Epidemiology, Etiology, And Pathophysiology
Fever in the elderly is a common complaint. Approximately 10% of elderly patients will have a fever when presenting to the ED.4 Of these, 70-90% will be admitted, and 7-10% will die within one month, as compared to less than 1% of inpatients 17-59 years old.4,5 Infectious disease is the most common cause of fever in the elderly patient presenting to the ED and is the most common reason for admission to the hospital in this population.4,6
Fever in the elderly should be regarded with concern. Its presence usually presages serious disease. Most fevers in the aged are caused by infections, and even in chronic fevers (fevers of unknown origin), more than one-third are due to microbes. Unlike fever in younger patients who often harbor a benign viral syndrome, fever in the elderly is typically associated with bacterial disease.7,8 (See Table 1.)
Appreciating the geriatric physiology helps explain the diminished fever response noted in this population. Fever occurs when the hypothalamic set point is fixed to a higher temperature. This is a response to cytokines, such as IL-1, IL-6, and TNF, which are released by leukocytes in the presence of infection, neoplasm, toxins, drugs, or immune complexes.9,10 The hypothalamus releases prostaglandin E in response to these cytokines. The cascade affects vasomotor centers, sympathetic nerves, and vasculature to decrease heat dissipation and to increase body temperature.11
Many of these pathways, illustrated in Figure 1, are blunted in the elderly.7,12 Aging and vascular changes may also affect the hypothalamic circulation and interleukin response, resulting in a diminished fever.7 In addition, altered mental status and malnutrition, both common in elderly patients, are associated with a decreased fever response.13,14 Hippocrates wrote in Aphorism, "The fevers of old men are less acute than others, for the body is cold."15 In fact, elderly people often have a lower baseline temperature.16,17 This, in addition to the blunted fever response, makes an elderly patient less likely to reach a temperature traditionally considered a fever.
Though less likely to have a fever, older patients are more likely to develop an infection than younger adults. The explanation for this increased susceptibility is multifactorial. First, elderly patients have decreased natural barriers to infections. Fragile skin with decreased vasculature and less subcutaneous tissue contributes to slower wound healing and increased risk for skin infections.18 A less vigorous cough and decreased mucociliary clearance may predispose to pneumonia, particularly in patients with COPD.19 Comorbid illnesses also contribute to increased susceptibility. Diabetes mellitus and various malignancies can diminish the immune response. Impairments in cell-mediated immunity also contribute to increased infection rates in this population.
The elderly are also at increased risk for hyperthermia, defined as a temperature greater than 41°C. This is frequently due to high ambient temperatures, complicated by behavior deficits, medicines, and malnutrition.20 The impoverished or isolated elderly may be unable to escape the heat.21 Peripheral mechanisms of vasodilatation and sweating become insufficient or are overwhelmed by excess external or internal heat.22 Furthermore, many medications commonly prescribed for the elderly impair their ability to dissipate heat. These medicines include thyroid hormone, anticholinergics, phenothiazines, tricyclic antidepressants (TCAs), lithium, MAO inhibitors, and diuretics.6