This type of myoclonus was described by Lance and Adams in a group of patients who were recovering from hypoxic encephalopathy. When the patient is relaxed, the limb and other skeletal muscles are quiet (except in the most severe cases); only seldom does the myoclonus appear during slow, smooth (ramp) movements. Fast (ballistic) movements, however, especially when directed to a target elicit a series of irregular myoclonic jerks that differ from intention tremor. Only the limb that is moving is involved; hence it is a localized, stimulus-evoked myoclonus. Speech may be fragmented by the myoclonic jerks, and a syllable or word may be almost compulsively repeated, as in palilalia. Myoclonus of the axial muscles may make walking impossible.
Action myoclonus is almost always associated with cerebellar ataxia. The pathologic anatomy has not been entirely ascertained. Lance and Adams found the irregular discharges to be transmitted via the corticospinal tracts, preceded in some cases by a discharge from the motor cortex. Chadwick and coworkers postulated a reticular loop reflex mechanism, while Hallett and colleagues (1977) found that a cortical reflex mechanism was operative in some cases and a reticular reflex mechanism in others. Whether these are two aspects of one mechanism could not be decided.
Barbiturates and valproic acid have been helpful in some cases. Several clinical trials and case reports have suggested that the antiepileptic levetiracetam may be useful (Krauss et al, 2001). The use of 5-hydroxytryptophan alone or in combination with tryptophan or other drugs had been recommended in the past (van Woert et al). A combination of several of these medications is usually required to make the patient functional.
Pathophysiology of Myoclonus
It seems logical to assume that myoclonus is caused by abnormal discharges of aggregates of motor neurons or interneurons because of the enhanced excitability of these cells or the removal of some inhibitory mechanism. Sensory provocation may be a prominent feature of polymyoclonus, particularly those related to metabolic disorders. Flickering light, a loud sound, or an unexpected tactile stimulus to some part of the body initiates a jerk so quickly and consistently that it must utilize a direct sensorimotor pathway or the mechanism involved in the startle reaction. Repeated stimuli may recruit a series of incremental myoclonic jerks that culminate in a generalized convulsion, as often happens in the familial myoclonic syndrome of Unverricht-Lundborg.
Evidence implicating cortical hyperexcitability in myoclonus is indirect, being based mainly on the finding that the cortical components of the somatosensory evoked potential are exceedingly large and that in some instances, the myoclonic jerks have a strict time relationship (“time-locked”) to preceding spikes in the contralateral rolandic area (Marsden et al; Brown et al). It is possible that these potentials originate from subcortical structures that project both to the descending motor pathways and upward to the cortex. There is an indication, for example, that postanoxic action myoclonus has its basis in reflex hyperactivity of the reticular formation. Furthermore, the only consistent damage in some disorders such as postanoxic myoclonus is in the cerebellum rather than in the cerebral cortex. As already noted, several types of myoclonus are closely coupled with other cerebellar degenerations.