Spikes and epilepsy

Digital EEG analysis provides significantly more information to the clinical electroencephalographer (EEGer) for scalp as well as for intracranial monitoring than is currently being routinely utilized. When modern data analysis software is used, interictal spikes contain considerably more information than had previously been ascribed to them. To optimize the diagnostic value of the EEG, sleep recordings after sleep deprivation is valuable because focal spikes, unless abundant, are relatively rare in the waking state. Recording time should also be sufficiently long to allow spikes to emerge. Spikes are always pathologic and can be associated with impaired cerebral perfusion, metabolic changes and concomitant behavioral changes. They can also be separated into simple and complex forms which may allow prognostic statements. The simplest way to accomplish this is by placing a cursor on the peak of the spike and see whether or not other channels show latency differences. More precise methods are: comparisons of voltage maps with current source density maps, principal component analysis and distinctions between stationary versus moving dipoles. Averaging of spikes is valuable but care must be taken that only those spikes which have the same distribution are averaged, and when the average is obtained only from the spike peak, propagation may already have occurred. It has been recommended that the midpoint of the ascending negative phase be used as the point for averaging. In intracranial recordings the frequencies above the gamma range should also be assessed. Their small electrical field allows a differentiation between locally generated events from those which are volume conducted and can thereby more accurately reflect the epileptogenic zone(s). High frequency activity can also be recorded from foramen ovale electrodes which enhances their diagnostic utility. It is emphasized that for centers which perform pre-surgical evaluations the software supplied by instrument manufacturers is inadequate and needs to be supplemented by additional commercially available programs. Furthermore, archived data should be used for retrospective investigations and follow-up studies of patients who have undergone either excisions, resections, or multiple subpial transections to evaluate the success rates by taking into account all the properties of interictal and ictal recordings which are mentioned in this article.