Horner's Syndrome in Cats

Rhiannon Koehler, DVM
By Rhiannon Koehler, DVM on Sep. 1, 2023
vet placing eye drops into cat's eye

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What Is Horner’s Syndrome in Cats?

Horner’s syndrome in cats is a group of changes to the appearance of the eye that occurs because of damage to the nerves that supply the affected eye. Specifically, the nerves affected are part of the sympathetic nervous system. But what does that mean?

There’s a part of the nervous system that controls bodily functions without the cat’s control, and the same goes for us humans! This part of the nervous system is called the autonomic nervous system. Examples of functions controlled by the autonomic nervous system include pupil dilation or constriction, sweating, and intestinal movement.

The Sympathetic Nervous System and Parasympathetic Nervous System in Cats

The autonomic nervous system has two main branches: the sympathetic nervous system and the parasympathetic nervous system.

The parasympathetic system maintains a relaxed state and is thought of as the “rest and digest” branch. The sympathetic system, on the contrary, prepares the body for “fight or flight.” Both the sympathetic and parasympathetic nervous systems have an effect on the eyes.

The sympathetic nervous system dilates the pupil, opens the eyelid, and keeps the eye forward in its socket. Basically, it offers changes to the eye that help a cat in flight or fight see their surroundings better. On the other hand, the parasympathetic system constricts the pupil, raises the third eyelid, and retracts the eye into the socket. In a relaxed state, your cat doesn’t need a wide-open eye to survey its surroundings.

Both systems are always working in tandem, with the dominant system at any given moment determined by what is happening around the cat.

When damage occurs to the sympathetic nerves that control the eye, the parasympathetic nerves dominate, resulting in changes to the eye and creating Horner’s syndrome.

Horner’s syndrome in cats is not an emergency, but if you’ve noticed any signs of Horner’s, contact your veterinarian. Horner’s syndrome is common in cats and is easily recognized by most veterinarians.

Symptoms of Horner’s Syndrome in Cats

Horner’s syndrome can affect one or both eyes. Symptoms of Horner’s syndrome in cats may include:

  • One pupil more constricted than the other

  • Elevated third eyelid on the side with constricted pupil

  • Retraction of the same eyeball into the head

  • Slight drooping of the upper eyelid on the affected side

Causes of Horner’s Syndrome in Cats

The sympathetic nerves that control the eye begin in the brain, travel through the brainstem, then down the spinal cord in the neck, and then partway down the spinal cord in the chest. These nerves then course back up the neck (outside the spinal cord) near the middle ear, and then into the eye.

This is a very long nerve path, and damage anywhere along this path can lead to Horner’s syndrome. Examples of potential causes include:

  • Tumors of the brain, spinal cord, middle ear, or behind the eye

  • Stroke

  • Trauma that pulls on the foreleg, stretching the nerves in the armpit (brachial plexus avulsion)

  • Mass in the chest that damages the sympathetic trunk

  • Middle ear infection

  • Abscess behind the eye

  • Head trauma, including being hit by a car, being bitten, or falling from a significant height

In 42% of cases, Horner’s syndrome occurs with no known cause (referred to as “idiopathic”). Any breed of cat can develop Horner’s syndrome.

How Veterinarians Diagnose Horner’s Syndrome in Cats

Veterinarians diagnose Horner’s syndrome in cats with a physical examination. Once the veterinarian confirms that the cat has Horner’s, their next step is to identify which segment of the nerve pathway is affected. This information is key for prognosis and treatment.

The veterinarian may place a medicated drop in the affected eye. Based on whether the eye dilates, they may be able to narrow down which part of the long sympathetic nerve pathway is affected.

Other diagnostics could include:

  • Neurologic examination to help localize the lesion (by checking reflexes and responses and watching gait)

  • X-rays of the chest, neck, and skull

  • Ear examination (otoscopic exam) to look for a ruptured eardrum or evidence of a middle ear infection

  • Magnetic resonance imaging (MRI) if a brain or spinal cord injury is suspected

  • Computed tomography (CT) scans of the head, neck, or chest

Treatment of Horner’s Syndrome in Cats

Horner’s syndrome itself isn’t painful and doesn’t cause blindness. Therefore, the syndrome isn’t usually treated. Instead, the veterinarian treats the underlying cause of Horner’s syndrome if possible.

If the cause of Horner’s is within the brain or along the pathway leading up to the middle ear, it generally carries a less favorable prognosis. For example, tumors may require surgery, radiation therapy, and/or chemotherapy, which does not guarantee resolution of Horner’s syndrome.

If Horner’s syndrome is caused by damage along the pathway from the middle ear to the eye, the prognosis is much more favorable.

For middle ear infections, treatment involves antibiotics. The eardrum may be purposely punctured (myringotomy) to allow the veterinarian to collect a sample for culture. This enables the veterinarian to choose an effective treatment for the specific bacteria causing the infection. Antibiotics are given for six to eight weeks to ensure adequate treatment.

If the middle ear infection isn’t resolving, the veterinarian may recommend removing the ear canal and cleaning out the bony region that houses parts of the middle and inner ear, a procedure known as total ear canal ablation and bulla osteotomy, or TECA-BO. Unfortunately, this surgery can also result in Horner’s syndrome approximately 58% of the time. This is because during surgery there can sometimes be damage to the sympathetic nerves, which can also cause Horner’s syndrome.

Some veterinarians may consider prescribing phenylephrine drops for management of the clinical signs. This would work for animals whose nerve damage is between the middle ear and the eye.

Recovery and Management of Horner’s Syndrome in Cats

Idiopathic Horner’s syndrome usually spontaneously resolves on its own anywhere from several weeks to several months after first appearing. Remember that Horner’s syndrome is not painful or life-threatening, and most of these cats will live long lives, even if Horner’s syndrome doesn’t completely resolve. The caveat here is that some underlying causes of Horner’s syndrome, such as brain tumors, carry a poor prognosis.

In cases with a known cause, cats who will improve usually do so within two to six weeks after the inciting incident or after the cause has been treated.

Some cats may have improvement in some of the signs of Horner’s syndrome but won’t have complete resolution. Around 25% of cats who develop Horner’s syndrome after TECA-BO will have the condition permanently.

If your cat’s Horner’s syndrome is caused by a traumatic injury or an ear infection, make sure to speak with your veterinarian about pain management. Be sure to provide a safe and comfortable location for your cat’s recovery.

Horner’s Syndrome in Cats FAQs

Can Horner’s syndrome in cats be caused after cleaning their ears?

Yes, vigorous cleaning of a cat’s ears can lead to Horner’s syndrome. Accidentally puncturing the eardrum or putting cleaning materials into the ear when the eardrum is ruptured can result in Horner’s syndrome.

What is the most common cause of Horner’s syndrome in cats?

Very often the cause is never determined. However, some common known causes include head or neck trauma—such as a car accident, bite wound, or high-rise fall—and ear inflammation or damage from ear cleaning.

Is Horner’s syndrome in cats painful?

Horner’s syndrome is not painful, but underlying causes such as an ear infection or trauma can cause pain.

Featured Image iStock.com/Vonschonertagen

Rhiannon Koehler, DVM


Rhiannon Koehler, DVM


Dr. Rhiannon Koehler is a veterinarian and freelance medical writer. She received her Doctor of Veterinary Medicine and Master of Public...

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