SYMPTOM
One pupil larger than the other
The owner notices that the two pupils are not the same size in normal lighting, with one appearing wider or narrower than the other, sometimes alongside a drooping eyelid, a change in the third eyelid's appearance, or squinting of one eye.
Ocular Disease in the Affected Eye
Glaucoma typically produces a dilated pupil that is fixed or slow to respond, in an eye that is often painful, red and cloudy because of raised intraocular pressure, while uveitis more often produces a constricted pupil from inflammation within the eye. Raised pressure can rapidly lead to irreversible blindness, with most dogs with sudden primary glaucoma losing sight in the affected eye within hours to days, and the pressure can only be measured with a tonometer. In both cases, the abnormal pupil is on the same side as the underlying eye disease.
Corneal Injury
A corneal ulcer causes reflex constriction of the pupil in the affected eye through pain-related nerve pathways, and the eye is usually visibly uncomfortable with squinting or increased tear production.
Lens Displacement
A lens that slips out of its normal position can alter pupil shape and reactivity, and a lens displaced forwards produces sudden, severe signs, frequently with glaucoma and a swollen, cloudy cornea. This is particularly recognised in terrier breeds with a hereditary predisposition, such as the Jack Russell Terrier and Miniature Bull Terrier.
Horner's Syndrome
Disruption of the sympathetic nerve pathway supplying the eye produces a small, constricted pupil on the affected side, together with a drooped upper eyelid and protrusion of the third eyelid. In published series no cause was found in between about a quarter and a half of dogs, and in about four in ten cats, and these cases are termed idiopathic.
Middle Ear Disease
Otitis media can affect the sympathetic nerve supply to the eye where it runs close to the middle ear, producing Horner's syndrome as a secondary consequence of ear disease rather than a primary eye problem.
Central Neurological Disease
Lesions within the brain affecting the pathways that control pupil size, including intracranial masses, can produce unequal pupils (anisocoria), usually accompanied by other neurological signs rather than occurring in isolation. After a head injury, unequal pupils can indicate that one side of the brain is more affected than the other, and pupils that go on to widen or stop responding to light can reflect rising pressure within the skull, which is life-threatening and can progress within hours.
Age-Related Iris Change
In older dogs, gradual thinning or atrophy of the iris is a common, usually benign finding that can produce a mild, static difference in pupil size or shape, most noticeable in bright light and without accompanying pain or other signs.
Why timing matters
Early observation
A newly noticed, isolated pupil difference without pain, redness, or other visible signs is a useful early observation, even before the underlying cause is apparent. At this stage the difference may be subtle and only evident in certain lighting or on close comparison of photographs. The absence of obvious squinting does not rule out a painful eye, as glaucoma pain in dogs usually shows as a change in behaviour, and the pressure and inflammation inside the eye cannot be judged by looking at it.
Later presentation
A persistent or worsening difference, particularly one developing alongside pain, an eyelid droop, altered vision, or other neurological signs, suggests the underlying process remains active rather than having resolved. The combination of signs present at this later stage is often more informative than the pupil difference alone.
In sudden glaucoma or a forward lens displacement the signs come on abruptly and sight can be lost rapidly and permanently, so a painful, red or cloudy eye is not a pattern that photographs taken over several days can characterise. For an eye that appears comfortable and clear with no other signs, photographs of both eyes in consistent lighting show whether the difference is static, resolving, or progressing, although an eye that looks comfortable can still have raised pressure, which only measurement can exclude. Idiopathic Horner's syndrome is a recognised cause that resolves without treatment, over months and not days: within 11 to 20 weeks in one series of Golden Retrievers, and within 6 months in another.
Conditions commonly associated
Horner's Syndrome
A recognisable cause of anisocoria producing a small pupil, drooped eyelid, and third eyelid protrusion on the affected side, often linked to nerve damage along its path from the chest to the eye.
Uveitis
Inflammation within the eye typically produces a constricted pupil on the affected side, making this a main eye-related cause of unequal pupils.
Glaucoma
Raised intraocular pressure typically produces a dilated pupil in an often painful eye on the affected side, distinguishing it from the constricted pupil seen with uveitis.
Lens Luxation
Displacement of the lens can distort pupil shape and reactivity, particularly in predisposed terrier breeds.
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Corneal Ulcers
Pain from a corneal injury can produce reflex pupil constriction in the affected eye through associated nerve pathways.
The page on this condition is being checked again and is offline for now.
Otitis Media
Middle ear inflammation can damage the nearby sympathetic nerve pathway, producing Horner's syndrome as a secondary effect.
Brain Tumours
Intracranial lesions affecting the pathways controlling pupil size are a central neurological cause of anisocoria, usually with accompanying neurological signs.
Glaucoma in Dogs
Glaucoma can cause the pupil in the affected eye to become dilated and fixed, unable to respond to light as the elevated pressure inside the eye damages the structures that control pupil size. The difference in pupil size may be one of the earliest visible changes when one eye is affected.
When to explore further
A sudden change in pupil size in an eye that is also painful, red or cloudy, shown by squinting, eye rubbing or a blue-white haze over the surface, is the pattern of glaucoma, uveitis, lens displacement or a corneal ulcer, which are time-critical for sight and cannot be told apart by appearance at home.
Anisocoria appearing alongside a drooped eyelid, third eyelid protrusion, or a noticeably smaller pupil on the affected side fits a recognisable Horner's syndrome pattern that may benefit from characterising the underlying cause.
A history of recent ear infection, head shaking, or ear discharge alongside the pupil change may point towards a middle ear contribution worth exploring alongside the eyes themselves.
Unequal pupils after a blow to the head can reflect injury to the eye itself or to one side of the brain. Pupils that then widen or stop responding to light can reflect rising pressure within the skull, which is life-threatening and can progress within hours. Unequal pupils accompanied by other neurological signs, such as altered gait, a head tilt, or behavioural change, point to a cause in the brain, nerves or inner ear rather than in the eye itself.
A pupil difference noticed only in certain lighting, without other visible signs, is a more subtle presentation. Photographs over time show whether it is changing, but cannot show the pressure inside the eye.
Where the eye appears comfortable and clear and there are no other signs, photographing both eyes together in the same, consistent lighting allows the pupil sizes to be compared objectively over time rather than relying on memory, although a comfortable appearance does not exclude raised pressure, which only measurement can show. A painful, red or cloudy eye, or a pupil change after a head injury, is not a pattern that comparison over time can characterise. Noting any accompanying signs, such as squinting, redness, cloudiness of the eye, eyelid position, head tilt, or recent ear issues, adds useful context. Recording when the difference was first noticed and whether it has changed helps characterise the overall pattern.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS