Be Aware to Hypertermia

 

Hyperthermia is characterized by an unchanged (normothermic) setting of the thermoregulatory center in conjunction with an uncontrolled increase in body temperature that exceeds the body’s ability to lose heat. Exogenous heat exposure and endogenous heat production are two mechanisms by which hyperthermia can result in dangerously high internal temperatures. Excessive heat production can easily cause hyperthermia despite physiologic and behavioral control of body temperature. For example, work or exercise in hot environments can produce heat faster than peripheral mechanisms can lose it.

Although most patients with elevated body temperature have fever, there are a few circumstances in which elevated temperature represents not fever but hyperthermia. Heat stroke, caused by thermoregulatory failure in association with a warm environment, may be categorized as exertional or nonexertional. Exertional heat stroke typically occurs in younger individuals exercising at ambient temperatures and/or humidities that are higher than normal. In a dry environment and at maximal efficiency, sweating can dissipate ±600 kcal/h, requiring the production of>1 L of sweat. Even in normal individuals, dehydration or the use of common medications (e.g., over-the-counter antihistamines with anticholinergic side effects) may help to precipitate exertional heat stroke. Nonexertional or classic heat stroke typically occurs in either very young or elderly individuals, particularly during heat waves. According to the Centers for Disease Control and Prevention (CDC), there were 7000 deaths attributed to heat injury in the United States from 1979 to 1997. The elderly, the bedridden, persons taking anticholinergic or antiparkinsonian drugs or diuretics, and individuals confined to poorly ventilated and non-air-conditioned environments are most susceptible.

Drug-induced hyperthermia has become increasingly common as a result of the increased use of prescription psychotropic drugs and illicit drugs. Drug-induced hyperthermia may be caused by monoamine oxidase inhibitors (MAOIs), tricyclic antidepressants, and amphetamines and by the illicit use of phencyclidine (PCP), lysergic acid diethylamide (LSD), methylenedioxymethamphetamine (MDMA, “ecstasy”), or cocaine.

 

Malignant hyperthermia occurs in individuals with an inherited abnormality of skeletal-muscle sarcoplasmic reticulum that causes a rapid increase in intracellular calcium levels in response to halothane and other inhalational anesthetics or to succinylcholine. Elevated temperature, increased muscle metabolism, muscle rigidity, rhabdomyolysis, acidosis, and cardiovascular instability develop rapidly. This condition is often fatal. The neuroleptic malignant syndrome (NMS) occurs in the setting of neuroleptic agent use (antipsychotic phenothiazines, haloperidol, prochlorperazine, metoclopramide) or the withdrawal of dopaminergic drugs and is characterized by “lead-pipe” muscle rigidity, extrapyramidal side effects, autonomic dysregulation, and hyperthermia. This disorder appears to be caused by the inhibition of central dopamine receptors in the hypothalamus, which results in increased heat generation and decreased heat dissipation. The serotonin syndrome, seen with selective serotonin uptake inhibitors (SSRIs), MAOIs, and other serotonergic medications, has many overlapping features, including hyperthermia, but may be distinguished by the presence of diarrhea, tremor, and myoclonus rather than the leadpipe rigidity of NMS. Thyrotoxicosis and pheochromocytoma can also cause increased thermogenesis.

 

It is important to distinguish between fever and hyperthermia since hyperthermia can be rapidly fatal and characteristically does not respond to antipyretics. However, there is no rapid way to make this distinction. Hyperthermia is often diagnosed on the basis of the events immediately preceding the elevation of core temperature—e.g., heat exposure or treatment with drugs that interfere with thermoregulation. However, in addition to the clinical history of the patient, the physical aspects of some forms of hyperthermia may alert the clinician. For example, in patients with heat stroke syndromes and in those taking drugs that block sweating, the skin is hot but dry. Moreover, antipyretics do not reduce the elevated temperature in hyperthermia, whereas in fever—and even in hyperpyrexia—adequate doses of either aspirin or acetaminophen usually result in some decrease in body temperature.

Source: Harrison_s_Principles_of_Internal_Medicine_16th_Edition