Essential myoclonus

The clinical characteristics of 15 patients with essential myoclonus are evaluated. The course of illness was one of initial worsening followed by a stable or improved state. Only two patients had positive family histories of involuntary movements. Nine patients had segmental distribution despite a careful search for etiology; in two of these patients, myoclonus was rhythmic. The trunk and proximal limbs were the most frequently affected body regions. Clonazepam improved myoclonus in 10 of 13 patients, two of whom had complete resolution of symptoms. On review of the salient features of the myoclonus, we discerned four phenomenological subcategories: (a) oscillatory myoclonus, (b) rhythmic segmental myoclonus, (c) nonrhythmic segmental myoclonus, and (d) nonrhythmic multifocal myoclonus.