It is estimated that approximately one-third of patients with sporadic aniridia will develop a Wilms tumor (nephroblastoma), while this risk increases to approximately one half of patients if aniridia, genitourinary anomalies (eg, hypospadias), and mental retardation are simultaneously present. The association with genitourinary anomalies and mental retardation is known as ☞WAGR Syndrome. Most patients have prominent lips and macrognathia. Other features may include congenital cataracts, nystagmus, ptosis, and blindness. The presence of ptosis and generalized hypotonia in a subgroup of patients reportedly suggests a susceptibility to malignant hyperthermia.
Estimated between 1:60,000 and 1:100,000 live births. All races are affected.
Both, hereditary and sporadic (approximately 30% of cases) forms exist. The usual form of inheritance is autosomal dominant, but autosomal recessive transmission has been suggested for the rare ☞Gillespie Syndrome. Four types of aniridia have been described, however there are two genetic loci for aniridia, Aniridia Type I (ANI) and Aniridia Type II (ANII) and both have been mapped to mutations in the PAX6 (paired box gene family) gene on chromosome 11p13. In Aniridia I, ocular and central nervous system anomalies exist, while in Aniridia II the anomalies are typically limited to the eyes (see “Clinical aspects”). Aniridia as an isolated ocular malformation is an autosomal dominant disorder. Patients with a positive family history for aniridia (hereditary form) do not have an increased risk for Wilms tumor; however, patients without a positive family history for aniridia (sporadic cases) have an up to 67 times higher risk of developing Wilms tumor, but also require the deletion of Wilms Tumor suppressor gene 1 (WT1).
Aniridia is related to a primary arrest in the neuroectodermal development and a secondary disturbance of the three neural crest waves from the mesenchyme. The functional development of the anterior eye segment is based on a complex interaction between neural ectoderm and the neural crest waves mesenchyme. Cellular and/or biochemical aberrations may result in malformation or regression of different layers of the anterior segment of the eye. In aniridia, the iris stroma is hypoplastic and the extent of the defect can range from slit-like lesions in the iris stroma only visible under slit-lamp examination to coloboma-like defects and almost complete absence of the iris stroma. The root of the iris, however, is most often visible on gonioscopy. Aniridia has been induced experimentally in mice with maternal vitamin A deficiency.
At birth, the iris is reduced to a small tube. In general, vision is decreased due to numerous other contributing anomalies, including corneal and lenticular opacities, severe glaucoma, nystagmus, optic nerve agenesis, foveal hypoplasia, and light scatter.