Hyperthermia is a high body temperature.  It happens when a person is not able to control their body temperature.  This can happen for many reasons, like very hot weather, fever, and some medicines or illegal drugs.

Doctors define hyperthermia as a body temperature that is over 101 degrees Fahrenheit (equal to 38.3 degrees Celsius). This is higher than the average human body temperature, which is normally around 98.6 degrees Fahrenheit (37 degrees Celsius). When a person gets hyperthermia, they can have heat-related injuries, where the high body temperature hurts the body.

There are three forms of heat-related injuries caused by hyperthermia: heat cramps (the least serious), heat exhaustion, and heat stroke (the most serious).

Causes of hyperthermia 

All types of hyperthermia can be caused by the same things.  Some common causes are:
 Weather: Weather that is very hot, sunny, and humid
 Activity: Exercising or working a lot, especially if it is hot out
 Older people and infants can get hyperthermia even if they are resting inside, if the weather outside is hot and humid, and they are not getting enough cool air.  This can happen because very old and very young people have trouble controlling their body temperatures.
 Illegal drugs, especially ecstasy and amphetamines, can make the body's temperature get very hot, very quickly

Other things that make a person more likely to get hyperthermia, especially if they are exercising or working, include:
 Clothing: Wearing dark clothing, hats or helmets, or padded clothing (like football pads)
 Body weight: Having more body fat makes it harder for the body to cool down
 Dehydration (not having enough fluids in the body): This makes it harder for the body to cool itself down by sweating
 Fever: Because the body's temperature is already higher than normal
 Medicines: Some medications, like beta blockers and antipsychotic medicines

Heat cramps 
Heat cramps are the least serious form of hyperthermia.  (A "cramp" is a sharp pain caused by a muscle getting tighter and shorter.)

Symptoms 
The major symptoms of heat cramps are:
 Painful cramps, usually in the legs or abdomen
 Sweating a lot, which can cause dehydration and loss of electrolytes (important salts that the body needs)

Treatment 
People with heat cramps usually do not need medical treatment.  The best treatments for heat cramps are:
 Moving to a cool place and resting
 Resting the muscles that hurt
 Drinking water or sports drinks to treat dehydration

Heat exhaustion 
Heat exhaustion is more serious than heat cramps.  If affects the whole body instead of just certain muscles, like heat cramps.

Symptoms 
Symptoms of heat exhaustion may include:
 Syncope (fainting) or dizziness
 Low blood pressure
 More serious dehydration
 The person may be orthostatic (this means they get dizzy or faint when they stand up; it is a sign of dehydration)
 Feeling very thirsty with a dry mouth
 Cool, clammy skin (because the body is trying as hard as it can to pull heat out of the body by sweating)
 Nausea or vomiting (because of dehydration)

Treatment 
People with heat exhaustion may need medical treatment.

First aid for people with heat exhaustion includes:
 Moving the person to a cool place
 Having the patient take off extra layers of clothes
 Cooling the patient down by fanning them and putting wet towels on their body
 Having them lie down and put their feet up if they are feeling dizzy
 Having them drink water or sports drinks - but only if they are awake, not confused, and not vomiting
 Turning the person on their side if they are vomiting
If a person with heat exhaustion gets medical treatment, EMTs or doctors may:
 Give them oxygen
 Give them water and electrolytes intravenously (through a needle placed into a vein) if they are too confused to drink, or are vomiting

Heat stroke 
Heat stroke is a medical emergency.  If it is not treated quickly enough, it causes brain damage and death.  It is one of the most common causes of death in sports that can be prevented.

Heat stroke happens when a person gets so hot that their body cannot do anything to bring their temperature down.  The body has tried every strategy it has to cool itself down.  But the body is so hot that none of those strategies work any more.  This causes the body temperature to rise very quickly.  The body gets so hot inside that its tissues, especially the brain, get damaged.  Usually, people with heat stroke have a body temperature of 104 degrees Fahrenheit or higher.  The brain cannot survive for long at these temperatures.  Heat stroke can very quickly cause damage to the brain, heart, kidneys, and muscles.

Symptoms

Early symptoms 
Often, the first signs of heat stroke are:
 Agitation (the person gets upset and restless)
 Confusion
 A strong, fast heart rate

Late symptoms 
As heat stroke gets worse, symptoms that can kill the person start to appear.  For example:
 Seizures, especially status epilepticus
 Very low blood pressure (too low to get blood and oxygen to the brain)
 A weak, slow heart rate (this is a sign that the heart cannot beat strongly enough to get blood and oxygen to the body)
 Delirium or coma (caused by the brain getting so hot that it cannot work, and not getting enough oxygen)
 The patient's skin will also be flushed (red), hot, and dry (because the body is no longer able to cool itself by sweating)

Treatment 

People with heat stroke always need emergency medical treatment as soon as possible.  If a person might have heat stroke, 9-1-1 or another emergency telephone number should be called right away.  The emergency medical dispatcher can explain what to do to help the person until an ambulance gets there.

Once the person with heat stroke gets to an ambulance or a hospital, treatments for heat stroke may include:
 Cooling the person down as fast as possible.  Ways to do this include:
 Taking the person's clothes off
 Covering the person with wet towels
 Turning up the air conditioning or turning on a fan, if possible
 Putting ice packs in the person's armpits, on the back of their neck, and in their groin
 Putting the person into an ice bath 
 Giving cold intravenous fluids, both to help cool the person down and to help with dehydration
 Giving benzodiazepines to stop shivering from being cooled down so quickly (shivering makes the body even warmer)
 Giving medicines to stop seizures
 Giving medicines to help heart problems caused by the heat stroke
 Giving oxygen, or putting a tube down the person's throat to help them breathe (this is called intubation)

Prognosis
The prognosis for people with heat stroke depends on how high their body temperature got, how quickly their body temperature increased, and how quickly they got treatment.

Up to 80% of people who do not get treatment for heat stroke right away die.  But cooling people with heat stroke down right away, and getting them treatment very quickly, can change this so that only 10% die.  But some people who survive have brain damage or other health problems caused by their heat stroke.

References

Health problems