Aniridia

 

How is it Diagnosed?
Aniridia can be diagnosed by an experienced ophthalmologist with a simple eye exam. There are genetic tests available as well, but they are not yet perfected. For more information on genetic testing for aniridia, contact a qualified genetic counselor. Genetic counselors are generally affiliated with universities and/or children’s hospitals.

What Causes Aniridia?

Vision Problems Associated with Aniridia
Although people with aniridia always have vision problems, the degree varies greatly. Some are legally blind, while others have vision that is good enough to drive a car. Most aniridics are able to read without using Braille.

1. The Iris. The iris is the colored part of the eye. It helps to control the amount of light entering the eye. Those with aniridia have no iris in either eye (sometimes there is a partial iris). People with aniridia are often sensitive to bright lights and glare, which is usually helped by wearing sunglasses.


2. The Cornea. The cornea is like the windshield of the eye. A healthy cornea is clear, allowing for light to pass through. By their mid-twenties, people with aniridia typically develop corneal pannus. Corneal pannus is scarring of the corneal tissue. In a normal eye, the cornea is able to refresh itself. For some unknown reason, the mutated PAX 6 gene in aniridics disrupts the signal for the cornea to refresh itself. Instead, the cornea begins to scar, changing the clear windshield into a window pane with the consistency of wax paper.


3. The Lens. The function of the lens is to focus light onto the macula. Some people with aniridia are born with small cataracts, which are little “dots” on the lens. Typically, these small dots do not impact one’s vision. However, as the aniridic eye becomes more affected by the other associated eye problems, such as glaucoma, the cataracts can grow and begin to impact the aniridic’s vision.


4. The Retina. The biggest challenge for children born with aniridia is their lack of detail vision. The fovea is the center of the macula – the area of the retina responsible for our central, sharpest vision. In a normal eye, the macula is the area of a person’s best vision that focuses on an image such as a person’s face. The fovea zeros in on detail such as seeing a freckle on someone’s face. In people with aniridia, the fovea is underdeveloped. This is known as foveal hypoplasia.


5. The Optic Nerve. The optic nerve is the connection from the eye to the brain. In some aniridics, optic nerve hypoplasia, where the nerve is smaller and underdeveloped, is present. Fortunately, this is not common in aniridia. The optic nerve is more commonly affected by the development of glaucoma (see Glaucoma section below). The danger in glaucoma is the increase in eye pressure. This increased pressure causes the nerve fibers of the optic nerve to die. Once the fibers die, the optic nerve stops functioning and the eye can no longer send signals to the brain.

Other Eye Problems Associated with Aniridia


Strabismus. This is a muscle imbalance of the eye which leads to crossing of the eyes or a “lazy eye.” In esotropia, the eye turns inward. In exotropia, the eye turns outward. This is correctable with glasses, eye-patches and/or surgery.

Glaucoma. Glaucoma can be present at birth (congenital glaucoma), but it typically arises after the teenage years in aniridics. Normally, a clear fluid, called the aqueous humor, provides nutrients to and carries waste products away from the lens and cornea. This fluid flows from behind the iris through the pupil and into the front (anterior) chamber of the eye, where it drains from the eye through a network of tissue called the trabecular meshwork. In glaucoma, this flow is disrupted, causing a build-up of fluid and, consequently, a build-up of pressure within the eye. There are 2 types of glaucoma, open-angle and closed-angle. Either type can develop in the aniridic. Open angle glaucoma occurs from an absent or dysfunctional aqueous drainage system. Closed-angle glaucoma occurs from a blockage of the iris tissue over the trabecular meshwork. The danger of glaucoma is due to the increase in intraocular pressure (IOP). This increased pressure causes the nerve fibers of the optic nerve to die, which in turn slows down and eventually stops the signals from the eye to the brain. Therefore, it is vital that the pressure within the eye of aniridics be closely monitored. There are many options for monitoring eye pressure and it is important that this be done at every ophthalmology appointment – including the very first one. For a small child with aniridia, sedation may be necessary and the exam may be performed in the operating room. Such a procedure is called an Examine Under Anesthesia or “EUA.” Typically, a reading of 20 or less is considered “normal.” A reading of greater than 20 could mean that glaucoma is present and a glaucoma specialist should be consulted. After the child reaches the age of three or four (depending on the child’s disposition), the pressure readings can be performed in the ophthalmologist’s office. Typically, a Tonopen is used to measure the pressure, although there are other instruments that can be used.

