Coloboma comes from the Greek word “κολοβωμα” that can be translated as curtailed, mutilated or defect. The iris is the anterior-most part of the uvea (middle layer of the eye). A coloboma is a hole in one of the structures of the eye, such as the iris. It can be present from birth (congenital) or the result of eye surgery or eye trauma. A coloboma can affect one or both eyes.
Congenital iris colobomas are caused by the failed or incomplete closure of the embryonic fissure, which normally closes around the 6th week of pregnancy. Traumatic iris colobomas are an acquired condition that can occur e.g. as a result of an accident when the iris is ruptured or after glaucoma surgery. In this case, the coloboma often develops at the top of the iris.
The iris controls the amount of light entering the eye by adjusting the size of the pupil through two muscles. When the iris is defective, it can no longer perform this control function. People with iris coloboma are more sensitive to glare since more scattered light can enter the eye through the “enlarged pupil”. Blue blocking filters can help counteract the effects.
An irregularly shaped pupil often indicates a tear at the base of the iris. If this tear occurs below the normal pupil, patients encounter double vision since the “second” pupil caused by the tear is not covered by the upper lid. The effects on vision depend on the severity and size of the coloboma.
Another aspect is the appearance, especially if the iris coloboma is large. Since we usually look first into the face of another person, it is difficult to hide this defect. Patients may feel stigmatised and suffer from psychological stress.
Depending on the severity of the iris coloboma, the following treatment options may be useful:
To alleviate the symptoms such as glare or reduced contrast vision, fully closed spectacles with blue blocking filters are recommended.
• Prosthetic contact lenses
Prosthetic contact lenses are tinted, hand-painted or printed contact lenses that mask the iris and can also occlude the pupil. In addition to the cosmetic correction, they can also help reduce glare. The colour of the contact lenses matches that of the patient’s natural eye colour. The contact lenses mimic the natural iris pattern and feature a pupil opening to create a new aperture for the eye. However, the new pupil cannot adjust to differing light conditions. The ideal size in terms of optics and appearance must be determined when designing the new lens. Pupil-occluding lenses are another type of contact lenses that have an opaque (black) pupil area. They can be used to help stop double vision that may already be present. Both types of lenses are available in the patient’s refractive power. Printed iris-occluding contact lenses are the low-cost alternative to custom-made hand-painted lenses.
• Surgery options: artificial iris lens or iris implant
During iris surgery, an artificial intraocular lens with an iris painted on is implanted. Similar to cataract surgery, the natural lens must be removed before the artificial iris-painted lens can be implanted. The implantation of artificial iris tissue custom-made from silicone to match the healthy eye is also an option. This operation is one of the most challenging in eye surgery. The results are quite impressive since the difference between the two eyes is barely detectable after successful operation. Another possibility is the implantation of a foldable artificial iris that requires only a small incision. During surgery, the entire iris can be implanted or just part of it to replace the missing tissue. The iris implant is rolled up into a special insertion device and “injected” through a tiny incision.