Chiari malformation presenting as loss of cerebellar substance on computed tomography.

Cerebral computed tomography (CT) is the primary imaging modality in the investigation of suspected cerebellar degeneration. A case is presented in which an inaccurate clinical and radiologic diagnosis of olivo-pontocerebellar degeneration resulted from CT imaging demonstrating loss of cerebellar substance. Subsequent clinical recognition of downbeat nystagmus led to a diagnosis of Chiari malformation. This malformation was classified as a "severe" Chiari type I via clivomyelography and angiography. Surgical decompression resulted in marked clinical improvement. Visualization of the loss of cerebellar substance should not be considered to exclude Chiari malformation.