allowed to cool down prior to measurement of the temperature (as may occur during transportation in a cool ambulance or evaluation in an emergency department), the measured temperature may be much lower than 41°C, making the temperature criterion relative. Similarly, some patients may retain the ability to sweat, removing anhidrosis as a criterion for the diagnosis of heatstroke. Therefore, strict adherence to the definition is not advised because it may result in dangerous delays in diagnosis and therapy.

Clinically, 2 forms of heatstroke are differentiated. Classic heatstroke, which occurs during environmental heat waves, is more common in very young persons and in the elderly population and should be suspected in children, elderly persons, and individuals who are chronically ill who present with an altered sensorium. Classic heatstroke occurs because of failure of the body's heat dissipating mechanisms.

On the other hand, EHS affects young, healthy individuals who engage in strenuous physical activity, and EHS should be suspected in all individuals with bizarre irrational behavior or a history of syncope during strenuous exercise. EHS results from increased heat production, which overwhelms the body's ability to dissipate heat.

Exertional heatstroke

EHS is characterized by hyperthermia, diaphoresis, and an altered sensorium, which may manifest suddenly during extreme physical exertion in a hot environment.

A number of symptoms (eg, abdominal and muscular cramping, nausea, vomiting, diarrhea, headache, dizziness, dyspnea, weakness) commonly precede the heatstroke and may remain unrecognized. Syncope and loss of consciousness also are observed commonly before the development of EHS.

EHS commonly is observed in young, healthy individuals (eg, athletes, firefighters, military personnel) who, while engaging in strenuous physical activity, overwhelm their thermoregulatory system and become hyperthermic. Because their ability to sweat remains intact, patients with EHS are able to cool down after cessation of physical activity and may present for medical attention with temperatures well below 41°C. Despite education and preventative measures, EHS is still the third most common cause of death among high school students.

Risk factors that increase the likelihood of heat-related illnesses include a preceding viral infection, dehydration, fatigue, obesity, lack of sleep, poor physical fitness, and lack of acclimatization. Although lack of acclimatization is a risk factor for heatstroke, EHS also can occur in acclimatized individuals who are subjected to moderately intense exercise.

EHS also may occur because of increased motor activity due to drug use, such as cocaine and amphetamines, and as a complication of status epilepticus.

Nonexertional heatstroke

Classic NEHS is characterized by hyperthermia, anhidrosis, and an altered sensorium, which develop suddenly after a period of prolonged elevations in ambient temperatures (ie, heat waves). Core body temperatures greater than 41°C are diagnostic, although heatstroke may occur with lower core body temperatures.

Numerous CNS symptoms, ranging from minor irritability to delusions, irrational behavior, hallucinations, and coma have been described.

Anhidrosis due to cessation of sweating is a late occurrence in heatstroke and may not be present when patients are examined.

Other CNS symptoms include hallucinations, seizures, cranial nerve abnormalities, cerebellar dysfunction, and opisthotonos.

Patients with NEHS initially may exhibit a hyperdynamic circulatory state, but, in severe cases, hypodynamic states may be noted.

Classic heatstroke most commonly occurs during episodes of prolonged elevations in ambient temperatures. It affects people who are unable to control their environment and water intake (eg, infants, elderly persons, individuals who are chronically ill), people with reduced cardiovascular reserve (eg, elderly persons, patients with chronic cardiovascular illnesses), and people with impaired sweating (eg, patients with skin disease, patients ingesting anticholinergic and psychiatric drugs). In addition, infants have an immature thermoregulatory system, and elderly persons have impaired perception of changes in body and ambient temperatures and a decreased capacity to sweat.

Vital signs

Temperature: Typically, the patient's temperature exceeds 41°C, but, in the presence of sweating, evaporating mechanisms, and the initiation of cooling methods, body temperatures lower than 41°C are common.

Pulse: Tachycardia to rates exceeding 130 beats per minute is common.

Blood pressure: Patients commonly are normotensive, with a wide pulse pressure; however, hypotension is common and is due to a number of factors, including vasodilation of the cutaneous vessels, pooling of the blood in the venous system, and dehydration. Hypotension also may be due to myocardial damage and may signal cardiovascular collapse. This will usually correct with normalization of the body temperature.

Central nervous system

Symptoms of CNS dysfunction are present universally in persons with heatstroke. Symptoms may range from irritability to coma.

Patients may present with delirium, confusion, delusions, convulsions, hallucinations, ataxia, tremors, dysarthria, and other cerebellar findings, as well as cranial nerve abnormalities and tonic and dystonic contractions of the muscles.

Patients also may exhibit decerebrate posturing, decorticate posturing, or they may be limp.

