Heterochromia, where a person’s eyes are two different colors or contain patches of contrasting color, affects about 1% of people worldwide. Most cases are present from birth and completely harmless, caused by uneven distribution of pigment during fetal development. But heterochromia can also develop later in life from eye injuries, certain medications, or underlying health conditions.
How Iris Color Works
Eye color comes down to one pigment: melanin. A blue eye doesn’t actually contain blue pigment. Instead, a small quantity of scattered melanin granules reflects light as a blue color, the same way suspended particles in the atmosphere make the sky look blue. Brown and black eyes simply have more melanin packed into the iris. Light brown eyes fall somewhere in between.
Melanin is produced by specialized cells called melanocytes that migrate into the iris during fetal development. If those cells distribute unevenly, or if something interrupts their migration to one eye or one section of an eye, the result is heterochromia. The nervous system plays a direct role here: sympathetic nerves have what researchers describe as a “trophic influence” on melanin development. Without proper nerve signaling, melanocytes fail to migrate fully and the iris stays lighter than it otherwise would.
Three Types of Heterochromia
Not all heterochromia looks the same. The three recognized types describe where the color difference appears:
- Complete heterochromia: each eye is an entirely different color, like one brown eye and one blue eye.
- Sectoral heterochromia: a wedge or patch of a different color appears in one iris, so part of the eye is brown while the rest is green, for example.
- Central heterochromia: a ring of a different color surrounds the pupil, distinct from the outer iris color. This is the most common form and often goes unnoticed.
Born With It: Congenital Causes
Most people with heterochromia have had it since birth or early infancy. In the majority of these cases, it’s simply a random variation in how melanocytes settled during development. No disease, no genetic syndrome, just a quirk of biology. This is sometimes called “simple” heterochromia, and it doesn’t affect vision or eye health.
In rarer cases, congenital heterochromia signals a genetic condition. Waardenburg syndrome is the most well-known example. People with this condition often have very pale blue eyes, two different-colored eyes, or one eye with segments of two different colors. It’s caused by mutations in genes involved in melanocyte development, including PAX3 and MITF among others. Because melanocytes also play a role in inner ear function, Waardenburg syndrome can cause hearing loss alongside the eye color differences. Other features may include patches of white or prematurely gray hair and lighter skin in certain areas.
Congenital Horner syndrome is another cause. This condition results from damage to the sympathetic nerves that run from the brain down through the neck and into the eye. Because those nerves control iris pigment development in infants, disrupting them early in life can leave one eye permanently lighter than the other. The affected eye also typically has a smaller pupil and a slightly drooping eyelid.
Developing It Later: Acquired Causes
When heterochromia appears for the first time in an adult or older child, the cause is almost always identifiable, and it’s worth investigating. The eye can gain or lose pigment from a range of conditions.
Eye Injury or Trauma
A blow to the eye, a penetrating injury, or even certain surgical procedures can damage the iris enough to change its color. This may happen through direct destruction of melanocytes, scarring of the iris tissue, or iron deposits from bleeding inside the eye. The color change is usually permanent.
Inflammation
Fuchs heterochromic iridocyclitis is a chronic, low-grade inflammation inside one eye that gradually breaks down iris pigment. Between 75% and 90% of people with this condition develop noticeable heterochromia. The affected eye typically becomes the lighter one as the iris loses its surface architecture and takes on what clinicians describe as a “moth-eaten” appearance. An early theory blamed sympathetic nerve dysfunction for this depigmentation, but later research found that only about 1.4% of cases were associated with nerve problems. The condition can also lead to cataracts and glaucoma over time.
Other inflammatory eye conditions, including uveitis and Posner-Schlossman syndrome, can similarly alter iris color through repeated bouts of swelling and pigment disruption.
Glaucoma Medications
One of the more surprising causes of acquired heterochromia is glaucoma eye drops. Prostaglandin analog medications, commonly prescribed to lower eye pressure, list permanent eye color change as a common side effect. These drops stimulate melanin production in the iris, gradually darkening the treated eye. If you’re using drops in only one eye, or if your eyes respond differently, the result can be noticeable heterochromia that persists even after stopping the medication.
Tumors and Serious Conditions
Intraocular melanoma, a cancer that develops from pigment cells inside the eye, can darken a section of the iris or change its overall color. Neuroblastoma, a cancer that primarily affects young children, can damage sympathetic nerves and produce heterochromia along with other signs like a drooping eyelid and constricted pupil (essentially causing an acquired Horner syndrome). Central retinal vein occlusion, where blood flow from the retina is blocked, is another recognized cause.
Why Timing Matters
The single most important distinction with heterochromia is whether it’s been present since infancy or appeared recently. Lifelong heterochromia with no other symptoms is almost always benign. A new color change in one eye, on the other hand, points to something actively happening: inflammation, pressure changes, medication effects, or in uncommon cases, a tumor.
Subtle shifts in eye color can be easy to miss. Some people only notice the change when someone else points it out or when they compare old and recent photographs. Any new or changing heterochromia in an adult, especially if accompanied by pain, vision changes, or a visible difference in pupil size, warrants an eye examination to identify the underlying cause.