Heterochromia is a variation in the hair, eyes, or skin color determined by melanin’s production, delivery, and concentration. The term is most often used to describe the iris color. Heterochromia iridis is one of the three types of heterochromia. Individuals with this condition have differently colored eyes or an eye with more than one hue. It is seen in humans and some breeds of domesticated animals and can be inherited.

Types and symptoms
The only symptom of this heterochromia is the iris color variation. It takes on three main forms:

• Complete heterochromia, or heterochromia iridis: This form shows a complete distinction in the iris color of the two eyes. It is the least common form and manifests as two completely ‘mismatched’ eye colors.
• Segmental heterochromia or heterochromia iridium: In this condition, the color variation can be seen within one iris itself, with different parts colored differently. It takes on a ‘wedge’ or ‘slice’ pattern. It is the most common form of heterochromia. It can affect a single eye or both.
• Central heterochromia: In this condition, the outer ring area of your iris is different in color from the rest. It usually occurs in both eyes. Eyes with this condition start with one color at the center near the pupil, which then turns into another shade as the edge approaches.

As there are various forms of heterochromia, it is possible that, at times, it can also be confused with anisocoria. This condition makes the two eyes appear to be different colors, though the only difference is in pupil size. This makes it look like one eye is darker than the other.

Causes
The main cause behind the difference in color is disproportionate melanin deposition. Melanin is a pigment that delivers color to the eyes, be it brown, green, blue, or hazel. In general, lighter eye color results from less melanin, while darker shades stem from a higher pigment concentration. A lack of genetic diversity is primarily believed to be the reason behind the genetic condition arising from a gene mutation. There are two ways in which individuals can have heterochromia iridis: either acquired (after you’re born) or congenitally( since birth).

The causes behind congenital heterochromia involve the following:

• Benign heterochromia
• Piebaldism
• Hirschsprung disease
• Bloch-Sulzberger syndrome
• Von Recklinghausen disease
• Bourneville disease
• Waardenburg syndrome
• Sturge-Weber syndrome
• Parry-Romberg syndrome
• Horner’s syndrome
• Bleeding in the eye
• Eye surgery
• Fuchs’ heterochromic cyclitis
• Latisse (a repurposed glaucoma medication used cosmetically to thicken eyelashes)
• Pigment dispersion syndrome
• Ocular melanosis
• Posner-Schlossman syndrome
• Iris ectropion syndrome
• Benign and malignant tumors of the iris
• Diabetes mellitus
• Central retinal vein occlusion
• Chediak-Higashi syndrome

The causes behind acquired heterochromia include the following:

• Glaucoma
• Eye injury
• Neuroblastoma
• Eye cancer
• Swelling due to uveitis or iritis
• Reactions to certain treatment methods

Treatment
Generally, congenital heterochromia does not require any treatment. However, if the condition develops in individuals after infancy, i.e., leading to acquired heterochromia, they should be referred to an ophthalmologist who would confirm it and then get to the root cause of the change. Generally, color alteration does not occur due to any concerning diseases. However, if any underlying condition is the reason, treatment for that illness will be initiated.