Presurgical evaluations for epilepsy surgery: State-of-the-art 2019

Abstract

Epilepsy can be treated with antiepileptic drugs in 60％-70％ of affected patients. However, the remaining 30％-40％ of patients with epilepsy become medically intractable. For patients with intractable focal epilepsy, freedom from seizures can be achieved with epilepsy surgery, namely, resection of the epileptic focus. In this review, we introduce the important concepts for presurgical evaluation for epilepsy surgery: Epileptogenic zone, seizure onset zone, irritative zone, ictal symptomatogenic zone, functional deficit zone, and epileptogenic lesion. We present illustrative cases to show how to identify the epileptogenic zone (epileptic focus). We introduce both fundamental diagnostic procedures and state-of-the-art techniques, such as wideband electroencephalographic (EEG) analysis (from ictal direct current [DC] potentials to high-frequency oscillations), analysis of cortico-cortical evoked potentials, and stereotactic EE (SEEG). In view of the concordance of the presurgical evaluations, we strongly recommend discussion among a comprehensive team comprising adult and pediatric neurologists, neurophysiologists, neuroradiologists, and neurosurgeons to make a good rational working hypothesis of the epileptic focus. When noninvasive presurgical workups yield concordant results in patients with epileptogenic lesions visible on magnetic resonance imaging (MRI; i.e., “MRI-positive” patients), presurgical invasive evaluations can be omitted. Invasive evaluations with either subdural electrodes or SEEG are needed when (1) the working hypothesis needs further validation by recordings of epileptic electrocorticographic activities, especially in cases of “MRI-negative” patients or (2) the epileptogenic zone (focus) is presumably located near the eloquent cortices, and detailed functional cortical/network mapping is therefore necessary for preservation of important brain functions.