Obtaining an accurate history from the patient, family, bystanders, or first responders is of pivotal importance as this can help determine what the patient was doing when symptoms started and in what environment they were found. Symptoms of heat illness vary depending on which form of heat injury has occurred. The following are a list of various heat-related injuries[17]:

• Heat edema: swelling and possible discomfort in the distal extremities due to vasodilation and gravity-dependent pooling of fluids. An example of this would be an elderly individual who has recently arrived at an environment hotter than previously accustomed to with dependent swelling in the extremities after one to two days. Individuals may complain that gloves or shoes/boots feel tight due to this acral swelling.[18]
• Heat rash (also known as miliaria or prickly heat): caused by obstruction of eccrine sweat glands during exposure to hot climates. The use of restrictive clothing or occlusive objects like bandages may exacerbate heat rash. The various types include:
• Miliaria crystallina: this is the most common and mild form and involves the superficial stratum corneum. It typically develops on the face and trunk as small, clear vesicles. This condition can also appear during the neonatal period.
• Miliaria rubra: this involves the deeper epidermis. It presents as small, non-follicular erythematous macules and papules topped by a punctate vesicle, usually on the neck and upper trunk. These may be pruritic or feel like a burn. The onset is typically days to weeks after heat exposure and can progress to a chronic, pustular state known as miliaria pustulosa.
• Miliaria profunda: this more severe form involves the deeper, dermal-epidermal junction and presents as larger white vesicles that are more commonly asymptomatic due to their location deeper in the dermis.
• Heat cramps: involuntary spasmodic contractions of large muscle groups (as opposed to an isolated muscle spasm/cramp that can also occur during or after exertion). This condition is due to a relative deficiency of sodium, potassium, chloride, or magnesium. Other symptoms may include nausea, vomiting, fatigue, weakness, sweating, and tachycardia.
  • There has previously been a distinction between muscle cramping due to heat-exposure and cramping caused by exertion, also known as exercise-associated muscle cramps (EAMCs). The reason behind this is because research has shown that exertion-related cramping is not necessarily related to elevated body temperature, it does not occur with passive heating at rest, and it can be present during exercise in cool, temperature-controlled environments. Regardless, the treatment, as described below, is the same for both cramping conditions.[19]
• Heat syncope: temporary, self-limited dizziness, weakness, or loss of consciousness during prolonged standing or positional changes in a hot environment, including during physical activity. The thinking is that it is due to a combination of dehydration, pooling of blood in the venous system, decreased cardiac filling, and low blood pressure, which leads to decreased cerebral blood flow.[19] 
• Heat stress or heat exhaustion: symptoms usually include any combination of nausea, vomiting, dizziness, light-headedness, irritability, headache, thirst, weakness, excessive sweating, muscle cramps, and decreased urine output in the setting of high body temperatures. External temperatures may be more moderate if associated with intense physical exertion. Survey-based data has shown that some of the most common symptoms are headache, exhaustion, or a combination of symptoms.[20] Body temperature is not usually elevated at or above 40.5 C [105 F]. Heat exhaustion can progress to life-threatening heatstroke if immediate interventions, including removal from the hot environment, are not carried out.
• Heat injury: presents with similar symptomatology as heat exhaustion and includes organ injury or dysfunction (cardiac, renal, hepatic, gut, or musculoskeletal injury).

Central nervous system involvement with symptoms or signs that may include seizures, gait abnormalities, delirium, coma, etc. in the setting of elevated temperature cause concern for heatstroke, as described elsewhere.

History and physical examination may be all that is necessary to make the diagnosis, and it is not uncommon that testing, including lab work, is normal. If a patient's presentation is concerning for heat stress, heat injury, or heatstroke, a core temperature should be obtained with rectal thermometry if clinically possible.[21] Laboratory tests are not typically necessary unless there is a concern for electrolyte imbalance, significant dehydration, or organ injury or to investigate other diagnoses. Because of the risk for critical cardiovascular collapse found with progression to heatstroke, further testing may be warranted depending on the severity of clinical presentation. Episodes that are not consistent with the above presentations should obtain a referral for further workup.[22][23] Tests to consider include:

• CBC and clotting factors as a significant systemic inflammatory response may be present that can progress to disseminated intravascular coagulation in severe cases
• Electrolytes and magnesium levels due to volume status abnormalities. These may have developed if there was an unbalanced replacement of fluid losses, i.e., drinking large quantities of water with no electrolytes
• Kidney and liver functioning testing due to the increased risk of organ injury found with significant heat illness
• Creatinine kinase levels to evaluate for skeletal muscle injury and rhabdomyolysis
• Urinalysis to further evaluate the health of the kidneys and presence of myoglobin
• EKG, as heat illness, can provoke cardiac arrhythmias or unmask arrhythmias that may warrant expert consultation.[24][25]
• Glucose levels as patients with heat illness frequently have hypoglycemia and may require carbohydrate replacement

Patients may also have elevated levels of lactic acid secondary to decreased organ perfusion. Other tests, including a lumbar puncture or CT scan of the head, might be indicated if concern exists for other causes of a patient's presentation.

