Relationships between interictal epileptic spikes and ripples in surface EEG

Objective: Ripples (80-250. Hz) have been shown to be a more specific biomarker for the epileptogenic zone than epileptic spikes in intracranial EEG and even surface EEG. Ripples often co-occur with spikes. We investigated the spatiotemporal relation between spikes and ripples, and differences between spikes that do and do not co-occur with ripples. Methods: We marked 50 time points with spikes in bipolar surface EEG during NREM sleep in patients with focal or multifocal epilepsy. We marked ripples that occurred with spikes and calculated parameters relating spikes and ripples: the duration, amplitude and slope of spikes, the timing of the start of ripples and spikes and the proportion of overlap. Results: In total 219 ripples and 5995 individual spikes were marked in 31 patients. Spikes with ripples were on average shorter, had higher amplitude and higher slope than spikes without ripples. 64% of ripples started before spikes started. Spikes occurred on 13 (5-26) channels per patient, and ripples on 3 (0-14) channels, which were also spike channels. Conclusion: Ripples precede rather than follow spikes, so ripples are unlikely to result from spikes. Significance: Ripples and spikes seem not one-on-one coupled, but certain states of the brain can accommodate both.