Heterochromia: Different eye colors explained (iris heterochromia)

One blue and one brown eye – sounds like something out of a fantasy novel, but it also happens in real life! Iris heterochromia makes for fascinatingly different eye colors, but what’s behind it? Is it simply a genetic coincidence or can it also indicate a disease? In this article, you will find out what forms of heterochromia there are, what causes it and when an ophthalmological examination is advisable – because sometimes our eyes tell us more than we think!

What is heterochromia?

Iris heterochromia refers to a difference in eye color between or within the two eyes. This phenomenon is caused by uneven pigmentation of theiris and can be genetic or acquired as a result of a disease. Depending on how much melanin is present in the iris, the eyes appear blue, brown or in different color combinations. While many cases are harmless, an examination by an ophthalmologist may be useful if there is an underlying eye disease such as Fuchs uveitis syndrome or inflammatory iritis.

What forms of iris heterochromia are there?

Formally, a distinction is made between three “types” or forms of heterochromia.

Complete heterochromia

In complete iris heterochromia, a person has two different eye colors, meaning that each eye has a completely different pigmentation. For example, one eye may be blue and the other brown. This form of heterochromia is rare and is usually genetically determined.

Partial heterochromia

In partial iris heterochromia, also known as sectoral heterochromia, one eye has two different colors. This means that one section of the iris has a different pigmentation to the rest. For example, a predominantly brown eye may have a blue or green sector. This difference in eye color is caused by an uneven distribution of melanin in the iris.

The causes are often genetic, but inflammatory diseases such as anterior uveitis, injuries or viral infections can also cause partial heterochromia.

Central heterochromia

Central iris heterochromia is a different coloration within an eye, whereby the color around the pupil is different to the outer edge of the iris. This results in a striking, often ring-shaped color gradient that makes the eye appear particularly unique.

This form of heterochromia in humans is usually genetically determined and is caused by an uneven distribution of melanin in the iris. The more melanin there is, the darker the iris appears, which in turn protects against UV rays.

Causes of different eye colors:

Heterochromia in humans is a rare phenomenon. It is estimated that only around 0.6 to 1 % of the world’s population have two different eye colors. The exact frequency varies according to region and genetic predisposition. The main causes are

Genetic factors

In most cases, iris heterochromia is genetically determined and is caused by a congenital change in pigmentation in the iris.

Sometimes heterochromia occurs in humans as an isolated trait, but it can also be associated with certain genetic syndromes, including:

• Waardenburg syndrome – can cause deafness as well as two different eye colors.
• Heterochromic cyclitis – a rare form of chronic inflammation of the iris that causes color changes.
• Neurofibromatosis – a genetic disease that can lead to heterochromia in rare cases.

As genetic factors play a decisive role, heterochromia can be inherited over several generations in some families.

Diseases such as Fuchs uveitis syndrome

Diseases that can lead to heterochromia:

• Fuchs uveitis syndrome (FUS) – A chronic inflammation of the iris that can lead to whitening of the affected iris. Often only one eye is affected, accompanied by blurred vision and an increased risk of glaucoma.
• Horner’s syndrome – Nerve damage can lead to altered pigmentation of the iris, often in combination with a reduced pupil on the affected side.
• Anterior uveitis – An inflammatory disease of the eye that can lead to different eye colors. Especially if the inflammation of the iris lasts longer, the pigmentation can change.
• Neurofibromatosis type 1 – This genetic disease can cause heterochromia and other changes in the iris.
• Waardenburg syndrome – A rare genetic mutation that can cause heterochromia, deafness and changes in skin and hair pigmentation.
• Multiple sclerosis – In rare cases, this disease of the nerve tracts can also have an effect on the pigmentation of the iris.

Injuries or medication

Eye injuries or certain medications in particular can changethe pigmentation of the irisand lead to two different eye colors.

Injury-related heterochromia

• Trauma to the eye – A severe bruise or cut can affect melanin production in the iris, causing the eye to appear lighter or darker.
• Eye operations – After an operation, e.g. to treat glaucoma, pigment changes may occur in rare cases.
• Bleeding in the anterior chamber of the eye – Can lead to permanent darkening of the iris.

Medication that can change the color of your eyes

• Glaucoma medication (prostaglandin analogs) – eye drops such as latanoprost or bimatoprost for the treatment of glaucoma can cause the iris to darken – often only in one eye.
• Corticosteroids – Long-term use may in rare cases cause pigment changes in the iris .

Is heterochromia a disease?

Basically, heterochromia is not an independent disease, but a phenomenon of eye color that is usually genetically determined. In most cases, people with two different eye colors have no health restrictions and their vision is not impaired.

However, a sudden onset of heterochromia can be an indication of an underlying eye disease. Causes such as Fuchs’ uveitis syndrome, anterior uveitis or Horner’s syndrome can lead to a change in the pigmentation of the iris. Injuries or medication can also trigger heterochromia in humans.

When should I see an ophthalmologist for an examination? Effects on vision

• When the eye color suddenly changes.
• If additional symptoms such as blurred vision, pain or sensitivity to light occur.
• If the heterochromia is associated with other symptoms such as deafness (e.g. Waardenburg syndrome).