Horner's Syndrome in Cats: Signs, Causes & Diagnosis

Horner's syndrome in cats — a drooping eyelid, sunken eyeball, small pupil, and raised third eyelid on one side — is not a disease itself but a neurological sign pointing to a disruption of the sympathetic nerve pathway to the eye. The cause can be as benign as an ear infection or as serious as a chest tumor. Finding out which one requires prompt evaluation.

Last reviewed: June 2026

What Is Horner's Syndrome in Cats?

Horner's syndrome is a clinical sign complex caused by interruption of the sympathetic nervous supply to the eye and surrounding structures. The sympathetic pathway travels from the hypothalamus, down the spinal cord, across the chest, up the neck, through the middle ear, and into the orbit — making it vulnerable to lesions at many levels, as described in Côté's Clinical Veterinary Advisor.

The classic four signs on the affected side are:

1. Ptosis — drooping of the upper eyelid (the lid narrows the eye opening)
2. Miosis — abnormally small, constricted pupil (the iris dilator muscle is denervated)
3. Enophthalmos — apparent sinking of the eyeball into the orbit
4. Protrusion of the third eyelid (nictitating membrane) — the pale membrane passively rises as the retractor bulbi muscle loses tone

All four signs occur on the same side as the lesion. The eye itself is not injured — the changes are entirely neurological.

Common Causes in Cats

The cause depends on where along the sympathetic pathway the injury occurred:

First-order (central) lesions — brain or cervical spinal cord:

• Cervical intervertebral disc disease
• Spinal cord tumors or lymphoma
• Brainstem disease (rare)

Second-order (preganglionic) lesions — chest, neck, brachial plexus:

• Brachial plexus avulsion from trauma (common in cats hit by cars)
• Thoracic mass or mediastinal lymphoma (especially in young cats)
• Bite wound injury to the neck

Third-order (postganglionic) lesions — middle ear, orbit:

• Otitis media or interna (middle/inner ear infection) — the most common cause of Horner's syndrome in cats; the sympathetic fibers pass through the middle ear
• Nasopharyngeal polyp compressing the Eustachian tube
• Retrobulbar abscess or mass

In cats, middle ear disease accounts for a large proportion of Horner's cases, and treatment of the ear condition often resolves the syndrome.

What Your Vet Will Do

The physical exam — comparing pupil size in different lighting, assessing the degree of ptosis, and palpating the neck and prescapular lymph nodes — localizes the lesion before imaging.

Key diagnostic steps:

1. Otoscopic exam under sedation — direct visualization of the tympanic membrane; polyps or fluid behind the membrane are visible or palpable
2. CT scan of the skull and neck — best modality to evaluate middle ear disease, nasopharyngeal polyps, and cervical cord lesions; far superior to radiographs for this anatomy (ACVS Surgical Guidelines)
3. Chest radiographs (3 views) — screens for mediastinal mass (mediastinal lymphoma is common in young cats with Horner's)
4. Pharmacological localization — dilute phenylephrine (1%) or hydroxyamphetamine eye drops can help differentiate first-, second-, and third-order lesions based on how quickly and fully the pupil dilates
5. MRI of the brain and spinal cord — for first-order lesion localization when spinal cord or brainstem disease is suspected
6. Fine-needle aspirate of enlarged lymph nodes or masses

Treatment

Treatment is directed at the underlying cause:

• Otitis media: Long-course antibiotics guided by culture, or surgical bulla osteotomy for chronic/refractory middle ear disease; most Horner's cases from ear infection resolve within weeks to months of successful otitis treatment
• Nasopharyngeal polyp: Surgical traction-avulsion, with or without bulla osteotomy; Horner's typically resolves after polyp removal
• Mediastinal lymphoma: Chemotherapy (often L-asparaginase, vincristine, cyclophosphamide protocols); the AAFP Feline Retrovirus Guidelines, 2020 recommend FeLV/FIV testing in any young cat with mediastinal disease
• Brachial plexus avulsion: Unfortunately, nerve avulsion injuries rarely recover; physiotherapy preserves limb function but the Horner's signs may be permanent
• Idiopathic Horner's: In some cats no cause is found; many resolve spontaneously over 6–16 weeks

The eye itself generally does not require treatment — topical lubricants can help if incomplete eyelid closure causes corneal dryness.

When to See a Vet

Call your vet today if:

• You notice a drooping eyelid, small pupil, sunken eye, or visible third eyelid on one side of your cat's face
• The signs appeared suddenly — within hours to a day
• Your cat is also shaking its head, pawing at its ear, or has a head tilt
• You notice any facial asymmetry in addition to eye changes

Go to the ER immediately if:

• Signs appeared after trauma (car accident, fall from height, bite wound)
• Your cat also has hind-limb weakness, inability to walk, or pain along the spine
• Signs are accompanied by difficulty breathing (suspect mediastinal mass)
• Rapid deterioration or progressive neurological signs

Frequently Asked Questions

Is Horner's syndrome in cats painful? The four classic eye signs themselves are not painful — the eye is anatomically normal. However, the underlying cause (otitis, spinal cord compression, trauma) may cause significant pain. Cats with middle ear disease frequently have concurrent head shaking and apparent discomfort. Neurological pain from cord compression can cause cervical hyperesthesia. Treating the cause addresses the pain even when the Horner's signs persist temporarily.

Can Horner's syndrome in cats resolve on its own? Yes — idiopathic Horner's syndrome, for which no cause is found after thorough investigation, resolves spontaneously in most cats over 6–16 weeks. When the cause is a treatable middle ear infection or removable polyp, resolution typically follows successful treatment. Brachial plexus avulsion injuries are the main category where permanent signs are expected.

Does Horner's syndrome affect a cat's vision? Horner's syndrome itself does not impair vision. The pupil is smaller than normal (miosis) but the cat can still see normally. The drooping eyelid and third eyelid protrusion can slightly reduce the visual field but are rarely clinically significant. If the cat is behaving normally aside from the eye appearance, vision is almost certainly intact.

How much does diagnosing Horner's syndrome cost in cats? A focused diagnostic workup — exam, otoscopy under sedation ($150–300), CT of the skull ($800–2,000), chest radiographs ($150–400), and bloodwork ($100–250) — typically runs $1,200–2,950. Pharmacological testing adds minimal cost. If lymphoma is suspected, lymph node aspirate and cytology add $200–400. Treatment costs vary widely from $200 (short antibiotic course for otitis) to $5,000+ (surgery plus chemotherapy for mediastinal lymphoma).

What is the prognosis for a cat with Horner's syndrome? Prognosis depends entirely on the underlying cause. Middle ear disease and polyp-related Horner's carry an excellent prognosis after successful treatment. Mediastinal lymphoma responds well to chemotherapy in many cats with remission rates of 60–80%. Brachial plexus avulsion carries a poor prognosis for neurological recovery but a good quality-of-life prognosis with supportive care. Idiopathic cases almost always resolve without treatment.

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