Introduction

The investigation of the surgical patient is essentially the search for the epileptic focus.

The neurosurgeon must understand that the clinical manifestations of a seizure result from activation of a brain region, the so-called symptomatic zone, which may be distant from the actual epileptic focus, the so-called ictal zone. EEG, as well, must be interpreted carefully for its localization value since the abnormal activity first detected by the electrode may in fact have originated from a distant brain site that may not be sampled adequately with an EEG recording electrode. Much effort is put into the search for an imaging abnormality since a lesion seen on MRI has considerable value in the localization of the seizure focus. In surgery for epilepsy, it is the true epileptic focus or ictal zone that is essential to recognize and target with treatment to realize the best results in seizure control.

Rasmussen has developed a useful concept of localization to orient the investigation, determine the modalities of resection, and explain success or failure of resective surgery. It is based on three levels of localization that constitute the surgical hypothesis or the deinition of the epileptogenic zones and the area of resection that is likely to abolish the seizures.