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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.)

Secondary epileptogenesis is a well-established phenomenon in laboratory research. Focal epileptic seizures may be caused in a specific area of a test animal's brain by the chronic application of some stimulus, often a toxic substance, to that part of the brain. Signs of epileptic activity begin in the affected area, but then also emerge in the corresponding area on the other side of the brain. If the noxious stimulus is removed early, the seizure area on the opposite side of the brain disappears. If too much time elapses before the noxious stimulus is removed, the secondary epileptic focus on the opposite side of the brain has established itself and does not disappear. This secondary epileptogenesis, in which epileptic foci arise not from the primary cause but from the effects of other epileptic foci, has been difficult to ascertain in human patients, since toxic substances causing epilepsy can not be applied to and removed from human brains. However, in the December, 1991 issue of Archives of Neurology, researchers present findings which suggest that secondary epileptogenesis may occur in humans as well. The researchers studied patients whose epilepsy resulted from conditions such as fever and infection, which affect both sides of the brain, and conditions such as the growth of a tumor, in which only a specific area is affected. The researchers anticipated that epileptic seizure foci would occur in both temporal lobes more commonly among the patients who suffered fever or infection than among the patients with a brain lesion limited to one side. In fact, just the opposite was observed. Epileptic electrical activity was observed on both sides of the brain more commonly among the patients with one-sided brain lesions. Since the sequence of events can not be observed in a retrospective study of case records, it is impossible to say with certainty that the epileptic activity on the opposite side of the brain in these patients resulted secondarily from the primary epileptic focus near the primary cause. Nevertheless, these findings are very suggestive of secondary epileptogenesis as a cause of epileptic foci in humans as well as in animal models of epilepsy. (Consumer Summary produced by Reliance Medical Information, Inc.)

The brainwave tracings on an electroencephalogram are so complicated that it is almost impossible to tease out the information of direct concern from the background of electrical activity. One method of improving the recovery of useful information is to examine the evoked potential. Some sensory stimulus is applied, and in the moments that follow a series of brain events are electrically recorded that relate directly to the stimulus. One such event is the so-called P300. The P300 is defined as a positive wave which occurs at least 250 milliseconds (0.25 seconds) after the stimulus, and usually between 300 and 600 milliseconds after. At this time delay, the P300 does not represent the response of the brain to the stimulus itself; that process occurs much more rapidly. Rather the P300 relates in some fashion to the brain 'making sense' of the stimulus it has received. What, exactly, contributes to the P300 remains a matter for discussion, however. Even without understanding its precise source, P300 is nevertheless a useful tool for examining brain function in patients with various neurological impairments. P300 function was examined in 20 epileptic patients who were scheduled to undergo temporal lobectomy (removal of part of the temporal lobe). Ten patients were scheduled for left temporal lobectomy and 10 for right. Five healthy control subjects were also evaluated; all the control subjects were strongly right-handed. A printed word was presented to either the left or right visual field; in healthy subjects with a dominant left hemisphere, the response to a verbal stimulus is strongly dependent upon left or right presentation. However, in the epileptic patients there was no significant pattern of lateralized scalp P300 activity. Patients with left temporal lobe epilepsy showed reduced P300 activity compared with patients with right temporal disease, but it is thought that if the appropriate non-verbal stimulus were chosen, the right-hemisphere patients would have the reduced P300 response. In some patients, a P300 response could not be detected at all. Previous studies have examined the P300 response of epileptic subjects using depth electrodes; the present study not only confirms the abnormalities of the epileptic patients, but also demonstrates that it is possible to obtain useful recordings from the surface, as well. (Consumer Summary produced by Reliance Medical Information, Inc.)

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.)