Other Medical Problems
There are several other medical problems that are associated with aniridia. The deletion of the 11p13 section of the PAX 6 gene causes WAGR Syndrome. To be diagnosed with this syndrome, two of four possible conditions must exist. These include Wilms' tumors (cancerous tumors of the kidneys), Aniridia, Genital abnormalities and mental Retardation. Genetic testing is recommended for all children born with aniridia to see if the PAX 6 gene has a deletion present. A deletion is when part of the gene is missing. If no deletion is found, the PAX 6 gene only has a mutation, which is a small change in the sequence. With just a mutation, the possibility of WAGR syndrome is remote. However, pediatricians still recommend that all aniridic children with a mutated PAX 6 be tested for Wilm’s tumors every 3-6 months with a non-invasive ultrasound of the kidneys (which takes less than a half hour). Other medical problems associated with aniridia are diabetes or glucose intolerance. Also, some people with aniridia have reported a decreased sense of smell.

Treatments
There is currently no cure for aniridia. However, there are some treatment options available:

1. Glaucoma:
   Frequent eye examinations are very important for those with aniridia to monitor for glaucoma. Increased eye pressures in glaucoma can be managed by eye drops and surgeries. There are many types of eye drops on the market to help lower eye pressure. There are also many surgical options including a trabeculectomy, trabeculotomy, laser surgery and shunt implantations. A trabeculectomy is a surgical procedure in which the trabecular meshwork is opened up, allowing the aqueous humor to drain more easily. This operation can be performed several times in different areas of the meshwork. A trabeculotomy is a more invasive procedure in which a new drainage angle is made to allow the aqueous humor to drain more effectively. Although this is an outpatient surgery, the eye must be monitored by the surgeon on a daily basis for at least two weeks. Another surgical option is laser surgery, where a laser is used to destroy the ciliary body, which produces the aqueous humor. This slows down the inflow of the fluid thereby reducing eye pressure. When these measures fail, a glaucoma surgeon can surgically insert a shunt (there are several different kinds) which is implanted into the front (anterior) chamber. Because aniridics have a risk for aniridia fibrosis (see below), any implantation procedure should be carefully monitored.


2. Lack of Iris
   The lack of iris can be corrected with an iris implantation surgery, although it is not yet approved by the FDA. This can help control the amount of light entering the eye. Some aniridics have been successful with the iris implant operation. Others have had problems associated with the implant, specifically “aniridia fibrosis.” Aniridia fibrosis means a fibrotic tissue forms in the anterior chamber. The hypothesis is that this fibrotic tissue is formed because implanted devices are near or touching immature vessels. Such a condition is NOT always present with implanted devices in aniridic patients; however, it certainly should be monitored. Should the fibrosis become present, early surgical intervention is recommended.


3. Corneal Pannus
   For corneal pannus, stem cell transplants can be performed to refresh the cornea and prevent further scarring. If the scarring has been very pervasive (a stem cell transplant was not performed during the early onset of the growing pannus), a corneal transplant can be performed to clear the cornea.


4. Cataracts
   For cataracts, the lens can be removed and either a contact lens can be used or a lens can be surgically inserted into the eye. Most ophthalmologists do not recommend contact lens usage on aniridic patients because of cornea sensitivity. However, implanted devices within the aniridic eye can lead to aniridia fibrosis as described above