Specific Imaging Findings

MR imaging in rhombencephalosynapsis (RES) shows a characteristic fusion of the cerebellar hemispheres, without separation by a cleft or the vermis. The vermis is absent while the cerebellar dentate nuclei, as well as the middle and superior cerebellar peduncles are fused. The coronal view is most helpful, showing abnormal horizontal orientation of the cerebellar folia that are continuous across the midline, while the typical vermian folial pattern is missing in the midline. The single lobed cerebellum is small, with decreased transverse diameter. In the sagittal plane, the primary fissure is missing and the festigial process of the fourth ventricle is rounded. In the axial plane, the fused structures arch in a horseshoe shape across the midline, resulting in a narrowed and posteriorly pointing fourth ventricle, described as a keyhole, diamond, tear drop, or square. This appearance may suggest RES on CT images; however, the diagnosis is made with MRI. DTI depicts vertically oriented fibers in the midportion of the cerebellum. RES should be considered whenever fetal MRI is performed for the most common indication – ventricular enlargement on prenatal ultrasound.

Pertinent Clinical Information

The clinical presentation of RES is variable, usually including ataxia, hypotonia, involuntary head movements, abnormal eye movements, and/or seizures, and the symptoms may range from mild to debilitating. In a majority of cases, cognitive development is impaired, but this is also markedly variable and ranging from severely handicapped to normal. Obsessive–compulsive disorder, self-mutilation and depression have been observed, while attention deficit and hyperactivity disorders are frequent behavioral problems. The outcome of RES is accordingly unpredictable – some patients die in childhood due to severe disabilities, while others have either subtle or nonexistent clinical signs and the diagnosis is made incidentally in adulthood. The prognosis may depend on the associated abnormalities, particularly supratentorial anomalies and hydrocephalus.