The presence of short and sharp MEG spikes implies focal cortical dysplasia

This study focused on the characteristic needle-like epileptic spikes of short duration and steep shape seen on magnetoencephalography (MEG) in patients diagnosed with focal cortical dysplasia (FCD) morphologically. We aimed to validate the analysis of MEG spike morphology as a noninvasive method of identifying the presence and location of FCD.

MEG was collected by 204-channel helmet-shaped gradiometers. We analyzed MEG spike sources for 282 patients with symptomatic localization-related epilepsy. MEG showed clustered equivalent current dipoles when superimposed on their three-dimensional-magnetic resonance images (MRI) in 85 patients. Fifty-seven patients were excluded from our study, because they had destructive brain lesions or an insufficient number of spikes for statistical analysis. Twenty-eight patients (18 males, 10 females; aged 1-34 years) were finally matched to our inclusion criteria, and were categorized into three groups: FCD (7 patients), non-FCD (10 patients), and non-lesion (11 patients), based on the MRI findings. We measured the duration, amplitude, and tilt manually for at least 15 spikes per patient, and compared the three groups using a one-way analysis of variance, followed by the Tukey test when statistically significant (p < 0.05). In 17 patients with visible MRI lesions, we investigated the correlation between the depth of the lesion and the tilt using the Pearson product moment correlation.

The average spike duration was significantly shorter in the FCD and non-lesion groups than in the non-FCD group (p < 0.05). The average amplitude was not significantly different between the three groups. The average spike tilt was significantly steeper in the FCD group than in the non-FCD group (p = 0.0058). There was no significant difference between FCD and non-lesion patients in both duration and tilt. Our additional study revealed a significant negative correlation between the depth of the lesion and the average tilt (p = 0.0009).

MEG epileptiform discharges of short duration and steep tilt characterize FCD, especially when located at the superficial neocortical gyrus. We speculate that this particular spike morphology results from the intrinsic epileptogenicity of FCD. Morphological analysis of MEG spikes can evaluate the etiology of epileptogenic lesions and detect a strong, localized epileptogenic focus such as that typically observed in FCD.