Myoclonus in corticobasal degeneration.

Myoclonus occurs in about 50% of cases of corticobasal degeneration. The myoclonus is focal, affecting one arm or less commonly a leg. Action 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 myoclonus. In contrast to the recognized forms of cortical reflex myoclonus, the myoclonus of corticobasal degeneration is not associated with enlargement of the secondary component of the cortical somatosensory evoked potential (SEP) or backaveraged electroencephalographic (EEG) spikes 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 differences may be the preferential enhancement of different components of long-latency transcortical reflexes in different diseases. Changes in the sensory input to the cortex, alterations in the balance of local cortical excitatory and inhibitory influences, and therefore cortical excitability are likely to be important factors in determining the physiological characteristics of myoclonus of cortical origin.