INTERICTAL INHIBITORY MECHANISMS IN PATIENTS WITH CRYPTOGENIC MOTOR CORTEX EPILEPSY - A STUDY OF THE SILENT PERIOD FOLLOWING TRANSCRANIAL MAGNETIC STIMULATION

The silent period (SP) following transcranial magnetic stimulation (TM S) of the motor cortex is mainly due to cortical inhibitory mechanisms . The aim of the present study was to investigate these inhibitory phe nomena in primary motor cortex epilepsy. We studied the TMS-induced SP in both the first dorsal interosseous (FDI) muscles in 8 patients who suffered from cryptogenic partial epilepsy with seizures starting wit h clonic movements of the right upper limb. All patients were on chron ic medication with antiepileptic drugs. Therefore, besides contrasting the results with 16 age-matched normal controls, we also studied 10 p atients receiving similar antiepileptic treatments who suffered from c ryptogenic partial epilepsy with seizures characterised by the absence of clonic manifestations. The duration of the SP was bilaterally incr eased in the patients with clonic seizures when compared with the two other groups of subjects. The SP was longer in the left FDI muscle (co ntralateral to the side of the clonic manifestation in all the patient s). Our findings likely indicate enhanced interictal inhibitory mechan isms in patients with partial epilepsy involving the primary motor cor tex. The resulting inhibitory effect could be greater in the intact he misphere rather than in the affected one, in which the hyperexcitabili ty of the epileptic focus had to be counterbalanced. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.