Central heterochromia (CH) is a variation in eye pigmentation that results in two distinct color zones within the same iris. This condition is a form of heterochromia, specifically referring to a concentric pattern of coloration. Determining the true percentage of the population with central heterochromia is complex.
Visual Characteristics of Central Heterochromia
Central heterochromia presents as a sharp, two-toned appearance in the iris, where the color closest to the pupil is different from the color of the outer ring. This central zone is often a warmer shade, such as copper, gold, or light brown, due to a localized concentration of pigment. The outer portion of the iris, which represents the individual’s true eye color, typically displays a lighter hue like blue, green, or gray. This strong contrast creates a distinct bullseye or target pattern. This characteristic sharp boundary differentiates central heterochromia from hazel eyes, where the colors tend to blend gradually throughout the iris.
The Challenge of Determining Prevalence
Determining a precise percentage of the global population with central heterochromia is challenging because large-scale, dedicated epidemiological studies are scarce. Since CH is generally considered a benign variation in eye color and is not associated with disease, it is not consistently tracked by public health organizations.
Many available estimates rely on smaller observational studies or photographic surveys. Some anecdotal sources suggest that fewer than one percent of the population has this condition. One study that examined all forms of noticeable heterochromia found the overall prevalence was approximately 0.06% of the population, a figure that includes complete and sectoral forms as well.
The subjective nature of defining the exact boundaries of central heterochromia further complicates data collection. The trait can range from a very subtle difference in shade to a dramatic contrast, and there is no universal standard for classification versus simply having complex or mixed eye color, like hazel. Experts generally agree that central heterochromia is more common than complete heterochromia, where each eye is an entirely different color.
Genetic and Developmental Factors
The appearance of central heterochromia is primarily rooted in the uneven distribution of melanin within the iris stroma, the layer of tissue responsible for eye color. Melanin concentration dictates the final eye color. In central heterochromia, the melanocytes in the inner ring of the iris produce or retain a slightly different amount of pigment compared to the melanocytes in the outer ring.
This pigment variation is typically a congenital trait. It is usually a polygenic condition, meaning it is influenced by the interaction of multiple genes, rather than a single genetic mutation. Genes such as OCA2 and HERC2 are known to influence melanin production and distribution, and minor variations in these or other genes can lead to the two-toned pattern. While most cases are harmless and developmental, a change in eye color later in life can be a form of acquired heterochromia, sometimes caused by eye injury, inflammation, or certain glaucoma medications.
How Central Heterochromia Differs from Other Types
Central heterochromia is one of three main visual patterns of heterochromia, defined by the location of the color difference within the eye. The condition is distinct from complete heterochromia, which involves two entirely different colored eyes, such as one blue eye and one brown eye.
Central heterochromia also differs from sectoral heterochromia, sometimes called partial heterochromia. Sectoral heterochromia features a wedge or pie-slice-shaped segment of a different color within one iris. This segment does not form the concentric ring around the pupil that characterizes central heterochromia.