If one of your cat’s eyes looks sunken in and droopy, it could be Horner’s syndrome. This condition is caused by damage to some of the nerves surrounding the eye.
Horner’s syndrome can be caused by a variety of underlying problems, but it is not painful in and of itself. Usually, it will resolve on its own with resolution of its primary cause.
Signs of Horner’s Syndrome
Signs of Horner’s syndrome include:
- Sunken eye
- Small, constricted pupil even in low light
- Droopy eyelids
- Elevated third eyelid
Horner’s syndrome usually only affects one eye, but can occur in both at the same time.
Why It Happens
To understand Horner’s syndrome, we must look at your cat’s nervous system. “The autonomic nervous system, which regulates involuntary body processes like heart rate and respiration, is divided into two parts: sympathetic and parasympathetic,” says Erin Scott, VMD, board-certified veterinary ophthalmologist and associate professor of ophthalmology at Cornell University’s College of Veterinary Medicine.
“The sympathetic nervous system is responsible for the ‘fight or flight’ reflexes in the body, which allows the eyes to come forward, the eyelids to open wide, and the pupils to dilate,” says Dr. Scott. “With Horner’s syndrome, the sympathetic nerve supply to the eye is disrupted, resulting in your cat’s eye sinking back into the orbit (enophthalmos), causing the third eyelid (membrane at inner corner of eye) to elevate and partially cover the eye. The upper eyelid also becomes droopy (ptosis), and the pupil is smaller than normal (miosis).”
Basically, your cat’s eye is in rest mode. The nerves that tell the eye to be alert and take in as much light as possible are not working.
What Causes Horner’s Syndrome
“The sympathetic nervous system runs down the neck, through the chest, back up the neck, into the head near the ear, and finally to the eye,” says Dr. Scott. “Horner’s syndrome can be due to inflammatory, infectious, cancerous, or traumatic conditions in any of these locations (neck, chest, or head).”
Trauma, such as being hit by a car, is one of the most common causes of Horner’s syndrome in cats. If your cat is showing signs of Horner’s, your veterinarian will ask if your cat goes outside and if you know of any physical trauma that he has endured.
“To help diagnose Horner’s syndrome, we can place an eye drop that temporarily replaces the sympathetic nervous system in your dog or cat’s eye,” says Dr. Scott. “If a quick response is noted, it is more likely that your pet has a lesion closer to their eye, which is typically a benign or idiopathic form of Horner’s syndrome. We may also recommend routine laboratory work and chest x-rays to rule out other causes of Horner’s syndrome, such as a mass in the chest. In many cases we do not find a cause for this condition.”
You may choose to take your cat to an ophthalmology or neurology specialist, or your regular general practice veterinarian can do a variety of tests to figure out what is causing your cat’s Horner’s syndrome. Tests that might be recommended include:
- Radiographs (x-rays) to look for signs of trauma (such as broken bones) or tumors
- Bloodwork to identify infection or inflammation
- Checking your cat’s temperature to make sure he doesn’t have a fever
- Testing reflexes around both of your cat’s eyes and elsewhere on the body to assess the extent of the damage
- Examination of your cat’s ears to check for an ear infection
Treat the Underlying Issue
Since Horner’s syndrome is a manifestation of many different problems, there is no specific treatment for it. Instead, your veterinarian will treat the primary cause, if identified.
If trauma is to blame, your cat may receive medications for pain and inflammation, and might need surgery to stabilize broken bones.
If an infection is the cause, your cat will be given topical, oral, and/or injectable antimicrobials to address his specific infection.
If inflammation surrounding part of the sympathetic nervous system is the problem, your cat may be treated with anti-inflammatory drugs.
If a tumor is found, your veterinarian may recommend surgical removal, if possible.
In some cases, no obvious cause of Horner’s syndrome is found. This can be frustrating, but even without knowing what caused it, your cat may improve on his own over time. “If Horner’s syndrome has not resolved in six months, it is unlikely to resolve,” says Dr. Scott.
Erin Scott, VMD, DACVO, is an associate professor of ophthalmology at Cornell University’s College of Veterinary Medicine.
