Isolated posterior cerebellar vermal defect: a morphological study of midsagittal cerebellar vermis in 4 fetuses--early stage of Dandy-Walker continuum or new vermal dysgenesis?

This report is a neuropathological description of posterior cerebellar vermis agenesis/hypoplasia at midgestation. This defect was demonstrated by prenatal ultrasound in four 21- to 24-week-old fetuses. Neuropathological findings were characterized macroscopically by hypoplasia of the posterior vermis with normal cerebellar hemispheres and brainstem; hypoplasia of the posterior vermian lobules 6 to 10, mildly cystic dilatation of the ventricular cavity, and a flat profile of the roof of the fourth ventricle also were demonstrated. Microscopically, a hypercellular and abnormally located germinal matrix and abnormal migration of external granule cell precursors into the meningeal tissue above the outer surface of the cerebellum were found. These abnormalities might result in delayed growth and foliation of the posterior vermian lobules. This feature might be the neuropathological pattern of the early stage of the Dandy-Walker continuum, although it cannot be excluded as a consequence of a primary developmental failure of the vermal primordium.