Jace fans, this one’s for you. 🖤 

July 12 was Different Eye Colour Day (yes, that is a thing) and we thought, what better way to celebrate than to dive into the magical world of heterochromia? The fascinating condition that manifests different eye colours in a single individual. Whether you consider it magical or mundane, this condition is pretty enthralling.

What is Heterochromia?

Heterochromia is a condition that results in someone having two different coloured eyes, or more than one distinct colour in a single eye, though it’s also been used to describe those with different shades of hair or skin tone. This is because it’s connected to the body’s production of melanin (our body’s source of natural pigment). It’s a rare condition, affecting only an estimated 1% of the world.

What Determines Eye Colour?

It was once believed that eye colour was determined strictly by direct genetics (brown-eyed parents = brown-eyed baby), but now we know there are many genes that affect the eyes and determine their colour. Darker and brown eyes are generally considered more dominant when it comes to any part heredity may play, and they’re the result of more melanin being present in the iris. They’re especially common in warmer climates of the world, where humans’ natural melanin production is higher. Blue eyes appear that colour due to light scattering across the iris from lack of melanin in the stroma (the front layer of the iris). A combination of melanin and scattering light result in green eyes, the rarest eye colour in the world, manifesting in only about 2% of people (hazel comes in second at about 5% of people). 

Our eye colour is usually settled by the time we’ve reached a year old, though in some cases, the eyes can darken slightly during puberty, pregnancy, or the later stages of life (lighter eyes later in life is more to do with cataract formation in the lens than lack of melanin in the iris). 

People’s eyes may appear to change colour based on one general factor: light. The dilation and constriction of our pupils depends most frequently on light, and the expansion and contraction causes the pigments in our irises to coalesce or spread apart, creating illusions of colour change. Another way our eyes can “appear” to change colour, is how the light around us reflects in or through them. 

David Bowie is often described as having heterochromia, but this actually isn’t true. Bowie had a condition known as anisocoria, meaning his pupils were different sizes. As a result of a fistfight in school, Bowie was struck in the left eye and the muscles responsible for expanding and contracting his pupil were paralyzed, leaving the pupil permanently dilated, forcing the pigments of that iris to remain close together and reflect light differently than his right eye. This gave the Thin White Duke the appearance of having one blue eye and one black, a feature he often glamorised in his media.

Types of Heterochromia

There are actually several types of heterochromia:

Acquired Heterochromia

This occurs when the irises change colour after infancy (meaning you weren’t born with two different coloured eyes, but they developed later). 

Central Heterochromia

This occurs when the pigment around the edge of your iris is a distinctly different colour than the rest of the iris. This is not the same as hazel eyes, which contain a mixture of colours throughout the iris.

Complete Heterochromia

Also known as heterochromia iridis, this occurs when one of your irises is an entirely different colour than the other iris, such as someone with one brown eye and one blue.

Congenital Heterochromia

Simply means you were born with heterochromia.

Segmental Heterochromia

Also known as heterochromia iridum, this occurs when there is more than one distinct colour present in a single iris. 

What Causes Heterochromia?

There are many things that can cause heterochromia to manifest. The distinct difference in tracing the cause lies in whether the individual was born with heterochromia (congenital) or if it manifested later in life (acquired). 

Congenital Heterochromia Causes

• Benign Heterochromia – When the condition is caused by a natural genetic mutation, not linked to any disease or illness. 
• Bloch-Sulzberger Syndrome – Also known as Bloch–Siemens syndrome, incontinentia pigmenti is a rare hereditary skin disease passed on genetically.
• Bourneville Disease – Also known as tuberous sclerosis complex, this disease is a rare genetic disease that causes non-cancerous tumours to grow throughout the vital organs.
• Horner’s Syndrome – A rare condition that develops from damaged nerves called cervical sympathetics, that affects the nerves in the eyes and face.
• Parry-Romberg Syndrome – A rare condition that affects the structure of the face’s soft tissues.
• Piebaldism – A condition that affects the body’s production of melanin.
• Sturge-Weber Syndrome – A neurological condition that causes too much blood vessel growth. The overpopulation of blood vessels can create too much pressure in the eye and on the optic nerve, resulting in glaucoma.
• Von Recklinghausen Disease – A genetic condition with varying symptoms per case, including benign tumour growth and pigmentation changes.
• Waardenburg Syndrome – A group of genetic conditions that affect hearing and pigmentation of the eyes, hair and skin.

