Abstracts

We have shown that both the scalp EEG initial ictal discharge (IID) frequency and the intracranial EEG onset site are related to degree of hippocampal pathology in temporal lobe epilepsy (TLE). The goal of this study was to determine whether the intracranial EEG IID frequency (IF) is related to site of onset or to degree of hippocampal or neocortical pathology. Patients with TLE on scalp EEG monitoring and varying degrees of hippocampal atrophy (HA) on MRI were studied prospectively with longitudinal depth electrodes and multiple subdural strip electrodes. IF and quantitative HA were measured as described (Vossler et al. [italic]Ann Neurol[/italic] 1998;43:756-62). HS was graded using the Watson scheme, and neocortex pathology was examined by counting the number of astrocytes per 400X field in a survey of 10 random fields of layers III-V of the mid portion of the middle temporal gyrus (Vossler et al. [italic]Epilepsia[/italic] 2004;45(5)). 36 patients had depth + strip electrodes; 9 had only strips. Four of the depth + strip and 6 of the strip only patients had substantial HA. 13 depth patients had grade 0-II and 6 had grade III-V HS. 32 of the 45 subjects had intracortical gliosis measured. The hippocampus (HF) IF was slower when the IID was confined to the HF than when it was in the HF + paleocortex +/- neocortex (8.7 vs. 16.5 Hz, [italic]p[/italic]=0.06). The neocortical IF was faster when the IID was confined to the neo- or paleocortex than when when the IID was over the whole lobe (25 vs.12 Hz, [italic]p[/italic][lt]0.03). IIDs confined to neo- or paleocortex were faster than those confined to the HF ([italic]p[/italic]=0.09). The IF in the HF when the IID was in the HF +/- paleo- or neocortex was not significantly faster in patients with little HA vs. substantial HA (16 Hz vs. 9.2 Hz) or with lower grade than higher grade HS (17 Hz vs. 15 Hz). Neocortical astrocyte count did not correlate with: 1. the neocortical frequency first attained regardless of the site of IID or 2. the neocortical IF when the IID was only in the neocortex. However, astrocyte counts were significantly higher in patients whose IID was in only the lateral neocortex vs. either the HF alone or any sites other than the lateral neocortex (p=0.05 and 0.01). This study suggests that when seizure onset is in the HF, the HF IF is slower and is not affected by degree of HS or HA . We previously showed the the scalp EEG IF is related to degree of HS or HA. Because scalp EEG IIDs are typically delayed after the IID begins in the HF, perhaps HS affects the frequency eventually seen at the scalp as the ictal discharge evolves. Neocortical IFs are faster than HF-only IFs, but are not correlated with degree of intracortical gliosis. The greatest astrocytosis occurred in seizures beginning only in lateral neocortex.