Last updated: 11/24/2009

Today, an iridectomy is most often performed to treat closed-angle glaucoma or melanoma of the iris. An iridectomy performed to treat glaucoma is sometimes called a peripheral iridectomy, because it removes a portion of the periphery or root of the...

iris.

In some cases, an iridectomy is performed prior to cataract surgery in order to make it easier to remove the lens of the eye. This procedure is referred to as a preparatory iridectomy.

Closed-angle glaucoma

Closed-angle glaucoma is a condition in which fluid pressure builds up inside the eye because the fluid, or aqueous humor, that is produced in the anterior chamber at the front of the eye cannot leave the chamber through the usual opening. This opening lies at the angle where the iris meets the cornea, which is the clear front portion of the exterior cover of the eye. In closed-angle glaucoma, the fluid is blocked because a part of the iris has moved forward and closed off the angle. As a result, fluid pressure in the eye rises rapidly, which can damage the optic nerve and lead to blindness. About 10% of all cases of glaucoma reported in the United States is closed-angle. This type of glaucoma is also called angle-closure glaucoma, acute congestive glaucoma, narrow-angle glaucoma, and pupillary block glaucoma. It usually develops in only one eye at a time.

There are two major types of closed-angle glaucoma: primary and secondary. Primary closed-angle glaucoma most commonly results from pupillary block, in which the iris closes off the angle when the pupil of the eye becomes dilated. In some cases, the blockage happens only occasionally, as when the pupil dilates in dim light, in situations of high stress or anxiety, or in response to the drops instilled by a doctor during an eye examination. This condition is referred to as intermittent, subacute, or chronic open-angle glaucoma. In other cases, the blockage is abrupt and complete, leading to an attack of acute closed-angle glaucoma. In primary glaucoma, the difference between the chronic or intermittent forms and an acute attack is usually due to small variations in the anatomical structure of the eye. These include an unusually shallow anterior chamber; a lens that is thicker than average and situated further forward in the eye; or a cornea that is smaller in diameter than average. Any of these differences can narrow the angle between the iris and the cornea, which is about 45° in the normal eye. In addition, as people age, the lens tends to grow larger and thicker; this change may cause fluid pressure to build up behind the iris. Eventually, pressure from the aqueous humor may force the iris forward, blocking the drainage angle.

Secondary closed-angle glaucoma results from changes in the angle caused by disorders, medications, trauma, or surgery, rather than by the anatomy of the eye itself. In some cases, the iris is pulled up into the angle by scar tissue resulting from the abnormal formation of blood vessels in diabetes. Another common cause of secondary closed-angle glaucoma is uveitis, or inflammation of the uvea, which is the covering of the eye that includes the iris. Cases have been reported in which uveitis related to HIV infection has led to closed-angle glaucoma. Melanoma of the iris has also been associated with closed-angle glaucoma.

Any medication that causes the pupil of the eye to dilate may cause an acute attack of closed-angle glaucoma, including antihistamines and over-the-counter cold preparations. Medications that are given to treat anxiety and depression, particularly the tricyclic antidepressants and the selective serotonin reuptake inhibitors (SSRIs), may trigger the onset of closed-angle glaucoma in some patients. In other instances, anesthesia for procedures on other parts of the body produces an acute attack.

In terms of trauma, a direct blow to the eye can dislocate the lens, bringing it forward and blocking the angle; overly vigorous exercise may have the same effect. Lastly, certain types of eye surgery performed to treat other conditions may result in secondary closed-angle glaucoma. These procedures include implantation of an intraocular lens; cataract surgery; scleral buckling to treat retinal detachment; and injection of silicone oil to replace the vitreous body in front of the retina following a vitrectomy.

Melanoma of the iris

Melanoma of the iris is a malignant tumor that develops within the pigmented cells of the iris; it is not a cancer that has developed elsewhere in the body and then spread to the eye. Melanoma of the iris can, however, enlarge and gradually destroy the patient's vision. If left untreated, it can also metastasize or spread to other organs—most commonly the liver—and eventually cause death.