The contribution of MRI in the differential diagnosis of posterior fossa damage.

In multiple sclerosis patients, magnetic resonance imaging (MRI) frequently detects lesions in the brain stem and cerebellum. However various pathologies that have a predelection to occur in posterior fossa parenchyma may share similar features with inflammatory-demyelinating lesions. In this paper, we review the contribution of MRI to the differential diagnosis of posterior fossa pathology. Vascular lesions due to chronic hypoperfusion and arteriolosclerosis or occlusion of the main supplying arteries of the posterior circulation leading to acute infarction frequently produce characteristic pontine or cerebellar lesions. Neoplastic disease, in particular pontine gliomas in younger patients may have similar MRI features and may be difficult to distinguish from inflammatory-demyelinating lesions. Central pontine myelinolysis usually occurs in severely ill patients but the pontine MRI changes have an overlapping profile with inflammatory demyelination. Diffuse axonal injury of the midbrain and brainstem after head trauma and atrophy of posterior fossa structures in degenerative diseases may appear similar on MRI to tissue changes also seen frequently in MS. Analysis of the MRI appearance and clinical information is most often useful to narrow the fairly long list of differential diagnoses of posterior fossa pathology.