Spatiotemporospectral characteristics of scalp ictal EEG in mesial temporal lobe epilepsy with hippocampal sclerosis

The aim of present study was to identify the common propagation pattern of ictal discharges in mesial temporal lobe epilepsy (TLE). Pre-surgical ictal scalp EEG recordings were collected from patients with TLE associated with hippocampal sclerosis. Two types of ictal onset patterns were identified based on spatial and spectral characteristics of initial ictal discharge waveforms: (a) a sustained regular 5- to 9-Hz rhythm with a restricted temporal or subtemporal distribution (type 1); and (b) an irregular 2- to 5-Hz rhythm with a widespread distribution (type 2). Scalp EEG data were decomposed into temporally independent, spatially fixed component by independent component analysis. The identified source activities corresponding to ictal discharges from each seizure were localized by dipole source localization, and dipole sources with similar spatial locations were clustered. To identify the sequence of propagation among component clusters during the progress of seizures, event-related spectral perturbation by wavelet transform was used. Fifty-five seizures (22 seizures in four Type 1 patients and 33 seizures in eight Type 2 patients) in 12 patients were analyzed. Ictal discharges associated with type 1 seizures arose from both the anterior temporal region and basal ganglia, and then spread into medial frontal region. The dominant frequency of ictal rhythm was in the theta range and remained relatively constant until the middle portion of seizures. Type 2 seizures developed bilaterally or predominantly in the ipsilateral medial temporal region, followed by the medial frontal region and basal ganglia. The dominant frequency of ictal activity at the onset of seizure was in the delta range. However, rhythmic theta activities with decreasing tendency ensued rapidly after seizure onset. These findings suggest that TLE associated with hippocampal sclerosis may have preferential propagating patterns according to the type of the ictal onset pattern.