Heterochromia means “different (hetero-) colours (-chromia).” Usually, the term is used to describe the condition where a person has different coloured eyes — one blue eye and one green eye, for example.
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 eyes, green eyes, hazel eyes or brown eyes. Blue eyes have the least amount of melanin in the iris; brown eyes have the most.
Heterochromia usually is benign. In other words, it is not an eye disease, and it does not affect visual acuity.
Benign heterochromia can give a person a captivating, even exotic, appearance. In fact, a number of celebrities — including Dan Aykroyd, Kate Bosworth, Henry Cavill, Alice Eve, Josh Henderson, Mila Kunis, Jane Seymour and Christopher Walken — have heterochromia.
Heterochromia also occurs in animals. Breeds of dogs that commonly exhibit heterochromia include a Siberian husky, an Australian shepherd, a border collie, a collie, a Shetland sheepdog, a Welsh corgi, Great Dane, a dachshund and Chihuahua. Such cat breeds include Turkish Van, Turkish angora, Japanese bobtail and sphynx. Often such “odd-eyed cats” have been bred specifically to have this feature.
Types Of Heterochromia
There are three types of heterochromia, based on where the different colours are located:
1. Complete heterochromia. This is where the iris of one eye is a completely different colour than the iris of the other eye.
2. Partial heterochromia (or sectoral heterochromia). This is where only a portion (or sector) of the iris of one eye has a different colour than the rest of the iris of that eye. Partial heterochromia can occur in one eye or both eyes.
3. Central heterochromia. In this type of heterochromia, the iris has a different colour near the border of the pupil (compared with the colour of the rest of the iris), with spikes of the central colour radiating from the pupil toward the middle of the iris.
Something that’s often confused with heterochromia is a benign growth called an iris nevus. A pigmented nevus in the iris usually is round in shape and brown in colour. Usually, only one iris nevus is present, but it’s possible to have more.
Though a person might argue that a brown iris nevus on a blue, green or hazel eye is a type of partial heterochromia, the term heterochromia usually isn’t used when the cause of the colour variation in the iris is a nevus.
Iris nevi (plural of nevus) typically remain stable in size. If you have an iris nevus, your eye doctor usually will want to see you every six months (for a while, at least) to measure its size and rule out any growth that could indicate malignancy.
What Causes Heterochromia?
As already mentioned, most cases of heterochromia are benign. An infant can 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.
Usually, congenital heterochromia is a genetic trait that is inherited. Benign heterochromia also can occur as the result of a genetic mutation during embryonic development.
In some cases, heterochromia is a symptom of another condition that’s present at birth or develops shortly thereafter.
One example of a condition that causes heterochromia is Horner’s syndrome. This is the 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.
Latisse, a repurposed glaucoma medication now used primarily as a cosmetic agent to thicken eyelashes, also can cause the iris to change colour.
David Bowie’s Eyes: Anisocoria, Not Heterochromia
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 characterized by unequal pupil sizes. It affects about 20 per cent of the population.
In most cases, anisocoria is present at birth and is perfectly harmless. Also, the difference in pupil sizes usually is small — less than a millimetre difference between the right and left eye.
