A dog closing one eye is almost always a sign of pain or irritation in that eye. The squinting reflex, called blepharospasm, is your dog’s way of protecting an eye that hurts. The causes range from something as simple as a grass seed stuck under the eyelid to something as serious as glaucoma, so the specific symptoms alongside the squinting matter a lot.
Something Stuck in the Eye
Foreign bodies are one of the most common reasons a dog suddenly starts holding one eye shut. The usual culprits are organic materials like grass seeds, plant awns, and small twigs, though sand, dirt, and even tiny bits of metal or glass can lodge on the surface of the eye or tuck behind the third eyelid. Your dog will typically squint, paw at the eye, and tear up excessively. The irritation can also trigger inflammation inside the eye itself, causing redness and swelling that makes things look worse than the original debris might suggest.
Corneal Ulcers
A corneal ulcer is essentially a wound on the clear surface of the eye, and it’s extremely painful. Dogs with a corneal ulcer keep the affected eye tightly shut, rub it against carpet or furniture, and often have discharge collecting in the corner of the eye or running down the face. The eye may look cloudy or lose its normal transparency, and you might notice tiny blood vessels spreading across the surface.
Superficial ulcers typically heal within about two weeks with treatment. Deeper ulcers take longer, with a median healing time around 19 to 21 days, and older dogs (over 10 years) can take closer to a month. Deep ulcers are considered an emergency because they risk perforating the eye entirely.
Eyelid Problems
Entropion is a condition where the eyelid rolls inward, causing the fur and lashes on the outside of the lid to scrape directly against the cornea. It’s as uncomfortable as it sounds. Dogs with entropion squint, hold the eye shut, and produce excessive tears or thick, goopy discharge. If the eyelid has been rubbing against the eye for a long time, blood vessels or dark pigment can grow across the cornea and start blocking vision. Entropion often affects just one eye and can develop gradually, so you might notice the squinting getting worse over weeks or months.
Blepharitis, or inflammation of the eyelid itself, is another possibility. The eyelid looks red, swollen, and itchy, and it can result from bacterial infections, allergies, insect bites, mites, or even environmental irritants like tobacco smoke.
Glaucoma and Internal Eye Inflammation
Glaucoma happens when pressure builds up inside the eye. Normal eye pressure in dogs falls between 15 and 25 mmHg; glaucoma is diagnosed when pressure exceeds 30 mmHg. A dog with glaucoma may squint, and the affected eye often looks red, cloudy, or visibly swollen compared to the other eye. Glaucoma can cause permanent vision loss, so it requires urgent treatment.
Uveitis, or inflammation inside the eye, causes similar symptoms: pain, squinting, redness, and cloudiness. It can develop on its own or as a complication of another problem like a dislocated lens or an eye injury. Glaucoma can actually develop as a consequence of uveitis, so the two conditions sometimes overlap.
Flat-Faced Breeds Are More Vulnerable
If your dog is a French Bulldog, Pug, Shih Tzu, English Bulldog, Pekingese, Boxer, or Boston Terrier, eye problems are significantly more likely. These breeds have round skulls and shallow eye sockets that cause the eyes to sit more prominently on the face. Their eyelid openings are often too wide to fully cover and lubricate the eye, a condition called lagophthalmos. Some can’t even close their eyelids completely.
This combination of bulging eyes and poor eyelid coverage leaves the cornea chronically exposed. The result is a higher rate of corneal erosion, ulcers, and trauma. In one study of 93 brachycephalic dogs, 53% of Pugs had corneal pigmentation (a sign of chronic irritation), and 36% of Shih Tzus had corneal scarring. If you own one of these breeds, take any squinting seriously, because their eyes have less natural protection to begin with.
What You Can Safely Do at Home
If your dog just started squinting and you suspect something might be in the eye, you can try a gentle rinse with sterile saline eye wash, the kind sold over the counter for wound irrigation. Do not use contact lens solution. It contains enzymes that can damage the surface of the eye.
A clean, warm, wet washcloth held gently against the closed eye for 5 to 15 minutes can help soothe muscle spasms, soften any crusty discharge, and bring healthy blood flow to the area. You can repeat this two to four times a day. Be patient and wait until debris softens before trying to wipe it away. If your dog won’t let you near the eye or seems in significant pain, don’t force it.
Signs That Need Prompt Veterinary Attention
Some patterns signal a problem that shouldn’t wait. A cloudy or hazy eye can indicate glaucoma, a dislocated lens, or internal inflammation. A visibly bulging or swollen eye suggests dangerously elevated pressure or, in cases of trauma, the eye pushing out of the socket. Red eyes with any discharge beyond clear tears point to infection, ulceration, or deeper inflammation. Any sudden loss of vision in the affected eye is an emergency.
If the squinting doesn’t resolve within a few hours after a saline rinse, or if your dog is rubbing the eye aggressively, showing colored discharge, or refusing to open the eye at all, these are signs of something beyond a minor irritant.
What Happens at the Vet
A veterinary eye exam typically starts with a few quick, painless baseline tests. A fluorescein stain is a drop of bright green dye placed on the surface of the eye. It sticks to damaged areas of the cornea, making ulcers and scratches glow under a blue light. This test doesn’t require numbing drops and gives an immediate answer about whether the corneal surface is intact.
Tonometry measures the pressure inside the eye using a small instrument that briefly touches the surface (usually after a numbing drop). This is how glaucoma is detected or ruled out. A tear production test, called a Schirmer tear test, uses a small paper strip placed at the edge of the eye to measure whether the eye is producing enough moisture. Together, these three tests cover the most common causes of one-sided squinting and guide what comes next.