In most mammals and birds, the iris is a thin, annular structure in the eye that contains the muscles that control the pupil. It can be classified as one of six colors: brown, blue, amber, hazel, green, or gray.
About 70 to 80% of the world's population have brown or darkly pigmented irises, making brown the most common eye color. In most cases, both eyes have matching colors, but some people suffer from heterochromia, where one eye is an entirely different color than the other.
What is Heterochromia?
Heterochromia is a rare condition in which an individual has two different-colored irises. Although this color variation is mostly used to refer to eye color differences, it can also be applied to hair or skin color variations.
Color variation of the iris can appear in three different visual patterns. In complete heterochromia (heterochromia iridium), one eye is an entirely different color than the other. There is also sectoral or partial heterochromia (Heterochromia iridis), where one iris has a section that is a different color from the rest. Finally, there is central heterochromia, where one iris has an inner ring that is a different color from the rest.
Due to inbreeding, heterochromia is quite common in horses, cats, dogs, and cattle. Some dog breeds known to have this condition are the Australian sheepdogs and Dalmatians. Meanwhile, variation in eye color is uncommon in humans, affecting only 1% of people worldwide. In the US, fewer than 200,000 people are known to have heterochromia.
Doctors do not provide a specific treatment for heterochromia since it is usually a harmless variation in eye color. Sometimes, healthcare providers treat the underlying conditions that cause heterochromia if they are ever-present.
If an infant is found to have heterochromia, it must be examined by an ophthalmologist, who will confirm the appearance of the color variation and look for any underlying causes. If the heterochromia is acquired later in life, a detailed eye exam must be performed to develop a necessary treatment plan.
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What Causes Heterochromia?
A harmless genetic mutation often causes variation in eye color. It may also result from congenital and acquired conditions, eye injury, and some eye drops. An eye care specialist can diagnose or rule out the underlying causes.
The isolated genetic mutations that cause heterochromia affect the genes instructing the body to make, transport, and store melanin. Melanin is a natural pigment that gives color to the eyes.
Some people are born with mutations that affect eye color for no reason. For others, the mutation was inherited as an autosomal dominant trait. Either way, the genetic variants do not cause other symptoms.
Melanin production may also be affected by conditions that harm melanocytes, such as Horner syndrome. Some infants are born with this condition, while some adults develop it later.
Other causes of eye color change include bleeding or hemorrhage, foreign objects in the eye, glaucoma, injury, or mild inflammation. Neurofibromatosis and Waardenburg syndrome are also related conditions that could cause color change in one eye.
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