How large must an epileptic focus be to cause an electrographic status epilepticus--a case report

Based on experimental data from animal studies different theories regarding the size of an epileptic focus have been postulated which range from single pacemaker cells to extended neuronal networks. We report a case which gives further information about the size of a human epileptic focus which can trigger manifest epileptic seizures. We report a 22-year-old man with medically refractory temporal lobe epilepsy. This patient suffered from brief complex partial seizures and frequent epigastric auras. To differentiate a mesiotemporal from a temporolateral seizure origin the patient was implanted with a 10 contact depth electrode from a posterior approach into the right hippocampus, and additional temporobasal/temporolateral subdural strip electrodes. Depth recordings revealed an electrographic status with continuous rhythmic sharp wave activity (1 Hz), the field of which was confined to a diameter of less than 1 cm in the anterior hippocampus, whereas temporobasal subdural strip electrodes did not display this activity. Periodically, spread of this activity occurred to the amygdala, to the posterior part of the hippocampus, and less often to the temporobasal cortex. Most seizure patterns remained subclinical, few of them became symptomatic as partial seizures. This case demonstrates that a hippocampal epileptic focus causing electrographic focal status epilepticus may be limited to a volume of less than 1 cm in diameter. This observation is discussed with regard to implantation strategies and to possible superselective resective or modulatory approaches in the treatment of such limited epileptogenic areas.