SYMPTOM
A change in eye colour
Owners notice one or both eyes looking a different shade than before, a cloudy blue-grey haze centred on the pupil, a darkening or lightening of the iris itself, a reddish tinge, or a patch of brown pigment spreading across the surface of the eye.
Normal Ageing Lens Change (Nuclear Sclerosis)
Nuclear sclerosis is a near-universal ageing change in the lens, appearing as a hazy blue-grey tint centred on the pupil in dogs and cats typically from around six to seven years onward. It results from the lens continuing to lay down new fibres throughout life, compressing and hardening the older central fibres, and light still passes through relatively normally so vision is largely preserved. This is the benign end of the eye-colour-change spectrum and is often mistaken by owners for a cataract, which it is not.
Cataract Formation
A cataract is a true opacity within the lens itself, appearing as a denser white or crystalline patch rather than the even blue-grey haze of nuclear sclerosis, and can be focal or progress to involve the whole lens. Diabetes is the leading cause in dogs, where cataracts can develop within weeks of poorly controlled blood glucose; inherited cataracts also occur in numerous breeds at younger ages. Cataracts are less common as a primary finding in cats, where they more often follow uveitis or trauma.
Intraocular Inflammation (Uveitis)
Inflammation inside the eye alters iris colour by causing protein and inflammatory cells to accumulate in the fluid in front of the lens, giving the eye a duller, muddier, or darker appearance than its normal colour, sometimes with visible redness or a constricted pupil. Uveitis has a wide range of underlying triggers, infectious, immune-mediated, traumatic, or secondary to other eye disease, and in cats is a recognised feature of several systemic viral and infectious conditions.
Elevated Pressure or Corneal Change (Glaucoma)
Glaucoma raises pressure within the eye, and the resulting corneal swelling can produce a bluish haze across the front of the eye that owners often describe as a colour change rather than cloudiness. This can develop gradually or, less commonly, quite quickly, and is more frequently unilateral at onset, meaning the two eyes look visibly different from each other, which is itself a useful observation.
Iris Pigment Change
A gradual increase in brown pigment spreading across the iris surface is recognised particularly in older cats, most often those with orange, tortoiseshell, or amber-eyed colouring, and is usually a slow, flat, benign change. It is distinguished from a more concerning pigmented lesion by remaining flat against the iris surface and by its gradual, even progression rather than a raised or irregular patch.
Corneal Pigmentation or Surface Disease
Chronic irritation, reduced tear production, or an immune-mediated corneal condition can cause the surface of the eye itself, rather than the iris or lens behind it, to develop a brown or pigmented film, most recognised in breeds prone to pannus or dry eye. This changes the apparent colour of the eye from the outside in, rather than from a structure deeper within it.
Why timing matters
Early observation
Early on, many colour changes are subtle enough to be noticed only in certain lighting, or when comparing a recent photo to an older one, and the two main benign patterns, nuclear sclerosis and iris pigment spread, both begin this gradually.
Later presentation
Later in a pathological process, the colour change tends to become more obvious, may be joined by other signs such as squinting, redness, or a visible size difference between the pupils, and any asymmetry between the two eyes tends to become more pronounced over time.
Because nuclear sclerosis and benign iris pigmentation both progress extremely slowly over months to years, a colour change that has visibly worsened within a period of weeks reflects a different underlying tempo and is worth documenting with dated photographs to establish the actual rate of change.
Conditions commonly associated
Nuclear Sclerosis
The most common cause of an owner-noticed eye colour change, reflecting a normal ageing process of the lens rather than disease.
Cataracts in Dogs
Lens opacity can alter the eye's appearance and, unlike nuclear sclerosis, can progress to affect vision.
Uveitis
Intraocular inflammation commonly changes iris colour and clarity and can arise from a wide range of underlying triggers.
Glaucoma
Raised intraocular pressure can produce a bluish corneal haze that presents to owners as a colour change.
Corneal Ulcers
Surface injury or infection of the cornea can alter the eye's clarity and apparent colour, often with discomfort.
Pannus (Chronic Superficial Keratitis)
Progressive pigmentation across the cornea changes the eye's surface colour, recognised particularly in predisposed breeds.
Keratoconjunctivitis Sicca (Dry Eye)
Chronic reduced tear production can lead to corneal pigmentation and a darkened eye surface over time.
Sudden Acquired Retinal Degeneration Syndrome (SARDS)
While primarily causing vision loss, the associated pupil changes can alter the eye's overall appearance.
When to explore further
A colour change confined to one eye, or a difference in appearance between the two eyes where previously they matched, is a distinguishing feature worth describing precisely, since several of the categories above more often start unilaterally while normal ageing lens change is typically symmetrical.
A change that develops within days rather than months, particularly alongside squinting, visible redness, excess tearing, or pawing at the eye, reflects a different tempo than the slow, even progression typical of nuclear sclerosis or benign pigment change.
A raised, irregular, or asymmetric pigmented patch on the iris, as opposed to a flat and evenly spreading one, is a distinction that benefits from characterisation, since it separates the common benign pigment pattern from a less common progressive one.
A colour change occurring in a diabetic dog, or developing rapidly alongside increased thirst and urination, adds context linking the eye change to a metabolic process rather than a primary eye condition.
Any accompanying change in behaviour suggesting visual impairment, bumping into furniture, hesitance on stairs, or reluctance to move in dim light, provides useful additional detail regardless of which colour pattern is present.
A series of close-up photographs taken in consistent, even lighting, ideally without flash, which can distort apparent eye colour, over several weeks gives a much clearer picture of whether a change is progressing than memory alone. Comparing both eyes side by side in the same photograph, and noting whether the change is centred on the pupil, spread across the iris, or affecting the clear surface of the eye, helps characterise which structure is involved.
Last reviewed: 12 September 2026 · Dr Alastair Greenway MRCVS