Heterochromia Iridis (or just heterochromia) is medically defined by Merriam-Webster as “A difference in color between the irises of the two eyes or between parts of one iris.” In other words, a person’s eyes have some sort of color mutation. I actually have heterochromia myself, but it isn’t always completely noticeable, and sometimes I forget that it’s there. Now, I was born this way, but are there exceptions to this rule? Do some people just “get” Heterochromia or are genetics always a factor?

As it turns out, Heterochromia is “caused by too much or too little melanin (pigmentation)” (Source), and there are three different types. Central Heterochromia is when the middle of the iris has two different colors. Sectoral Heterochromia is when the same iris contains completely different colors, and Complete Heterochromia is when both eyes are completely different colors. All three cases of Heterochromia are actually extremely rare. So rare, in fact, that “six out of 1,000” (1% of the population) people are born with it, and “in most cases, it is hardly noticeable and unassociated with any other abnormality” (Source). Therefore, more often than not, Heterochromia Iridis is acquired.

How you ask?

Well, Heterochromia can happen for a multitude of reasons. When it comes to being born with Heterochromia, a person can get it through Waardenburg Syndrome, Sturge-Weber Syndrome, Parry-Romberg Syndrome, and Horner’s Syndrome–to name a few (Source). I won’t explain those though because they’re beside the point. Heterochromia can develop after the time of birth and childhood from a multitude of difficulties.

A few examples include:

Pigment Dispersion Syndrome: When the outer surface of the iris loses pigment, and then said pigment is dispersed into the inner part of the eye. This causes the anterior part of the iris to darken.

Trauma: If a blunt or sharp object hits, penetrates, scratches, or damages the eye in any way, it can cause one side of the eye (the hurt part) to lighten. This occurs because the damaged part of the eye has lost cells.

Medication: Xalatan, Lumigan, Travatan which are eyedrops used for Glaucoma and Latisse used for cosmetic purposes can darken the iris because of “stimulation of melanin production.”

Other causes of Heterochromia include (but are not limited to): inflammation from any source, Glaucoma, iron, tumors, scarring, bleeding, clouding of the cornea, malignant melanoma, diabetes, injury/disease affecting the sympathetic nervous system, genetic disorders, and other specific diseases. (Source)

This goes to show that things are not always what they seem. Most people who inherited heterochromia are actually pretty lucky, and are among a small group of the Heterochromia population–which is something I know I never would have guessed. So the next time you come across a person with Heterochromia, don’t think about how they look cool or different, but rather, the story behind the color because it was, more than likely, completely beyond their control.

Fun fact: Inherited Heterochromia is more common in dogs, cats, horses, cows, and water buffalo than it is in humans.