Late EEG responses triggered by transcranial magnetic stimulation (TMS) in the evaluation of focal epilepsy.

Purpose: To evaluate the use of EEG responses to transcranial magnetic stimulation (TMS-EEG responses) as a noninvasive tool for the diagnosis of focal epilepsy.

Methods: Fifteen patients and 15 healthy subjects were studied. TMS at an intensity set at resting corticomotor threshold were delivered at the standard EEG electrode positions. For each position, EEG responses to TMS were evaluated before and after averaging EEG recordings synchronized with the TMS pulse.

Results: Two types of TMS-EEG responses were seen: (A) early responses: consisting of a single slow wave seen after the TMS pulse; and (B) late TMS-EEG responses, which were subclassified into (b.1) delayed responses: waveforms resembling interictal epileptiform discharges induced by TMS; or (b.2) repetitive responses: onset of a new rhythym induced by TMS. Early responses were observed in patients and healthy subjects when stimulating at various sites and were considered normal responses to TMS. Late TMS-EEG responses were not seen in healthy subjects, whereas they were seen in 11 of the 15 epileptic patients. Late TMS-EEG responses occurred when stimulating the epileptogenic side in eight out of the nine patients who had lateralized late TMS-EEG responses. The combined use of late TMS-EEG responses and interictal scalp EEG would have suggested the diagnosis of focal epilepsy in all patients, despite the absence of late TMS-EEG responses in four patients and the presence of normal interictal scalp EEG in three.

Conclusions: TMS-EEG responses can identify epileptogenic cortex and may substantially improve the diagnosis of focal epilepsy, particularly, if combined with standard EEG studies.