Figure 12-4 The choroid is a vascular, pigmented structure present between the retinal pigment epithelium and the sclera. The layer closest to the pigment epithelium is composed of capillaries and is known as the choriocapillaris (arrowheads).

(Courtesy of Nasreen A. Syed, MD.)

Congenital Anomalies

Aniridia

True aniridia, or complete absence of the iris, is rare. Most cases of aniridia are incomplete, with a narrow rim of rudimentary iris tissue present. 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 with 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.

Both autosomal dominant and sporadic inheritance patterns for aniridia 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. Microcephaly, mental retardation, and genitourinary abnormalities have also been described in association with aniridia.

See also BCSC Section 2, Fundamentals and Principles of Ophthalmology, and Section 6, Pediatric Ophthalmology and Strabismus.

Hingorani M, Hanson I, van Heyningen V. Aniridia. Eur J Hum Genet. 2012;20(10): 1011–1017.

Coloboma

A coloboma—the absence of part or all of an ocular tissue—may affect the iris, ciliary body,