We have a deep appreciation for eyes, as some of you may have only guessed. It’s not all just about glasses, contact lens, and making your eyes feel better and see clearer (although that is a major component). Having an appreciation for eyes also means having an appreciation and understanding of some of the eye’s most unique traits, even those seen in more than just humans.
One of the rarer eye conditions is one that can also be rather eye-catching: heterochromia. In heterochromia, the irises of the eyes are different colors than each other. Most cases are either segmented heterochromia or central heterochromia, only affecting portions of the iris. These cases tend to be much less noticeable than the rarer and more apparent complete heterochromia, which affects only six in every 1,000 people.
Like our skin, the color of our irises depends on different amounts of melanin. Brown eyes, for example, have more melanin pigmentation than green or blue eyes, which have none. Colors like hazel eyes have some melanin present, but not as much as brown eyes.
Eye color is an inherited trait, but can usually be more complex than a single gene would allow. A whole group of genes are responsible for giving you a specific pattern or placement of pigmentation in your iris. This is why when you look in a mirror, your eyes aren’t just one uniform shade. Most of the time, your eyes appear to have a complex (and often mesmerizing) pattern within the iris.
Most of the time, this unique pattern and pigmentation is the same for both eyes. However, in heterochromia one iris will receive more or less melanin than the other, resulting in a different color or pattern.
Most importantly, heterochromia is a mostly benign mutation. This means it’s usually not related to an underlying disorder or condition and does not inhibit a person’s eyesight. In rare cases, having heterochromia can point towards the presence of Waardenburg Syndrome, which can affect a person’s hearing and has been related to partial albinism.
Aside from that, heterochromatic people lead perfectly normal, if not eye-catching lives.
Also called Heterochromia iridium, complete heterochromia occurs when the irises are two completely different colors. This is the rarest form of heterochromia, and is seen in a variety of mammals, from humans to cats and dogs.
Otherwise known as Heterochromia iridis, segmented heterochromia is when small patches of your iris are different colors. Say, for example, you come across someone who has green eyes, but has a patch of hazel in one eye without being completely hazel or brown.
This form of heterochromia, much like segmented heterochromia, only affects certain areas of the iris instead of the entire iris. However, instead of patches, central heterochromia appears like a ring, causing either the inner iris, middle iris, or outer iris to be a different color.
Acquired Heterochromia (Anisocoria)
While it’s usually an inherited condition, there is a form of heterochromia that is caused by external trauma called Anisocoria. With this, the iris or eye has experienced some form of physical damage, leaving one eye with a different appearance than the other. David Bowie is notable for having anisocoria, with one pupil more dilated than the other. This was caused by a fist fight he got into as a teenager with a lifelong friend of his, George Underwood. Later in life, Bowie thanked Underwood, citing his anisocoria as adding “a kind of mystique”.
For those that enjoy the holiday classic “A Christmas Story”, one of the deterrents of buying a Red Ryder BB Gun is that one will shoot their eye out. Comedic effect aside, ricochets BBs and other projectiles have also been known to cause this form of heterochromia, and makes it worth reiterating the importance of wearing safety glasses.
Another uniqueness to anisocoria comes from Horner’s syndrome — a lesion in the oculosympathetic pathway. This lesion can be central, preganglionic or postganglionic, characterized by a triad of ptosis, miosis, and anhidrosis where it’s been affected. In layman’s terms, nervous system damage in the face can cause pupils to constrict, the upper eyelid to droop, and a lack of sweating from the face. When it manifests early in life congenitally, the pigment of the iris stroma will fail to develop and will display as heterochromia.
While it manifests in these different ways, heterochromia is a mostly benign condition. While usually recognize right after birth, we can look for and identify central and segemented heterochromia during a comprehensive eye exam. It’s always good to know what your eyes do and what makes them unique to you! It also makes for good conversation ice breakers!