Etiologic factors for unitemporal vs bitemporal epileptiform discharges

Many patients with epilepsy have abnormal patterns of brain electrical activity that are present even when seizures are not occurring. (The electrical activity is measured by electroencephalography or EEG.) These interictal epileptiform discharges, or IEDs, are present on both sides of the brain in 20 to 35 percent of patients with temporal lobe epilepsy. Curiously, the precise sources of the abnormal activity, or foci, are often symmetric. It might be anticipated that this bitemporal activity is more common in patients whose epilepsy has resulted from causes that are more global in nature. For example, the effects of a high fever affect all parts of the brain and might therefore be more likely to cause bitemporal epilepsy than, say, a brain tumor which exerts its effect in a limited region on one side of the brain. Researchers set out to confirm this hypothesis by comparing the case records of 30 patients with unitemporal IEDs with those of 30 patients with bitemporal IEDs. The researchers did not anticipate finding the exact opposite to be the case. Seizures resulting from fever were actually more common among the patients with the unitemporal foci (40 percent) than with the bitemporal foci (17 percent). Conversely, 7 of the 10 patients with tumor masses had bitemporal rather than unitemporal IEDs. This suggests that the occurrence of epileptic foci in both temporal lobes of the brain is not more likely to have resulted from fever or infection, pathologic conditions with effects spanning the brain. These results hint that the bitemporal foci may actually result from secondary epileptogenesis. This pathologic process, which has been demonstrated in animals but remains unconfirmed in human patients, occurs when the initial epileptic focus in one area stimulates the formation of new epileptic foci elsewhere. These new foci did not result directly from the original cause, hence the name secondary. This process would provide an explanation for the symmetry that is often seen in bilateral temporal foci, since corresponding areas on the left and right sides of the brain are connected with nerve fibers. (Consumer Summary produced by Reliance Medical Information, Inc.)