Moderate severity

Horner's Syndrome

A cluster of eye and facial signs on one side of the face, caused by disruption to the sympathetic nerves supplying the eye. It is a sign of an underlying problem rather than a disease in itself.

⚠️ Common symptoms

Constricted pupil on one sideDrooping upper eyelidSunken-looking eyeRaised third eyelidSlightly warm, pink ear on the affected side

Horner's syndrome is not a disease in its own right but a recognisable set of signs that appear when the sympathetic nerve supply to one eye is interrupted. That nerve pathway is unusually long: it begins in the brain, travels down the spinal cord into the chest, then loops back up the neck alongside major blood vessels and passes through the middle ear before finally reaching the eye. Damage at any point along this route produces the same characteristic picture on the affected side.

The classic signs are a constricted pupil (miosis), a drooping upper eyelid (ptosis), an eye that appears sunken into the socket (enophthalmos) and a raised or prominent third eyelid. Owners often simply notice that one eye looks 'smaller' or 'half closed'. The affected ear and side of the face may also feel a little warmer and look pinker, because the same nerves help control blood flow to the skin.

Because the pathway passes through the middle ear, ear disease is one of the most common identifiable causes in cats, along with trauma such as road accidents or bite wounds. Vets work methodically through the anatomy to localise the problem, sometimes using diagnostic eye drops and imaging of the ears, chest or head. In a large proportion of feline cases, however, no cause is found at all — so-called idiopathic Horner's syndrome — and these cats typically recover spontaneously over a few weeks to a few months.

Management is aimed squarely at any underlying condition: treating an ear infection, addressing an injury, or investigating a suspected mass. The eye signs themselves rarely need treatment and are not painful, and the overall outlook is generally good, especially for the common idiopathic and ear-related forms.

🔍 Causes

Horner's syndrome results from damage anywhere along the long sympathetic nerve pathway to the eye. Common triggers include middle or inner ear disease, trauma or bite wounds to the head, neck or chest, tumours pressing on the nerve, and occasionally injury during ear cleaning or surgery. In many cats no cause is ever found — these idiopathic cases are common and generally carry a good outlook.

🛡 Prevention

There is no specific prevention, but promptly treating ear infections, protecting cats from road-traffic and fall injuries, and handling the head and neck gently during grooming and cleaning all reduce the risk. Early veterinary assessment of any ear disease is worthwhile.

💊 Treatment

Directed at the underlying cause — for example treating an ear infection or an injury. The eye signs themselves often improve or resolve over weeks to months once the cause is addressed, and many idiopathic cases recover on their own without specific treatment.

When to see a vet: Book a veterinary appointment whenever one of your cat's eyes suddenly looks different — a smaller pupil, a drooping lid or a raised third eyelid on one side. It is not usually painful or an emergency, but it signals a nerve problem that needs investigating to rule out ear disease, trauma or a mass.

⚕️ Medical disclaimer

  • This information is educational only and is not a substitute for veterinary advice.
  • Symptoms overlap between conditions — only a vet can diagnose accurately.
  • Never withhold treatment or self-medicate based on information found online.
  • When in doubt, call your vet. Early intervention almost always leads to better outcomes.

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