Memory function during low intensity hippocampal electrical stimulation in patients with temporal lobe epilepsy

Hippocampal memory dysfunction is a main symptom of mesiotemporal lobe epilepsy (TLE). It may increase after temporal lobectomy for seizure relief. The aim of this study was to assess the ability of psychometry during hippocampal electrical stimulation to predict post-operative memory deficits and thereby to contribute to lateralization of the epileptogenic zone. A computerized memory test was performed during low intensity hippocampal stimulation in nine patients with TLE during invasive presurgical evaluation. The following results were obtained: speech dominant hippocampal stimulation induced subtle verbal memory deficits in patients with the epileptogenic zone in the speech dominant hemisphere and normal baseline memory performance. Verbal memory deficits could not be induced in patients with the epileptogenic zone in the speech dominant hemisphere and pre-existing memory deficits, or if the seizure origin was contralateral to speech dominance. Thus it was possible to lateralize the epileptogenic zone in patients with normal baseline memory performance by revealing hippocampal dysfunction only during electrical stimulation. Post-operative psychometric testing demonstrated that the individual risk for impairment of verbal memory performance following temporal lobectomy could be predicted correctly. In conclusion, psychometry during hippocampal stimulation may allow prediction of memory deficits following temporal lobectomy and improve determination of the epileptogenic zone.