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Squinting one eye means that eye hurts. The medical term is blepharospasm, and when it's on one side the trouble is in that eye rather than something general. Most of the time it's a corneal ulcer, glaucoma, anterior uveitis, dry eye or something stuck under the lid. Glaucoma is the one that can take the sight from that eye in a day, so a new squint gets seen the same day.
Key Takeaways
Come in now if the eye looks cloudy or blue, the pupil is bigger than the other side, the eyeball looks enlarged, or your dog won't open the eye at all. Call us at (519) 896-0532, or use our emergency page after hours.
The lid clamps shut because the eye is painful, and that's all it tells you. It doesn't say which structure is involved, only that something in or on that eye hurts enough to override the urge to see out of it.
One-sided is useful information on its own. Conditions that come from inside the body tend to hit both eyes, so a single squinting eye moves the odds toward a surface injury, a foreign body, or pressure in that one globe.
A lid held tightly shut for long enough can roll inward, and once it does, the hair on it rubs the cornea and causes more pain.7 Now the squint feeds itself, which is one reason these rarely settle on their own.
A corneal ulcer is a break in the clear surface at the front of the eye, and it's the most common reason a dog suddenly holds one eye closed.
Diagnosis takes about ten seconds. A drop of fluorescein dye goes on the eye, and it sticks to any area where the surface layer is missing.8 A simple surface ulcer gets a broad-spectrum antibiotic drop every four to eight hours, plus atropine to settle the painful spasm inside the eye.8 The antibiotic doesn't heal the ulcer. It keeps infection out while the eye closes the gap itself.
Indolent ulcers, also called recurrent corneal erosions, are the stubborn version. They show up in middle-aged and older dogs and are most common in Boxers. Drops alone won't close them, so your vet debrides the loose edge and follows it with a diamond burr or grid keratotomy, sometimes more than once.8
How deep it goes decides the rest. An ulcer that has eaten through at least half the corneal thickness needs an ophthalmologist to assess it for a graft.8 A melting ulcer, where enzymes are actively dissolving the cornea, needs antiprotease treatment including topical serum.8
Brachycephalic dogs had 11.18 times the odds of corneal ulcers compared with crossbreds, and spaniel types 3.13 times, in a study of 104,233 dogs under primary care in England. The authors put it down to the shape of the eye socket and lids in those breeds.4
Glaucoma is pressure building inside the eye, and it's the reason a new squint shouldn't wait until morning.
In an acute attack the pressure is usually above 40 to 60 mmHg.1 Alongside the squint you'd expect red whites of the eye, a diffusely cloudy cornea, a pupil that's dilated rather than small, and vision loss in that eye. If it goes on, the globe itself stretches and becomes enlarged.1
Once vision is gone, the prognosis for getting it back is guarded.1 An experimental study shows how fast that can happen. Fourteen beagles had eye pressure raised above systolic blood pressure for 60 minutes, and a day later every one of them was blind in the treated eye. By day seven, 4 of 14 had a menace response back.5 Those pressures ran higher than a typical clinical case, so the numbers don't transfer straight across, but one hour was enough to do lasting damage.
Primary angle-closure glaucoma is recognized in the American Cocker Spaniel, Basset Hound, Chow Chow, Chinese Shar-Pei, Samoyed and Siberian Husky. Beagles and Norwegian Elkhounds get the open-angle form instead.1
If one eye is confirmed as primary glaucoma, the other eye gets treated too. The unaffected eye goes on preventive drops to hold off an attack on that side.1
Anterior uveitis is inflammation inside the front chamber of the eye, and it's close to the mirror image of glaucoma. The pupil constricts where glaucoma dilates it, and the pressure usually drops where glaucoma drives it up.2
The finding that settles it is aqueous flare. Protein and cells leak into the normally clear fluid, so a fine beam of light shone across the front of the eye looks like a headlight in fog.2 Alongside that you might see a red eye, a cloudy cornea, or in a bad case blood or pus pooled at the bottom of the front chamber.2
Every uveitis case gets a fluorescein stain too, because an ulcer and uveitis can sit together and the treatment for one is wrong for the other.2
Uveitis also raises a question about the rest of the dog. When both eyes are involved, systemic disease is the first suspicion, including infections, autoimmune disease and cancer that has spread.2 A one-sided case is more often local, but it still earns a proper look.
Dry eye is usually a both-eyes problem, caused by the immune system destroying the tear glands, so one squinting eye makes it a less likely answer.6
One form does show up in a single eye. Neurogenic dry eye comes from nerve damage to the tear supply, and those dogs have signs in one eye plus a dry nostril on the same side.6 If one eye is gummy and the nostril above it is crusted and dry, say so when you call.
A Schirmer tear test measures it in a minute. Normal is 15 mm of wetting or more, 11 to 14 is early dry eye, 6 to 10 is moderate and under 5 is severe.9 Treatment is cyclosporine, which increases tear production in around 80% of dogs that still have some tear function left, with tacrolimus as the next step when cyclosporine isn't enough.6
A grass seed or a piece of grit produces the same clamped-shut eye, and it hides better than owners expect.
