One-Sided Facial Droop in Cats: Horner Triad

A cat with one small pupil, one drooping upper lid, and one eye that sits 1 to 2 mm deeper in the socket has Horner's syndrome until proven otherwise. Add a droopy lip and a droopy ear, and the facial nerve is involved too, which points at the middle ear. Get an exam within 24 hours: the common causes in cats are otitis media, head or neck trauma, and masses in the neck or chest, and several of them worsen fast.

The 3 signs that make the triad

Horner's syndrome is loss of sympathetic tone to the eye, and it produces a fixed set of 3 signs on the same side. Miosis: the pupil measures 2 to 3 mm in a lit room while the other measures 6 to 8 mm. Ptosis: the upper lid drops 2 to 4 mm and the eye opening narrows by 30 to 50 percent. Enophthalmos: the globe sits 1 to 2 mm back, which pushes the third eyelid forward 2 to 5 mm. The third eyelid covering 30 to 50 percent of the eye is often what an owner notices first.

Measure all 4 readings before you call

Photograph both eyes in the same light with the flash off, then measure across the pupil in each photo. Room light matters: a pupil that is 2 mm in a bright room and 5 mm in a dim one is responding, which is different from a fixed small pupil. Compare lid opening in millimeters and note whether the third eyelid is visible at rest. Write down the onset — hours or days — because a sign that appeared 6 hours ago reads very differently from one present for 3 weeks.

Sign	What to measure	Typical reading	Timeline to watch
Miosis	Pupil width, both eyes, same light	2 to 3 mm vs 6 to 8 mm	Minutes to 2 hours
Ptosis	Palpebral opening height	2 to 4 mm narrower, 30 to 50 percent	1 to 6 hours
Enophthalmos	Globe position, third eyelid cover	1 to 2 mm back, 2 to 5 mm lid cover	1 to 24 hours
Facial nerve signs	Lip and ear position, blink reflex	2 to 5 mm lip droop, no blink	1 to 48 hours
Nystagmus or head tilt	Head angle in degrees, eye movement	10 to 45 degrees tilt	Hours to 3 days

Facial nerve versus sympathetic nerve

These are 2 different nerves and they fail for different reasons, so separate them at the exam table. Facial nerve (cranial nerve VII) weakness gives a droopy ear, a droopy lip, drooling on 1 side, and an eye that does not blink, which risks exposure keratitis within 24 to 48 hours. Sympathetic loss gives the eye signs and nothing on the lip. When both appear together on the same side, the lesion is usually in the middle ear or the petrous bone, where the 2 nerves run within 1 cm of each other.

Where the lesion sits: 3-neuron chain

The sympathetic path to the eye runs in 3 neurons: brain to spinal cord at T1 to T3, spinal cord up the neck to the cranial cervical ganglion, then along the skull to the eye. First-order lesions follow brain or neck-cord disease and often come with weakness in 2 or 4 limbs. Second-order lesions follow neck or chest masses, and 2 to 3 percent of them sit behind the chest wall. Third-order lesions are the most common in cats and cluster around otitis media, skull trauma, and ear polyps. A vet can narrow the level with 1 or 2 topical drugs in about 30 to 60 minutes.

Why 24 hours, not 2 weeks

Otitis media in a cat can move to the inner ear and the vestibular system in 3 to 7 days, and a cat with a head tilt, nystagmus, or vomiting needs imaging, not drops. Neck masses and chest masses grow silently, so a 2-week delay can turn a 2 cm removable mass into something that is not removable. Trauma cases can hide a 5 mm skull fracture, and 1 in 4 cats with head trauma also has a bruised lung that shows up on day 2. Book the exam for the first 24 hours, and keep the photos on your phone for comparison.

What the exam usually includes

Expect an otoscopic look at both ears, a neurologic exam of 12 cranial nerves, blood pressure, and a retinal exam, because hypertension and retinal detachment can mimic part of this picture. If the ear is abnormal, the next step is imaging of the bulla, and owners should budget for CT rather than plain films: CT finds middle-ear disease in roughly 90 percent of confirmed cases against 50 to 60 percent for radiographs. Bloodwork covers thyroid and kidney values before any anesthetic.

When to go now, tonight

Go immediately for a pupil that is fixed and 1 to 2 mm in both eyes together with collapse, for facial droop with vomiting or a 30 degree head tilt, for any droop after a fall from 2 floors or higher, for bleeding from the ear or nose, or for a cat that cannot close 1 eye and has a cloudy cornea. Also go now if the droop came with exposure to lily, permethrin, or organophosphate: check /tools/toxic/ and bring the product. A cat that stops eating for 24 hours or drinks 100 ml per kg per day alongside the droop needs bloodwork the same day — see /symptom/cat-drinking-more-water/ — and any cat that has not passed stool for 48 hours needs the constipation check at /symptom/cat-constipation-straining/.

5 steps for the first 24 hours

• Photograph both eyes in even light, flash off, and measure pupil width in millimeters.
• Note the onset hour and whether the lip, ear, or blink is affected on the same side.
• Check the ear canal for discharge or odor, and check for a head tilt of 10 degrees or more.
• Do not use old steroid or antibiotic drops, and do not wipe the eye with anything but saline.
• Book the exam inside 24 hours with photos on your phone; sooner if any red flag is present.

Terms mentioned in this article

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