Heatstroke is a condition caused by your body overheating, usually as a result of prolonged exposure to or physical exertion in high temperatures. This most serious form of heat injury, heatstroke, can occur if your body temperature rises to 104 F (40 C) or higher. The condition is most common in the summer months.

Heatstroke requires emergency treatment. Untreated heatstroke can quickly damage your brain, heart, kidneys and muscles. The damage worsens the longer treatment is delayed, increasing your risk of serious complications or death.

Heatstroke signs and symptoms include:

• High body temperature. A core body temperature of 104 F (40 C) or higher, obtained with a rectal thermometer, is the main sign of heatstroke.
• Altered mental state or behavior. Confusion, agitation, slurred speech, irritability, delirium, seizures and coma can all result from heatstroke.
• Alteration in sweating. In heatstroke brought on by hot weather, your skin will feel hot and dry to the touch. However, in heatstroke brought on by strenuous exercise, your skin may feel dry or slightly moist.
• Nausea and vomiting. You may feel sick to your stomach or vomit.
• Flushed skin. Your skin may turn red as your body temperature increases.
• Rapid breathing. Your breathing may become rapid and shallow.
• Racing heart rate. Your pulse may significantly increase because heat stress places a tremendous burden on your heart to help cool your body.
• Headache. Your head may throb.

When to see a doctor

If you think a person may be experiencing heatstroke, seek immediate medical help. Call 911 or your local emergency services number.

Take immediate action to cool the overheated person while waiting for emergency treatment.

• Get the person into shade or indoors.
• Remove excess clothing.
• Cool the person with whatever means available — put in a cool tub of water or a cool shower, spray with a garden hose, sponge with cool water, fan while misting with cool water, or place ice packs or cold, wet towels on the person's head, neck, armpits and groin.

Heatstroke can occur as a result of:

• Exposure to a hot environment. In a type of heatstroke, called nonexertional (classic) heatstroke, being in a hot environment leads to a rise in core body temperature. This type of heatstroke typically occurs after exposure to hot, humid weather, especially for prolonged periods. It occurs most often in older adults and in people with chronic illness.
• Strenuous activity. Exertional heatstroke is caused by an increase in core body temperature brought on by intense physical activity in hot weather. Anyone exercising or working in hot weather can get exertional heatstroke, but it's most likely to occur if you're not used to high temperatures.

In either type of heatstroke, your condition can be brought on by:

• Wearing excess clothing that prevents sweat from evaporating easily and cooling your body
• Drinking alcohol, which can affect your body's ability to regulate your temperature
• Becoming dehydrated by not drinking enough water to replenish fluids lost through sweating

Anyone can develop heatstroke, but several factors increase your risk:

• Age. Your ability to cope with extreme heat depends on the strength of your central nervous system. In the very young, the central nervous system is not fully developed, and in adults over 65, the central nervous system begins to deteriorate, which makes your body less able to cope with changes in body temperature. Both age groups usually have difficulty remaining hydrated, which also increases risk.
• Exertion in hot weather. Military training and participating in sports, such as football or long-distance running events, in hot weather are among the situations that can lead to heatstroke.

• Sudden exposure to hot weather. You may be more susceptible to heat-related illness if you're exposed to a sudden increase in temperature, such as during an early-summer heat wave or travel to a hotter climate.

  Limit activity for at least several days to allow yourself to acclimate to the change. However, you may still have an increased risk of heatstroke until you've experienced several weeks of higher temperatures.

• A lack of air conditioning. Fans may make you feel better, but during sustained hot weather, air conditioning is the most effective way to cool down and lower humidity.

• Certain medications. Some medications affect your body's ability to stay hydrated and respond to heat. Be especially careful in hot weather if you take medications that narrow your blood vessels (vasoconstrictors), regulate your blood pressure by blocking adrenaline (beta blockers), rid your body of sodium and water (diuretics), or reduce psychiatric symptoms (antidepressants or antipsychotics).

  Stimulants for attention-deficit/hyperactivity disorder (ADHD) and illegal stimulants such as amphetamines and cocaine also make you more vulnerable to heatstroke.

• Certain health conditions. Certain chronic illnesses, such as heart or lung disease, might increase your risk of heatstroke. So can being obese, being sedentary and having a history of previous heatstroke.

Heatstroke can result in a number of complications, depending on how long the body temperature is high. Severe complications include:

• Vital organ damage. Without a quick response to lower body temperature, heatstroke can cause your brain or other vital organs to swell, possibly resulting in permanent damage.
• Death. Without prompt and adequate treatment, heatstroke can be fatal.

