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What to Know About Horner’s Syndrome in Cats: Signs, Diagnosis & Prognosis

Does your cat have a sunken eye, droopy eyelids, an exposed third eyelid or one pupil that appears smaller than the other? It might be Horner’s syndrome. Let’s learn about the symptoms, diagnosis and treatment for Horner’s syndrome in cats.

A sick cat lying down with eyes half closed.
A sick cat lying down with eyes half closed. Photography ©tverkhovinets | iStock / Getty Images Plus.
Last Updated on November 28, 2023 by Catster Editorial Team

They say a cat’s eyes are the window to her soul, but sometimes, the eyes are a window to a cat’s health. If you ever notice that one of your cat’s eye’s looks different than the other — sunken eye, droopy eyelids, exposed third eyelid or one pupil that looks smaller than the other — it could mean she has Horner’s syndrome. Horner’s syndrome in cats is a complex neurological disorder affecting the eyes and muscles of the face.

What is Horner’s syndrome?

Closeup of a cat eye.
Does one of your cat’s pupils look different than the other? Is it Horner’s syndrome? Photography © Bloodsuker | iStock / Getty Images Plus.

“Horner’s syndrome is only a collection of symptoms, not an actual disease itself,” explains Michelle Murray, DVM, MS, Dipl. ACVIM (Neurology), CCRT, owner of NEST Veterinary Neurology in San Clemente, California. “The goal is to diagnose and treat the underlying disease process if possible.”

In a normal cat, muscles behind each eyeball keep them in place in the forward part of the eye socket. The eyelids are open wide, the third eyelid is retracted in the corner of the eye where you can’t see it, and the pupil dilates when exposed to darkness or low light. In a cat with Horner’s syndrome, damage to the sympathetic nervous system (the part of the nervous system that produces the “fight or flight” response) causes issues with the muscles behind the eye, the pupil, the third eyelid and the upper and lower eyelids.

The pathway of the sympathetic nervous system to the eye is extremely long and travels throughout the body, so there are many different places where it can become damaged. “Any injury or disease process that disrupts the sympathetic pathway to the eye can cause the symptoms of Horner’s syndrome, including the brainstem, spinal cord anywhere down to the third thoracic vertebra, a disease outside the vertebral column in the neck down to the level of T3, middle ear disease or disease in the tissues behind the eye,” Dr. Murray says.

What are the symptoms of Horner’s syndrome in cats?

Horner’s syndrome in cats causes some distinctive symptoms. “There are four things that you might notice, usually all on the same side of the body,” Dr. Murray says. “A slightly drooping upper eyelid, making the eye on that side appear slightly smaller; the eyeball might appear slightly sunken in the eye socket; the third eyelid is often protruding and partially covering the inner part of the eye; and the pupil looks smaller than in the other eye and doesn’t dilate completely in dim or dark lighting. In rare cases, the affected side is also warmer to the touch, and the skin may appear ‘pinker’ than the unaffected side.”

How is Horner’s syndrome in cats diagnosed?

To make a diagnosis of Horner’s syndrome in cats, the vet will do a complete physical exam and neurologic evaluation, looking for the telltale signs. “Occasionally, if only a few of the symptoms are present, we need to confirm the diagnosis by using various eye drops to try to dilate the pupil,” Dr. Murray explains. “This helps us to determine whether Horner’s syndrome is present, and in some cases, can also help localize what part of the pathway is affected.”

If the cat has other symptoms, these will further help the vet narrow down the cause of the Horner’s syndrome symptoms. Other tests the vet might want to run include bloodwork and urinalysis, x-rays or advanced diagnostic imaging like MRI or CAT scan.

What is the prognosis for Horner’s syndrome in cats?

The prognosis for a cat with Horner’s syndrome is entirely dependent on the cause. “If the underlying cause is treatable, then the prognosis is good,” Dr. Murray says. “If the underlying cause is a severe neurologic injury, cancer or other serious disease, then the prognosis may be poor.” Sometimes, even if the underlying cause is identified and treated, the Horner’s syndrome symptoms don’t go away.

Thumbnail: Photography ©tverkhovinets | iStock / Getty Images Plus.

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About the Author

Jackie Brown
Jackie Brown

Pet expert Jackie Brown has spent 20 years following her passion for animals as a writer and editor in the pet publishing industry. She is contributing writer for National Geographic’s Complete Guide to Pet Health, Behavior, and Happiness: The Veterinarian’s Approach to At-Home Animal Care (April 2019) and author of the book It’s Raining Cats and Dogs: Making Sense of Animal Phrases (Lumina Press, 2006). Jackie is a regular contributor to pet and veterinary industry media and is the former editor of numerous pet magazines, including Dog World, Natural Dog, Puppies 101, Kittens 101 and the Popular Cats Series. Prior to starting her career in publishing, Jackie spent eight years working in veterinary hospitals where she assisted veterinarians as they treated dogs, cats, rabbits, pocket pets, reptiles, birds and one memorable lion cub. She lives in Southern California with her husband, two sons and miniature poodle Jäger. Reach her at

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