CYSTIC MALFORMATIONS OF THE POSTERIOR-FOSSA - DIFFERENTIAL-DIAGNOSIS CLARIFIED THROUGH EMBRYOLOGIC ANALYSIS

Cystic or cystlike malformations of the posterior fossa represent a sp ectrum of disorders, including the Dandy-Walker malformation, vermian- cerebellar hypoplasia, mega cisterna magna, and arachnoid cyst. Differ entiation of these lesions may be difficult with routine cross-section al imaging; however, an accurate diagnosis is essential for proper tre atment planning and genetic counseling. Dandy-Walker malformation is e asily diagnosed on the basis of the classic triad: complete or partial agenesis of the vermis, cystic dilatation of the fourth ventricle, an d enlarged posterior fossa. Vermian-cerebellar hypoplasia is a general classification that describes congenital malformations with a normal- sized posterior fossa, varying degrees of vermian and cerebellar hypop lasia, and a prominent retrocerebellar cerebrospinal fluid space that communicates freely with a normal or dilated fourth ventricle. Mega ci sterna magna can be asymmetric and can manifest apparent mass effect, simulating the appearance of an arachnoid cyst; therefore, ventriculog raphy or cisternography may be needed to demonstrate communication of the cystic mass with the subarachnoid space. A careful review of the e mbryologic development is essential in understanding these malformatio ns and in making a more accurate radiologic diagnosis.