Coma also may be caused by electrolyte abnormalities, hypoglycemia, hepatic encephalopathy, uremic encephalopathy, and acute structural abnormalities, such as intracerebral hemorrhage due to trauma or coagulation disorders.

Cerebral edema and herniation also may occur during the course of heatstroke.

Eyes

Examination of the eyes may reveal nystagmus and oculogyric episodes due to cerebellar injury.

The pupils may be fixed, dilated, pinpoint, or normal.

Cardiovascular

Heat stress places a tremendous burden on the heart. Patients with preexisting myocardial dysfunction do not tolerate heat stress for prolonged periods.

Patients commonly exhibit a hyperdynamic state, with tachycardia, low systemic vascular resistance, and a high cardiac index.

A hypodynamic state, with a high systemic vascular resistance and a low cardiac index, may occur in patients with preexisting cardiovascular disease and low intravascular volume. A hypodynamic state also may signal cardiovascular collapse.

The central venous pressure generally is within the reference range or elevated unless the patient is severely volume depleted.

High-output cardiac failure and low-output cardiac failure may occur.

Pulmonary

Patients with heatstroke commonly exhibit tachypnea and hyperventilation caused by direct CNS stimulation, acidosis, or hypoxia.

Hypoxia and cyanosis may be due to a number of processes, including atelectasis, pulmonary infarction, aspiration pneumonia, and pulmonary edema.

Gastrointestinal

Gastrointestinal hemorrhage occurs frequently in patients with heatstroke.

Hepatic

Patients commonly exhibit evidence of hepatic injury, including jaundice and elevated liver enzymes.

Rarely, fulminant hepatic failure occurs, accompanied by encephalopathy, hypoglycemia, and disseminated intravascular coagulation (DIC) and bleeding.

Musculoskeletal

Muscle tenderness and cramping are common; rhabdomyolysis is a common complication of EHS.

The patient's muscles may be rigid or limp.

Renal

Acute renal failure (ARF) is a common complication of heatstroke and may be due to hypovolemia, low cardiac output, and myoglobinuria (due to rhabdomyolysis).

Patients may exhibit oliguria and a change in the color of urine.

Increased heat production

Increased metabolism

• Infections
• Sepsis
• Encephalitis
• Stimulant drugs
• Thyroid storm
• Drug withdrawal

Increased muscular activity

• Exercise
• Convulsions
• Tetanus
• Strychnine poisoning
• Sympathomimetics
• Drug withdrawal
• Thyroid storm

Moderate physical exercise, convulsions, and shivering can double heat production and result in temperature elevations that generally are self-limited and resolve with discontinuation of the activity.

Strenuous exercise and status epilepticus can increase heat production 10-fold and, when uninterrupted, can overwhelm the body's heat-dissipating mechanisms, leading to dangerous rises in body temperature.

Stimulant drugs, including cocaine and amphetamines, can generate excessive amounts of heat by increasing metabolism and motor activity through the stimulatory effects of dopamine, serotonin, and norepinephrine. The development of heatstroke in individuals intoxicated with stimulants is multifactorial and may involve a complex interaction between dopamine and serotonin in the hypothalamus and the brainstem.

Neuroleptic agents also may elevate body temperature by increasing muscle activity, but, occasionally, these agents may cause neuroleptic malignant syndrome (NMS). NMS is an idiosyncratic reaction characterized by hyperthermia, altered mental status, muscle rigidity, and autonomic instability and appears to be due to excessive contraction of muscles.

Certain drugs, such as inhaled volatile anesthetics and succinylcholine, may result in malignant hyperthermia. In contrast to heatstroke, malignant hyperthermia is believed to be induced by a decreased ability of the sarcoplasmic reticulum to retain calcium, resulting in sustained muscle contraction.

Decreased heat loss

Reduced sweating

• Dermatologic diseases
• Drugs
• Burns

Reduced CNS responses

• Advanced age
• Toddlers and infants
• Alcohol
• Barbiturates
• Other sedatives

Reduced cardiovascular reserve

• Elderly persons
• Beta-blockers
• Calcium channel blockers
• Diuretics
• Cardiovascular drugs - Interfere with the cardiovascular responses to heat and, therefore, can interfere with heat loss

Drugs

• Anticholinergics
• Neuroleptics
• Antihistamines

Exogenous factors

• High ambient temperatures
• High ambient humidity

Reduced ability to acclimatize

• Children and toddlers
• Elderly persons
• Diuretic use
• Hypokalemia

Reduced behavioral responsiveness

Infants, patients who are bedridden, and patients who are chronically ill are at risk for heatstroke because they are unable to control their environment and water intake. To compound matters, comorbidities and polypharmacy in the elderly can compromise their recovery.