The clinicopathologic spectrum of focal cortical dysplasias: a consensus classification proposed by an ad hoc Task Force of the ILAE Diagnostic Methods Commission

Focal cortical dysplasias (FCDs) are localized regions of malformed cerebral cortex and are very frequently associated with epilepsy in both children and adults. A broad spectrum of histopathology has been included in the diagnosis of FCD. Characteristic findings include aberrant radial or tangential lamination of the neocortex (FCD Type I) and cytological abnormalities (FCD Type II). An ILAE task force has re-evaluated available data and proposes a clinico-pathologic classification system of FCDs. The major change since a prior classification represents the introduction of FCD Type III, which occurs in combination with Hippocampal Sclerosis (FCD Type IIIa), or with epilepsy-associated tumors (FCD Type IIIb). FCD Type IIIc is found adjacent to vascular malformations, whereas FCD Type IIId can be diagnosed in association with epileptogenic lesions acquired in early life (i.e., traumatic injury, ischemic injury or encephalitis). Hence, FCD Type I will now refer to isolated lesions, which present either as radial (FCD Type Ia) or tangential (FCD Type Ib) dyslamination of the neocortex, microscopically identified in one or multiple lobes. FCD Type II is an isolated lesion characterized by cortical dyslamination and dysmorphic neurons without (Type IIa) or with balloon cells (Type IIb). Architectural abnormalities adjacent to or within gross malformations of cortical development are frequently observed and not distinguished as a specific FCD variant. This three-tiered classification system will help to better characterize specific clinico-pathological entities and is an important basis to further explore imaging, electro-clinical features, and postsurgical seizure control as well as underlying molecular pathomechanisms.