Heterochromia: Why do some people have different coloured eyes?
27 May 2021
What is heterochromia?
The term heterochromia describes a condition whereby a person has different colours in their eyes. In Ancient Greek, ‘heteros’ meant different and ‘chroma’ meant colour, so this is one of those wonderful cases where something is succinctly and accurately described by its name!
The iris (the coloured part of your eye) is usually just made up of one colour, such as brown, green, blue or hazel, but some people experience multiple shades at once – whether that’s different distinct colours within a single iris, or each iris having its own colour entirely.
Heterochromia affects around 0.06% of the population (based on 200,000 affected people in the US which has a total population of 328 million).
What are the different types of heterochromia?
There are three different types of heterochromia; complete, central and sectoral.
Complete heterochromia is perhaps the most commonly-known form of heterochromia. This is where each eye has its own coloured iris, meaning that someone could have one blue eye and one brown eye.
Central heterochromia is more subtle than complete heterochromia as the two eyes generally match each other, rather than the stark difference between the two eyes that is characteristic of complete heterochromia. The difference in colour with central heterochromia comes when the middle of the iris surrounding the pupil is a different colour to the outer edge of the iris.
Sectoral heterochromia is similar to central heterochromia in that a single eye can feature more than one colour, but the colour difference is often presented as a wedge-type shape in the iris. This type of heterochromia can be present in one eye or both eyes. If it occurs in both eyes, they can either match or appear different to each other.
What causes heterochromia?
Our eye colour is dependent on the amount of melanin in our irises. Heterochromia occurs when one part of the iris, or the whole iris, has a different level of melanin to the rest of the iris/other eye’s iris. The more melanin you have in your iris, the darker your eyes will be. Therefore, people with less melanin in one or both eyes will likely have blue irises, while people with more melanin will usually have brown irises.
The variation in melanin levels is usually a genetic mutation, leading people to be born with the condition. Most forms of heterochromia are congenital which means that a person is either born with it, or develops it in infancy. In these cases, heterochromia is benign and isn’t something to worry about. However, heterochromia which develops later in life (known as acquired heterochromia) might be a sign of something more sinister and requires a full check from an optometrist.
Acquired heterochromia can develop as a result of:
- Eye injuries
- Glaucoma (and certain medications used for its treatment)
- Tumours in the iris (both benign and malignant)
- Eye surgery
Variation in eye colour later in life can be a sign of a more serious disease like cancer or illnesses such as diabetes, so it’s important to book an appointment with your eye doctor if you notice any new differences in your eye colour.
Does heterochromia require treatment?
Congenital heterochromia requires no treatment at all, as it is merely a variation in melanin between the two eyes (or within an eye itself). It is only if it appears later in life that it might be a symptom of something more serious and, in that case, treatment for the heterochromia will centre around treatment of the underlying cause of the condition.
This could be treatment for diabetes or glaucoma, or treatment for a tumour found within the iris. In the case of heterochromia occurring after eye surgery or after trauma to the eye, there is no existing treatment to reverse the effect. Actress Mila Kunis acquired heterochromia in this way after an eye injury in her childhood and now has two subtly different coloured eyes, as well as partial blindness in the affected eye. However, actress Kate Bosworth (pictured above) was born with the condition and therefore has unaffected vision, requiring no treatment at all!
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