Heat-related illness should be an easy diagnosis, but it is not that simple, and there are a number of tripwires.
The common pathophysiology for most heat-related illnesses is heat generated from muscular activity that accumulates faster than can be dissipated via increased skin blood flow and sweating, resulting in exertional hyperthermia. Part of the challenge for clinicians is that heat illness is a continuum with a significant overlap of signs and symptoms. Granted, heat rash, heat cramps, and heat edema aren't that confusing, but diagnostic accuracy can be a little more challenging at the other end of the spectrum.
Syncope, a diagnostic element of heat syncope, a milder form of heat-related illness, can also occur in heat exhaustion and heat stroke. (Wilderness Environ Med. 2014;25[4 Suppl]:S55.) Altered mental status is the defining symptom of heat stroke, but heat exhaustion and heat syncope also can have brief periods of altered mental status. Other minor neurological signs and symptoms, including dizziness, anxiety, vomiting, headache, and fatigue, also occur across the heat illness spectrum. An elevated temperature also occurs across the spectrum of heat illness.
The diagnosis of heat stroke depends on only two of the multiple signs and symptoms associated with this condition: a core temperature of higher than 40°C and CNS abnormalities such as the three Cs (confusion, convulsions, and coma). The documented temperature should be a esophageal or rectal temperature. Unfortunately, a core temperature may not be feasible or practical in some prehospital or austere settings. Oral, axillary, and tympanic temperatures can provide clues, but they are considered vulnerable to environmental influences. It is possible for a heat stroke patient to have a temperature below 104°F (40°C) if the patient was treated aggressively in the prehospital setting. Consequently, heat stroke can initially be confused for heat exhaustion and heat syncope without the core temperature.
The diagnosis of these heat-related illnesses is not based on any specific laboratory testing, though it will show end-organ injury in the most serious cases. It is mandatory that treatment start long before any laboratory tests are available to the clinician.
Heat injury causing end-organ damage without any significant neurologic manifestations can occur. A condition that looks like simple heat exhaustion may in fact have evidence of end-organ damage. It is a clinical judgment call when to evaluate patients for end-organ damage (vital signs, urine output, liver function tests, prothrombin time, urinalysis including urine myoglobin, BUN, creatinine, and creatine kinase).
Heat stroke also has two similar presentations, exertional and nonexertional or classic and exertional heat stroke. Classic or nonexertional heat stroke usually occurs in the elderly patient with underlying chronic medical conditions that impair thermoregulation, inadequate hydration, or lack of access to cooling. Sweating v. no sweating is another area of confusion in these two presentations. It is dangerous to assume that a sweating patient cannot be experiencing heat stroke. Sweating commonly occurs in heat stroke, especially in the exertional type. In fact, the overheated athlete suffering from heat stroke will often demonstrate profuse sweating.
The most-recommended method for cooling the hyperthermic patient is cold water immersion therapy. A short stent immersed in ice water also appears to cool patients rapidly. Some evidence shows that immersion initiates vasoconstriction and shivering through its effect on skin temperature, but it is still considered the best option. (Clin Infect Dis. 2000;31 Suppl 5:S224; http://bit.ly/2Irtt3q.) Contrary to common assumptions, the endpoint of therapy is not a normal temperature. There is no specific endpoint for stopping cooling measures, but most sources recommend stopping around 100.4-102°F (38-39°C) to avoid an unwanted hypothermic overshoot.
Heat-related illness is a bona fide spectrum illness, and tripwires in diagnosis and management exist. The patient shown in the video is a good example of a heat illness diagnostic challenge. Based on his presentation, we couldn't come to a firm conclusion whether he had simple heat syncope or heat exhaustion.
Watch this video of a boy who experienced heat illness while running.