Almost all humans have two eyes that are the same color. Encountering someone with two different colored eyes is rare. In fact, only about 6 in every 10,000 people in the United States have different colored irises, a phenomenon known as heterochromia. We will discuss why these differences occur and if treatment is needed for this rare condition.
What Is Heterochromia?
The iris is the colored part of the eye that surrounds the pupil. Heterochromia is an umbrella term used to describe different colored irises in people. The name heterochromia comes from the Greek words “heteros”, which means different, and “chroma”, which means color.
People with heterochromia may have one iris that is a different color from the other or they can have part of their iris that is a different color from the rest of the iris.
What Does the Iris Do?
When people discuss eye color, they are referring to the iris. The iris surrounds the pupil (the black opening at the center of the eye), and regulates the amount of light that enters the eye by adjusting the size of the pupil.
The iris has two layers – a front layer (stroma) and a back layer (pigment epithelium). Your eye color is determined by the amount of brown pigment (melanin) in the stroma. The more melanin present, the darker the eye color. However, the amount of melanin varies in each person, which is why no two people have the same exact eye color.
Types of Heterochromia
There are three types of heterochromia: complete heterochromia, partial heterochromia, and central heterochromia.
Complete heterochromia: This type of heterochromia involves a different colored iris in each eye. For example, the iris in your left eye may be blue and the iris in your right eye may be brown.
Partial heterochromia: Also called segmental heterochromia, this type presents as different sections of color within the same iris. Often, this can appear as a slice or a wedge on the iris. For example, some people may have a blue iris with a small patch of brown.
Central heterochromia: This type of heterochromia involves the inner ring of the iris having a different color than the outer ring. For example, the center of the iris near the pupil may be blue and the outer ring may be brown. Central heterochromia usually affects both eyes.
How Rare is Heterochromia?
Heterochromia is a rare eye condition with less than 1% of the population having different colored eyes. An older study of more than 25,000 people concluded the occurrence rate for any type of heterochromia was 0.26%. According to the National Institutes of Health, Office of Rare Diseases, the occurrence of complete heterochromia in the U.S. population is only about 0.06%
What Causes Two Different Colored Eyes?
At birth, the color of your eyes is determined by genetics. Two major genes – HERC2 and OCA2 – are important in determining whether you will have brown, blue, hazel, amber, or green eyes. Sometimes, defects in these genes can produce heterochromia.
Most cases of congenital heterochromia (meaning the condition is present at birth) are passed down from parent to child via an autosomal dominant trait; meaning only one parent needs to have a mutated gene to pass on the trait.
In other cases, older children and adults can acquire heterochromia due to an injury or eye disease that develops over time.
Congenital Heterochromia
Congenital heterochromia can sometimes be linked to conditions that produce an abnormality in which the iris is darker (increased pigmentation) or lighter (lack of pigmentation).
Congenital conditions that can produce a darker iris include:
- Sturge-Weber syndrome
- Ocular melanosis
- Pigment dispersion syndrome
Congenital conditions that can produce a lighter iris include:
- Waardenbug syndrome
- Piebaldism
- Hirschsprung disease
- Parry-Romberg syndrome
Acquired Heterochromia
There are also several ways in which heterochromia can be acquired that might darken or lighten the color of the iris.
Acquired conditions that can produce a darker iris include:
- Eye trauma: Any type of eye trauma that causes bleeding or swelling can lead to iron deposition that makes the iris darker.
- Glaucoma: Medicated eye drops are routinely used to treat glaucoma, and these medications (called prostaglandin analogue eye drops) can cause your eye to darken over time. These glaucoma medications have also been repurposed to create a product called Latisse®, which is used to thicken eyelashes. The use of this product can also cause your eyes to darken.
- Pigment dispersion syndrome: This condition causes pigment to come off a tissue layer at the back of the iris due to rubbing against the lens. While rare, the pigment that rubs off may turn the iris slightly brown.
- Diabetes: Diabetes can affect the blood vessels in your eyes and cause the iris to darken.
- Eye tumors (both benign and malignant): Eye tumors can cause tiny dots to appear in your iris, which changes its color.
Acquired conditions that can produce a lighter iris include:
- Iritis (or Anterior Uveitis): This swelling or irritation of the iris can lead to color change.
- Fuchs’ heterochromic cycl*tis: This condition is chronic type of uveitis in which one eye has inflammation in the eye wall (uvea). It causes one eye to look lighter than the other does, although the changes may be subtle.
- Acquired Horner’s syndrome: This rare neurological syndrome causes damage to the nerves that connect the brain to the eye and can lead to a lightening of the iris.
- Posner-Schlossman syndrome: This condition causes recurrent acute elevation of intraocular pressure. Repeated occurrences can lighten the iris.
Is Treatment for Heterochromia Necessary?
Having a different colored iris may sound like a bad thing, but this eye condition is usually harmless and typically does not require treatment.
As mentioned above, however, there are certain instances in which heterochromia can be indicative of a larger health problem such as glaucoma or chronic eye inflammation. Treatment for these types of conditions involves treating the underlying cause, not the colored iris.
That is why it is important to contact your eye doctor if you notice any changes to your eye color or that of your loved ones.
