Myoclonus is characterized by sudden unpredictable synchronous contractions of functionally synergistic muscles, causing a sudden jerk of a segment of the limb, head, or part of the trunk, which is characterized by abruptness and brevity.5 The movements involve mainly the proximal limb muscles and are usually bilateral. They may involve parts of a muscle, entire muscles, or several independent muscle groups and may occur every few seconds or as often as 50 times per minute. The movement may be precipitated by peripheral stimuli, and generally disappear during sleep. The myoclonic jerks are frequently activated on attempting movement or walking, although they may be elicited also by visual, auditory, or proprioceptive stimuli or may occur spontaneously. The severity of myoclonus is extremely variable; it may be manifested by only a small twitch of a single muscle group or by a generalized sudden spasm of many muscles of the body. Although the muscles involved and the extent of the body involved are quite variable from patient to patient, a pattern may remain relatively consistent once it is established. Myoclonus is not to be confused with myoclonic epilepsy, which involves sudden jerking movements as a form of seizure, and can be differentiated by the electroencephalogram, wherein diffuse bursts of electrical activity are seen in concert with the muscle movements in myoclonic epilepsy. It was demonstrated during stereotactic surgery on a patientnwith myoclonus that the myoclonic bursts were initiated in the motor cortex, followed immediately by a burst of activity in the ventrolateral nucleus of the thalamus, which suggests that the movements originate in the cortex and are propagated via the thalamus, although that patient had no benefit from a ventrolateral thalamotomy. Myoclonus may be found after many disorders of the central nervous system, including encephalitis, meningitis, anoxia, degenerative disease, or vascular conditions.  It may be caused by meta­bolic disturbances, slow virus infection, or degenerative processes. Myoclonic movements may appear in many diseases, including Creutzfeldt-Jakob disease, leucodystrophies, lipidoses, or as a familial disease similar to the progressive familial myoclonic epilepsy of Unverricht.5 However, in the majority of cases there is no demonstrable etiology. As might be anticipated in an entity of diverse origin, neuropathological findings are likewise diverse. Most often the dentate or olivary nuclei show abnormalities, particularly if the myoclonus is secondary to vascular disease.5 Other findings are variable, and lesions can be demonstrated anywhere in the neuraxis, but no con­sistent lesion has been found to explain the characteristic involun­tary movement. Our ignorance about the etiology and pathophysiology is so complete that there is no accepted classification system for the many types of myoclonus, other than to recognize that some myoclonic jerks may occur as a manifestation of epilepsy. There is some evidence that the serotoninergic system may be involved in myoclonus. There are decreased levels of 5-hydroxyindoleacetic acid (5-HlAA) in the cerebrospinal fluid of patients with post hypoxic myoclonus.n Treatment with 5-hydroxytryptophan or its levorotatory form may improve myoclonus in some patients. The indication for surgery is myoclonus severe enough to be disabling. On the other hand, it should be clear that the disability is due to the myoclonic jerks and not from other symptoms of progressive neurological degeneration, particularly dementia. Electroencephalographic evidence of myoclonic epilepsy should be ruled out, but other diffuse electroencephalographic abnormalities do not constitute a contraindication.The target involves the same areas as for other motor disorders, that is, the ventrolateral nucleus of the thalamus or Fore!'s field.74.82 bilateral lesions may be necessary, but the second lesion should not be made until 3 to 12 weeks after the first. The initial lesion should be made contralateral to the more severe movement disorder. Results are generally good, although no series is large enough for statistical analysis. Interestingly, myoclonic epilepsy may also respond to lesions in Fore!'s field, but with somewhat less consistency than non epileptic myoclonus.