SYMPTOM
Squinting or holding eye closed
Partial or complete closure of one or both eyes, sometimes accompanied by visible discomfort or resistance to opening the affected eye.
Corneal injury or ulceration
Squinting is one of the hallmark responses to corneal pain. Even a superficial scratch on the corneal surface, which is among the most densely innervated tissues in the body, can produce pronounced blepharospasm. The degree of squinting is not a guide to the depth of the injury: sensory nerve endings are concentrated near the corneal surface, so a superficial ulcer may be more painful than a deep one, and pain is not always apparent with a deep ulcer. An ulcer is confirmed with fluorescein dye, which stains the exposed tissue beneath the surface layer. A simple superficial ulcer usually heals within about a week, whereas a melting ulcer, in which the cornea softens and breaks down, can perforate in as little as 24 hours.
Conjunctivitis or ocular surface inflammation
Inflammation of the conjunctiva (the mucous membrane lining the eyelids and covering the white of the eye) can produce squinting alongside redness, discharge, and swelling. Causes range from allergic reactions and environmental irritants to viral and bacterial infections. The squinting may fluctuate with the degree of inflammation.
Uveitis (intraocular inflammation)
Inflammation within the eye itself can produce significant photophobia and squinting. Uveitis may develop secondary to infections, immune-mediated processes, trauma, or lens-related conditions. The pain is often deep and aching, producing persistent squinting that may worsen in bright light.
Glaucoma (elevated intraocular pressure)
Increased pressure within the eye is painful, and a sudden rise is more painful than long-standing glaucoma. Squinting from a sudden rise may be accompanied by a red eye, a dilated pupil that is fixed or slow to respond, and a cloudy cornea, while a visibly enlarged eye develops only with prolonged pressure. Most dogs with primary glaucoma lose vision within hours to days of a sudden rise in pressure. Glaucoma is diagnosed by measuring the pressure in the eye (tonometry) and examining the back of the eye.
Foreign body
Plant material, sand, hair, or other debris trapped beneath the eyelids or embedded in the conjunctiva can produce persistent irritation and squinting. The animal may show sudden-onset squinting following outdoor activity, and the discomfort may be most pronounced straight after the foreign body lodges.
Why timing matters
Early observation
Squinting that appears suddenly, particularly affecting only one eye, often indicates an acute event such as a corneal scratch, foreign body, or the onset of ocular inflammation. The sudden nature of the change distinguishes it from chronic conditions and may help localise the problem to a specific incident. Early squinting may be intermittent initially, worsening over hours as inflammation develops.
Later presentation
Squinting that has continued for days reflects an ongoing source of ocular pain. By this stage the eye may be increasingly inflamed, with redness, discharge and possible corneal changes. Chronic squinting that waxes and wanes may indicate a recurring condition or a persistent irritant such as an aberrant eyelash or entropion.
The trajectory of squinting depends on its cause. Foreign body-related squinting may resolve once the material is removed. Corneal ulcers may produce squinting that improves as healing progresses or worsens if infection develops, but an ulcer that deepens beyond the nerve endings near the surface can also become less painful. In dogs, glaucoma pain often shows as a change in behaviour more than as squinting. A reduction in squinting is therefore not a reliable sign that the underlying process is resolving.
Conditions commonly associated
Corneal Ulcers
Squinting is one of the earliest signs of corneal ulceration, reflecting the pain produced when the sensory nerve endings near the corneal surface are exposed.
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Glaucoma in Dogs
Squinting can accompany glaucoma, as raised pressure within the eye is painful and a sudden rise more so, although in dogs the pain often shows as a change in behaviour more than as squinting.
Conjunctivitis
Ocular discomfort and light sensitivity in conjunctivitis often manifest as partial or complete eyelid closure.
Keratoconjunctivitis Sicca (Dry Eye)
The chronic irritation and discomfort from inadequate tear film in dry eye often leads dogs to squint or hold the affected eye partially closed.
Entropion
The discomfort from hair rubbing against the cornea causes characteristic squinting or blepharospasm.
Cherry Eye
Irritation from the prolapsed gland and any secondary conjunctivitis may cause squinting or discomfort.
Uveitis
Pain from intraocular inflammation causes the eye to be held partially or fully closed.
Corneal Sequestrum in Cats
The dark, degenerating patch of corneal tissue is painful, prompting the affected eye to be held partly or fully closed.
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Distichiasis
Extra lashes emerging along the eyelid margin brush against the cornea with every blink, prompting the eye to close protectively.
Ectopic Cilia
A misplaced hair emerging through the conjunctiva scrapes directly across the cornea, causing marked, protective closure of the eye.
Eosinophilic Keratitis in Cats
The corneal surface changes in eosinophilic keratitis are uncomfortable, so the affected eye is often held partly or fully closed.
Horner's Syndrome
A drooping upper eyelid on the affected side can make the eye look partly closed or squinted, one of the more noticeable signs of disrupted nerve supply to the eye.
Hyphaema
Blood within the eye arises from causes such as uveitis, trauma and glaucoma, which are painful and can lead to the eye being held partly or fully closed.
Lens Luxation
Squinting reflects the discomfort caused by a lens that has slipped from its normal supporting position within the eye.
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When to explore further
Sudden-onset squinting in one eye, particularly following outdoor activity or an encounter with another animal, may indicate a corneal injury or foreign body.
Squinting accompanied by visible cloudiness or colour change in the eye, altered pupil size, or a visible change in the eye's shape or appearance matches the signs described for raised eye pressure or a deep corneal ulcer, not those of mild surface irritation.
Squinting is a sign of eye pain from its onset, and mild squinting does not indicate a mild cause: the corneal staining and pressure measurement that tell the causes apart cannot be carried out at home.
Squinting accompanied by thick or coloured discharge from the eye may suggest secondary infection that could complicate the underlying cause of the ocular discomfort.
Noting which eye is affected, when the squinting began, and whether it is constant or intermittent provides useful baseline information. Observing whether the animal is also pawing at the eye, showing increased tearing, or displaying sensitivity to light can help build a more complete picture. Forcing the eyelids open can increase discomfort and may worsen an underlying corneal injury.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS