Silent period measurement revives as a valuable diagnostic tool with transcranial magnetic stimulation

Following magnetic transcranial stimulation (TCS) a post-excitatory pause can be observed in surface electromyographic (EMG) recordings from pre-innervated muscles. We studied the duration of this silent period (SP) in the abductor pollicis brevis muscle while varying the stimulus intensity (SI) and the amount of the voluntary tonic contraction in 23 normal adults aged 20-78 years. A multivariate linear regression analysis revealed a positive correlation of SP with SI and a slight negative correlation with age. In 11 hemiparetic patients a relative increase of the SP was found on the affected side despite normal central motor conduction time. A marked shortening of the SP in relation to controls was observed in 6 parkinsonian patients.