Abstracts

In the presurgical EEG evaluation of patients with epilepsy it is often necessary to conduct intracranial recordings (IR), the purpose of which is to accurately localize the epileptogenic zone(s) when surface recordings are insufficient. Reported morbidity includes infection, cerebral edema and hemorrhage. However, the relevant literature has not emphasized the possibility of falsely localizing data produced by transient focal seizures (TFS) that may appear early in the post-implantation phase and spontaneously subside. Published reports are scant and sometimes in journals not likely to be read by epileptologists. Once aware of the possibility of transient epileptogenic foci associated with subdural electrode placement (Clin Neurophysiol 1999;110:419-23), we examined 26 consecutive patients undergoing IR. Visual analysis of the IR was performed by MdC and/or RW. The simultaneous video data was analyzed and patients were questioned about any symptoms surrounding the times during which TFS occurred. Brain CT/MRI and direct observation at the time of surgery was carried out to look for blood or any other change associated with the presence of TFS during the IR. Seventeen of the 26 patients were implanted with subdural grid and/or strip electrodes. The remainder had depth electrodes. Three patients developed TFS distant from the ultimately identified clinically relevant epileptogenic zone. Localized near the border regions of subdural grids, these TFS were very frequent at first and subsided within 3 days of implantation. Thin layers of blood were found underlying the grids on CT or at surgery. Another patient had morphologically distinct TFS recorded from a basal temporal subdural strip which subsided after evacuation of a subdural hematoma in the area. Clinically relevant seizures appeared 3 days later arising from the same region but exhibiting very different electrographic features. None of the 4 patients was symptomatic or demonstrated any behavioural changes during the TFS which lasted up to 8 minutes, preceeded by long periods of continuous rhythmic spike or sharp wave activity. Transient areas of focal epileptogenesis may develop as a discrete irritative phenomenon distant from areas of clinical relevance. This occurred in our patients in association with the presence of blood under subdural grids or strips. Recognition of this infrequent complication of IR will avoid mislocalization of areas to be included in surgical resection. It is also important that patients not be inappropriately denied surgery should TFS appear close to eloquent cortex or as one of several foci.
The occurence of frequent focal electrographic seizures preceeded by long periods of rhythmic spiking early in the post-implantation period should alert the electroencephalographer to this phenomenon and the likely presence of subdural blood in the region.