Aniridia: a rare eye disease

Aniridia: a rare eye disease

28/02/2019

Congenital aniridia (from the Greek an (without) iris) is a rare disease, which is mainly characterized by the absence of the iris, although it can be associated with alterations of other ocular structures and also of other organs of the body.

Aniridia is classified in two forms, type I aniridia, which is inherited from a father or mother affected by the disease, and type II aniridia, which appears sporadically, with no family history. In both cases, the genetic anomaly that produces the disease is known: deletion or mutation of the gene located on chromosome 11 called PAX6. The degree of mutation of this gene can cause different forms of the disease, from isolated partial aniridia to complete aniridia with cataract formation and corneal involvement. If the WT1 gene, which is located immediately adjacent to the chain of chromosomal DNA, is damaged, it can cause the so-called Wilms' tumor, a malignant tumor in the kidney that can cause death if it is not diagnosed and treated in time. Therefore, it is very important to make a genetic diagnosis to children born with this disease in order to assess the degree of genetic damage and, thus, to perform the required regular check-ups to diagnose possible associated pathologies in time.

Alterations in vision and photophobia

At the ocular level, the most outstanding feature is the absence of iris. The iris is a muscular tissue responsible for controlling the diameter of the pupil and thus the amount of light reaching the retina. The pupil contracts when there is a lot of light and dilates at night. In addition, on the back side of the iris there is a very dark layer of pigment that prevents light from passing through it. There are other diseases, as in the case of ocular albinism, in which this layer does not exist, so the color of the eyes is a very light blue or even pink, and important defects in vision occur. The patients with aniridia, not having the pupil diaphragm, cannot focus properly on the objects. The pupil works in the same way as the diaphragm of a camera; when closed it increases the depth of the focused field and when it opens decreases. In the absence of this mechanism, patients with aniridia fail to fully develop their vision in childhood. As a logical consequence of the absence of iris, patients also suffer from photophobia, which produces a defensive eyelid closure, head bending and the need to wear protective goggles at all times.

Implantation of an artificial iris

As we move forward in time, the appearance of other ocular complications such as cataracts, glaucoma and corneal involvement also called keratopathy is quite frequent. Cataracts can be removed with the usual phacoemulsification technique and at the same time to perform the the implantation of an artificial iris. There are artificial colored flexible irises that can be implanted through the small incisions of modern cataract surgery without the need for sutures.

Annual eye check-ups

The ophthalmologist must carry out an exhaustive annual follow-up to diagnose the presence of glaucoma as soon as possible, in case it occurs. Because patients with aniridia have an above-average corneal thickness, it is necessary to evaluate by means of other complementary explorations such as computerized campimetry, analysis of the retinal nerve fiber layer or optical tomography of the optic nerve, the possible presence of glaucomatous damage, since intraocular pressure, in isolation, is not a reliable parameter of diagnosis in these cases.

 Diagnosis and early treatment

In the cornea, due to premature failure of the corneal epithelial stem cells, a slow opacification occurs that severely affects vision. Although stem cell transplants from family donors, amniotic membrane implant and even artificial keratoprostheses have been tried, none of them has shown an efficacy that lasts over time. In recent years, the use of new generation artificial tears and blood derivatives such as plasma rich in growth factors (PRFC) have been introduced to improve the condition of the corneal epithelium, thus delaying corneal degeneration or its opacification.

Due to the intrinsic characteristics of this disease, it is important to emphasize that an accurate genetic diagnosis, a follow-up by specialized pediatricians and ophthalmologists and an early treatment of the possible associated pathologies, can increase the quality of life and the hope of maintaining a useful vision over time in these patients.

Dra. María Fideliz de la Paz

Dr. Juan Pedro Álvarez de Toledo

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