Asymmetry of silent period evoked by transcranial magnetic stimulation in stroke patients.

Abstract

Introduction— A transcranial magnetic stimulus delivered during voluntary activity produces a prolonged postexcitatory inhibition (silent period) of this activity. Besides more conventional measurements of central motor conduction time and amplitude of evoked potentials, we investigated silent period changes in patients with ischaemic brain lesions. We aimed at finding a more reliable method of data processing to differentiate healthy subjects from patients with motor disturbances due to stroke. Methods— Standardized transcranial magnetic stimulation during sustained muscle contraction was performed on the vertex. Electromyographic activity was recorded via surface electrodes placed over the mm. abductor digiti minimi on both sides. 25 patients with different degrees of impairment due to stroke as well as 75 healthy control subjects were examined. Results— In the control group there was no statistically relevant interside difference with respect to silent period duration, whereas a marked interindividual variation was found. In the group of patients, we found mostly a significant prolongation and sometimes a shortening of the postexcitatory inhibition recorded from the affected side compared with the healthy side. This interside difference of silent periods is significantly greater than in the control group. Even greater group differences were found if ratios of silent periods (longer/shorter) were calculated. Conventional parameters were clearly less sensitive. Conclusion— Our findings show that the measurement of the silent period elicited by transcranial magnetic stimulation is a very sensitive neurophysiologic parameter for the assessment of central motor system disturbances, even more so if ratios are considered.