In general, the initial treatment of heat illness involves stopping the activity and moving the individual to a cool environment as all of these conditions will benefit from cooling.[26] Supportive care is the recommended strategy for the more benign conditions described above, including heat edema, syncope, rash, and cramps. For more serious presentations, including heat exhaustion, resuscitation should occur as typical to address concerns that may exist for airway, respiratory, or cardiovascular compromise and, if present, may warrant emergent transportation to a higher level of care. Otherwise, oral rehydration with water or an electrolyte-containing drink can commence on-site. Studies have previously shown that oral and IV rehydration are equivalent for the treatment of heat illness.[27][28] Care should be taken not to overhydrate patients as some may be sensitive to significant volume status changes, including patients with a decreased cardiac function. Specific strategies are as follows:

• Heat edema: elevation of the affected limbs and the use of compression garments can help with dependent swelling. There is no role for diuretic medications for heat edema.
• Heat rash/miliaria: a staple of treatment includes the avoidance of tight-fitting clothing. Symptoms are treatable with topical lotions like calamine or steroids of mid-potency like triamcinolone 0.1% cream, which will also help with inflammation. If there is any concern for bacterial superinfection, as may be seen in the pustulosa form or the development of impetigo, topical antibiotics should be considered with selection towards those with adequate staphylococcal coverage like mupirocin. 
• Heat cramps: The patient should cease physical activity and initiate oral rehydration. Rest and massage or passive stretching of the muscle groups have also demonstrated to be helpful. Additional treatment with medications, including diazepam, magnesium, or calcium, has not shown to be effective.[29]
• Heat syncope: the treatment also includes replacement of fluid losses with a preference towards oral rehydration over parenteral rehydration. Supine positioning with leg elevation above the level of the heart can also be helpful. Orthostatic vital sign measurements can be considered an adjunct to monitor for adequate replacement in addition to the resolution of symptoms during positional changes.
• Heat stress, exhaustion, and injury: more aggressive measures at cooling should take place, including lukewarm water sprayed over the skin with forced air conduction to promote evaporation and consideration towards parenteral rehydration if the patient cannot tolerate oral rehydration due to symptoms or degree of injury. Further, the correction of electrolyte and divalent disturbances may also require intervention.

There is no role for antipyretics or muscle relaxants like dantrolene in heat illness. 

More dramatic measures are necessary if the patient has symptoms and signs consistent with heatstroke; the management of heatstroke is beyond the scope of this article.

The differential diagnosis for these presentations includes:

• Infectious etiologies like tetanus, sepsis, or meningoencephalitis, which are similar concerning alterations in mental status, body temperature setpoints, and focal neurologic changes that may mimic heat-related injury.
• Primary metabolic abnormalities, including endocrine dysfunction like thyroid storm or diabetic ketoacidosis
• Neurologic insults, including stroke or primary seizure disorder.
• Toxicologic ingestions including anticholinergic toxidrome, neuroleptic malignant syndrome, serotonin syndrome, and withdrawal symptoms

Specific heat illnesses can also have symptoms that mimic other syndromes. For example, heat edema should not be mistaken for swelling caused by other medical conditions, including hypervolemia from congestive heart failure or liver dysfunction or a deep vein thrombosis of the extremity. Heat syncope should not be confused for syncope of other etiology, including cardiogenic or with different pathologies that lead to a transient change in consciousness (e.g., pulmonary embolism, rupture of an ectopic pregnancy, aortic dissection, or some environmental gas exposures).

The prognosis for the more benign heat illnesses described above is excellent. They are self-limited and resolve with removal from the hot environment and supportive care with rehydration and electrolyte replacement.

The minor heat illnesses are benign and self-limited with no on-going complications. For the more moderate heat illnesses, including heat exhaustion/stress and heat injury, complications are exacerbated by the patient's age, comorbidities, the presence of organ injury, or if severe electrolyte disturbances have occurred. These patients may benefit from admission to a tertiary care center for further stabilization before discharge. Predictive factors for hospitalization include age of equal to or greater than 65 years, the level of temperature elevation, alterations in mental status, and elevations of creatinine from baseline.[30]

Public education should focus on preventative measures that are possible to ensure people understand the risks of heat illness. Education should include the importance of maintaining an adequate level of cooling during the hottest of days and the importance of staying shaded. Utilizing the wet-bulb globe temperature (WBGT) index, which is a composite index of temperature, humidity, and solar radiation is an excellent assessment of the environmental risk of developing a heat-related illness.[22] If not available, heat index, which accounts for temperature and humidity, can be used as a slightly less effective surrogate. Screening tools also exist both at the personal and at the population-level to evaluate for susceptibility to heat illness, although there is conflicting evidence about their effectiveness.[31][32] Other preventative strategies include utilizing air-conditioning, taking breaks in shaded or cooled areas during particularly hot days, and avoidance of excessive exertion during the hottest periods of the day. Acclimation in the days to weeks before performing maximal exertion at a hot location or time of day is one of the most successful strategies to prevent heat-related injuries. Previous recommendations include an acclimation period of 7 to 14 days with maintenance as decay can occur over 2 to 3 weeks.[19] Appropriate hydration is crucial in hot environments, with one recommendation in adolescents being to consume 240 mL (8 ounces) of fluid for every pound lost during exercise.[15]

An interprofessional approach to patient care is unavoidable as there must be coordination between bystanders or staff at the scene of the injury and first responders when they arrive. Treatment for these conditions can occur at the scene and may include removal from a hot environment, supine positioning, and rehydration. Once evaluated by an initial medical professional like an on-site physical trainer or by emergency medical services, transport concerns may require discussion with an online medical control physician depending on the stability of the patient. Also, specific centers may be more appropriate for the care of the patient depending on the availability of additional resources, including critical care units. If transport is necessary for a patient, different treatment modalities may need to be employed, including EKG testing, glucose point-of-care testing, and IV placement by technicians or nursing staff, or more invasive cooling measures depending on the severity of the patient's illness that may require specialist consultation. Emergency department nurses are involved in triage, initial care, patient monitoring, and education. Depending on the degree of dysfunction, specialists in critical care may also need to be consulted, although this would be more consistent with heatstroke. With an interprofessional approach to these cases, heat-related illness can achieve optimal patient outcomes. [Level 5]