Horner's Syndrome in Cats

By Tammy Hunter, DVM; Ryan Llera, BSc, DVM; Cheryl Yuill, DVM, MSc, CVH

What is Horner's syndrome?

Horner's syndrome is a common neurological disorder of the eye and facial muscles, caused by dysfunction of the sympathetic nervous system. The condition usually occurs suddenly and typically affects one side of the head but can be bilateral (affect both sides of the head) in rare cases.

What is the sympathetic nervous system?

The sympathetic nervous system is one part of the autonomic nervous system. The autonomic nervous system controls automatic or involuntary functions in the body. The autonomic nervous system helps control normal functions of the eyes and facial muscles, including blinking, muscle tone, the amount of dilation or constriction of the pupils, for example.

What are the clinical signs of Horner’s syndrome?

The most common clinical signs of Horner's syndrome are:

  • the upper eyelids droop on the affected side (ptosis)
  • the pupil of the eye on the affected side is constricted, or smaller than usual (miosis)
  • the eye of the affected side often appears sunken (enophthalmos)
  • the third eyelid of the affected side may appear red and raised or protruded (conjunctival hyperemia)

What causes Horner's syndrome?

Horner's syndrome is due to a dysfunction of the sympathetic nerves of the eyes and surrounding facial muscles. If the sympathetic nervous system that supplies the eyes is damaged or is malfunctioning, the parasympathetic system “takes over”, and signs of Horner's syndrome appear. The damage or malfunction can occur in the areas of the neck, the ear, or the eye. Damage may occur because of blunt force trauma, such as that caused by an automobile accident, or because of a bite wound from another animal.

Other problems that can cause damage or inflammation of the nerves in this area include diseases within the eye or in the retrobulbar area (behind the eye), problems in the middle ear (otitis media), or tumors in the chest, neck, or brain.

In dogs, about half the cases of Horner's syndrome are idiopathic, meaning it has no known cause. In cats, however, a cause is virtually always found, and idiopathic Horner's syndrome is very rare. With many cats that develop Horner's syndrome, there is a recent history of trauma, particularly being hit by a car.

"With many cats that develop Horner's syndrome, there is a recent history of trauma, particularly being hit by a car."

Depending on your cat's recent history and other physical findings on examination, your veterinarian may recommend a series of diagnostic tests to determine if there is an underlying cause. Initial tests usually include a neurologic evaluation, an otoscopic examination (examination of the ears), and X-rays of the chest and neck area.

Are there any other problems that could look like Horner's syndrome?

In some cases, the only obvious symptom is elevation of the third eyelid. If this is the case with your cat, your veterinarian will rule out the possibility of problems such as facial paralysis (common with severe ear infections), severe dehydration, or Haw's paralysis. With Haw's paralysis, cats will elevate their third eyelids in response to illness, particularly intestinal irritation; the third eyelids may remain elevated for up to 4-6 weeks but will eventually go back to normal.

There is a rare disorder in cats called feline dysautonomia (Key-Gaskell syndrome) that has symptoms of constricted pupils, elevated third eyelids, urinary retention, constipation, and other problems related to severe disruption of the sympathetic nervous system.

What is the treatment?

Most cases of Horner's syndrome resolve spontaneously. Since your cat may have an inability to blink normally, your veterinarian may recommend symptomatic treatment in the form of eye lubricants to minimize the development of corneal ulcers from what is called “exposure keratitis”.

Phenylephrine drops in the affected eye may also be recommended to dilate the pupil. However, these drops can be irritating to the cornea. If an underlying disease is identified, it is important to treat that disease.

What is the prognosis and recovery rate?

The prognosis depends on the underlying cause. Patients with chest trauma tend to have a quicker recovery rate (days to weeks) than patients with other lesions. If the underlying problem is feline dysautonomia, the prognosis is poor.

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