Abstracts

Rationale: Infraslow activity (ISA), here defined as activity <0.1 Hz in frequency, has been demonstrated to have localizing value in focal epilepsies. The majority of work to date has been on ictal ISA, the so called DC shift first described by Ikeda et al. (1996) using subdural electrodes. Recent work has extended this analysis to the interictal period (Constantino and Rodin 2012), again using subdural recordings. To date ISA has not been effectively demonstrated using stereoelectroencephalography (SEEG) technique. A retrospective review of archived intracranial data was conducted to determine the localizing value of ictal and interictal ISA.Methods: The entire video-EEG record on six adult patients with platinum depth electrodes placed by SEEG methodology was performed. These records were originally sampled at 1000 Hz using a Nihon Khoden system with a high pass filter of 0.016 Hz (time constant of 10 seconds). Data was down sampled to 250 Hz (for ictal review) and 10 Hz (for interictal review) and analyzed using BESA software. To isolate ISA, a band pass of 0.02-0.2 Hz was used and the viewing window was adjusted accordingly (10 minute pages for interictal review). Ictal ISA was defined as positive or negative deflections of > 300 V lasting greater than 3 seconds and occurring in conjunction with conventional frequency activity (CFA). The timing and spatial extent of ictal ISA and CFA were compared. Analysis of interictal ISA was facilitated by fast Fouier transform (FFT) estimation of the power in the ISA band as a function of time.Results: In all patients consistent and characteristic ictal ISA (DC shifts) were seen. As previously described, these occurred in close temporal proximity to ictal onset by CFA and were often more spatially restricted. As seizures progressed, ISA was noted to propagate to regions outside of the ictal onset zone. The same regions displaying ictal ISA were found to show interictal ISA and prominent fluctuations in power occurred over the course of the recording period. Two patients had concomitant subdural strips allowing comparison between the two recording techniques. Conclusions: ISA can be measured by SEEG technique and complements conventional analysis of seizure onset. However, the spatial extent of ictal and interictal ISA argue that it demarcates regions beyond that of the ictal onset zone alone. The implication of this finding requires further study.