Heterochromia is an anatomical condition that happens when there is an imbalance in melanin levels in the body. Melanin is a pigmentation compound, and symptoms of the condition are usually quite obvious. In the most extreme cases, impacted people or animals have differently colored eyes: one is usually light, like blue or green, while the other is dark, usually brown. Rings of different colors within the irises of individual eyes is also possible, and the condition can also manifest as different skin pigmentations in different parts of the body. Sometimes two different colors of hair will grow, too. Most of the time the condition is genetic, and is present from birth. It doesn’t usually present with any medical complications in these cases, and isn’t usually a sign of any problem or issue. In rarer instances pigmentation changes can come about after a head trauma, and in these cases it can be concerning because of the risk of brain injury.
The condition is almost always related to melanin levels. Melanin is a chemical pigment in humans, animals, and even plants that controls the expression of color — particularly when it comes to how darkly or intensely colors appear. The higher the pigmentation compound, the darker the expressed color. Melanin is stored in different places, which is how people can have very dark eyes and hair but pale skin; also how some people with dark skin and hair have light eyes.
Heterochromia happens when melanin levels are distorted, confused, or misinterpreted. This can happen for a couple of reasons, but in the vast majority of cases it is a genetic abnormality. It can be inherited, but it can also be a unique mutation.
The condition can also happen as a result of injury, particularly when it comes to eye color shifts. A hemorrhage or foreign object in the eye that disrupts pigment production is a common culprit. Glaucoma, certain glaucoma medications, and neurofibromatosis can be causes, too; even mild inflammation in one eye can sometimes cause pigmentation shifts, though these may not be permanent. Genetic variations aren’t usually anything that doctors worry about so long as the impacted parts of the body still function. When eyes suddenly change color later in life, though.
As Concerns the Eyes
Eye color variations are by far the most common manifestation of the condition, and are sometimes more specifically referred to as cases of heterochromia iridum. The eyes can have either complete heterochromia, meaning each eye is a different color, or partial or sectoral heterochromia, meaning that there are two different colors within one iris. Complete is the more common variety, and is usually immediately noticeable because of how striking it can be to see two differently-colored eyes on the same face.
The partial or sectoral version usually comes from inherited conditions, such as Waardenburg syndrome and Hirschsprung's disease. In these instances, the pupillary part of the iris is a different color than the mid-peripheral or ciliary part, forming a central "ring" around the eye's pupil. This type is most common in irises that contain low levels of melanin. The true color is actually the outer ring, whereas the central ring displays the color affected by the pigmentation problem.
Manifestations in Animals
Entirely different colored eyes is most common in cats, though it can occur in humans. Most frequently, the so-called "odd-eye" cat has one blue eye and one brown or golden eye. It also occurs among dogs, particularly in Siberian husky and Dalmatian breeds, some horses, cattle, water buffalo, and certain ferrets. Partial manifestations is most common in dogs from specific breeds, including the border collie and Australian shepherd.
Broader Health Implications
Medical advice from a health care provider should be sought when any person notices a changes in the color of one eye. He or she will typically need a complete eye examination and a report of any other symptoms to determine the underlying problem. If an infant has two different colored eyes, a pediatrician should be consulted, who will likely want the child to see an ophthalmologist. In genetic cases the eyesight isn't usually impacted by the pigmentation, but in the case of injury this may not be the case.