What is Aniridia?
Aniridia is a genetic condition and eye disorder that affects people at birth and refers to the absence of the iris. The term “aniridia” literally means “without iris” (the colored part of the eye), which is generally the first indication that an individual has aniridia. It may look like the child has an enlarged pupil. In cases of aniridia, the disorder is defined by an underdevelopment of the iris tissue. The iris controls the amount of light entering the eye and individuals with aniridia generally have increased light sensitivity (also known as photophobia) and issues with glare. While the name ‘aniridia’ focuses on the iris, the condition is pan-ocular, meaning it usually affects other components of the anatomy of the eye including the cornea, the fovea, or retina, the optic nerve, the anterior chamber, as well as the lens. As a result, ocular conditions can include glaucoma, foveal hypoplasia, involuntary eye movements (nystagmus), strabismus, dry eye, corneal degeneration, and clouding of the lens (cataracts). Most people with aniridia have at least one of these associated ocular conditions that impact their vision, and in some cases cause poor vision or vision loss.
Aniridia affects approximately 1 in 60,000 births. However, the eye problems associated with aniridia mentioned above are quite common. What is rare is to have the potential for all of these conditions present in one individual.
Although people with aniridia always have vision problems, the degree varies greatly and is dependent upon which complications an individual has or may acquire over time. Generally, individuals with aniridia have a visual acuity measurement between 20/80 and 20/200. Some are legally blind, while others have vision good enough to drive a car. Most individuals with aniridia read without using Braille, especially in today’s technically advanced environment of e-Readers and smart devices.
It is important to note that some of the conditions related to aniridia are non-degenerative (meaning they do not get worse over time) and others are degenerative. Conditions that can degenerate the vision of an individual with aniridia include corneal keratopathy, aniridic glaucoma, and cataracts. For more information on these conditions and treatments, please refer to the ‘Aniridia’s Impact on Vision’ section below.
Recent research indicates that other medical problems may be associated with aniridia (all types). These include: glucose intolerance (thought to be a precursor to diabetes in some individuals), central auditory processing disorder (difficulty with discriminating and interpreting sounds), decreased or absent sense of smell, and subtle abnormalities in the structure of the brain, such as decreased size or absence of the anterior commissure and/or the pineal gland. When aniridia occurs as part of WAGR syndrome, there may be medical problems in addition to those listed above. About 50% of children with WAGR syndrome will develop Wilms Tumor. More information on these can be found here: http://www.wagr.org/
Aniridia is generally diagnosed by a pediatric ophthalmologist. Many times, during a routine medical exam, a pediatrician will notice that a baby with aniridia does not have a pupil reaction to light (i.e. the iris is not contracting or expanding). Or a parent may notice or question a large pupil area. A qualified ophthalmologist usually provides a diagnosis of aniridia during eye exams. In addition, a blood test performed by a geneticist can confirm the genetic mutation. For more information on genetic testing, contact a qualified genetic counselor. Genetic counselors are generally affiliated with universities and/or children’s hospitals. Read more here.
Genetics of Aniridia
Simply put, genes hold the information to build and maintain an individual’s cells. All organisms have many genes corresponding to various biological traits, some of which are immediately visible, such as eye color or hair color. Minute changes in these genes (mutations) can have big consequences. In the vast majority of cases, aniridia is caused by a mutation in the PAX6 gene located on the short arm of chromosome 11 (11p13). If an individual has a normal PAX6 gene, another gene mutation may be mutated. Possible genes include: PITX, FOXC1, cypB, GDF6, or other. In relation to aniridia, most information and data is known about the PAX6 gene.
Inheritance and the Genetic Types of Aniridia
Every individual has two copies of every gene: one from their father and one from their mother. Unlike most genetic conditions, aniridia is dominant. It only takes one copy of the mutated gene to have the condition. Consequently, if a person with aniridia has a child, there is a 50% chance the child will have aniridia. There are four basic types of aniridia:
1. Familial aniridia (autosomal-dominant) is the most common form of aniridia. Familial aniridia is inherited directly from a parent who has aniridia. In familial aniridia, there is a mutation of the PAX6 gene.
2. Sporadic aniridia is the second most common form of aniridia. In this case, both parents have normal chromosomes and genes. However, in the affected child, a new change (mutation) of the PAX6 gene, whose cause is unknown, occurred before or soon after conception.
3. WAGR syndrome is a very rare form of sporadic aniridia (both parents have normal genes) where there is not only a mutation of the PAX6 gene but also involves deletion of the neighboring genes on chromosome 11. In addition to the common traits of aniridia, children with WAGR have a high risk for developing Wilms tumor and other medical complications such as genital abnormalities and learning and behavioral difficulties. Early diagnosis of WAGR is essential for monitoring and treating Wilms tumor and other complications. WAGR syndrome stands for Wilms Tumor, Aniridia, Genitourinary Abnormalities, and Range of Developmental Delays.
4. Gillespie syndrome is an extremely rare (there are only a few documented cases) type of aniridia and is not thought to be associated with a PAX6 mutation. This type of aniridia is characterized with a particular appearance of the iris remnant (described as having a “scalloped” border) mental retardation, and cerebellar ataxia, (muscle incoordination).