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, often painful and reddened eye due to raised intraocular pressure, while uveitis more often produces a constricted pupil from inflammation within the eye. 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
Lens luxation disrupts the normal position and support of the lens, which can distort pupil shape and reactivity. 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. Around half of canine cases and a significant proportion of feline cases are idiopathic, with no identifiable cause found.
Middle or Inner Ear Disease
Otitis media can affect the nearby sympathetic nerve trunk as it passes 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 anisocoria, usually accompanied by other neurological signs rather than occurring in isolation.
Age-Related Iris Change
In older dogs, and less commonly cats, 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. Early identification, before other signs accumulate, can make later characterisation more straightforward.
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. A difference that has remained static over time, by contrast, carries different implications from one that continues to change.
Comparing photographs of both eyes, taken in consistent lighting over several days, can reveal whether the difference is static, resolving, or progressing. This trajectory materially narrows which broad category of underlying cause, ocular, neurological, or related to the ear, is most likely. Spontaneous resolution over some weeks is itself a recognised pattern for one specific and common cause.
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 key ocular differential for anisocoria.
Glaucoma
Raised intraocular pressure typically produces a dilated, often painful pupil 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.
Corneal Ulcers
Pain from a corneal injury can produce reflex pupil constriction in the affected eye through associated nerve pathways.
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.
Progressive Retinal Atrophy
Progressive retinal atrophy causes gradual loss of the light-sensing cells at the back of the eye, and as retinal function declines the pupils' response to light can become uneven between the two eyes, contributing to a visible size difference.
When to explore further
A sudden change in pupil size accompanied by evident ocular pain, such as squinting, redness, or eye rubbing, may warrant distinguishing an ocular from a neurological origin, as the two are approached quite differently.
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 toward a middle ear contribution worth exploring alongside the eyes themselves.
Anisocoria accompanied by other neurological signs, such as altered gait, head tilt, or behavioural change, adds information suggesting a more central origin.
A pupil difference noticed only in certain lighting, without other signs, is a more subtle presentation that may still benefit from being documented and monitored for change over time.
Photographing both eyes together in the same, consistent lighting allows the pupil sizes to be compared objectively over time rather than relying on memory. Noting any accompanying signs, such as squinting, redness, 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: 27 August 2026 · Dr Alastair Greenway MRCVS