This subject was introduced at the beginning of this chapter, where it is pointed out that everyone at some time or other experiences muscle cramps. Cramps often occur during the night, after a day of unusually strenuous activity; less frequently they occur during the day, either during a period of relaxation or occasionally after a strong voluntary contraction or postural adjustment. A random restless or stretching movement may induce a hard contraction of a single muscle (most frequently of the foot or leg) that cannot be voluntarily relaxed. The muscle is visibly and palpably taut and painful, and the condition is readily distinguished from an illusory cramp, in which the sensation of cramp is experienced with little or no contraction of muscle. The latter phenomenon may occur in normal persons as well as in those with peripheral nerve diseases. Massage and vigorous stretch of the cramped muscle will cause the spasm to yield, although for a time the muscle remains excitable and subject to recurrent cramps. Visible fasciculations may precede and follow the cramp, indicating excessive excitability of the terminal branches of motor neurons supplying the muscle. Sometimes the cramp is so intense that the muscle is injured; it remains sore to touch and painful upon use for a day or longer. Cramps of precordial chest muscles or diaphragm may arouse fear of heart or lung disease. In the EMG, the cramp is marked by bursts of high-frequency, high-voltage action potentials, and the precramp phase shows runs of activity in motor units. Why cramps should be painful is not known; probably the demands of the overactive muscle exceed metabolic supply, causing a relative ischemia and accumulation of metabolites. Overwork of muscle with or without impairment of circulation is also painful. Between cramps, the muscles are normal clinically and electromyographically.
Cramps are known to increase in frequency under certain conditions and with certain diseases. They are common during pregnancy for reasons not fully understood. Dehydration and excessive sweating predispose to cramping and are a constant threat to athletes. Exertional cramps are elicited more easily than usual in motor system disease, hypothyroidism, and in chronic polyneuropathies. Focal cramping occurs after partial nerve or root injury. For example, the calf muscle on one side is subject to severe recurrent cramps after decompression of the S1 root for lumbar disc disease; in extreme cases, the muscle hypertrophies after long periods of intermittent cramping. Patients undergoing hemodialysis are subject to cramps, which can be suppressed by intravenous hypertonic saline or hypertonic glucose.
The mechanism of muscle cramping is obscure. Several enzymes have been implicated, controversially but often included among these is myoadenylate. This enzyme, which is present in high concentration in muscle, is thought to function primarily during aerobic exercise and facilitate the regeneration of ATP from ADP through the action of adenylate kinase. However, low levels of this enzyme are not specific, occurring in such unrelated disorders as hypokalemic periodic paralysis and spinal muscular atrophy (see Layzer for details).
4 h if necessary, or 300 mg tid for idiopathic diurnal cramping) had been an effective medication but is no longer widely used because of a low risk of ventricular arrhythmia. Some patients, nonetheless, seek relief from the use of quinine water, some brands of which still contain the chemical. Diphenhydramine hydrochloride (Benadryl) 50 mg or procainamide 0.5 to 1.0 g are alternatives. Phenytoin, carbamazepine, and other antiepileptic drugs, and clonazepam may be useful in alleviating repeated daytime cramping.
Tetany, Pseudotetany, and Related Cramp Syndromes