Interictal epileptic spikes averaging in scalp EEG

Interictal epileptic spikes (IES) were recorded in 3-8 channels and averaged off-line in 5 adult patients suffering from the focal epilepsy, and spatiotemporal correlations between spikes were analysed. The averaged IES were compared in all channels whilst always one of them was used alternatively as the triggering one. The relationship between IES in two channels was considered as significant only when it was observed also at exchanged roles of the triggered and the triggering channel. In contrast to the authors' preceding work in which only 18% of the significant correlations between two channels were found at intracerebral IES analysis, processing of scalp EEG signals showed 86% significant correlations. The results were further processed and the epileptic focus and propagation of spikes on the scalp were determined. This reliability of the method is, nevertheless, limited, especially when the results are further automatically processed, because the information can be distorted due to the IES relative heterogeneity. It is also dependent on the selected type of montage. Both mentioned disadvantages can be, nevertheless, quite easily eliminated if suitable statistical methods are used and the original EEG signals are assessed carefully. Averaging of the large number of interictal epileptic spikes can suitably complete the assessment of their basic spatiotemporal relations in patients showing frequent interictal spikes and help in the noninvasive presurgical evaluation in focal epilepsy.