Persistent Bilateral Mydriasis associated with a Pituitary Adenoma in a Horse

Abstract Severe bilateral mydriasis and bilaterally decreased vision observed in a 23-year-old crossbred warmblood gelding with a history of pituitary pars intermedia dysfunction. Ten years after the onset of clinical signs it was killed humanely due to worsening of clinical signs and loss of therapeutic responsiveness. Postmortem examination of the head was performed in order to confirm the suspected pituitary neoplasm and to investigate secondary oculomotor lesions. Pathomorphologic examination revealed an expansile space-occupying pituitary adenoma and degenerative changes in the preganglionic oculomotor nerve, ciliary ganglion and optic chiasm. The ocular clinical findings share features of a cavernous sinus syndrome.