Dysplastic disease of the cerebellum of an adult horse.

A 4-year-old horse was evaluated at the Colorado State University Veterinary Teaching Hospital for rapidly progressing cerebellar disease. Euthanasia was elected and at postmortem examination a proliferative mass encompassing the right side of the cerebellum was discovered. The lesion was characterized by large, convoluted, vascular folia and absence of the core of central white matter. Histologically, there was a diminution or loss of the internal granule cell layer, cavitation of the central white matter, and absence of Purkinje cells. The molecular layer was thickened with myelinated axons originating from large neurons aberrantly located in this zone. The pathology of this cerebellar mass closely resembles that seen in the cerebellum of humans with Lhermitte-Duclos disease. Mature adults are most commonly affected and clinical signs associated with this disease are often vague. The disease is thought to result from hypertrophy of granule cells possibly due to an early maturation or migration defect. Because of the late age of onset and the rarity of the disorder, its pathogenesis has been difficult to discover. Awareness that other mammalian species may be affected with a similar disorder may lead to an understanding of the causative developmental defect.