Heatstroke is predictable and preventable. Take these steps to prevent heatstroke during hot weather:

• Wear loosefitting, lightweight clothing. Wearing excess clothing or clothing that fits tightly won't allow your body to cool properly.
• Protect against sunburn. Sunburn affects your body's ability to cool itself, so protect yourself outdoors with a wide-brimmed hat and sunglasses and use a broad-spectrum sunscreen with an SPF of at least 15. Apply sunscreen generously, and reapply every two hours — or more often if you're swimming or sweating.
• Drink plenty of fluids. Staying hydrated will help your body sweat and maintain a normal body temperature.
• Take extra precautions with certain medications. Be on the lookout for heat-related problems if you take medications that can affect your body's ability to stay hydrated and dissipate heat.

• Never leave anyone in a parked car. This is a common cause of heat-related deaths in children. When parked in the sun, the temperature in your car can rise 20 degrees F (more than 11 C) in 10 minutes.

  It's not safe to leave a person in a parked car in warm or hot weather, even if the windows are cracked or the car is in shade. When your car is parked, keep it locked to prevent a child from getting inside.

• Take it easy during the hottest parts of the day. If you can't avoid strenuous activity in hot weather, drink fluids and rest frequently in a cool spot. Try to schedule exercise or physical labor for cooler parts of the day, such as early morning or evening.
• Get acclimated. Limit time spent working or exercising in heat until you're conditioned to it. People who are not used to hot weather are especially susceptible to heat-related illness. It can take several weeks for your body to adjust to hot weather.
• Be cautious if you're at increased risk. If you take medications or have a condition that increases your risk of heat-related problems, avoid the heat and act quickly if you notice symptoms of overheating. If you participate in a strenuous sporting event or activity in hot weather, make sure there are medical services available in case of a heat emergency.

It's usually apparent to doctors if you have heatstroke, but laboratory tests can confirm the diagnosis, rule out other causes for your symptoms and assess organ damage. These tests include:

• Rectal temperature to check your core body temperature. A rectal temperature is the most accurate way of determining your core body temperature and is more accurate than mouth or forehead temperatures.
• A blood test to check blood sodium or potassium and the content of gases in your blood to see if there's been damage to your central nervous system.
• A urine test to check the color of your urine, because it's usually darker if you have a heat-related condition, and to check your kidney function, which can be affected by heatstroke.
• Muscle function tests to check for serious damage to your muscle tissue (rhabdomyolysis).
• X-rays and other imaging tests to check for damage to your internal organs.

Heatstroke treatment centers on cooling your body to a normal temperature to prevent or reduce damage to your brain and vital organs. To do this, your doctor may take these steps:

• Immerse you in cold water. A bath of cold or ice water has been proved to be the most effective way of quickly lowering your core body temperature. The quicker you can receive cold water immersion, the less risk of death and organ damage.
• Use evaporation cooling techniques. If cold water immersion is unavailable, health care workers may try to lower your body temperature using an evaporation method. Cool water is misted on your body while warm air is fanned over you, causing the water to evaporate and cool your skin.
• Pack you with ice and cooling blankets. Another method is to wrap you in a special cooling blanket and apply ice packs to your groin, neck, back and armpits to lower your temperature.
• Give you medications to stop your shivering. If treatments to lower your body temperature make you shiver, your doctor may give you a muscle relaxant, such as a benzodiazepine. Shivering increases your body temperature, making treatment less effective.

Home treatment isn't enough for heatstroke. If you have signs or symptoms of heatstroke, seek emergency medical help. Others should take steps to cool you off while waiting for emergency help to arrive. Don't drink any fluids while waiting for medical assistance.

If you notice signs of heat-related illness, lower your body temperature and prevent your condition from progressing to heatstroke. In a lesser heat emergency, such as heat cramps or heat exhaustion, the following steps may lower your body temperature:

• Get to a shady or air-conditioned place. If you don't have air conditioning at home, go someplace with air conditioning, such as the mall, movie theater or public library.
• Cool off with damp sheets and a fan. If you're with someone who's experiencing heat-related symptoms, cool the person by covering him or her with damp sheets or by spraying with cool water. Direct air onto the person with a fan.
• Take a cool shower or bath. If you're outdoors and not near shelter, soaking in a cool pond or stream can help bring your temperature down.
• Rehydrate. Drink plenty of fluids. Also, because you lose salt through sweating, you can replenish salt and water with some sports drinks. If your doctor has restricted your fluid or salt intake, check with him or her to see how much you should drink and whether you should replace salt.
• Don't drink sugary or alcoholic beverages to rehydrate. These drinks may interfere with your body's ability to control your temperature. Also, very cold drinks can cause stomach cramps.