Heterochromia Iridis

Asymmetry in the pigmentation of the irides probably occurs as an isolated phenomenon inherited as a dominant (Calhoun, 1919). Whether hereditary heterochromia iridis ever exists independent of Horner syndrome (143000), Waardenburg syndrome (193500), or the piebald trait (172800) is not clear. The melanocytes of the uveal trait constitute a branching pseudosyncytium richly innervated by sympathetic nerves. Pigmentation of the iris does not occur in the absence of this innervation. Sympathetic fibers leave the lateral horn of the gray matter of the first and second thoracic segments, pass out in the anterior roots, and join the lateral sympathetic chain via the white rami communicantes. They then proceed to the superior cervical ganglion and along the distribution of the carotid artery to the head. Congenital (or at least connatal) Horner syndrome with associated heterochromia iridis can be produced by birth injury to the lower roots of the brachial plexus (Klumpke palsy). Byrne and Clough (1992) discussed the occurrence of hypochromia iridis following acquired Horner syndrome in a 40-year-old man who had suffered brachial plexus trauma in a motorcycle accident 23 years earlier. Heterochromia iridis (singular) is the designation that the purist reserves for different pigmentation in sectors of 1 iris, whereas heterochromia iridum (plural) is the term used when the 2 irides are of different color.

Morrison et al. (2000) examined 75 children with iris colobomata (120200). In 13 (17.3%) patients, noticeable iris heterochromia was present. In patients with unilateral coloboma, the heterochromia was characterized by the darker iris being the one affected with coloboma. In cases of bilateral iris colobomata with clinical microphthalmos and reduced corneal diameter, the variation in iris color was inconsistent. A fundus coloboma was not always present. The authors postulated that the iris coloboma-iris heterochromia association may result from the abnormal closure of the embryonic fissure, resulting in irregular or excessive migration of neural crest cells into the iris stroma. In addition, the high frequency of iris heterochromia-iris coloboma with microphthalmia suggests that an increased density of pigmented cells within the iris stroma may be a contributing factor.