EFFECTS OF PHENYTOIN ON CORTICAL EXCITABILITY IN HUMANS

We studied the effects of a loading dose of phenytoin on motor cortex excitability in five healthy volunteers. Phenytoin elevated motor thre sholds to transcranial magnetic stimulation (TMS) in all subjects, but had no effects on motor-evoked potential amplitudes, silent period du rations, and intracortical excitability tested by paired TMS during re st and voluntary muscle activation. These results are consistent with the hypothesis that blockade of voltage-gated sodium channels decrease s membrane excitability and elevates the threshold to TMS, but will no t reduce intracortical excitability.