[Electrophysiological study of a case of clinically diagnosed corticobasal degeneration with rhythmic myoclonus]

A 66-year-old man with clinically diagnosed corticobasal degeneration was studied electrophysiologically. The patient had bilateral forced grasping, rigidity, bradykinesia and hyperreflexia which were predominant on the right side, motor aphasia, constructional apraxia, forced laughing, dysequilibrium and myoclonus of the right upper extremity. Several anti-parkinsonism drugs were ineffective. Brain MRI revealed cortical atrophy of the fronto-temporo-parietal lobes with left predominance. On single photon emission computed tomography, cerebral blood perfusion was decreased, especially on the left side in the fronto-temporal lobes, basal ganglia and thalamus. Myoclonus was distal dominant, worse on action or posture, and was rhythmic, mimicking a tremor. On surface EMG recording of the myoclonus, agonist and antagonist muscle pairs were activated simultaneously and rhythmic activities with frequencies ranging from 7 to 8 Hz were seen. The patient had an enhanced C reflex with a relatively short latency (41.0 ms) after median nerve stimulation only at the right wrist. Additionally, during voluntary contraction, the time-constant EMG silence lasting for about 80-90 ms followed the C reflexes. On somatosensory evoked potentials (SEPs) to the median nerve stimulation, N20 latencies were normal and P25 and N33 amplitudes were not giant. There was no premovement corticat spike when a jerk-locked averaging method was used. Regarding motor evoked potentials (MEPs) elicited by magnetic brain stimulation, central motor conduction times were normal. The estimated cortical delay between the arrival of a somatosensory volley and the motor cortical discharge responsible for C reflex was 1.0 ms, which was shorter than those (3.1 +/- 0.9 ms) estimated in five patients with typical cortical reflex myoclonus. A conditioning stimulation (C) of the right median nerve produced marked facilitation of MEPs following magnetic stimulation of the left motor cortex, at conditioning-test intervals (C-T intervals) of 20-22 ms, whereas a conditioning stimulation of the left median nerve did not produce the same effect. These C-T intervals were thought to be very short, considering that N20 latency was 19.6 ms in this patient. The duration of the EMG silence following the C reflex corresponded to that of the EMG silence between muscle activities during his rhythmic myoclonus, and also the myoclonus was reset by occurrence of the C reflex. These electrophysiological findings indicate that his myoclonus was based upon the enhancement of direct sensory input from the thalamus to the motor cortex. Moreover, it is suggested that the existence of the time-constant EMG silence following the C reflex was related to the myoclonal rhythm.