Focal Cortical Dysplasia

Focal cortical dysplasia was originally described in patients who were treated surgically for drug-resistant epilepsy. The histologic abnormalities are usually restricted to one lobe or a smaller segment and include local disorganization of the laminar structure, large aberrant neurons, isolated neuronal heterotopia in subcortical white matter, balloon cells, giant and odd macroglia, foci of demyelination, and gliosis of adjacent white matter. One or more of the above components might not be present, and three main subtypes of focal cortical dysplasia are recognized, which might correspond to the different times of embryologic origin. A first type is characterized by architectural dysplasia, with abnormal cortical lamination and ectopic neurons in the white matter. A second type, defined as cytoarchitectural dysplasia, is characterized by giant neurofilament-enriched neurons in addition to altered cortical lamination. A third type, corresponding to Taylor-type cortical dysplasia, is characterized by giant dysmorphic neurons and balloon cells associated with cortical laminar disruption. Focal cortical dysplasia can occur in any part of the cortex. Magnetic resonance imaging (MRI) can be unrevealing in up to 34% of patients. The histologic characteristics of Taylor-type dysplasia are indistinguishable from those of hemimegalencephaly. Focal cortical dysplasia usually presents with intractable focal epilepsy, which can start at any age but generally before the end of adolescence. Seizure semiology depends on the location of the lesion, and focal status epilepticus has been reported frequently. Electrocorticographic seizure activity shows spatial colocalization with the lesion. Complete resection of the tissue producing ictal electrocorticographic discharges is essential for good seizure outcome. Intrinsic epileptogenicity of the dysplastic tissue has been confirmed with depth electrode studies. The mechanisms underlying the epileptiform activity generated by dysplastic neocortex remain to be elucidated. The different histologic subtypes of focal cortical dysplasia can carry different chances of seizure freedom after surgery. Most descriptions of focal cortical dysplasia and hemimegalencephaly are based on studies from epilepsy surgery centers (LINK with epilepsy surgery program). Thus, the clinical and electrophysiologic features described are likely to be typical only of the most severe cases. Our experience indicates that there are some patients with well-controlled seizures in whom MRI shows focal abnormalities identical to those present in patients with histologically proven focal cortical dysplasia.

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