Surgical removal of epileptogenic brain is indicated for treatment of many medically refractory focal seizure disorders. One of the important factors in providing good results from surgery is the accuracy of identifying the epileptogenic focus. However, accurate localization may be difficult when only standard scalp recordings are used. Many epilepsy centers have used intracranial recording techniques to better define regions of cortical epileptogenicity . Although subdural strip electrodes were first utilized many years ago, the more popular method of intracranial recording has been by intracortical depth electrodes. The authors present their method of placing subdural strip electrodes for extensive recordings from the cortex. To date, this method has been used to provide continuous monitoring of the electrocorticogram in 28 patients for periods up to 3 weeks, with only two minor complications. This procedure is relatively safe and a valuable alternative to placing intracortical depth electrodes.

Preoperative variables from a full range of medical specialties were used to predict degree of seizure relief from cortical resection surgery as treatment for epilepsy in 100 patients. General, seizure history, electroencephalographic (EEG), radiological, surgical, and psychological/neuropsychological data were considered. The patients were divided into one large predictive group (n = 75) and a smaller independent cross-validation sample (n = 25). Eight predictive variables emerged: single EEG focus; anterior-midtemporal lobe discharges; discharges only from the side of surgery; rate of occurrence of discharges in surgical area; Wechsler Adult Intelligence Scale Digit Symbol subtest; Marching Test, preferred hand, time; Minnesota Multiphasic Personality Inventory (MMPI) Hysteria scale score; and MMPI Paranoia scale score. By use of multivariate procedures, increased predictability of surgical outcome was obtained not only with the predictive group but with the independent cross-validation sample. The results demonstrate that predictions of seizure relief from epilepsy surgery can be made with 80% accuracy using multiple, rather than single, predictors.

A total of 90 neurons were recorded extracellularly from 17 awake patients undergoing craniotomy for excision of epileptogenic cortex. Relationships between single-unit activity and gross epileptiform spikes recorded locally by the microelectrode or from the immediate overlying cortical surface by electrocorticography (ECoG) were examined. Similar relationships were also sought between interictal bursts from nearby cells when action potentials from several neurons had been recorded simultaneously by the tungsten electrodes. Although 40 single units fired action potentials in some relation to ECoG spikes, the relationships were variable between units and, often, for the same unit. For many units, action potentials were more consistently related to one phase of the local field potential recorded through tungsten microelectrodes than to the ECoG recorded from the overlying cortical surface. Synchronous firing between single units recorded simultaneously by the same microelectrode was rarely seen except at the onset of an ictal event. In addition, a high degree of synchrony between unit firing and local ECoG spikes was recorded in a few patients, but these patients had frequent focal spontaneous seizures. The data imply that in human epilepsy, unlike some animal models of the disorder, relationships between surface epileptiform events and single-unit burst firing are not easily found in interictal recordings. The data also suggest that synchrony between unit and surface events requires a high degree of synchrony among neurons within the epileptogenic focus.

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