Benign fasciculations, a common finding in otherwise normal individuals, can be identified by the lack of muscular weakness and atrophy and by the small-size muscle fascicles involved and repetitive appearance in only one or a few regions. The recurrent twitches of the eyelid or muscles of the thumb experienced by most normal persons are often referred to inaccurately as “live flesh” or myokymia but are benign fasciculations of this type. Individuals with truly benign fasciculations have normal EMGs (i.e., they have no fibrillations) as demonstrated in a large series of such patients studied and followed for many years by Blexrud and colleagues. Myokymia is a less common condition, in which there are repeated twitchings and rippling of a muscle at rest.
Muscle cramps, despite their common occurrence, are a poorly understood phenomenon. They occur at rest or with movement (action cramps), and they are frequently reported in motor system disease, tetany, dehydration after excessive sweating and salt loss, metabolic disorders (uremia and hemodialysis, hypocalcemia, hypothyroidism, and hypomagnesemia), and certain muscle diseases (e.g., rare cases of Becker muscular dystrophy and congenital myopathies). Gospe and colleagues reported a familial (X-linked recessive) type of myalgia and cramps associated with deletion of the first third of the dystrophin gene, which is the one implicated in Duchenne dystrophy; strangely, there was no weakness or evidence of dystrophy. Lifelong, severe cramping of undetermined type has also been seen in a few families. The dramatic Satoyoshi syndrome, is characterized by continuous, painful leg cramps, alopecia universalis, and diarrhea, is described further on.
Far more frequent than all these types of cramping, and experienced at one time or another by most normal persons, is the benign form (idiopathic cramp syndrome) in which no other neuromuscular disturbance can be found. Most often benign cramps occur at night and affect the muscles of the calf and foot, but they may occur at any time and involve any muscle group. Some patients state that cramps are more frequent when the legs are cold and daytime activity has been excessive. In others, the cramps are provoked by the abrupt stretching of muscles, are very painful, and tend to wax and wane before they disappear. The EMG counterpart is a high-frequency discharge. Although of no pathologic significance, the cramps in extreme cases are so persistent and readily provoked by innocuous movements as to be disabling. Cramps of all types need to be distinguished from sensations of cramp without muscle spasm. The latter is a dysesthetic phenomenon in certain polyneuropathies. The disorders that simulate cramps, such as stiff-man syndrome and other forms of continuous muscle fiber activity that have various bases, are discussed in Chap. 46.
Contrasted to cramp is the already described physiologic contracture, observed in McArdle disease and related metabolic myopathies, in which increasing muscle shortening and pain gradually develop during muscular activity. Unlike cramping, it does not occur at rest, the pain is less intense, and the EMG of the contracted muscle at the time is relatively silent. Continuous spasm intensified by the action of muscles and with no demonstrable disorder at a neuromuscular level is a common manifestation of localized tetanus and also follows the bite of the black widow spider. There may also be difficulty distinguishing cramps and spasms from the early stages of a dystonic illness.