Scope and implications of MRI-negative refractory focal epilepsy

In the absence of a demonstrable epileptogenic lesion, epilepsy is often referred to as “nonlesional epilepsy.” In this book, we preferentially use the term “MRI-negative epilepsy” instead of “nonlesional epilepsy.” Our reason for this preference is that MRI of patients with refractory epilepsy not infrequently shows structural lesions or alterations which are not the immediate cause of the epilepsy. Some of these lesions or abnormalities that are noncausative for epilepsy are cerebral atrophy, nonspecific white matter signal changes, and slight asymmetry in size or shape of regions in the brain. In these situations, the MRI cannot be said to be normal. Therefore, we avoided the use of the term “epilepsy with normal MRI.” Another reason for our preferential use of the term “MRI-negative epilepsy” is that histopathological examination of resected tissues has revealed lesions in as many as 50% of nonlesional MRI patients, especially neuronal migrational abnormalities such as microdysgenesis and focal cortical dysplasias [1]. Conversely, histopathologically proven cortical dysplasia lesions are undetectable by MRI in 30% of the patients [2]. For these reasons, the term “nonlesional epilepsy” would be literally and technically incorrect. The term “MRI-negative epilepsy” better conveys the context in which it is used, in that the presurgical MRI is devoid of a structural abnormality as the probable cause of the epilepsy, and for which epilepsy surgery evaluation could be considered.