Heterochromia: Definition & Causes

Instructor: Danielle Haak

Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences

Have you ever seen a dog or cat with two differently colored eyes? This is called heterochromia. Read this lesson to learn more about this fascinating condition and how heterochromia happens!

What Is Heterochromia?

Heterochromia is a condition that causes an organism to have differently colored eyes. This makes sense if we look at the word heterochromia. Hetero means different, and chromia means color. Sometimes, the term heterochromia may be used to refer to different coloration in the hair or skin, but usually it is used to describe the condition in the eye.

Heterochromia causes two differently colored irises.

In some rare forms of this condition, one iris has multiple colors in it. This is called segmental heterochromia. In this case, just one eye might have multiple colors, or both might, such as in the image below.

Segmental heterochromia leads to different colors within an individual iris. It can occur in one or both eyes.

What Causes Heterochromia?

The part of the eye that has color is called the iris. Usually, the color of the irises matches, at least in humans. In fact, it's estimated that only six out of every 1,000 people have heterochromia. So what causes a person to have two different eye colors? Most cases of heterochromia are hereditary, meaning the condition is coded into the person's DNA and passed down from parent to offspring.

In certain conditions, such as Waardenburg syndrome, Sturge-Weber syndrome, von Recklinghausen disease, Parry-Romberg syndrome, and Horner syndrome, heterochromia can be one of the symptoms. In these cases, heterochromia is not congenital.

Heterchromia may also be caused by an illness or injury, bleeding, cancer, or inflammation around the eye. Inflammation can be an effect of a foreign object in the eye, among other causes. Another potential source of heterochromia is medication used to treat glaucoma, an eye condition characterized by damage to the optic nerve. This medication can create a side effect that leads to a darkening of the iris and inconsistent coloring.

Ultimately, the color of the iris is dependent on the amount of melanin contained within the iris. You may have heard of melanin in terms of other parts of the body--it's the pigment responsible for determining hair and skin colors in all organisms. The more melanin concentrated in your iris, the darker your eyes will be.

Heterochromia Symptoms and Treatment Options

Other than the different coloration of the eyes, there often aren't any other symptoms present with heterochromia. If it's caused by another medical condition, there may be symptoms affiliated with that condition, but not with heterochromia.

If treatment is necessary, it is usually to treat the underlying condition, not the heterochromia itself. Generally, heterochromia doesn't require any type of treatment.

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