Transcranial magnetic stimulation reveals an interhemispheric asymmetry ofcortical inhibition in focal epilepsy

Transcranial magnetic stimulation (TCS) was applied to both hemispheres of 16 patients affected by criptogenic focal epilepsy to evaluate the interhem ispheric symmetry of the motor cortex excitability. The amplitude of the mo tor evoked potentials (MEPs) and the duration of the post-MEP silent period (SP) were measured at threshold (THR) and at increasing TCS stimulation in tensities. The THR was significantly higher in patients than in 16 age-matc hed control subjects (p < 0.01). No interhemispheric differences were found in MEP amplitude. In controls, the correlation between SP duration and inc reasing TCS stimulus intensity was linear with a symmetrical progression of the SP duration over the two hemispheres. In patients this linear SP progr ession was lacking on the 'epileptic' hemisphere: the SP duration did not i ncrease following TCS >40% above THR, indicating abnormal interhemispheric asymmetry. This finding suggests a selective dysfunction of inhibition in t he epileptic hemisphere as signaled by an abnormal SP duration in response to progressively higher TCS intensities. NeuroReport 11:701-707 (C) 2000 Li ppincott Williams & Wilkins.