Heterochromia is a condition where a person’s eyes are different colors, or where one eye contains more than one color. It affects roughly 0.25% of the population and is usually harmless on its own, though in some cases it can signal an underlying health condition. The difference in color comes down to melanin, the same pigment that determines skin and hair color, being unevenly distributed between or within the irises.
How Melanin Creates Eye Color
Your eye color depends on how much melanin sits in the front layers of the iris. A large amount produces brown eyes, while very little produces blue. Green and hazel fall somewhere in between. Up to 150 genes play a role in this process, with two genes on chromosome 15 doing most of the heavy lifting. These genes control how pigment-producing cells mature and how much melanin they store.
In heterochromia, something disrupts the even distribution of melanin across one or both irises. One eye may end up with significantly more pigment than the other, or pigment-producing cells may cluster unevenly within a single iris. The result is a visible color mismatch that can range from subtle to striking.
Three Types of Heterochromia
Heterochromia takes three distinct forms, depending on where the color difference appears:
- Complete heterochromia: Each eye is a completely different color, such as one brown eye and one blue eye. This is the most recognizable type.
- Sectoral heterochromia: A single iris contains two distinct colors, like a wedge of brown in an otherwise blue eye. The differently colored section typically has a clear boundary.
- Central heterochromia: A ring of one color surrounds the pupil, while a different color fills the rest of the iris. A common example is a gold ring around the pupil with blue or green further out.
Born With It: Congenital Causes
Most people with heterochromia have had it since birth or early childhood. In fact, about five out of six cases are identified between ages 2 and 19. The most common form is benign heterochromia, meaning it occurs on its own with no associated health issues and no effect on vision. It can run in families through a dominant inheritance pattern, so a parent with heterochromia has a reasonable chance of passing it on.
Less commonly, heterochromia appears as part of a broader genetic condition. Waardenburg syndrome is one of the more well-known examples. It combines heterochromia with hearing loss, a widened nasal bridge, and unusually close-set inner eyebrows. Congenital Horner syndrome can also produce heterochromia, typically making the affected eye lighter, along with a smaller pupil and a slightly drooping eyelid on the same side. Parry-Romberg syndrome, which causes progressive shrinking of tissue on one side of the face, is another rare cause.
Sturge-Weber syndrome, recognizable by a port-wine birthmark on the face, can produce a darker iris on the affected side and carries a risk of glaucoma. Neurofibromatosis type 1 can create small pigmented nodules on the iris that may look like color variation.
Acquired Heterochromia: Eye Color Changes Later in Life
When someone’s eye color changes after childhood, that warrants attention. Several conditions can alter iris pigmentation over time.
Fuchs uveitis syndrome is a form of chronic, low-grade inflammation inside the eye that gradually lightens the color of the affected iris. It often goes unnoticed for years because it typically causes no pain or redness. The real concern is its complications: cataracts develop in anywhere from 23% to over 90% of patients, and secondary glaucoma occurs in 15% to 59%. Both can cause permanent vision loss if untreated, but the overall outlook is good when these complications are caught and managed.
Eye injuries, including blunt trauma and penetrating wounds, can damage pigment cells in the iris and change its color. Iron deposits from bleeding inside the eye can darken the iris on that side. Tumors of the iris, while rare, can also create a color change that mimics heterochromia.
Glaucoma Eye Drops and Iris Darkening
Certain glaucoma medications, particularly a class of drops called prostaglandin analogues, can gradually darken the treated eye. These drugs stimulate melanin production in iris pigment cells. If you’re using these drops in only one eye, the treated eye may slowly become darker than the other, creating a noticeable mismatch. People with hazel or mixed-color eyes seem especially prone to this effect. The color change is likely permanent or extremely slow to reverse, even after stopping the medication.
When Heterochromia Needs a Closer Look
Heterochromia that’s been present since birth and hasn’t changed is almost always benign. The eye color difference itself doesn’t affect vision or eye health.
The situations that call for an eye exam are more specific. A new color change in one eye at any age is the most important red flag, since it can signal inflammation, trauma-related damage, or, rarely, a tumor. In infants and young children, heterochromia combined with other signs (a drooping eyelid, a smaller pupil on one side, hearing problems, a facial birthmark, or skin changes) may point to one of the genetic syndromes described above. An ophthalmologist can typically determine the cause through a detailed eye exam, and in many cases, old photographs showing when the color difference first appeared help establish whether it’s congenital or acquired.
For the vast majority of people, heterochromia is simply an uncommon and harmless variation in how their body distributed pigment during development. It’s one of the rarer traits in human genetics, but on its own, it’s a quirk of biology rather than a medical problem.