Foreign bodies sit on the surface of the conjunctiva, hide in the pocket behind the third eyelid, or embed in the cornea itself.10 Expect a squint, watering, and often a secondary reaction inside the eye with a small pupil and low pressure.10 Any dog with a corneal ulcer gets checked for a plant awn, because the ulcer can be the consequence rather than the whole story.10
Home removal is where real damage gets done. Your vet establishes the depth before touching anything, and if there's any doubt it's treated as having reached the front chamber of the eye.10 Anything embedded comes out under general anesthetic.10
Three of the five look the same from across the room: a squinting, red, watering eye. The pupil separates them, and you can check it at home with a phone photo of both eyes together.
| Condition | Pupil | Pressure | When |
|---|---|---|---|
| Corneal ulcer | Normal or small | Normal | Same day |
| Glaucoma | Large1 | High1 | Now |
| Anterior uveitis | Small2 | Low2 | Same day |
Pressure is the confirmation, and it needs a tonometer. In an acute glaucoma attack it usually runs over 40 to 60 mmHg, while uveitis pushes it the other way.1,2 The cornea helps as well: an ulcer holds fluorescein dye, glaucoma clouds the whole cornea, and uveitis can cloud it without holding any dye unless there's an ulcer alongside.1,2,8 Watering and face rubbing put the problem on the surface. Flare in the fluid, or blood or pus in the front chamber, puts it inside.2
Those five account for most cases. These come up often enough to recognize. Blunt trauma belongs here too: any knock to the head followed by a squint is urgent until someone has examined the eye.
| Cause | What you'd notice | Important Info |
|---|---|---|
| Entropion | The eyelid rolls inward so hair rubs the surface | The most frequent inherited eyelid defect in dogs.7 It affected 15.41% of Shar-Peis and 9.28% of Chow Chows in a study of 2,250,417 UK dogs, against 0.33% overall11 |
| Ectopic cilia | Severe pain with nothing obvious to see | Hairs that grow through the inner surface of the lid onto the cornea. They cause profound pain and corneal ulceration7 |
| Distichiasis | Mild irritation, sometimes none at all | Extra lashes emerging from the lid margin. Often needs nothing, treated with cryotherapy when it does7 |
| One-sided conjunctivitis | Redness and discharge, less hard clamping | Conjunctivitis in one eye alone points back to a foreign object, tear sac inflammation or dry eye12 |
Three tests answer most of it, and they're quick. Most dogs need no sedation.
The Schirmer tear test goes first, before any drops, because anything put in the eye changes the reading. Fluorescein stain comes next to find an ulcer.8 Then a tonometer reads the pressure, and that's the test that separates glaucoma from uveitis when both eyes look equally angry.1,2
If the eye is swollen rather than squinting, or the lids themselves look thickened, the workup shifts. That's covered in causes of swollen eyes in dogs.
Treat it as one until a vet has looked at the eye. Squinting is a pain response, and two of the common causes, glaucoma and a deep corneal ulcer, can cost the eye its sight. Come in straight away if the eye is cloudy or blue, the pupil is larger than the other side, the eyeball looks bigger, or your dog won't open it at all. Anything milder still needs to be seen the same day rather than left overnight.
No. Steroid eye drops are contraindicated when a corneal ulcer is present, because they hold the surface back from healing and can drive a melting ulcer. Topical anti-inflammatory drops carry the same warning. Since you can't tell from the outside whether there's an ulcer under the squint, leftover drops from a previous problem are a genuine risk. Bring your dog in and let us look before anything goes in the eye.
Some do, which is why waiting feels reasonable. A mild surface scratch can settle, but a squint from glaucoma or a grass seed under the lid gets worse while you wait, and a lid held shut for long enough can roll inward and start rubbing the cornea, which adds a second problem. There's no way to sort the harmless from the serious by looking. A fluorescein stain and a pressure reading take a couple of minutes and answer it.
Still get it checked. Two of the more painful causes are almost invisible to an owner. Ectopic cilia are hairs growing through the inner surface of the eyelid onto the cornea, and they cause severe pain with nothing obvious to see from the outside. A grass seed sits in the pocket behind the third eyelid, out of view. A vet can lift the third eyelid under topical anesthetic and see what you can't.
A plant awn is the first thing to rule out. Grass seeds lodge on the surface of the conjunctiva, in the pocket behind the third eyelid, or embedded in the cornea itself, and they produce a hard squint with watering. Don't try to pull it out. The depth has to be established first, because something that looks superficial can be sitting inside the front chamber of the eye, and anything embedded comes out under general anesthetic.
Pupil size and eye pressure. Glaucoma dilates the pupil and pushes pressure up, usually above 40 to 60 mmHg in an acute attack. Uveitis constricts the pupil and pressure is typically low. A tonometer gives the reading in seconds. Check your dog's pupils against each other before you come in. The difference is often visible in a photo, and it tells us how fast we need to see you.
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