Aniridia and the Iris
The term aniridia is Greek for "without iris." It is a congenital, bilateral (both eyes) condition characterized by the complete or partial absence of the iris.
The iris is the colored part of the eye surrounding the pupil. The iris contains muscles that allow the pupil to become larger (open up or dilate) and smaller (close up or constrict). The iris regulates the amount of light that enters your eye by adjusting the size of the pupil opening.
The iris also determines your eye color. People with brown eyes have heavily pigmented irises, while people with blue or lighter-colored eyes have irises with less pigment.
What Is Aniridia?
Aniridia is a genetic condition, caused by a mutation in the PAX6 gene on chromosome 11:
- In approximately 2/3 of people with aniridia, the condition is inherited in an autosomal dominant fashion, meaning that an individual has to inherit only one abnormal gene, from either parent, in order to manifest the disease.
- In approximately 1/3 of people with aniridia, the genetic mutation is sporadic, meaning that it is not inherited from either parent, but developed spontaneously in the person.
- Rarely, aniridia can be inherited in an autosomal recessive fashion, meaning that an individual has to inherit two abnormal copies of the gene, one from each parent, in order to exhibit the disorder. These individuals may also have other symptoms, including ataxia (problems with balance and movement) and mental handicaps.
Aniridia and Wilms' Tumor
Approximately 30% of persons with sporadic aniridia can have WAGR syndrome. Wilms' tumor is a rare kidney cancer that primarily affects children. WAGR is an acronym for a constellation of conditions that commonly occur together in this syndrome:
- W: Wilms' tumor
- A: Aniridia
- G: Genitourinary abnormalities, i.e. abnormalities in the reproductive and/or urinary organs
- R: mental Retardation
Therefore, it is important for your eye doctor to evaluate your family history if you have aniridia. If you have sporadic aniridia, regular ultrasounds are necessary to screen for Wilms' tumor.
How Does Aniridia Affect the Eyes?
While aniridia is named for its classic effect upon the iris, the disorder actually involves widespread abnormal development of many structures inside the eye, all of which can impair vision.
The effects of aniridia can vary significantly from person to person. Some individuals exhibit only mild effects from the disorder, while others have profound ocular abnormalities:
Some persons have only minimal underdevelopment of the iris that is barely noticeable to the untrained eye, while others have partial absence of the iris, and still others have total absence of the iris. Even in persons with "total" absence of the iris, there is usually a very rudimentary iris tissue remnant that can be seen during a careful eye examination under the microscope.
Since the iris helps block and absorb some of the light entering the eye, people with aniridia may be very light sensitive (photophobic) and report symptoms of glare.
Either abnormal development of the angle (the area of the eye responsible for draining aqueous fluid from the eye), blockage of the angle from the rudimentary iris stump, or narrowing of the angle can lead to glaucoma.
People with aniridia may have a deficiency in the limbal stem cells. These are stem cells that arise in the limbus. The limbus is an area at the border of the conjunctiva, which is the thin translucent tissue over the white surface of the eye, and the cornea, the transparent dome-shaped tissue that forms the front part of your eye, through which you can see the colored iris. These stem cells are responsible for maintaining the health and integrity of the cornea.
As a result of a deficiency in the limbal stem cells, the conjunctiva can grow over the cornea, the cornea cannot heal easily from injuries or scratches, and the cornea ultimately can undergo scarring or vascularization (growth of abnormal blood vessels into the normally clear cornea). These corneal changes can impair vision.
People with aniridia are also at higher risk of abnormalities in the lens, including cataracts and dislocation of the lens.
Individuals with aniridia often have foveal hypoplasia, meaning underdevelopment of the fovea, which is the part of the retina responsible for fine visual acuity; they may also have hypoplasia of the optic nerve, which transmits visual information from the eye to the brain. "Hypoplasia" is a term that refers to a deficiency or underdevelopment of a tissue or body structure.
Infants with anirida may exhibit abnormal, involuntary, rapid side-to-side or up-and-down movements of the eyes called nystagmus.
How Is Aniridia Treated?
Since aniridia can affect the eyes in a multitude of ways, aniridia treatment is similarly multifaceted:
- Opaque or colored contact lens can give the appearance of an iris to improve one's appearance, or for cosmetic purposes, as well as to improve vision and reduce glare/photophobia.
- Sunglasses can also help with symptoms of photophobia and glare.
- Some individuals also may be candidates for surgical placement of an artificial iris, though this procedure may be associated with complications.
- Lubrication with artificial tears can help maintain the health of the cornea.
- Significant corneal disease may require more aggressive surgery, including transplants and/or stem cell transplantation to replace some of the missing stem cells.
- Individuals who have cataracts may require surgery to remove the cataract.
- People with aniridia should be monitored closely for glaucoma and those who develop it should be treated accordingly, with medications, lasers, and/or surgery.
- A comprehensive low vision examination, along with optical and non-optical low vision devices and vision rehabilitation services and training, can be helpful for some individuals with aniridia.