Cortical excitability in cryptogenic localization-related epilepsy: Interictal transcranial magnetic stimulation studies

Purpose: To assess whether single- and paired-pulse transcranial magnetic stimulation (TMS) can measure the interictal brain excitability of medicated patients with cryptogenic localization related epilepsy (CLE). Changes in the balance between excitation and inhibition are the core phenomena in focal epileptogenesis. TMS can assess this balance in the primary motor cortex. Methods: We selected 18 patients with CLE and similar clinical features in whom we located the epileptogenic area reliably, with 11 age- and sex-matched healthy controls. For both motor cortices, we determined the threshold to TMS, the duration of the cortical silent period, and the corticocortical inhibition and facilitation curve. Results: TMS was safe. The more antiepileptic drugs (AEDs) taken by the patients, the higher their threshold to TMS. The silent period duration failed to show significant changes. On paired TMS, a cluster analysis identified a homogeneous subgroup of patients (n = 7) who showed a significantly defective corticocortical inhibition and excess facilitation. With respect to the epileptogenic area, the phenomenon was bilateral in four of these patients, ipsilateral in two, and contralateral in one. The phenomenon was independent of AEDs and many other clinical variables. However, this patient group had a higher seizure frequency and a higher proportion of electroencephalograms (EEGs) showing interictal generalized epileptic discharges than the rest of the patients. Conclusion: Paired TMS provided a valuable pathophysiologic insight into the interictal excitatory state of the cortex in CLE. This method can potentially supply useful prognostic clinical information.