Heat cramps are severe muscle spasms resulting from a combination of prolonged exercise, heavy sweating, and excessive water replacement in extreme heat.

During sweating, salts (electrolytes) and fluids are lost, but drinking large quantities of water dilutes the salts, causing cramps. Heavy sweating is most likely to occur on warm days, especially during strenuous exertion. Heat cramps are common among all of the following:

• Manual laborers, such as engine-room personnel, steelworkers, roofers, and miners
• Athletes, especially mountain climbers or skiers, whose many layers of clothing may keep them from noticing their heavy sweating, and tennis players and runners who do not take time to replace salts lost in sweat
• Military trainees

Heat cramps are strong contractions in muscles of the hands, calves, feet, thighs, or arms. The contractions cause muscles to become hard, tense, and painful. Pain can be intense. Fever does not occur.

Mild heat cramps can be treated by drinking beverages that contain salt or by eating salty food. Drinking 1 to 2 quarts (about 1 to 2 liters) of a sports drink or water containing 2 teaspoons of salt is usually enough. Severe heat cramps are treated with fluids and salts given intravenously. Stretching the involved muscle often gives immediate relief of pain.

Spotlight on Aging
There are several reasons why older people have particular difficulty when the temperature is high. As people are exposed to long periods of high heat and humidity, their body gradually adjusts (acclimates) and is better able to maintain usual body temperatures. Older people, however, are not as able as younger people to acclimate to higher temperatures and humidity. Older people tend to have difficulty increasing the flow of blood to all skin surfaces, and thus their body does not cool itself as readily. Certain drugs, such as antipsychotics and antidepressants, and some disorders that affect the skin, such as systemic sclerosis and psoriasis, can also interfere with sweating. Other disorders such as heart failure can interfere with the body's ability to cool itself. Aging also affects thirst; older people do not get thirsty as readily as younger people. Thus, older people tend to get dehydrated, which in turn means they are less able to sweat in warm surroundings.

Last full review/revision February 2009 by James P. Knochel, MD