Abstracts

Rationale: Slow waves are observed commonly in subdural electrode recording (SER). The slowing may be present prior to, during and after ictal period. Although the postictal slowing is well recognized, the significance of peri-ictal slow activity (PISA) is unclear. The purpose of this study is to characterise the slow wave pattern observed on subdural recordings and its relationship to seizure onset. Methods: 12 adult patients underwent SER from July 2008 to May 2009. All patients underwent prolonged video scalp EEG monitoring (VEM) before the invasive recording using the international 10-20 electrode system, with or without additional electrodes. 7 patients had clear seizure onset and PISA. 5 Patients with unclear seizure onset were excluded. 1 patient had simple partial seizures alone (SPS); 4 had complex partial seizures (CPS) and 2 patients had secondary generalized seizures. There were 3 cases with mesial temporal lobe epilepsy, 1 case with neocortical temporal epilepsy, 2 with occipital lobe and 1 with parietal lobe epilepsy. Habitual seizures with clear subdural seizure onsets were analyzed for PISA. PISA was defined as delta activity (<=3 Hz), related to ictal activity, but without direct evolution and was not previously observed during interictal periods. PISAs were compared with previous VEM recordings. Results: SER: 29 habitual clinical seizures were analyzed in 7 patients during the invasive monitoring. There were three types of PISAs observed. Type I: PISA amplitude(P) > epileptiform build up amplitude(B) then followed by B>P Type II: B>P in amplitude all the time Type III: P>B in amplitude all the time There were 19 type I, 7 type III and 3 type II PISAs. VEM and SER onset: there was localization discrepancy between VEM onset and SER onset in 5 patients out of the 6 patients with ictal onset identified on VEM. 1 patient with SPS had no ictal scalp EEG change identified. Discrepancy between VEM seizure onset and subdural seizure onset were mostly seen in the patients who had type I and type III PISAs. Location of PISA and VEM onset were concordant in 4 patients who had VEM onset and SER onset location discrepancy. Conclusions: The irregular, semi-rhythmic slow activity at peri-seizure onset on scalp EEG could be reflection of PISA. Often PISA’s, with higher amplitude than fast activity at seizure onset, can point to scalp EEG onset.