Figure 6.

Congenital anomalies of the cerebellum in three patients. First, a 53-year-old female with extension of cerebellar hemisphere folia continuously across midline with absent vermis indicating classic rhombencephalosynapsis, an abnormality of dorsoventral patterning (a–c).⁸ The superior cerebellar peduncles are fused posteriorly (white arrows, a) and the deep cerebellar nuclei appear fused (white arrows, b). Inferiorly, the vermis is completely absent (c), although in cases of incomplete rhomboencephalosynapsis, the inferior vermis is present. This condition is associated with highly variable cognitive and motor deficits. This patient was a high-functioning professional with mild ataxia on exam. Another case demonstrates vermian hypoplasia in a 5-year-old (d). The vermis is small with superiorly positioned inferior border (white arrows); a dorsally directed white matter stem consistent with a stem to lobules VI, VIIa, and VIIb flanked by the prominent primary and pre-pyramidal/pre-biventral fissures are present while the lobules while the number of white matter stems to the remaining areas of the anterior and posterior lobe are more difficult to identify. On overall semblance of typical lobulation remains present, compatible with hypoplasia whereas lack of normal lobulation would indicate dysgenesis. A third case demonstrates hypoplasia of both the cerebellum and pons in a 1-year-old (e–f). The vermis is hypoplastic and the pons is markedly hypoplastic (e). Laterally on the right, the cerebellar hemisphere is also small, although the fissures are not widened (f); decreased size due to widening of otherwise normal fissures would indicate atrophy rather than hypoplasia. Note that the white matter stems to Crus I and crus II remain identifiable. Hypoplasia of the pons frequently accompanies that of the cerebellum due to shared developmental origins and/or white matter circuitry.⁸