Why is one eye darker than the other?

It is usually the pupil, not the eye itself

When people notice that one eye looks darker than the other, they are nearly always observing a difference in pupil size rather than a true difference in the colour of the eye. The pupil is the dark central opening in the coloured iris. A larger pupil lets in more light and appears as a wider, darker circle; a smaller pupil creates a narrower dark centre with more of the coloured iris visible around it.

Around one in five people have a measurable difference in pupil size between the two eyes as a completely normal finding. This is called physiological anisocoria. The difference is usually small, often less than one millimetre, and the pupils still respond normally to light. If you have always had this, or if someone else in your family has it too, it is almost certainly of no consequence.

The key distinction is whether the asymmetry is new or longstanding. A difference you have had for years is very unlikely to represent anything serious. A difference that has appeared recently, particularly if accompanied by other symptoms, is worth investigating.

Horner syndrome

Horner syndrome occurs when the sympathetic nerve supply to one eye is disrupted. The sympathetic nervous system controls both pupil dilation and partial elevation of the upper eyelid, so when this pathway is interrupted, the affected eye develops a characteristic cluster of signs: a smaller pupil on that side, a slightly droopy upper eyelid, and sometimes a barely perceptible elevation of the lower lid.

The eye itself is otherwise normal in Horner syndrome. Vision is not affected. There is no pain associated with the condition itself, though the underlying cause may sometimes produce pain or other symptoms.

If Horner syndrome is the explanation for the asymmetry, it is worth finding out why, because the sympathetic nerve supply runs a long route from the brain down through the neck and chest before reaching the eye. A new Horner syndrome warrants investigation, as it can occasionally be caused by a problem in the apex of the lung, the carotid artery in the neck, or the brain itself.

Third nerve palsy: the important emergency

A dilated (large) pupil on one side, particularly one that is accompanied by a drooping eyelid, double vision, and pain around or behind the eye, is a neurological emergency. This combination of signs can indicate a third cranial nerve palsy caused by compression of the nerve, most seriously from an aneurysm at the base of the brain.

The third nerve controls four of the six muscles that move the eye, the muscle that lifts the eyelid, and the muscle that constricts the pupil. When the nerve is compressed externally, the pupil-constricting fibres, which run on the outer surface of the nerve, are typically affected first, causing the pupil to dilate.

Differences in iris colour: heterochromia

Heterochromia is a genuine difference in iris colour between the two eyes, or sometimes between different parts of the same iris. The word sounds alarming, but congenital heterochromia, present from birth, is almost always a benign finding with no effect on vision. It is simply the result of variation in the amount of melanin pigment in the iris tissue, determined at the time the eye was developing.

Acquired heterochromia, which develops later in life in a previously normally coloured eye, is a different matter. This can be caused by inflammation inside the eye, glaucoma, medications applied to the eye (such as prostaglandin eye drops used to treat glaucoma), or, rarely, infiltrative conditions. If you notice a change in the colour of one iris that has developed over weeks or months, it is worth having the eye examined.

Cataract in one eye

A cataract is a clouding of the natural lens inside the eye. When a cataract develops in one eye but not the other, people may notice that the affected eye appears slightly different in colour, particularly if the cataract has a brownish or golden tinge. More commonly, the predominant symptom is a difference in vision quality between the two eyes, with colours appearing less vivid or the image less sharp on the affected side.

Cataracts nearly always develop in both eyes eventually, but they frequently affect one eye first and progress at different rates on each side. This is one of the most common reasons for asymmetric vision, and it is very treatable with a straightforward surgical procedure.

Red flags that need prompt assessment

Most causes of one eye appearing darker than the other are benign, but certain patterns of asymmetry warrant same-day or urgent assessment.

A longstanding, stable difference in eye appearance that you have simply never paid attention to before is rarely a cause for concern. If you are uncertain whether the asymmetry is new or longstanding, it can be helpful to look at old photographs to compare.