My sister and I have the same eyes. That was the family line for 30 years, both of us “hazel”, both of us from the same 2 parents, and it took a phone photo at her kitchen table last Christmas to settle that it was not true. I had zoomed in on my own eye to check a stye and there was a copper ring around the pupil, a millimetre or so wide, then a hard edge, then grey green out to the white. She took the same photo of hers. Flecks of gold and green and brown all the way across, no ring, no edge, colours running into each other like wet paint. Same parents. Different eyes. Hers are hazel. Mine, it turns out, are central heterochromia, and I had been misnaming them my whole life.
That edge is the whole definition. Heterochromia is an iris with 2 distinct colour zones, an inner ring around the pupil and a different colour outside it, with a clear line where one stops and the other starts. The inner ring is nearly always warm, gold, amber, copper or brown. The outer zone is nearly always cool, blue, grey or green. The line between them does not blur and it does not move when the light changes, and that last part is the test, which I will come back to.
Where The Ring Comes From
The iris has 2 zones anatomically, whether or not you can see them. The pupillary zone is the inner band right around the pupil, and the ciliary zone is everything outside it, and the boundary between them is a slightly raised, wavy ridge called the collarette. In most people both zones carry roughly the same amount of melanin, the pigment that colours eyes, hair and skin, so the iris reads as 1 colour and the collarette is just a texture you notice on a close photo.
The pupillary zone has more melanin than the ciliary zone. Melanin is brown, and where there is a lot of it the iris looks brown or gold, and where there is very little the iris looks blue or grey for the same reason the sky does, the light scatters off the fibres instead of being absorbed. So the pigment rich inner zone reads warm and the pigment poor outer zone reads cool, and the collarette becomes the border between 2 colours instead of a ridge inside 1. That is all it is. Nobody has extra pigment cells or a second iris layer. The melanin is spread unevenly across a structure that was always there.
I find that oddly comforting. My eyes are not doing anything unusual. They are just showing the seams.
Central Heterochromia Or Hazel Eyes
This is the question that brings most people to a page like this one, and it is the one my sister and I argued about over the turkey. The 2 get confused for a good reason. Both involve a warm colour near the pupil and a cooler colour further out, and both are most common in people of European ancestry with mid range pigment. The difference is in how the colours meet.
- In central heterochromia the 2 colours are solid and separate, with a visible edge between them, and the pattern is the same in bright sun, in a dim room, in a green jumper or a blue one.
- In hazel eyes the colours are mixed through the iris in flecks, streaks and marbling, they fade into each other with no line, and the overall shade appears to shift with the lighting and with what you are wearing, because the eye is genuinely a blend and different parts of the blend catch the light differently.
If you want the long version, with photographs side by side and the full set of tells, The Touring Eye has the most careful comparison I have read, and it is the page that ended the argument, because it has the 2 patterns next to each other and there is no reading them any other way. She was quite put out.
How To Check Your Own Eyes In A Mirror
- Stand in daylight, near a window, and get close enough to the mirror that you can see the texture of the iris.
- Look at the edge around the pupil and try to point to the line where the inner colour stops.
- If you can point to the line, it is central heterochromia. If you cannot find a line because the colours melt into each other, it is hazel.
- If you are still not sure, take a photo with the flash on, because the flash flattens the reflections that make the iris hard to read by eye.
Why Some People Are Born With It
Nearly everyone with central heterochromia was born with it, and in that case it is a trait and not a condition. It runs in families in a loose way, the same way eye colour in general does, which is how 2 sisters from the same parents end up with 1 ring and 1 blend. The genes involved are the ones that decide how much melanin the iris makes. The 2 that come up most in the research are OCA2, which makes a protein involved in producing melanin HERC2, which sits next to it and controls how much OCA2 is switched on. A small difference in either can leave the outer iris short of pigment while the inner ring keeps a normal amount, and the ring appears.
It does not affect vision. It is not linked to any disease when it has been there since childhood. It does not need treatment and there is no treatment to have.
How rare is it? Nobody knows, and I would treat any percentage you read as invented. 2 eyes of fully different colours, is rare enough that it gets photographed and written about, and the usual estimate for it in the US is well under 1 in 100 people. Central heterochromia is far more common than that, partly because so many people who have it call it hazel and never get counted. Mine had been in front of opticians for 30 years and it took a stye and a phone camera to find it.
When A Colour Change Later In Life Means Something
This is the part I would not skip. Central heterochromia you were born with is harmless. An iris that changes colour after childhood, or develops a ring or patch that was not there before, needs an eye doctor, and soon. Acquired colour change in one eye has a short list of causes and most of them are treatable if caught.
- Inflammation inside the eye, particularly a long running low grade form called Fuchs heterochromic iridocyclitis, which lightens the affected iris and is often found only because someone noticed the 2 eyes no longer matched.
- Horner’s syndrome, damage to the nerve supply on one side of the face, which can lighten the iris on that side along with a drooping eyelid and a smaller pupil.
- Glaucoma eye drops of the prostaglandin family, latanoprost and its relatives, which darken the iris in a proportion of people who use them and are the commonest cause of acquired heterochromia in older adults.
- Injury to the eye, bleeding inside it, a retained metal fragment, or, rarely, a growth on the iris.
None of those look like the neat copper ring of the congenital kind, but people cannot always tell, and the rule is simple. Born with it, fine. Appeared, changed, or came with pain, redness, a drooping lid or any change in vision, get it looked at that week.
The 3 Types, And Which One You Have
Central Heterochromia
The subject of this piece: 2 colours in 1 iris arranged as an inner ring and an outer zone, usually in both eyes and usually symmetrical. It is the common one and the quiet one, and it hides inside “hazel” for years.
Sectoral (Partial) Heterochromia
A wedge or patch of a different colour within 1 iris, like a slice of brown in a blue eye, and it can be tiny or take up half the iris. It is congenital in most cases, occasionally linked to a few inherited syndromes when it appears alongside other signs, and it is the one that tends to be in only 1 eye.
Complete Heterochromia
2 eyes of 2 different colours, 1 blue and 1 brown being the classic pairing. It is the rarest, the most photographed, and the one that makes casting directors phone your agent, and it is also the form most likely to be acquired rather than inborn, which is why an adult who suddenly has it should be seen.
Once you have found the line around your pupil you will look for it in everyone you meet. I do it on trains now. It is about 1 in 8 people on the Northern line, by my unscientific count, and most of them, like me until last Christmas, have no idea.
