Focal Cortical Dysplasias: clinical implication of neuropathological classification systems

Focal Cortical Dysplasias (FCDs) are highly epileptogenic brain lesions and are a frequent cause for drug-resistant focal epilepsies in humans. FCDs present with variable histopathological patterns, including architectural, cytoarchitectural or white matter abnormalities. Pathomechanisms compromising neuroblast proliferation, migration, or differentiation are likely to play a role in the etiology of FCD variants. FCDs were subsumed, therefore, into the broad spectrum of malformations of cortical development. The most frequent subtype comprises FCD Type II, which in general occurs as isolated lesion in extratemporal location and is histopathologically characterized by dysmorphic neurons (Type IIA) and balloon cells (Type IIB). Neuroimaging hallmarks include hyperintense T2-signaling and a "transmantle sign". Electrophysiological recordings show peculiar interictal spike patterns and complete surgical resection results in favorable seizure control. In contrast, FCD Type I can be identified in young children with severe epilepsy and psychomotor retardation. Parietal, temporal, and occipital lobes may be involved in seizure generation, although neuroimaging often reveals normal contrast intensities. Surgical resection strategies ameliorate seizure frequencies in many children, whereas complete seizure relief can be achieved only in rare cases. According to the currently used FCD classification system, the same histopathological FCD Type I variant can be diagnosed as associated lesion in the large cohort of epilepsy patients with hippocampal sclerosis, low-grade glio-neuronal tumors, vascular malformations, or glial scarring. MRI is often not helpful to detect the dysplastic cortical areas. In addition, there is no specific electrophysiological pattern for an associated dysplastic lesion. Surgical resection of the epileptogenic area results, however, in favorable seizure control. These findings argue for a revised neuropathological classification system that distinguishes isolated versus associated FCD variants to obtain a better correlation with electro-clinical findings and prediction of postsurgical seizure control.