Transcranial magnetic stimulation reveals an interhemispheric asymmetry of cortical inhibition in focal epilepsy.

Transcranial magnetic stimulation (TCS) was applied to both hemispheres of 16 patients affected by criptogenic focal epilepty to evaluate the interhemispheric symmetry of the motor cortex excitability. The ampitude of the motor evoked potentials (MEPs) and the duration of the post-MEP silent period (SP) were measured at threshold (THR) and at increasing TCS stimulation intensities. The THR was significantly higher in patients than in 16 age-matched control subjects (p 40% above THR, indicating abnormal interhemispheric asymmetry. This finding suggests a selective dysfunction of inhibition in the epileptic hemisphere as signaled by an abnormal SP dut action in response to progressively higher TCS intensities.