Persistent Bilateral Mydriasis Associated With a Pituitary Adenoma in a Horse.

This research article recites a case of a 23-year-old horse suffering from severe eye dilation and vision decrease linked to a pituitary tumor. The tumor’s secondary effects and related ocular issues were confirmed after 10 years of consistent symptoms, upon the horse’s death and subsequent autopsy.

Case Overview

• The horse, a 23-year-old crossbred warmblood gelding, had chronic pituitary pars intermedia dysfunction. Over ten years, it showed signs like severe eye dilation (bilateral mydriasis) and decreased vision in both eyes. Due to the worsening of the clinical signs and the animal’s lack of response to the treatment, it was humanely euthanized.

Postmortem Examination

• The researchers performed a postmortem examination on the horse’s head to confirm the suspected pituitary neoplasm, or tumor and to investigate any secondary effects related to the oculomotor nerve responsible for eye movements.
• This examination revealed the presence of an expansile, space-occupying pituitary adenoma – a type of noncancerous tumor.
• Alongside the pituitary tumor, the autopsy also uncovered degenerative changes in the preganglionic oculomotor nerve (which sends signals to muscles enabling most eye movements), the ciliary ganglion (a group of neurons involved in the control of eye muscles), and the optic chiasm (the part of the brain where the optic nerves partially cross).

Connection with Cavernous Sinus Syndrome

• The ocular clinical findings in this case bear similarities with a condition known as cavernous sinus syndrome. This syndrome is characterized by multiple cranial nerve palsies and is commonly caused by any mass effect in the cavernous sinus region, such as tumors.

Despite the horse’s unusual condition, the study provides significant insights into the link between pituitary adenomas and mydriasis and could potentially further the understanding of similar conditions in other species.