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 (TMS) of the motor cortex is mainly due to cortical inhibitory mechanisms. The aim of the present study was to investigate these inhibitory phenomena 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 with clonic movements of the right upper limb. All patients were on chronic medication with antiepileptic drugs. Therefore, besides contrasting the results with 16 age-matched normal controls, we also studied 10 patients receiving similar antiepileptic treatments who suffered from cryptogenic partial epilepsy with seizures characterised by the absence of clonic manifestations. The duration of the SP was bilaterally increased in the patients with clonic seizures when compared with the two other groups of subjects. The SP was longer in the left FDI muscle (contralateral to the side of the clonic manifestation in all the patients). Our findings likely indicate enhanced interictal inhibitory mechanisms in patients with partial epilepsy involving the primary motor cortex. The resulting inhibitory effect could be greater in the intact hemisphere rather than in the affected one, in which the hyperexcitability of the epileptic focus had to be counterbalanced.