The development of independent foci in epileptic patients

Although many different factors can cause epilepsy, epilepsy with a secondary focus is thought to always have one origin, which is the epileptic discharges in another part of the brain. A special case of secondary discharges is the mirror focus, in which the primary and secondary discharges arise in the same place in the two opposite brain hemispheres. A secondary focus may become independent; that is, its discharge may no longer be time-related to discharges in the primary focus. To determine the incidence of independent secondary foci, and to identify any distinguishing characteristics, the electroencephalograms of 309 epileptic patients were examined. Of the total group, 41 patients had definite single one-sided foci and served as a comparison group. A total of 33, or 10.7 percent, had independent secondary foci. In 25 of these, the focus was of the 'mirror' variety. The records indicated that in 12 patients, the secondary focus started activity roughly at the same time as the primary focus. Most prevailing notions about the cause of the secondary focus attribute it to changes induced over time by epileptic activity in the primary focus. However, the observations in this study suggest that this explanation is not adequate to account for the existence of independent secondary epileptic foci. (Consumer Summary produced by Reliance Medical Information, Inc.)