Neonates with cerebellar hypotonia usually have no other neurological manifestations nor any clinical signs that suggest that the cerebellum is the affected organ, except for occasionally demonstrating an unusual respiratory pattern. Evidence of an abnormal cerebellum in a brain imaging study is often the first indication of cerebellar pathology. The structures of the cerebellum are easily identifiable by MRI. Magnetic resonance imaging of the brain is better than CT scan for evaluating the posterior fossa. Magnetic resonance imaging provides good anatomical information and allows cerebrospinal fluid flow evaluation. The anatomy of the posterior fossa can be schematically represented by four parenchymal structures and two cerebrospinal fluid spaces. The parenchymal structures are: (1) brainstem, (2) cerebellar vermis, (3) right cerebellar hemisphere, and (4) left cerebellar hemisphere. The cerebrospinal fluid spaces are: (1) the fourth ventricle, and (2) the cisterna magna (Figure 116.1). Cerebrospinal fluid flows from the third ventricle through the aqueduct of Sylvius into the fourth ventricle, and from the fourth ventricle to the cisterna magna through the laterally placed foramina of Luschka and the midline foramen of Magendie.

Figure 116.1.— Structures of the posterior fossa. The cerebellum is the largest structure of the posterior fossa. The cerebellum has two components: the cerebellar hemispheres and the vermis. Ventral to the cerebellum is the fourth ventricle and the brainstem (midbrain, pons, and medulla). Dorsal to the cerebellum is the cisterna magna.