Cortical dysplasia: neuropathological aspects.

Introduction: Malformations of the cerebral cortex are a frequent cause of pharmacoresistant epilepsies and developmental disorders.
Epidemiology and Genetics: The incidence of cortical dysplasias in epilepsy surgical series varies from 12 to 40% and focal cortical dysplasias (FCD) are one of the most common neuropathological findings in resection specimens from pediatric patients undergoing cortical resections for the treatment of refractory epilepsy.
Macroscopy and Histopathology: Surgical specimens in FCD may appear normal macroscopically, but in some cases, widening of the cortex with poor demarcation from the underlying white matter is noted. In milder dysplasias, the main pathological feature is disorganization of the cortical architecture ("dislamination") with less striking neuronal and glial cytopathology. Histopathology shows an excess of neurons in layer I, including Cajal-Retzius cells, clusters of neurons, marginal glioneuronal heterotopias, and a persistent subpial granule cell layer. The hallmarks of FCD are disorganization of the laminar architecture and of the cytology of individual neurons. In many cases, layer I remains hypocellular and distinct from deeper laminae, but lower cortical layers may be ill-defined or broken up by the presence of many large and randomly located abnormal and cytomegalic neurons; depending on their morphology, referred to as "giant neurons," "immature neurons," or "dysmorphic neurons." The other pathognomonic cell type associated with FCD is the "balloon cell." These cells were originally considered to be of astrocytic lineage; however, there is evidence that they are in effect "balloon neurons."
Immunohistochemistry and Structural Findings: Immunohistochemistry is not essential in making the diagnosis of FCD or microdysgenesis but allows further characterization of cell types.