Heat Cramp

A heat cramp consists of spasmodic muscle pain typically associated with exercise and physical exertion in, but not limited to, hot environment. Individuals that are most susceptible to cramping include athletes, miners, and military personnel. Heat cramps occur as a result of excessive sweating, thus loss of water and sodium occur, with little or no replacement of sodium, or both sodium and water [1] [2]. This leads to hyponatremia. Cramps may start abruptly and last from several minutes to hours. The muscles in the limbs are usually affected.

Heat cramps are distinguished from cramps due to muscle fatigue by their unresponsiveness to measures such as stretching and cooling, as well as more severe pain [3] [4] [5]. The mechanism by which these cramps occur is thought to be a contraction of the extracellular compartment, which concentrates neurotransmitters and ions, leading to hyper-excitable nerve terminals. They usually affect alternating groups of voluntary muscle, resulting in the perception of what is sometimes described as a 'wandering' cramp. Some patients may experience prodromal muscle twitching shortly before the full onset of cramps. As an episode of cramping continues, it becomes more widespread, involving a greater number of muscles. Heat cramps may be seen as a form of heat illness; some literature suggests that the former may be considered as one of the symptoms of heat exhaustion.

Heat cramps in certain occupations have been successfully treated by drinking saline. Similarly, athletes can use this method to avoid cramping [6]. The same protective effect is not achieved with hydration alone. This provides further evidence that the heat cramps occur due to loss of sodium in sweat. Resting and decreasing physical activity at the first signs of cramping can also prevent heat cramps [7].

The diagnosis is based on the clinical signs and symptoms, as well as the history surrounding the occurrence of the heat cramp. Other tests that may be carried out include:

• Urea and electrolytes: Results often show normal or high sodium levels. This should not be misinterpreted to mean that plasma sodium is present in adequate amounts. On the contrary, it is a result of the body's compensatory mechanisms, in which it tries to keep the extracellular environment constant despite losses through sweat.
• 24-hour urine collection: This may be useful in predicting the likelihood of an individual suffering from future cramps, especially if the kidneys' excretion of sodium in the urine is reduced.
• Electromyography: This test proves that heat cramps are disorganized over-activity of nerve endings and muscle, as a result of a shrunken extracellular compartment and low sodium. It does so by the positive findings of abnormal electrical activity and fasciculations [8].

1. Bergeron MF. Heat cramps during tennis: a case report. Int J Sport Nutr. 1996;6(1):62-68.
2. Stofan JR, Zachwieja JJ, Horswill CA, Murray R, Anderson SA, Eichner ER. Sweat and sodium losses in NCAA football players: a precursor to heat cramps? Int J Sport Nutr Exerc Metab. 2005;15(6):641-652.
3. Jung AP, Bishop PA, Al-Nawwas A, Dale RB. Influence of hydration and electrolyte supplementation on incidence and time to onset of exercise-associated muscle cramps. J Athl Train. 2005;40(2):71-75.
4. Bentley S. Exercise-induced muscle cramp: proposed mechanisms and management. Sports Med. 1996;21(6):409-420.
5. Schwellnus MP, Derman EW, Noakes TD. Aetiology of skeletal muscle ‘cramps’during exercise: a novel hypothesis. J Sports Sci. 1997;15(3):277-285.
6. Bergeron MF. Heat cramps: fluid and electrolyte challenges during tennis in the heat. J Sci Med Sport. 2003;6(1):19-27.
7. Donoghue A, Sinclair MJ, Bates GP. Heat exhaustion in a deep underground metalliferous mine. Occup Environ Med. 2000;57(3):165-174.
8. Layzer RB. The origin of muscle fasciculations and cramps. Muscle Nerve. 1994;17(11):1243-1249.