True aniridia, or complete absence of the iris, is rare. Most cases of aniridia are incomplete, with a narrow peripheral rim of rudimentary iris tissue present (Fig 12-5). Aniridia is usually bilateral, though sometimes asymmetric. Histologically, the rudimentary iris consists of underdeveloped ectodermal/mesodermal neural crest elements. The angle is often incompletely developed, and peripheral anterior synechiae that have an overgrowth of corneal endothelium are often present, most likely accounting for the high incidence of glaucoma associated with aniridia. Other ocular findings in aniridia include cataract, corneal pannus, and foveal hypoplasia.
Figure 12-5 Aniridia, with a rudimentary iris present (visible near the limbus in this photograph). Cataract is also present.
(Courtesy of Daniella Bach-Holm, MD.)
Figure 12-6 Choroidal coloboma. Note the RPE hyperplasia at the margin of the colobomatous defect and the fibroglial tissue within the coloboma (arrow).
(Courtesy of Adela Castaño, COA, and Robert H. Rosa Jr, MD.)
Both autosomal dominant and sporadic inheritance patterns have been described. An association between sporadic aniridia and Wilms tumor has been linked to 11p13 deletions and to mutations in the PAX6 gene, located in the same region of chromosome 11. Microcephaly, cognitive impairment, and genitourinary abnormalities have also been associated with aniridia.
Excerpted from BCSC 2020-2021 series: Section 4 - Ophthalmic Pathology and Intraocular Tumors. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.