Acquired Heterochromia Causes

• Acquired Horner’s Syndrome – As mentioned above, someone can be born with Horner’s Syndrome, but it can also develop following a stroke, tumours and other conditions.
• Eye Cancer – In rare cases, melanoma (skin cancer) can affect the eyes, as it affects our melanin (which we now know contributes to eye colour). Common symptoms include blurry vision, sudden loss of vision, or a dark spot appearing in the iris that wasn’t there before. 
• Eye Injury – We talk about eye injury a lot on this blog. Any trauma to the eye can have surprise results, in this case including changing eye pigmentation.
• Glaucoma and certain medicines commonly used to treat it, such as bimatoprost (Latisse, Lumigan) and latanoprost (Xalatan) – Latisse is a glaucoma medication that was repurposed cosmetically to help grow eyelashes. It’s available by prescription only and has been known to affect iris colour after use. 
• Iris Ectropion Syndrome – A rare congenital anomaly caused by the development plateau of the neural crest.
• Neuroblastoma – This is a cancer that develops in early nerve cells. Normally these cells mature into working cells, but sometimes they grow out of control and become cancerous, forming a tumour. It’s responsible for 15% of cancer deaths in children. See a doctor immediately if your child’s eye colour changes, just to be safe.
• Ocular Melanosis – A hereditary condition that causes an accumulation of pigmented cells in the eye. These cells can eventually block fluids from draining from the eye, resulting in increased eye pressure (which leads to glaucoma). 
• Pigment Dispersion Syndrome – A condition that occurs when pigment cells detach from the back of the iris and float around inside the eye. Like ocular melanosis, these free-floating cells can eventually block the eye’s drainage system and increase eye pressure. 
• Posner-Schlossman Syndrome – Also known as glaucomatocyclitic crisis, this rare acute condition involves mild attacks of  granulomatous anterior uveitis (inflammation of the uveal tract at the front of the eye) and raised eye pressure. This condition is also sometimes referred to as secondary inflammatory glaucoma.
• Swelling – This can occur due to iritis (inflammation of the iris) or uveitis (inflammation of the uvea). 

How is Heterochromia Viewed?

Like everything else in our complex world, beauty is in the eye of the beholder. While many view heterochromia as something beautiful, there are also those who consider it a “defect”, “flaw”, or “imperfection”. Nature is full of imperfections; it’s what makes it so perfect. The soul is always intact, despite what we see on the outside. 

Throughout history, heterochromics have been viewed as other-worldly, whether positively or negatively. Pagans of Eastern Europe once believed children born with heterochromia had “witch eyes” or “hag’s eyes”, due to a witch stealing one of their eyes at birth and swapping it for another. Some Native American cultures believed that someone with two different coloured eyes had the ability to see both the spiritual and earthly planes, calling them “ghost eyes”. Heterochromics in Asia and Africa were once believed to be shapeshifters or witches. In Ancient Greece, some believed that those with blue and green eyes could see the future. 

The Greek historian Arrian of Nicomedia, who wrote The Anabasis of Alexander (considered one of the best sources of Alexander the Great’s campaigns), described Alexander as having “one eye dark as the night and one blue as the sky”. Another described, “the right one was heavy lidded and black, and the left one was blue; and his teeth were sharp as fangs, and he looked upon a defensive attack the same as a lion would.” It seems Alexander’s condition only added to his powerful reputation.

While we now know the origins of heterochromia aren’t rooted in folklore, still, even if all the stories aren’t true, it adds another layer of mysticism to this already compelling condition.

Final Thoughts

Whether you were introduced to the world of heterochromia through little Sally Owens’ amas veritas love spell (“…and he’ll have one green eye… and one blue.”) or everyone’s favourite Wayland/Lightwood/Morgenstern/Herondale (#iykyk), I’m sure you can agree that the idea of unusually coloured eyes has always held a touch of magic.

It was important with this post to clarify that someone with two different coloured eyes doesn’t necessarily have a “disease” or anything “wrong” with them; their condition doesn’t even affect their vision. For many, it’s simply a fascinating circumstance of birth. For others, however, it can be related to a post-infancy condition, or a sign of a greater underlying issue. Regardless, regular eye exams are always our best defence against ocular surprises. If your eyes (or your child’s) suddenly have a change of pigmentation in the iris, definitely book in to see your optometrist and make sure everything is working as it should.

Wishing you all the best in wherever your path takes you today,

Sydney

Sydney Gallant, CCOA

Virtual Clinic Assistant

Shelburne Optometry