Ictal onset slow potential shifts recorded with hippocampal depth electrodes

Reports of direct current shifts at the onset of scalp-recorded seizures prompted us to inspect depth-recorded seizures for the presence of similar slow potential shifts at the onset of the seizure to determine whether slow potential (SP) shifts actually occur at the onset of depth-recorded seizures and if these shifts can facilitate localization of the seizure focus.

With the low frequency filter "opened" (LLF=0.1 Hz, HLF=70 Hz, 3 dB/octave), 32 seizures recorded with hippocampal depth and subdural electrodes were visually inspected to identify an SP shift at the onset of the seizure. A seizure was considered as having an SP shift when the slow potential waveform was > 1.5 sec in duration and > 100 microV in amplitude. Seizures were obtained from 5 subjects; 4 underwent epilepsy surgery (3=Engel I, 1=Engel II) and one received VNS. SP shift duration, peak voltage and polarity were measured for each seizure. The ability to identify seizures based on SP shift configuration was also evaluated.

In 84% of the seizures, ictal onset was associated with a localized SP shift. Shift duration ranged from 1.5 sec to 11.5 sec (96% > 2 sec, 62% > 5 sec). The maximum shift ranged from 139 microV to 2305 microV (mean = 1123 microV, SD = 660 microV). In all the seizures, polarity was positive at the point of maximum shift. By visually examining the SP shift, seizures could be identified as originating from the same focus or from different foci.

The onset of depth-recorded seizures appears to be commonly associated with a localized positive SP shift. An SP shift at the onset of depth-recorded seizures is likely to be a useful visual aid for localizing electrographic seizure onset.