Lateralizing Cortical Excitability in Drug Naïve Patients with Generalized or Focal Epilepsy

Abstract

Background and purpose: Numerous transcranial magnetic stimulation (TMS) studies have defined the characteristic features of TMS in epilepsy. TME parameters were expected to classify the epilepsy syndrome or drug responses. However, the results such as cortical silent periods (CSP) are variable according to conditions of patients. Here, we investigate whether specific TMS parameters have localizing or lateralizing values in drug-naïve epilepsy patients.

Methods: We recruited 148 consecutive untreated patients with epilepsy (idiopathic generalized epilepsy (IGE) 38, focal epilepsy (FE) 110, mean age 31.4 years) and 38 age- and gender-matched normal subjects. We obtained resting motor threshold (RMT), motor-evoked potential (MEP), CSP, short interval intracortical inhibition (SICI, inter-stimuli interval 2-5 ms), and intracortical facilitation (ICF, inter-stimuli interval 10-20 ms). TMS were performed during a seizure-free state of more than 48 h.

Results: In IGE, no interhemispheric difference in CSP was found (p > 0.05). However, the mean CSP was longer in IGE patients than in normal controls at all stimulus intensities (p < 0.05). The mean CSP in ipsilateral hemisphere (IH) of FE was significantly longer at all stimulus intensities than that in normal controls (p < 0.001). The CSP in IH was longer than that in the contralateral hemisphere of FE. There was no significant difference in CSP between FE and IGE. SICI was significantly reduced only in the IH of FE versus normal subjects. RMT, MEP amplitudes, and ICF did not differ among IGE, FE, and normal controls.

Conclusions: We found that prolonged CSP and reduced SICI in FE indicate asymmetrically increased cortical inhibition and excitation in the epileptic hemispheres. It suggests that CSP among TMS parameters has a crucial role to lateralize the epileptic hemisphere in FE.

Keywords: Cortical silent period; Epilepsy; Intracortical inhibition; Transcranial magnetic stimulation.

Figure 1.

Cortical silent periods (CSP) in IGE (DH) vs. Control. The mean CSP was longer in dominant hemispheres of IGE patients compared with normal controls at all stimulus intensities.

Figure 2.

cortical silent periods (CSP) in FE (IH or CH) vs. Control. The mean CSPs were significantly longer in the IH of FE at all intensities than normal controls (p < 0.001) (A). The mean CSPs were significantly longer in the CH of FE at 120–140% of RMT than normal controls (p < 0.05) (B).

Figure 3.

cortical silent periods (CSP) in FE (IH) vs. FE (CH). The mean CSP was significantly longer in IH than that in CH at all intensities (p < 0.05).

Figure 4.

cortical silent periods (CSP)s in each group at all stimulation intensities. Between FE and IGE patients, there was no significant difference of CSP at any stimulus intensity.

Figure 5.

SICIs in FE (IH or CH) vs Controls at all stimulus intervals. SICI (MEP inhibition at ISI) was significantly reduced in IH of FE patients compared to normal controls (p < 0.05).