Have you ever met someone with two different colored eyes and wondered why? This rare and captivating phenomenon is known as heterochromia, a condition that causes variations in the color of the irises. While it’s often considered a striking and beautiful trait, heterochromia also offers a fascinating insight into genetics, pigmentation, and sometimes underlying medical conditions.
In this blog, we’ll break down the types, causes, and scientific background of heterochromia, and why it happens.
What Is Heterochromia?
Heterochromia is a condition in which a person (or animal) has two different colored eyes or variations in color within the same eye. It occurs due to differences in the amount and distribution of melanin, the pigment that gives color to the skin, hair, and eyes.
Melanin is produced by specialized cells called melanocytes. When these cells deposit different amounts of melanin in each eye—or in different areas of the same eye—it results in heterochromia.
Types of Heterochromia
1. Complete Heterochromia
- Definition: Each eye is a completely different color.
- Example: One eye is blue, and the other is brown.
- Common Cause: Usually congenital (present from birth), but can also result from injury or certain diseases.
- Famous Example: Actress Kate Bosworth has one blue eye and one hazel eye.
2. Sectoral Heterochromia (Partial Heterochromia)
- Definition: A segment of one iris is a different color from the rest of that same eye.
- Example: An eye that is mostly green with a brown segment.
- Appearance: It often looks like a “slice” or “wedge” of a different color.
- Common Cause: Can be genetic or due to conditions like Horner’s syndrome or injury.
3. Central Heterochromia
- Definition: The area around the pupil (the central iris) is a different color from the outer part of the iris.
- Example: Eyes that appear green with a gold or hazel ring around the pupil.
- Appearance: Usually radiates outward in a ring or starburst pattern.
- Common Cause: Often genetic and typically benign. It is more common than the other two types.
What Causes Heterochromia?
Heterochromia occurs when there is a difference in melanin (the pigment responsible for color) in the irises. The cause can be genetic, developmental, or due to external factors like disease, injury, or medication. Broadly, it is classified into two main categories: congenital and acquired.
1. Congenital Heterochromia (Present at Birth)
This type is usually genetic and benign. It occurs when genes that control eye color and melanin distribution are expressed differently in each eye.
Causes of Congenital Heterochromia:
- Benign genetic mutations
- Familial inheritance (runs in families)
- Genetic syndromes, such as:
- Waardenburg syndrome: affects pigmentation in eyes, hair, and skin.
- Sturge-Weber syndrome: a rare neurological condition.
- Hirschsprung disease: a bowel disorder that may also present with heterochromia.
In most cases, congenital heterochromia does not affect vision or eye health.
2. Acquired Heterochromia (Develops Later in Life)
Acquired heterochromia happens when external factors or medical conditions change the pigmentation of one or both eyes over time.
Causes of Acquired Heterochromia:
- Eye injury or trauma: damage to the iris can affect pigmentation.
- Certain medications, especially:
- Prostaglandin analogs used in glaucoma treatment (e.g., latanoprost).
- Inflammation of the eye (e.g., uveitis)
- Tumors in or near the eye (e.g., iris melanoma)
- Horner’s syndrome: affects nerves to the eye and face, leading to lighter iris color on the affected side.
- Fuchs’ heterochromic iridocyclitis: a form of chronic uveitis.
- Eye surgery or laser procedures: can sometimes alter eye pigmentation.
Is Heterochromia Dangerous?
In most cases, heterochromia is not dangerous, especially when it’s congenital—meaning a person is born with it. Congenital heterochromia is typically a harmless genetic variation and doesn’t affect vision or eye health. Many people live their entire lives with heterochromia without experiencing any problems.
However, if heterochromia develops later in life (known as acquired heterochromia), it could be a sign of an underlying medical issue. Conditions such as eye trauma, inflammation (like uveitis), glaucoma, or even tumors like iris melanoma can lead to changes in eye color. Some neurological disorders, such as Horner’s syndrome, may also cause the iris to lighten.
While the change in eye color itself isn’t inherently harmful, what causes the change could be. That’s why it’s important to seek medical attention if heterochromia appears suddenly, progresses, or is accompanied by other symptoms like pain, vision loss, or light sensitivity.
Heterochromia in Animals
Heterochromia is common in animals and usually harmless. It’s often seen in white cats like Turkish Vans and Persians, where one eye may be blue and the other green or amber. In some white cats, the blue eye may be linked to deafness on the same side. Certain dog breeds like Siberian Huskies, Australian Shepherds, and Dalmatians also frequently have heterochromia, which is typically genetic and doesn’t affect health. Though rare, it can appear in horses—especially Paint Horses and Appaloosas—with no impact on vision or behavior. If eye color changes suddenly, a vet should be consulted.
Final thoughts
Heterochromia is a rare and fascinating trait, affecting less than 1% of the population worldwide. Whether present from birth or acquired later in life, it adds a unique touch of individuality and beauty to those who have it. While usually harmless, any sudden changes in eye color should be checked by a medical professional to rule out underlying conditions. Embracing this rare genetic variation reminds us just how wonderfully diverse human—and animal—appearance can be.
