The United States’ International Classification of Diseases (ICD-10) codes for 2020 contain updated definitions of heat illness that go beyond heatstroke.¹ Here, we will discuss definitions and treatment implications.

Hyperthermia is defined as an elevated core body temperature related to thermoregulation failure. The body combats hyperthermia with thermoregulation, causing sweating and cutaneous vasodilation, maximizing evaporative heat loss. When these compensatory measures do not suffice, gut ischemia can occur, sometimes releasing cytokines, which can trigger cytokine-mediated systemic inflammatory response leading to multiorgan system failure. Environmental risk factors for developing heat illnesses include high temperature and humidity, lack of air movement, and presence of a heat wave.² The spectrum to heat illness carries a stepwise approach to treatment.¹

Definitions

Heat Edema: Heat edema is a mild illness caused by vasodilatation of the hands and feet. It typically occurs in unacclimatized elderly patients. Treatment is supportive with compressive stockings and relocation to a cooler environment.^2,3

Heat Cramp: Heat cramp is characterized by generalized muscle pain and persistent involuntary contraction of the muscles. Treatment is supportive and involves oral hydration, stretching, and muscle massage.^2,3

Heat Syncope: Also known as exercise-associated collapse, this clinical diagnosis typically occurs after completion of strenuous activity. The mechanism involves abrupt decrease in venous return once the activity has ceased, likely from peripheral venous pooling. It can occur in cold environments as well. Cardiac disease should be considered as part of the initial differential diagnosis, as this condition is correctly considered a diagnosis of exclusion. Treatment involves supine rest, elevation of the legs, and oral hydration.³

Heat Exhaustion: Heat exhaustion is an inability to continue adequate cardiac output resulting from strenuous exercise compounded by environmental heat stress. It typically presents as collapse during the exercise due to sodium derangements (versus heat syncope, which occurs after cessation of activity). Diagnosis is clinical and based on elevated core body temperature with signs of dehydration. Patients are typically anhidrotic. Treatment is the same as for heat syncope: supine positioning, elevation of the legs, and hydration. Cooling  the patient to prevent possible progression to heatstroke is important.^2–4 ICD-10 classifies heat exhaustion into subcategories of anhidrotic, salt depletion, and unspecified. It is important to note that the salt depletion category also includes heat exhaustion due to salt (and water) depletion.¹

Heat Fatigue, Transient: This is an ICD-10 code; however, it does not have a concurrent medical definition. Based on opinion, this code could be used for a patients not meeting criteria for nonexertional heatstroke (no end organ damage or CNS dysfunction) but still exhibiting transient symptoms of lethargy and weakness. 

Other Effects of Heat and Light: Miliaria rubra, otherwise known as heat rash, is included in this category. It is a benign, pruritic condition secondary to obstruction of eccrine sweat glands. It is treated with supportive measures and topical corticosteroids.²

Heatstroke and Sunstroke: Heatstroke is defined as a core body temperature >104ºF (40ºC) with CNS dysfunction due to a substantial environmental heat load that cannot be adequately dissipated.2,5  Exertional heatstroke typically affects younger, healthier individuals undergoing strenuous activity in a high-temperature and high-humidity environment. Individual risk factors include poor physical fitness, obesity, lack of acclimatization, underlying acute illness, initial dehydration, congenital disorders (eg, anhidrosis), alcohol use, and certain drugs and supplements, including amphetamines. Signs and symptoms include tachycardia, tachypnea, hypotension, nausea, vomiting, weakness, muscle flaccidity, ataxia, and encephalopathy. The differential diagnosis can include exertional hyponatremia and neuroleptic malignant syndrome.

Prehospital and hospital management centers on airway, breathing, and circulation. Rectal temperature and blood glucose should be monitored. Treatment prioritizes aggressive cooling measures within 30 minutes of presentation, which usually means initiation before transportation to a hospital.^2,3,5 There are limited studies surrounding first-line cooling measures, but based on the Wilderness Medical Society 2019 Update, ice water immersion carries a 1A recommendation (strong recommendation and high-quality evidence).³ Ice water immersion consists of removing all clothing and placing the patient in a tub of cold water. Second-line treatment consists of tarp-assisted cooling, a cold shower, and cold water from a hose.

Third-line treatment is evaporative cooling (1C recommendation, indicating a strong recommendation but low-quality or very-low-quality evidence). Evaporative cooling combines evaporation and convection and involves spraying the exposed patient with lukewarm water with fans blowing over the skin. Cooling should continue until a rectal temperature of 100.4°F (38ºC) is achieved. Shivering is a theoretical response found in normal patients but is not found in those who have heatstroke. If hypotension is present or there is concern for dehydration, patients can receive small isotonic crystalloid boluses while monitoring for signs of acute pulmonary edema.³

The fourth-line recommendation consists of ice packs to the entire body. If only chemical cold packs are available, application should to glabrous skin, palms, and soles only.^3,6 Cooling measures should be continued until core temperature has been lowered to 102.2°F (39ºC).^3,4 Dantrolene has been considered as a pharmacological treatment option, but data are limited, and no consistent improvement in outcomes has been described. It is currently not recommended as a therapy.^3,7

Nonexertional heatstroke is also a clinical diagnosis. This typically affects children who cannot escape hot environments and elderly people with impaired thermoregulation. Signs and symptoms are similar to exertional heatstroke: the characteristic sign is neurological dysfunction. Again, treatment is focused on aggressive cooling measures, along with rectal core body temperature and blood glucose measurements. Risk factors include lack of air conditioning, social isolation, underlying comorbid conditions, and certain drugs and supplements such as anticholinergics and beta-blockers. The differential diagnosis includes, but is not limited to, infection, endocrine dysfunction, or underlying CNS, toxic, and oncological processes.^2,5,7,8 Data are limited as to the best cooling technique, but experts recommend evaporative cooling, as most nonexertional heatstroke occurs in elderly patients with baseline comorbidities that require advanced monitoring. In addition, elderly patients may not be able to tolerate cold water immersion. This method, however, does cool at a slower rate in actual heatstroke victims (versus in study subjects) and is slower than cold water immersion.3,4 Antipyretics will not treat the cause of heatstroke and should be withheld.

In the hospital setting, patients experiencing exertional or nonexertional heatstroke should have a complete blood counts, basic metabolic panels, liver function tests, coagulation studies, and urinalyses. An ECG and chest X-ray are recommended for elderly patients. Administering a dose of antibiotics may be warranted if the differential diagnosis favors infection as a cause or contributing cause. Other laboratory tests to consider include creatine kinase, urine myoglobin, ECG, troponins, blood gas, lactate, toxicology screen, chest X-ray, and CT of the head as needed.

Complications of these illnesses include permanent neurological sequelae, seizures, noncardiogenic pulmonary edema, acute respiratory distress syndromes, arrhythmias, hypotension, gastrointestinal ischemia, hepatic injury, acute kidney injury, rhabdomyolysis, and disseminated intravascular coagulation. In most cases, admitting patients to the hospital for further observation is recommended.^3,5,7,8