Isolated focal cortical dysplasia type IIb

Isolated focal cortical dysplasia type IIb (FCD IIb) is a rare malformation of cortical development characterized by a localized area of abnormal brain architecture. In this condition, a region of the cerebral cortex contains disorganized layers (cortical dyslamination) along with two hallmark cell types: dysmorphic neurons and balloon cells. These abnormal cells result from disrupted neuronal migration and differentiation during fetal brain development. FCD IIb is classified under the International League Against Epilepsy (ILAE) focal cortical dysplasia classification system and is one of the most common causes of drug-resistant focal epilepsy, particularly in children and young adults. The primary clinical manifestation of isolated FCD IIb is epilepsy, often presenting with focal seizures that may secondarily generalize. Seizures frequently begin in childhood, though onset can occur at any age depending on the location and extent of the lesion. The seizure semiology depends on which brain region is affected. Patients may also experience developmental delay, cognitive difficulties, or focal neurological deficits, though some individuals have normal development apart from their seizures. On MRI, FCD IIb typically shows a thickened cortex with blurring of the gray-white matter junction and a characteristic "transmantle sign" — a hyperintense signal on T2/FLAIR sequences extending from the cortex toward the ventricle. The condition is considered sporadic in most cases, though somatic mutations in the mTOR signaling pathway (particularly in genes such as MTOR, TSC1, TSC2, and DEPDC5) have been identified in resected tissue, linking FCD IIb to the broader spectrum of mTORopathies. Treatment centers on seizure control. Many patients are refractory to antiseizure medications, making surgical resection of the dysplastic lesion the most effective treatment option, with seizure-freedom rates of 60–80% when complete resection is achieved. Presurgical evaluation typically involves video-EEG monitoring, high-resolution MRI, and sometimes invasive electrode monitoring to precisely delineate the epileptogenic zone.

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