Abstracts

Rationale: The significance of hippocampal sleep spindles and their relation to epileptic activity is still a matter of controversy. Hippocampal spindles have been considered a physiological phenomenon, an evoked response to afferent epileptic discharges, and even the expression of an epileptic manifestation. Because of these apparent contradictory findings, we aimed to study the presence and rate of hippocampal spindles in patients with temporal and extratemporal lobe epilepsy undergoing combined scalp and intracerebral EEG recording.Methods: Twenty-five patients (extratemporal epilepsy: n=6; temporal: n=15; and multifocal including the temporal lobe: n=4) were selected from 57 consecutive patients who underwent intracerebral EEG recording at our center between 01/2010 and 07/2014. Selected patients had ≥1 depth electrode in one hippocampus, a 3 T MRI acquisition for hippocampal volumetry, and a localizable epileptic generator. For scoring of sleep and identification of hippocampal spindles we selected the first available sleep recording after ≥72 hours post implantation. Hippocampal spindles were marked manually during N2/N3 sleep of the first sleep cycle. Interictal spiking was assessed qualitatively (4 categories: none, mild, moderate or severe spiking), and the seizure onset zone was defined as the depth electrode contacts showing the first unequivocal ictal EEG change at seizure onset. The hippocampus was automatically segmented using a surface-based multi-template algorithm. We analyzed associations between hippocampal spindles and hippocampal electrophysiological findings and MRI volumetry.Results: Sixteen of the 25 patients (64%) had hippocampal spindles (extratemporal epilepsy: 6/6; temporal: 10/15; and multifocal: 0/4; p=0.005; see Fig. 1). Median spindle rate was 0.6 (0.1–8.6)/min. Highest spindle rates were found in hippocampi of patients with extratemporal epilepsy (p<0.001; Fig. 2). Interictal hippocampal spiking, but not SOZ in the hippocampus or hippocampal atrophy, was associated with reduced spindle rates: 1) spiking (moderate or severe spiking category) vs. non-spiking or mildly spiking hippocampi [ 0.1 spindle/min (range, 0 - 1.7, n=30) vs. 1.7 (range, 0 - 8.6, n=8); p = 0.003]; hippocampus with SOZ vs. without SOZ [0.1 (range, 0 - 1.7, n=21) vs. 0.5 (range, 0 - 8.6, n=17); p = 0.114]; and atrophic vs non-atrophic hippocampus [0 (range, 0 - 1.7, n=8) vs. 0.3 (range, 0 - 8.6 / min, n=30); p = 0.195].Conclusions: Our findings support that hippocampal spindles represent a physiological rather than epileptic phenomenon, with an expression that is diminished in epilepsy affecting the temporal lobe; hippocampal spiking lowered the rate of hippocampal spindles. Some epileptic discharges may be a transformation of these physiological events, similarly to the hypothesis considering generalized spike-and-waves a transformation of frontal spindles. Acknowledgement: This work is supported by the Austrian Science Fund (Schrödinger fellowship J3485-B24) and the Canadian Institutes of Health Research (grant MOP-102710).