Merck Manual

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Heat Cramps


David Tanen

, MD, David Geffen School of Medicine at UCLA

Reviewed/Revised Mar 2023

Heat cramps are painful spasmodic muscle cramps that usually occur in heavily exercised muscles in hot and humid environments.

Although exertion may induce cramps during cool weather, such cramps are not heat related and probably reflect lack of fitness. In contrast, heat cramps can occur in physically fit people who sweat profusely and replace lost water but not salt, thereby causing hyponatremia Hyponatremia Hyponatremia is decrease in serum sodium concentration < 136 mEq/L (< 136 mmol/L) caused by an excess of water relative to solute. Common causes include diuretic use, diarrhea, heart failure... read more . Heat cramps are common among the following:

  • Manual laborers (eg, engine room personnel, steel workers, roofers, miners)

  • Military trainees

  • Athletes

Cramping is abrupt, usually occurring in muscles of the extremities. Cramping can begin during or after exercise. Severe pain and carpopedal spasm may incapacitate the hands and feet. Temperature is normal, and other findings are unremarkable. The cramp usually lasts minutes to hours. Diagnosis is by history and clinical evaluation.

Treatment of Heat Cramps

Heat cramps may be relieved immediately by firm passive stretching of the involved muscle (eg, ankle dorsiflexion for a calf cramp). The patient should rest in a cool environment. Fluids and electrolytes should be replenished orally (1 to 2 L water containing 10 g [2 level teaspoons] salt or sufficient amounts of a commercial sports drink) or, for more rapid relief or when oral repletion is not possible, IV (1 to 2 L 0.9% saline solution). Adequate conditioning, acclimatization, and appropriate management of salt balance help prevent heat cramps.

NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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