Abstract for presentation at The Australian and New Zealand Association of Neurologists Annual Scientific Meeting 2007

Purpose: In patients with medically refractory partial epilepsy, the topographic relationship between the location and the extent of the irritative zone, ictal onset zone, and the epileptogenic zone is controversial. Previous studies investigating the relationship between these zones have been based on a two-dimensional analysis of scalp or intracranial EEG recording, which can give misleading information regarding location of the underlying source generators. In this study patients who underwent chronic intracranial EEG monitoring with subdural electrodes, source localization was utilised using low-resolution electromagnetic tomography (sLORETA) and co-registration of MRI and CT images, to examine the three-dimensional topographic relationship of the source of the interictal spikes with the ictal onset zone and regions of immediate spread.
Method: Source localization and image co-registration was performed on six patients with the aid of the Curry™ software program. A sLORETA analysis was performed on each patient’s interictal spikes and seizures. A time window of one second was used to analyse the interictal spikes, onset and progression of seizures.
Results: Analysing the sites of the maximum current density foci, revealed a strong relationship between the interictal discharges in the irritative zone and the ictal onset region. In ten out of fourteen spike foci recorded, there was a topographically relation to the seizure onset focus. Two additional spike foci arose from regions lying in the path of seizure spread. The final spike foci were remote from the site of seizure onset or subsequent spread.
Conclusion: These results demonstrate that interictal spikes can topographically arise form the ictal onset zone, the path of seizure spread, or even occasionally from remote brain regions. It is therefore clinically important for the patients undergoing chronic intracranial EEG monitoring to have, where possible, both seizures and interictal activity recorded to allow a determination of their topographic relationships.