You may well be the first specialist to evaluate an elderly patient with accidental hypothermia, a severe medical illness, because patients with this condition may present initially with cognitive impairment and disruptive behavior. This problem is particularly evident when evaluating elderly patients. Accidental hypothermia commonly mimics major mental illness, may be induced or exacerbated by psychotropic medications, is commonly fatal, and may remain unrecognized without a high index of suspicion.
Hypothermia is defined as a fall in body temperature below 95°F or 35°C (Box 1). Clinical mercury thermometers commonly range between 96°F and 106°F. Thus, the family member or clinician may not suspect hypothermia after the initial temperature measurement.
The diagnosis of accidental hypothermia is straightforward if there is a history of environmental exposure, but such evidence is often lacking in urban settings and among the elderly. Also, particularly in the elderly, hypothermia may occur at room temperature, secondary to diseases that strike the hypothalamic thermoregulatory center.
Subjects with core body temperatures dropping from 95°F to 90°F develop amnesia, dysarthria, confusion, and disruptive behavior.1 Further cooling as the body temperature falls to 82.4°F yields stupor, paradoxical undressing, and hallucinations. These characteristics are illustrated in the accompanying vignette of Ms. B.
The body’s thermoregulatory center located in the hypothalamus normally maintains core body temperature between 97.5°F (36.5°C) and 99.5°F (37.5°C). When body temperature declines, heat production increases by shivering, and heat loss is reduced by decreasing cutaneous blood flow.
Accidental hypothermia is defined as an unintentional fall in body temperature below 95°F (35°C). The coordinated systems responsible for thermoregulation start to fail. Heat loss through radiation, conduction, convection, respiration, and evaporation occurs because compensatory physiologic mechanisms are both limited and impaired.
Schizophrenia and the hypothalamus
Over the course of 6 months before her death, Ms. B. showed evidence of both thermoregulatory dysfunction and autonomic nervous system instability. We do not know if these hypothalamic problems were separate from, or intrinsic parts of, her schizophrenia.
The hypothalamus regulates autonomic, endocrine, and visceral function. Hypothalamic dysfunction may be an intrinsic part of schizophrenia. Such dysfunction occurs most commonly in the periventricular and supraoptic nuclei of the hypothalamus.2 These areas are adjacent to hypothalamic areas regulating body temperature.3
Lesions in anterior parts of the hypothalamus (temperaturesensitive neurons in the preoptic nuclei—located close to nuclei controlling thirst and osmotic regulation) may induce hyperthermia, impairing heat-dissipating mechanisms including vasodilatation and sweating. Lesions in posterior parts of the hypothalamus may impair heat conservation and heat production mechanisms and induce hypothermia.4
Associated medical problems
Independent of drug treatment, metabolic and cardiovascular problems occur more frequently in patients suffering from schizophrenia than they do in the general population.5 Ms. B. developed hypertension, diabetes mellitus, dyslipidemia, and coronary artery disease.
Diabetes mellitus in particular is a risk factor for hypothermia and may be found in more than 10 percent of elderly patients who suffered thermoregulatory failure before dying.6 Diabetes may impair autonomic system vasomotor stability and the body’s ability to vasoconstrict to preserve body heat.
Dementia and hypothermia
Cognitive impairment is a core feature of schizophrenia,7 and dementia is a common outcome among elderly patients suffering with the disorder.8 We don’t know whether Ms. B.’s progressive cognitive deterioration derived from dementia associated with schizophrenia or from a separate process such as Alzheimer’s disease.
Alzheimer’s disease may limit behavioral responses to cooling or even recognition that the body temperature is dropping.9 This disease is associated with weight loss (and attendant loss of body fat that acts, in part, as insulation), hypothalamic pathologic changes, and decreased serotonin activity in the hypothalamus. The processes leading to Ms. B.’s progressive cognitive impairment most likely contributed to hypothalamic dysregulation and subsequent accidental hypothermia.
Ms. B.’s repeated disrobing during her stay at the adult care facility was ascribed to dementia. Serial body temperature measurements were not available, so we do not know the extent to which the disrobing may have been paradoxical—that is, undressing when cold rather than dressing more warmly. Paradoxical undressing is found during moderate (82.4°F to 90°F) hypothermia.1
Medications and hypothermia
Normally, mild hypothermia induces vasoconstriction and initial increases in heart rate and cardiac output. (The latter increase is principally driven by the accelerated heart rate rather than increased stroke volume.) These changes tend to protect the patient from further lowering of body temperature. But Ms. B.’s medications included the vasodilator, isosorbide dinitrate; the beta-blocker, metoprolol; and the angiotensin-converting enzyme (ACE) inhibitor, lisinopril. All these agents impaired her capacities to vasoconstrict and to increase cardiac output, thereby reducing her ability to conserve body heat.