Four signs on one side of the face add up to Horner syndrome: a pupil 2 to 3 mm smaller than the other, a lid that droops 2 to 3 mm, an eye that sinks 2 to 4 mm into the socket, and a third eyelid that slides across a third of the eye. The pupil is the giveaway in dim light, when the gap between the two eyes widens instead of closing. Horner syndrome is a sign, not a disease, and the work is finding which of 3 nerve segments failed: central, preganglionic, or postganglionic.
Why one side of the face changes
The sympathetic nerve that lifts the lid and dilates the pupil runs a long route: from the hypothalamus down the brainstem, out of the chest at the first 3 thoracic segments, up the neck beside the carotid artery, and into the eye. Any break along that route removes the tone that holds the eyelid up and the pupil open. Because the parasympathetic side keeps working, the pupil constricts and the third eyelid drifts up. The eye itself is healthy; only its innervation is not. That is why vision, pupil response to bright light, and the back of the eye usually stay normal, and why a cat with Horner syndrome alone is rarely in pain.
Sign 1 and 2: the small pupil and the drooping lid
Miosis is the most reliable sign. In a dark room after 15 to 20 minutes of adaptation, a normal cat's pupils reach 6 to 8 mm; the affected one stops at 2 to 3 mm. Ptosis is subtle and easy to miss in a fluffy face: the upper lid covers 2 to 3 mm more of the eye than on the other side, and the owner often reports that the cat looks sleepy or has a squint. Both signs are worse in dim light and can look almost normal in bright sun, which is why the exam should start in a dark room.
Sign 3 and 4: the sunken eye and the third eyelid
Loss of muscle tone behind the eye lets it settle 2 to 4 mm deeper, so the eyeball looks smaller. The third eyelid then loses the pressure that kept it folded back and covers roughly a third of the cornea, more when the cat is tired. In cats this is often what the owner notices first, and it is the last sign to resolve. A cat with all 4 signs and nothing else has a peripheral lesion until proven otherwise.
3 cause classes, and how they differ
Central lesions sit in the hypothalamus or brainstem. They usually come with other signs: weakness in 2 or 4 limbs, a head tilt, trouble swallowing, or a pupil that reacts poorly to light. Causes include inflammation, stroke, and tumor, and they are the group with the worst outlook. Preganglionic lesions sit between the chest and the neck: a mass in the chest, trauma to the neck, or damage at the first 3 thoracic segments. A cat with Horner syndrome plus a weak front leg, or a voice that has gone hoarse from laryngeal paralysis, belongs here. Postganglionic lesions sit in the middle ear, behind the eye, or along the carotid: ear infection, a polyp behind the soft palate, trauma, or a mass behind the eye. In cats, middle ear disease and nasopharyngeal polyps are the two most common structural causes found.
Locating it: the order that saves money
- Confirm the 4 signs in dim light, then measure pupil size with a ruler under the same light each visit.
- Check the rest of the nervous system: limb strength, spinal reflexes, swallowing, and the other eye. Any deficit moves the case toward central.
- Look in both ears with an otoscope, and look behind the soft palate for a polyp.
- Run the pharmacologic localization test: a dilute adrenergic drop in each eye, read at 20 minutes and again at 60 minutes. Early dilation points to one segment, late dilation to another, and no response to a third.
- Image what the test points at: 2 chest views, then CT or MRI of the head and neck, plus thyroid testing.
What the images and the clock say
| Lesion site | Numbers that go with it | First test | Recheck (days) |
|---|---|---|---|
| Central | Weakness in 2 or 4 limbs, tilt of 15 to 45 degrees | MRI of brain | 7 |
| Preganglionic | 1 weak front leg, hoarse voice for 3 or more days | 2 chest views, then CT | 7 to 10 |
| Postganglionic, ear | Ear discharge for 2 to 3 days, all 4 signs present | Otoscopy and ear flush | 14 |
| Postganglionic, orbital | Eye pushed out 2 to 4 mm, all 4 signs present | CT or MRI of the head | 14 |
| Idiopathic | All 4 signs, 0 abnormal findings elsewhere | Repeat exam at 3 weeks | 21 |
The timing matters as much as the test. A cat that develops Horner syndrome over hours, with a head tilt or a weak leg, needs imaging within 24 to 48 hours. A cat with the 4 signs and nothing else can be watched for 7 to 10 days, because many postganglionic cases improve on their own. A head tilt with a normal pupil reflex points the other way, toward the vestibular system, which is covered in vestibular disease in cats.
How long recovery takes
Idiopathic and traumatic postganglionic cases recover over 6 to 16 weeks, and the third eyelid is the last sign to fold back. Recovery order is usually pupil first at 2 to 4 weeks, then the lid, then the globe position, then the membrane. A pupil that stays at 2 mm for 6 months will probably stay that way. If the signs switch to the other eye, or a new weakness appears at week 3, the case goes back to imaging rather than more waiting. Cats with ear disease often need 4 to 6 weeks of treatment before the eye signs fade, and the ear must be rechecked even after the eye looks normal.
What to watch at home
- Photograph the eyes in the same dim light every 7 days; a phone photo at a fixed distance of 50 cm is enough.
- Watch for drooling, trouble swallowing, or a change in voice, which point to a wider nerve problem.
- Check for paw scuffing on the same side after 2 weeks; a weak front leg changes the plan.
- Keep the ear dry and do not use cotton swabs deeper than the visible canal.
- Note any new eye discharge, squinting in bright light, or swelling behind the eye.
Most of the cost of this work sits in the imaging, not the exam. Two chest views and an ear scope settle a good share of cases for a modest fee, while CT or MRI is the expensive step, and the cost breakdown is worth reading before booking. A cat with Horner syndrome plus neck pain or a weak leg should skip straight to imaging — waiting 3 weeks there costs more than the scan.
Terms mentioned in this article
Frequently asked
Is Horner syndrome in cats an emergency?
Sometimes. With only the 4 eye signs, it is not: book a visit within 2 to 3 days. With a head tilt, weakness in any limb, trouble swallowing, or signs that appeared over a few hours, it is urgent and needs imaging within 24 to 48 hours. A pupil that goes from 2 mm to 6 mm and back within minutes is a different problem and also needs a same-day look.
Will the eye go back to normal?
In postganglionic and traumatic cases, usually yes, over 6 to 16 weeks. The pupil returns first at 2 to 4 weeks, the lid next, then globe position, and the third eyelid last. A pupil stuck at 2 mm for 6 months is likely permanent. Central lesions have a worse outlook and often leave other neurologic deficits behind even when the eye recovers.
Why does the vet test with eye drops before imaging?
The drop tells you which nerve segment is broken, and that decides whether to scan the chest, the ear, or the brain. A response at 20 minutes points to one segment, a response at 60 minutes to another, and no response to a third. Skipping the test means paying for the wrong scan, and a chest CT in a cat with an ear problem answers nothing.