Heat illness spans a broad spectrum of disease, with outcomes ranging from benign rash to fatal heat stroke. Heat illness is broadly divided into 2 types: classic and exertional. Both types occur as a result of exposure to elevated temperature with inadequate thermoregulation; however, classic illness occurs without preceding physical activity. Treatment consists of rapid cooling, fluid replacement, and physiologic support. Other milder forms of heat illness include heat fatigue, heat syncope, heat edema, and heat rash. Drugs, drug combinations, drug side effects, and infections can also cause or complicate heat illness and these manifestations may not respond to standard cooling maneuvers and treatments alone; each requires specific additional therapy or antidotes to reverse the cycle of heat and organ damage. This review examines the physiology, diagnosis, and treatment of exertional, classic, and drug-induced hyperthermia. Field and prehospital diagnosis and treatment are also reviewed, with recommendations for rehydration and monitoring in rhabdomyolysis.
On a late summer afternoon shift, a 16-year-old adolescent boy presents to the ED via EMS after collapsing during football practice. The medics state that he complained of dizziness and staggered to the edge of the field before he fell to the ground. The medics call out vital signs: blood pressure, 102/60 mm Hg; heart rate, 120 beats/min; and respiratory rate, 16 breaths/min. They did not measure a temperature, but state that he feels hot to the touch. The patient is brought to the resuscitation room where you begin your evaluation. The patient is awake, but very lethargic and unresponsive to verbal commands. You notice he is flushed, his skin is hot to the touch, and he is sweating. The rectal temperature is 40.3°C. You begin volume resuscitation with normal saline, remove his athletic equipment and clothing, and order laboratory studies. As the nurse tells you that the patient is no longer responding to verbal or noxious stimuli, you wonder if you should intubate him. You consider the quickest and most effective way to cool the patient with the resources available in your department. As you assess what other treatments the patient may need during his resuscitation, the nurse tells you there is a new patient in the next room.
As you walk into the next room, you see a 35-year-old woman with her husband at her bedside. He says his wife was treated for nausea and vomiting at an urgent care center the previous day. When she woke up today, she was confused, sweating, and her arms and legs seemed "stiff.” As you begin your evaluation, you ask the ED clerk to obtain the records from the outside facility. On examination, the patient is febrile at 39.4°C, diaphoretic, and disoriented. Her heart rate is 133 beats/min and her blood pressure is 170/110 mm Hg. On motor examination, she exhibits cogwheel rigidity. You place an IV and administer a benzodiazepine, which does little to alleviate her symptoms. You ask yourself: What next?
Heat-related illnesses exist along a continuum from the less severe symptoms of heat edema to the potentially fatal syndrome of heat stroke. (See Table 1.) Heat-related illnesses are classically defined in review literature by exposure without alteration of hypothalamic thermoregulation.1,2 In contrast, fever is defined by changes to the hypothalamic set point by pyrogenic cytokines.1 Heat stroke is a life-threatening condition and requires the highest level of clinical attention. The cardinal sign of heat stroke is a change in mental status and it is associated with a significant mortality. Delirium, convulsions (particularly during cooling), multisystem organ failure, coma, and death can occur. Any substance or situation that alters thermoregulatory function can precipitate severe, life-threatening disease. As there are no tests to confirm the diagnosis of heat stroke, other conditions presenting in a similar way should be considered and investigated. A list of differential diagnoses is provided in Table 2. Heat-related illnesses occur when the body’s cooling mechanisms are unable to control the natural rise in body temperature caused by metabolism, physical activity, or exposure to warm temperature. The true prevalence is difficult to estimate, as many cases go unreported. Heat-related illnesses can be multifactorial, but are most commonly seen in improperly conditioned patients participating in physical activities in environments with hot, humid weather (exertional), and in patients unable to escape a heated environment (classic). The challenge for the emergency clinician is twofold: first, the severity of heat illness must be established and prompt cooling therapy must be started. Second, the emergency clinician must be aware of toxicological substances, infectious processes, and other medical conditions that can cause symptoms that mimic heat illness.
The majority of heat illness cases are benign and easily reversed. After evaluation, diagnosis, and cooling treatment, the vast majority of patients presenting with mild to moderate disease can be discharged from the emergency department (ED). Important exceptions include heat stroke and lesser forms of heat illness occurring in patients who are elderly or debilitated or who have multiple comorbidities. Such conditions may result in major physiologic stress in situations of passive ambient heat exposure. This issue of Emergency Medicine Practice reviews the types of heat illness, their diagnosis, and their management. Knowing how to rapidly cool the patient, what sequelae to anticipate, and which processes mimic heat stroke can prevent mortality.
A systematic search of the literature through May 2014 was undertaken using PubMed and the Cochrane Database of Systematic Reviews. The search was performed using the terms heat illness, heat stroke, neuroleptic malignant syndrome, serotonin syndrome, aspirin overdose, sympathomimetic overdose, rhabdomyolysis, and lithium toxicity. Limitations of the studies included poor study design, small sample sizes, and lack of appropriate controls in those studies examining treatment.
Evidence-based medicine requires a critical appraisal of the literature based upon study methodology and number of subjects. Not all references are equally robust. The findings of a large, prospective, randomized, and blinded trial should carry more weight than a case report.
To help the reader judge the strength of each reference, pertinent information about the study will be included in bold type following the reference, where available. In addition, the most informative references cited in this paper, as determined by the authors, will be noted by an asterisk (*) next to the number of the reference.
Jaron Santelli, MD; Julie M. Sullivan, MD, FACEP; Ann Czarnik, MD, FACEP; John Bedolla, MD
August 2, 2014