If you are a dog or a cat lover, you certainly have screened the internet for wallpapers featuring those cuties. You may have spotted that certain cats and dogs have two differently coloured eyes. Huskies among dogs and Japanese bobtails among cats, especially, exhibit this. This happens due to a certain mutation in the genes of these animals, and you would be surprised to know that this is seen in humans too. The phenomenon is called Heterochromia. Literally, Heterochromia means different (hetero-) colours (-chromia). This is not a mutation and does not affect the vision.
Other terms to describe different coloured eyes are Heterochromia Iridis and Heterochromia Iridum. “Iridis” and “iridum” refer to the iris of the eye. The iris is the thin, circular structure that surrounds the pupil and contains the pigment melanin, which gives our eyes their distinctive colour. The amount of melanin in the iris determines whether we have blue, green, hazel eyes, brown, or black eyes. Blue eyes have the least amount of melanin in the iris, black eyes have the most.
Benign Heterochromia can give a person a captivating, even exotic, appearance. Many celebrities including Henry Cavill, Mila Kunis have Heterochromia. David Bowie’s condition was not Heterochromia.
There are three types of Heterochromia:
- Complete Heterochromia is when the two iris have two different colours. Each iris has to be exactly of the same colour.
- Partial Heterochromia is when one or both iris have two or more colour variations in them.
- Central Heterochromia is when the central area of the iris surrounding the pupil is a different colour than the outer part.
Heterochromia must never be confused with Iris Nevus.
What causes Heterochromia?
Most cases of Heterochromia are benign. An infant may be born with benign Heterochromia, or it can become apparent in early childhood as the iris attains its full amount of melanin. These types are called Congenital Heterochromia.
Congenital Heterochromia is a genetic trait that is inherited. Benign Heterochromia also can occur as the result of a genetic mutation during embryonic development. There are some conditions that may be the reason for Heterochromia during embryonic development.
One of the conditions that causes Heterochromia is Horner’s syndrome. This is a combination of a constricted pupil, partial ptosis, and loss of the ability to sweat on half of the face, all caused by an interruption of certain nerve impulses to the eye.
Heterochromia that develops later in life is called acquired Heterochromia. Causes of acquired Heterochromia include eye injuries, Uveitis and certain Glaucoma medications.
Sometimes, a condition called Anisocoria can make people look like they have two different coloured eyes when they do not. Anisocoria is a common condition or mutation characterised by unequal pupil sizes. It affects about 20 percent of the population. In most cases, Anisocoria is present at birth and is harmless. Also, the difference in pupil sizes usually is small, less than a millimetre difference between the right and left the eye. When the difference is greater, it may be mistaken as Heterochromia.
Though most cases of Heterochromia are congenital and benign, if you or your child has different coloured eyes (or different coloured segments of one or both eyes), see your eye doctor for a comprehensive eye exam to rule out other causes.