Myoclonus specifies the very rapid, shock-like contractions of a group of muscles, irregular in rhythm and amplitude, and, with few exceptions, asynchronous and asymmetrical in distribution. If such contractions occur singly or are repeated in a restricted group of muscles, such as those of an arm or leg, the phenomenon is termed segmental myoclonus, whereas widespread, lightning-like, arrhythmic repeated contractions are referred to as polymyoclonus. In all forms of myoclonus, the muscle contraction is brief (20 to 50 ms)—that is, faster than that of chorea, with which it may be confused. The speed of the myoclonic contraction is the same whether it involves a part of a muscle, a whole muscle, or a group of muscles. The discussion that follows makes evident that each of the three phenomena has a distinctive pathophysiology and clinical implications.
A common and benign example of myoclonus, familiar to many persons, is the “sleep-start” that consists of a jerking of the body, particularly the torso, while falling asleep or occasionally, just prior to waking. Several other sleep-related syndromes involve repetitive leg movements that include an element of myoclonus. Rarely, the movements may extend to daytime behavior (Walters and colleagues). These sleep disorders are discussed in Chap. 18.
Several rapid movements of the limb or a part of a limb simulate myoclonus but have entirely different mechanisms and implications. For example, epilepsia partialis continua is a special type of epileptic activity in which one group of muscles—usually of the face, arm, or leg—is continuously (day and night) involved in a series of rhythmic monophasic contractions. These may continue for weeks, months, or years. The disorder appears to be cerebral in origin, but in most cases its precise anatomic and physiologic basis cannot be determined (see Chap. 15 for further discussion). The related term clonus designates another rapid rhythmic contraction and relaxation of a group of muscles. Reference has already been made in Chap. 3 to relationship of clonus to spasticity and heightened tendon reflexes in diseases affecting the corticospinal tract. It is most easily elicited by forcefully dorsiflexing the ankle; a series of rhythmic jerks of small to moderate amplitude result.
Focal, Segmental, and Regional Myoclonus
Patients with idiopathic epilepsy may complain of a localized myoclonic jerk or a short burst of myoclonic jerks, occurring particularly on awakening and on the day or two preceding a major generalized seizure, after which these movements cease. One-sided or focal myoclonic jerks are the dominant feature of a particular form of childhood epilepsy—the so-called benign epilepsy with rolandic spikes (see Chap. 15).
The notion that monophasic-restricted myoclonus always emanates from the cerebral cortex, cerebellum, or brainstem cannot be sustained, as there are forms that are traceable to a purely spinal cause. The problem takes the form of an almost continuous arrhythmic jerking of a restricted group of muscles, often on one side of the body. Such a subacute spinal myoclonus of obscure origin was described many years ago by Campbell and Garland, and similar cases continue to be cited in the literature. We have seen several in which myoclonus was isolated to the musculature of the abdominal or thoracic wall on one side, or to the legs; only rarely were we able to establish a cause, and the spinal fluid has been normal. This form has been referred to as “propriospinal” when it involves repetitive flexion or extension myoclonus of the torso that is aggravated by stretching or action.