What is the main abnormality in the posterior fossa? What is it called?
Absent vermis with fusion of the two cerebellar hemispheres. This is known as rhomboencephalosynapsis.
In what fundamental way does does rhomboencephalosynapsis differ from molar tooth abnormalities (Joubert syndrome)?
Although both have no vermis, in Joubert syndrome the hemispheres remain separated, whereas in rhomboencephalosynapsis they are fused.
What additional cortical midline abnormalities are present? What is this known as?
A band of white matter lined by grey matter fuses the cerebral hemispheres without a corpus callosum at this level, although corpus callosal fibres are seen anterior and posterior to it. This is known as syntelencephaly.
MRI of the posterior fossa demonstrates absence of the cerebellar vermis with complete fusion of the cerebellar hemispheres across the midline. Characteristic transverse folia inferiorly can be clearly seen crossing the midline. The fourth ventricle is abnormal in shape, elongated in the AP direction. Features are consistent with rhomboencephalosynapsis.
Additionally the corpus callosum is dysplastic and there is fusion of the cerebral hemispheres across the midline consistent with coexisting syntelencephaly.