MYOCLONUS IN CORTICOBASAL DEGENERATION

Myoclonus occurs in about 50% of cases of corticobasal degeneration. T he myoclonus is focal, affecting one arm or less commonly a leg. Actio n and reflex myoclonus, induced by sensory stimulation of the affected limb, are characteristic. Accordingly, the myoclonus of corticobasal degeneration has many of the clinical characteristics of cortical myoc lonus. In contrast to the recognized forms of cortical reflex myoclonu s, the myoclonus of corticobasal degeneration is not associated with e nlargement of the secondary component of the cortical somatosensory ev oked potential (SEP) or backaveraged electroencephalographic (EEG) spi kes preceding each myoclonic jerk. Furthermore, the latency of reflex myoclonus in corticobasal degeneration is some 10 ms shorter than that seen in cortical reflex myoclonus. One explanation for these differen ces may be the preferential enhancement of different components of lon g-latency transcortical reflexes in different diseases. Changes in the sensory input to the cortex, alterations in the balance of local cort ical excitatory and inhibitory influences, and therefore cortical exci tability are likely to be important factors in determining the physiol ogical characteristics of myoclonus of cortical origin. (C) 1996 Wiley -Liss, Inc.