MRI demonstration of subtotal agenesis of the cerebellum with closed-lip schizencephaly.

A 3-year-old female child, the product of a non-consanguineous marriage, presented with the chief complaints of developmental delay and generalized tonic–clonic seizures. Physical examination showed microcephaly, gross hypotonia, and nystagmus. MR imaging showed subtotal agenesis of the cerebellum, with rudimentary bean-like hemispheric remnants anterosuperiorly. The pons was hypoplastic, with no discernible ventral pontine eminence. The posterior fossa was fluid filled and of normal size. A few hyperintense foci were noted in the periventricular white matter on T2 and FLAIR images. Closed-lip schizencephaly was noted in the left frontal lobe. The diagnosis was subtotal cerebellar agenesis (Chiari IV malformation) with closed-lip schizencephaly.

Subtotal agenesis of cerebellum, also called Chiari IV malformation, is a very rare condition that is characterized by severely hypoplastic, rudimentary cerebellar hemispheres along with a normal-sized, fluid-filled posterior fossa and pontine hypoplasia. Clinical features vary from mild to severe.

The Chiari IV malformation is distinct from the Dandy–Walker malformation, the posterior fossa being of normal size in the former, but it is indistinguishable from primary cerebellar agenesis or pontocerebellar hypoplasia.1 Goasdoire et al have described the MR features of pontocerebellar hypoplasia. These include a thin flat pons, with loss of the anterior curve; a small cerebellum, with predominant flattening of hemispheres and shortened cerebellar fissures; and a normal-sized posterior fossa.2 Uhl et al have described similar features plus some other features like gyral atrophy, delayed myelination with no significant disorganization of brain architecture, or a corpus callosum defect.3

T1W sagittal image shows a small vermian remnant and a hypoplastic pons, with flattened anterior prominence. A fluid-filled posterior fossa can be seen.

Our patient also had closed-lip schizencephaly (which has not previously been described). The reason for pontine hypoplasia occurring with severe cerebellar hypoplasia is that the neurons that form the pontine nuclei and the cerebellar granular layer both originate from rhombic lips; in addition, several cerebellar nuclei are connected to the ventral pontine nuclei, and developmental abnormalities of one result in hypoplasia of the other.4 Subtotal cerebellar agenesis with pontine hypoplasia is an entity with distinctive MRI features but variable clinical, pathophysiologic, and etiological features.

Axial T2W FSE image shows rudimentary bean-like cerebellar hemispheric remnants and a hypoplastic pons.

T1W parasagittal image shows a holohemispheric schizencephalic cleft in the left mid-frontal region. A small right cerebellar hemispheric remnant is seen in the fluid-filled posterior fossa. The myelination pattern is normal for this patient's age.