The iris, the pigmented area surrounding the pupil, typically displays a uniform color. When a person has more than one color present in their eyes, such as a combination of blue and green, the variation is known as heterochromia. This difference in coloration is a relatively uncommon trait in humans, occurring in less than one percent of the population worldwide. Heterochromia is derived from ancient Greek words meaning “different color,” and it describes any variation in the distribution of pigment within the iris.
Defining Central and Sectoral Heterochromia
The appearance of blue and green within a single eye typically falls under the category of partial heterochromia, which includes both the central and sectoral types. Central heterochromia is characterized by a distinct inner ring of color that surrounds the pupil, contrasting sharply with the main color of the iris. For instance, a person might have an outer iris that is a clear blue or green, while the innermost ring near the pupil is a shade of gold, hazel, or amber.
Sectoral heterochromia presents as a distinct slice or wedge of color within one part of the iris that differs from the rest. This variation does not form a complete ring but instead looks like an irregular spot or segment cut into the eye’s main color. A person with sectoral heterochromia might have a predominantly blue iris with a clearly defined segment of green pigment. Both central and sectoral types are examples of heterochromia iridis.
Understanding Complete Heterochromia
When the color difference is not confined to a single iris but affects both eyes entirely, the condition is known as complete heterochromia, or heterochromia iridum. This form occurs when one eye is a completely different color than the other, such as one eye being blue and the other being green or brown.
In cases of complete heterochromia, the two irises have fundamentally different amounts of pigment. It is often a congenital trait, meaning it is present from birth, and is sometimes more commonly observed in certain domesticated animals like Siberian Huskies than in humans. While less common than the partial forms, it represents the most extreme example of pigment variability between the two eyes.
The Genetics of Eye Color Variation
The underlying mechanism for all forms of congenital heterochromia is rooted in the production and distribution of melanin, the pigment responsible for eye color. The amount of melanin primarily concentrated in the anterior stroma determines the visible color. More melanin results in darker eyes like brown, while less melanin allows for the scattering of light that produces blue or green coloration.
In people with heterochromia, the genetic instructions governing melanin are unevenly expressed or distributed during development. This unevenness is often the result of a spontaneous genetic mutation or polymorphism that affects the genes regulating melanin synthesis and transport. Specific genes, such as OCA2 and HERC2, are known to influence how much melanin is deposited in the iris tissue.
The result is that different areas of the iris receive different amounts of pigment, leading to the color variation. Most cases of congenital heterochromia are harmless and occur without any associated medical conditions.
When Color Differences Appear Later in Life
While most cases of heterochromia are present from birth, a difference in eye color that develops later in life is referred to as acquired heterochromia. The sudden appearance of a color change warrants a consultation with an eye care specialist to rule out potential medical causes.
Acquired heterochromia can result from physical trauma, such as an injury to the eye that damages the melanocytes. Certain inflammatory conditions, like uveitis, or specific syndromes, such as Horner’s syndrome, can also disrupt the normal pigmentation process. Furthermore, some medications used to treat glaucoma are known to stimulate increased melanin production, causing the treated iris to darken over time.