ELECTROPHYSIOLOGICAL STUDIES OF EARLY-STAGE CORTICOBASAL DEGENERATION

We conducted electrophysiological studies in two Asian patients with p robable corticobasal degeneration (CBD). The duration of illness from onset was 16 and 20 months, respectively. The clinical manifestations were markedly asymmetric and characterized by cortical sensory; loss, apraxia, action myoclonus, action tremor, and akinetic-rigid parkinson ism. Neither patient responded to levodopa therapy. Simple photon-emis sion computed tomography (SPECT) study showed significantly decreased regional cerebral blood flow in the frontoparietal areas and thalamus opposite to the predominantly affected Limb. A series of electrophysio logical studies failed to identify giant somatosensory evoked potentia ls (SEPs), enhanced long Latency electromyography (EMG) reflex, rind c ortical spikes preceding myoclonic jerk. However, the earliest cortica l component of the median nerve SEP was exclusively enlarged in one pa tient and preserved with depression of the subsequent components in th e other patient. Significantly shorter postmotor-evoked potential (MEP ) silent period was found after the transcranial magnetic stimulation of the motor cortex in both patients. CBD is a unique clinical entity characterized by action myoclonus probably the result of time patholog ic hyperexcitability of the motor cortex, based on a loss of inhibitor y input from the sensory cortex.