Focal cortical dysplasia (FCD)
General description
Focal cortical dysplasia (FCD) represents a spectrum of malformations of cortical development characterized by disruptions in neuronal proliferation, differentiation, and migration during embryonic and fetal brain development. The pathophysiology of FCD is intricately linked to aberrant molecular signaling pathways, particularly the mammalian target of rapamycin (mTOR) pathway, which serves as a central regulator of cell growth, proliferation, and energy metabolism during cortical development.
Clinical manifestations
FCD is highly associated with medication-resistant epilepsy and represents the most common cause of neocortical epilepsy in children. The clinical presentation varies significantly based on FCD subtype, with patients exhibiting FCD type II typically manifesting earlier seizure onset compared to type I . In the majority of patients, epilepsy begins within the first five years of life, although onset can occur throughout childhood and into adulthood.
Cortical lesion
FCD Type I lesions are generally subtle on MRI and may demonstrate only mild gray-white matter junction blurring due to neurons located within subcortical U-fibers, while FCD Type IIa demonstrates more pronounced imaging abnormalities including blurring of the gray-white matter junction, cortical thickening, and abnormal gyral or sulcal patterns.
Transmantle sign
FCD Type IIb presents the most distinctive imaging features, characterized by the same findings as type IIa with the addition of focal signal abnormality extending from cortex to ventricle, known as the transmantle sign.