The most common cerebellar lesions that cause neonatal hypotonia are Dandy-Walker syndrome, Joubert syndrome, and rhomboencephaloclasis. Cerebellar atrophy may also occur with fetal alcohol syndrome, cytomegalovirus infection, and hypothyroidism.

Dandy-Walker Malformation
Dandy-Walker malformation is most likely due to slow flow of cerebrospinal fluid from the fourth ventricle to the cisterna magna. Dandy-Walker malformation is characterized by agenesis or hypoplasia of the cerebellar vermis and cystic dilation of the posterior fossa (Figure 117.1).

A

B

Figure 117.1.— Schematic representation of Dandy-Walker malformation. Large posterior fossa due to a large cisterna magna and agenesis [A] or hypoplasia [B] of the cerebellar vermis.

The tentorium is positioned high because of the large size of the posterior fossa (Figure 117.2). Apneic spells and nystagmus may occur. Macrocephaly with prominent occiput may be present. Other congenital brain abnormalities that occur in association with Dandy-Walker syndrome are agenesis of the corpus callosum, neuronal heterotopia, and aqueductal stenosis. Hydrocephalus is not present at birth but usually develops after 3 months of age. Magnetic resonance imaging of the brain is diagnostic (Figure 117.2).

A	B

Figure 117.2.— Dandy-Walker malformation. [A] T1-weighted axial image demonstrates wide communication between the suspected region of the fourth ventricle and the large posterior fossa cyst. The cerebellar hemispheres are hypoplastic. [B] T1-weighted sagittal image demonstrates a large posterior fossa, superiorly rotated superior vermis, high-positioned tentorium, and torcula.