Heterochromia: Why Some People Have Two Eye Colours

Heterochromia is a condition where iris colour differs between a person's two eyes, or within different parts of the same eye. It happens because of an uneven distribution of melanin, the pigment that determines eye colour, and in most cases it's a harmless variation rather than a sign of disease.

Eye colour is determined mainly by the amount and type of melanin in the iris, largely controlled by a gene called OCA2, which regulates melanin production. More melanin produces brown eyes; less melanin allows more blue light to scatter and reflect, producing green, hazel, grey, or blue eyes.

There are three recognised types: complete heterochromia, where each eye is an entirely different colour; sectoral (or partial) heterochromia, where one section of an iris differs in colour from the rest of that same iris; and central heterochromia, where a ring of colour around the pupil differs from the rest of the iris, such as a gold-green centre with a brown outer ring.

Complete heterochromia is genuinely uncommon, affecting fewer than 1% of people worldwide. Partial and central heterochromia are somewhat more common but often go unnoticed since the colour difference can be subtle. It's also seen in various animals, particularly certain cat and dog breeds.

Congenital heterochromia is usually caused by normal genetic variation in how pigment cells (melanocytes) populate the iris during early development. It can also occur alongside specific genetic conditions such as Waardenburg syndrome (which may include hearing loss and a white hair streak), Horner's syndrome present from birth, piebaldism, or rarely, Hirschsprung disease. In these congenital cases, the colour difference is usually noticed in infancy and remains stable throughout life.

Acquired heterochromia can result from eye injury or trauma, chronic inflammation such as uveitis or iritis, Fuchs' heterochromic iridocyclitis, certain glaucoma medications (particularly prostaglandin-class eye drops, which can gradually darken the iris), pigmentary glaucoma, iris tumours, Horner's syndrome developing later in life, or siderosis from a retained metal foreign body in the eye. Any new colour change in adulthood deserves a proper eye examination.

No, this is a persistent online myth with no medical basis. Heterochromia results from normal genetic variation or specific underlying conditions, not from inbreeding. It can run in families when linked to a particular inherited syndrome, but on its own, having two different eye colours says nothing about a person's family background.

A sudden change in iris colour, as opposed to a difference present since birth, always deserves prompt attention. Possible causes include eye injury with bleeding or scarring, uveitis or iritis, Fuchs' heterochromic iridocyclitis, long-term use of certain glaucoma drops, an iris tumour, a retained metal foreign body, or a neurological condition such as Horner's syndrome. A full eye examination can usually identify the cause in a single visit.

This is one of the most widely repeated misconceptions about heterochromia. David Bowie's eyes were actually the same colour — his distinctive look came from anisocoria, a permanently dilated pupil in one eye following an injury in his youth, which made that eye appear darker. True heterochromia is a difference in iris colour, not pupil size, so his case is a classic example of the two conditions being confused.

No, heterochromia itself typically does not affect vision or overall eye health — it's simply a difference in iris pigmentation. If heterochromia is linked to an underlying condition, such as chronic inflammation or a neurological issue, that specific condition might affect vision, but the colour difference itself is not the cause.

Diagnosis usually starts with a detailed history, including when the colour difference was first noticed, family history, and any eye injury or surgery. A full eye examination follows, including a slit-lamp exam, checking pupil size and reaction, measuring eye pressure, and a dilated examination of the retina. If an underlying condition is suspected, imaging such as OCT or ultrasound, or in children, a broader systemic review, may be recommended.

Congenital heterochromia that is stable and isolated needs no treatment at all — it's simply a natural variation. When heterochromia results from an underlying cause, treatment targets that condition: managing inflammation or injury, reviewing glaucoma medications with your doctor, treating an iris tumour, or investigating a neurological cause like Horner's syndrome. Coloured contact lenses are a purely cosmetic option some people choose, fitted properly through an eye care professional.

Most children with isolated, stable heterochromia present since birth need only routine eye checks. It's worth mentioning to your doctor whether the colour difference has always been there, and watching for any associated features like hearing loss, a white patch of hair, lighter skin patches, or a difference in pupil size, since these can point toward an underlying genetic syndrome worth a specialist review.

Usually not — most heterochromia is a harmless, isolated variation. It can occasionally be associated with an underlying condition, particularly when it develops suddenly in adulthood or comes with other symptoms like eye pain, redness, vision changes, or a drooping eyelid. In these situations, prompt evaluation helps identify and address the underlying cause early.

Yes. Some people with heterochromia choose coloured contact lenses for cosmetic reasons, either to create a uniform eye colour or to enhance the natural difference. These should always be properly fitted through an eye care professional, since poorly fitted lenses or unverified "eye colour change" products can genuinely injure the eye.