Partial rhombencephalosynapsis: prenatal MR imaging diagnosis and postnatal follow up.

We present a case of partial rhombencephalosynapsis, diagnosed by magnetic resonance imaging (MRI), in fetus aged 27 gestational weeks, in a dizygotic twin pregnancy. The distinctive MRI features of this cerebellar malformation (segmental hypogenesis of the cerebellar vermis, partial fusion of the cerebellar hemispheres and dentate nuclei) without associated cerebral abnormalities were confirmed by 32-weeks prenatal and 3-months postnatal MRI studies. At the age of 12 months the affected twin had a slight delay in psychomotor development, mild hypotonia with normal cognitive development. To the authors' best knowledge, this is the first report of a fetal case with isolated partial rhombencephalosynapsis. Its MRI features enlarges the narrow spectrum of uncommon variants of rhombencephalosynapsis, and allow an accurate differentiation from other vermian and cerebellar anomalies with less favorable postnatal outcome.