Heat rash, also known as prickly heat, lichen tropicus, and miliaria rubra, occurs when sweat ducts are blocked by dead skin. Tiny vesicles form as sweat accumulates under the skin, resulting in purities. Chronic vesicles can rupture into the surrounding tissue, causing skin thickening and scarring. Treatment includes antihistamines for purities, cool baths, and time in a cool environment. Chronic heat rash will need dermatologic follow-up for treatment with salicylate gels to avoid scarring.
Heat edema occurs mostly in older individuals who are adjusting to an increased heat strain. Vasodilatation, in combination with relative venous stasis, causes blood pooling. No dehydration or salt imbalance is usually present, and diuretics are not warranted. This must be distinguished from more worrisome causes, including deep vein thrombosis (DVT), nephrotic syndrome, liver failure, and congestive heart failure. Heat edema is a benign condition that is self-limiting, and may be treated with elevation and compression stockings. Follow-up with a primary care provider is recommended in seven to 10 days (or sooner if the condition does not improve with conservative measures).
Heat syncope occurs when peripheral vasodilatation and impedance to venous return caused by posture combine to lower the blood pressure enough to interrupt cerebral blood flow. Classically, military personnel standing at attention with locked knees are at risk. Persons usually recover promptly with lowered head, elevated lower extremities, and a cooler environment. Differential diagnosis for syncope includes concerning entitities such as idiopathic hypertrophic cardiomyopathy, dysrhythmias, acute coronary syndrome, and cerebral vascular accident. In a young, non-exerting, otherwise healthy individual who responds to appropriate therapy, disposition may include discharge home. However, advanced age, co morbidities, and a history of exertion at time of syncope require further testing. In young patients with exertional syncope, restriction on activity level pending a referral to a cardiologist for further work-up is appropriate. Aggressive search for serious disease must be made in older patients and patients with co- morbidities, and admission or transfer to a higher level of care is appropriate.
Heat cramps are defined as motor unit hyperactivity in major muscle groups, usually thigh or leg, during or several hours after prolonged exertion under heat stress. The exact mechanism is not clear. It was originally thought that hyponatremia due to excess sweating and hydrating with water caused heat cramps. However, heat cramps can occur before any rehydration has taken place, and many patients with heat cramps have no electrolyte or serum osmolarity derangements. A spinal reflex caused by overexertion has also been proposed as a mechanism. The two most useful prophylactic steps to prevent heat cramps are heat acclimatization and consuming adequate water during exercise. Relative muscle dehydration appears before the subject experiences thirst, so the advice should be to consume water at regular intervals during exercise even if the athlete does not feel thirsty.
Treatment for heat cramps includes re-hydration with an oral salt solution or IV normal saline, as well as pain control, which may require narcotics. Electrolytes should be checked and replaced as needed. Typically, heat cramps respond rapidly to treatment, rarely lasting for more than 15 minutes during a flare-up. They can produce agonizing spasms during a flare-up, and can recur several times over the next 24 to 48 hours. During the recovery period, the patient should avoid exertion since the spasms can be triggered by a normal muscular contraction. Unfortunately, they can be triggered during sleep, and awaken the patient with severe pain. The painful contractions are usually in the flexor muscles, and hyperextension of the involved muscle may overcome the spasm. When they involve the hamstring flexors, extension of the hip and knee are useful, as is standing up, and slow mild pacing. Icing the involved muscles may provide the patient with pain relief, and mild analgesics are useful when the cramps subside. In the elderly patient who is perhaps already on a diuretic agent for hypertension, the cramps are worsened by any potassium imbalance. Many of these patients will obtain relief from potassium oral supplementation even when the serum electrolyte level of potassium is normal.
Heat exhaustion is generally a result of prolonged exertion or prolonged exposure to a higher heat index than normal. Symptoms are nonspecific, and can include any of the above syndromes, as well as lightheadedness, malaise, fatigue, headache, nausea, vomiting, decreased urine output, and thirst. Dehydration and electrolyte abnormalities are common. Patients with heat exhaustion need to be in a cool, air conditioned environment, and inappropriate extra clothing should be removed. Hydration can usually be accomplished with oral salt solution or normal saline, with electrolyte correction as necessary. Patients should respond to cooling and hydration; any patient with persistent symptoms or co morbidities should be admitted to the hospital. Prudent discharge requires that the patient have access to a cool, air conditioned environment for the next 48 to 72 hours, especially for those with risk factors for heat illness. Elderly patients, patients with limited mobility, and mentally ill or retarded patients need a caretaker or family member to check on them at least twice a day during periods of higher than normal heat or humidity. Close follow- up should be arranged. Workers and athletes likewise require 48 to 72 hours of decreased activity in a cool environment, and must re-acclimate.
These patients must be immediately transferred to a higher level of care. This is a true life-threatening emergency. During the course of weather episodes in which the daily high temperature might exceed 90o F, or 80o F (32 oC or 27oC, respectively) with high humidity, urgent care physicians can reduce the burden of heat illness with brief counseling, educational handouts, and posters. Excellent patient information sources are available. Patients should be counseled to hydrate liberally, unless specifically contraindicated. Inactive individuals need four liters of fluids or more daily during heat waves, and the exerting adult may need up to 10 liters daily. Thirst is an unreliable indication of hydration status, as it is mainly stimulated by hypernatremia, and hydration must often be scheduled. Exerting adults should drink 250 mL of fluid every 15 to 20 minutes during exercise, and children should drink 150 mL. It is often impossible to hydrate enough during exercise, and hydration must begin before activity and continue afterwards. Thirst is stimulated by eating; hydration at meals in addition to during activities is necessary. Electrolyte solution is generally unnecessary for people consuming a normal diet, and has only been shown to increase exercise tolerance during the first three days of acclimatization. However, if the taste is more tolerable, especially to children, this may encourage hydration. Patients not consuming a normal diet, exerting heavily for prolonged periods of time, or with gastroenteritis will benefit from electrolyte solutions, and there should be no hesitation to use intravenous fluids. Air conditioning for as little as three hours per day is the only intervention known to be protective against heat stroke during heat waves.16 For this reason, some urban areas have heat wave response plans that include heat shelters, such as malls. Fans have not been shown to be protective.17 Exertion should, if possible, occur in the early morning or late afternoon or evening, avoiding activity during the heat of the day. Outdoor laborers should be given regular rest and hydration breaks; an air conditioned rest area is protective against heat illness and maximizes exercise tolerance.