Occasionally, mild clonic manifestations such as subtle eyelid blinking or changes in postural tone occur. This EEG abnormality is activated by forced overbreathing. Further, these patients have normal intellectual and neurologic function. These patients may also have atonic and myoclonic seizures and may be mentally retarded. The motor movements of myoclonus are manifested as brief jerks or contractions of a specific muscle or group of muscles.
Hypnagogic myoclonus is both common and normal.
Focal myoclonus may be caused by destructive lesions of the brain stem or spinal cord. Metabolic disease, hypoxia, toxic processes, and infectious disease may cause focal or diffuse myoclonus. Myoclonus associated with epilepsy commonly is symmetric. Myoclonus is a component of several epilepsy syndromes and occurs before or as a part of both absence and generalized tonic-clonic seizures. One important syndrome to identify is juvenile myoclonic epilepsy because of the specificity of treatment with valproic acid.