Intracranial epidermoid cyst in a horse.

An intracranial epidermoid cyst was diagnosed in a Haflinger gelding aged 12 years suffering from episodes of neurological disturbance, apathy and fever. The extracerebral cyst was located in the region of the tentorium cerebelli and measured 9 x 8 x 5 cm. It displaced the cerebral hemispheres anteriorly and compressed the cerebellum and brainstem underneath. Histologically, the cyst was lined by keratinizing, stratified squamous epithelium and contained keratin scales, debris, focal haemorrhages, and degenerating inflammatory cells. The epithelium was supported by a vascularized fibrous capsule that firmly adhered to the dura mater. Multifocal haemorrhages and moderate lymphohistiocytic infiltrations were present in the capsule. The basement membrane was disrupted focally, and single or clustered cytokeratin-positive epithelial cells were detected in the fibrous capsule, suggesting a possible transition to malignancy.