OP03.19: Fetal ultrasound and magnetic resonance imaging in rhombencephalosynapsis

Abstract

Objective: Rhomboencephalosynapsis (RES) is a rare, but increasingly recognized malformation consisting of fused cerebellar hemispheres, dentate nuclei and superior cerebellar peduncles and absent vermis. RES is associated with significant neurocognitive delay, seizures, and movement disorder. Associated central nervous system abnormalities are common. We report five fetal cases and discuss the fetal ultrasound and magnetic resonance imaging (MR) findings. All cases presented with hydrocephalus. Methods: Institutional ethics approval was obtained. Prenatal ultrasound is available in five cases, prenatal MR imaging in two, postnatal ultrasound and MR in two and autopsy confirmation in three. Images were evaluated for presence and degree of vermian fusion, presence of primary fissure and fastigial point and associated central nervous system anomalies. Results: Fetal and postnatal MR readily identified the classic features of hypoplastic single lobed cerebellum with transverse folia and associated intracranial malformations, specifically within the realm of holoprosencephaly. Both also identified diamond shaped 4th ventricle on axial views, rounded ‘fastigial point’ and lack of primary fissure. Postnatal MR more readily identified fusion of the dentate nuclei. Fetal and postnatal ultrasound documented the transverse folia and narrow transverse diameter of the cerebellum. Conclusion: Rhomboencephalosynapsis is a rare lesion that is best imaged prenatally with fetal MR. Suspicious ultrasound features are cerebellar hypoplasia and absence of the vermis. The presence of these ultrasound findings should raise the clinical suspicion of RES, and be an indication for fetal MR if available.