The word heterochromia is a combination of two Greek words, which mean “different colors”. In the medical field, this term is sometimes used in connection with a skin or hair condition. However, most of the time heterochromia refers to a change in color of a part of the eye, called the iris. This condition is also know as “heterochromia iridum”.
Heterochromia dates back to ancient times. Aristotle described the condition initially and he referred to it as “heteroglaucos”. The eastern Emperor Anastasios was called Dicorus, meaning “two-pupiled”, because he had different colored eyes. The difference in color can be caused by distribution in asymmetric concentrations in the iris or due to varying amounts of melanin in the iris tissues.
The development of eye color is established by the time a child is 2 years old, meaning that the iris has become fully pigmented. There are 4 factors that determine eye color:
An alteration in any of these 4 factors can cause the color of the iris to change. Alterations can occur during fetal development (congenital heterochromia) and/or due to injury or diseases/conditions (acquired heterochromia) that impact any of the 4 factors. In most cases, the color change occurs bilaterally (it impacts both eyes). However, in rare cases, the change in color impacts only one eye. This is called heterochromia.
What are the types of heterochromia? Heterochromia can occur as either hypo-pigmentation (lack of pigmentation resulting in a lighter colored iris) or hyper-pigmentation (excess pigmentation resulting in a darker colored iris). The difference in the color of the iris is due to too much or too little pigmentation. It can occur in three degrees:
1. Complete Heterochromia
In complete heterochromia the iris of each eye is a completely different color. As you can see in the image below, one eye is light blue and the other is brown.
2. Sectoral/Partial Heterochromia
For it to be a sectoral/partial heterochromia, a segment or portion of contrasting colors in the iris must be present. In the image you can see spots of a light brown color in the iris.
3. Central Heterochromia
The characteristics in central heterochromia is that one color radiates from the pupil (the dark center of the eye), and then shifts to another color. You can see in the image that the pupil is surrounded by a yellow/brown color. This color radiates and shifts to a blue color
How rare is heterochromia? Interestingly enough, heterochromia commonly occurs in dogs! However in humans, it occurs very rarely and is officially classified as a “rare disease” by the Office of Rare Diseases at the National Institute of Health. It is difficult to report an accurate incidence rate since in many cases, the difference of color between the two eyes is so subtle and therefore it goes unnoticed. The NIH estimates that heterochromia iridis affects less than 200,000 people in the US. The estimated incidence of congenital heterochromia is approximately 6 in 1,000 live births.
What causes heterochromia? In most cases, heterochromia is hereditary. It is an autosomal dominant trait, meaning that it is likely one of the parents of an affected individual also has heterochromia. Several family members will also have heterochromia. People with hereditary (or congenital) heterochromia are born with different colored iris, called congenital heterochromia. Heterochromia can also be acquired later in life. This can be through an illness, injury, use of certain medications, or aging. For example, areas of iris hypopigmentation are commonly seen in older people; and this may be due to senile iris atrophy simply as part of the aging process. Heterochromia can occur by itself with no other disorders or problems, or it may occur as part of a syndrome or disease. It usually does not impact vision.
Congenital heterochromia has been associated with the following conditions:
Acquired heterochromia (acquired later in life, after birth) has been associated with the following syndromes or diseases:
What are the symptoms of heterochromia? There are no symptoms since heterochromia itself does not impact vision or the function of the eye. The primary way of detecting heterochromia is by observing a change in the color of the iris. Symptoms related to a disease or condition that is associated with heterochromia may occur, however, heterochromia by itself does not result in any particular symptoms.
How is heterochromia diagnosed? Usually the change in color of eyes is noted by the affected individual, by a pediatrician in cases of congenital heterochromia, or by close family/friends. A clinical will ask if the color change was first noticed at birth, during childhood, or recently (later in life)? The person affected should undergo an eye examination by an ophthalmologist to determine if there is any underlying disorder present. If an underlying disorder is suspected, the clinician will order further tests like chromosome/genetic testing or blood tests. If no underlying disorder is suspected, then further testing is not necessary.
What is the treatment for heterochromia? There is no treatment for heterochromia. Most people having heterochromia eyes seem to enjoy the unique color of their iris. For people with significantly different colored eyes or who are bothered by the difference in eye color may opt to wear colored contact lenses for cosmetic purposes.
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Imesch, P. D., et al. “The color of the human eye: a review of morphologic correlates and of some conditions that affect iridial pigmentation throughout life.” Survey of Ophthalmology. 41 (Suppl 2): S117-S123.
Olitsky, S. E., et al. “Abnormalities of pupil and iris.” In: Kliegman, R. M., et al. (Eds.), Nelson Textbook of Pediatrics. (Chapter 614) 19th Edition. Philadelphia, PA: Saunders Elsevier, 2011.