CLOSURE OF THE CEREBELLAR VERMIS - EVALUATION WITH 2ND-TRIMESTER US

PURPOSE: To evaluate with sonography the normal development of the fet al cerebellum in the second trimester. MATERIALS AND METHODS: The stud y included all patients undergoing second trimester genetic amniocente sis who had structurally normal fetuses, in whom the posterior fossa w as adequately seen and in whom the karyotype was later determined to b e normal. Views of the cerebellum were obtained prospectively, and the inferior vermis was determined to be open or closed at the time of th e initial scan. Follow-up was obtained by means of subsequent scans an d review of the pediatric record. RESULTS: Of 897 fetuses that met the inclusion criteria, 147 had an open vermis at initial scanning. At 14 weeks gestation, 56% of fetuses had an open vermis, decreasing to 23% at 15 weeks and 6% at 17 weeks. In utero follow-up sonograms were obt ained in 79% of fetuses with an initially open vermis and demonstrated closure in all but one fetus. The remaining 21%, without sonographic follow-up, were normal as newborns. CONCLUSION: The prenatal diagnosis of Dandy-Walker variant should not be made before 18 weeks gestation because the development of the cerebellar vermis may be incomplete at that time.