Background
In 1818, Barratta first described aniridia (Greek for absence of the iris). Aniridia is a rare, bilateral, panophthalmic disorder, of which iris hypoplasia is the most striking feature (see image below).
Aniridia. Note the almost complete absence of the iris.
Aniridia also has corneal, lens, optic nerve, and retinal manifestations. Foveal and optic nerve hypoplasia are often present, causing a reduction in visual acuity and congenital sensory nystagmus. Progressive worsening of vision may occur later in life as a result of cataracts, glaucoma, and corneal opacification. [1]
Pathophysiology
Aniridia may be familial or sporadic. It is caused by a mutation in the PAX6 gene on chromosome 11. [2]
Approximately 85% of cases are familial (designated AN1) with autosomal dominant inheritance with complete penetrance but variable expressivity. These familial cases usually have isolated ocular involvement.
The 15% of cases that are sporadic have a deletion or mutation on the short arm of chromosome 11. The same deletion is responsible for the development of Wilms tumor (nephroblastoma).
The exact pathogenesis of aniridia is unknown. After early reports of ocular colobomas in patients with aniridia, some authors proposed that it is a colobomatous disorder. Others attributed it to a failure of mesodermal development with involvement of the rim of the optic cup, causing iris hypoplasia. Still others advocated the neuroectodermal theory, which links the presence of retinal anomalies and iris muscular hypoplasia to a developmental failure of neuroectoderm.
Epidemiology
Frequency
United States
Aniridia is a very rare disorder. According to a population-based study in Michigan, it has been estimated to affect approximately 1 per 61,000 newborns.
International
A more recent study from Denmark reported the incidence as 1 per 96,000 live births.
Mortality/Morbidity
Aniridia itself is not a lethal disorder. However, miscarriages and a stillborn child have been the only results of a consanguineous mating between individuals with aniridia. A homozygous mutation of the aniridia gene may possibly be lethal. The morbidity of aniridia is significant because of the decreased vision and nystagmus.
Race
No racial predilection has been described.
Sex
The incidence of aniridia is equal in males and females.
Age
Aniridia is a congenital disorder apparent at birth. It is never acquired.
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Aniridia. Note the almost complete absence of the iris.
