Heterochromia iridis is characterized by color differences of the iris (the colored part of the eye), either between the eyes or within one eye. In people with complete heterochromia, the iris of one eye is a different color than the iris of the other eye. Segmental heterochromia occurs when areas of the same iris are different in color. Most cases of heterochromia iridis occur sporadically and are not associated with any other symptoms or problems. Rarely, heterochromia iridis is part of a congenital (present from birth) syndrome such as Waardenburg syndrome, Sturge-Weber syndrome, Parry-Romberg syndrome, or Horner's syndrome. Treatment for people with heterochromia iridis may only be needed if there is an underlying syndrome causing health problems.

Most cases of heterochromia iridis occur sporadically and are not associated with any additional symptoms or health problems. Acquired factors that may lead to heterochromia iridis include:

• Injury or trauma to the eye
• Melanoma of the eye
• Treatment of glaucoma with certain medications such as Latanoprost or Bimatoprost
• Neuroblastoma

Rarely, heterochromia iridis may be part of a congenital (present from birth) syndrome, such as Waardenburg syndrome, Sturge-Weber syndrome, congenital Horner's syndrome, or Parry-Romberg syndrome. Waardenburg syndrome (due to mutations in many different genes) and Sturge-Weber syndrome (due to somatic mutations in the GNAQ gene) have genetic causes. The exact underlying cause of Horner's syndrome and Parry-Rombery syndrome is currently unknown. Later-onset cases of heterochromia iridis may be associated with a syndrome called Fuchs heterochromic iridocyclitis, which also has an unkown cause.