Iridectomy
Definition
An iridectomy is a procedure in eye surgery in
which the surgeon removes a small, full-thickness piece of the iris, which is
the colored circular membrane behind the cornea of the eye. An iridectomy is
also known as a corectomy. In recent years, lasers have also been used to
perform iridectomies.
Purpose
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 angleclosure
glaucoma, acute congestive glaucoma, narrowangle 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 closedangle 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.
Demographics
Closed-angle glaucoma affects
between 350,000 and 400,000 people in the United States; in some Asian countries
such as China, however, it is more common than open-angle glaucoma.
Risk
factors for closed-angle glaucoma include:
• a family history of this type
of glaucoma
• farsightedness
• small eyes
• age over 40
•
scarring inside the eye from diabetes or uveitis
• a cataract in the lens
that is growing
• Eskimo or Asian heritage (Eskimos have the highest rate of
closed-angle glaucoma of any ethnic group)
Melanoma of the iris is a
relatively rare form of cancer, representing only about 10% of cases of
intraocular melanoma. The American Cancer Society estimates that about 220 cases
of melanoma of the iris are diagnosed in the United States each year. People
over 50 are the most likely to develop this form of cancer, although it can
occur at any age. It appears to affect men and women equally. Melanoma of the
iris is more common in Caucasians and in people with light-colored irides than
in people of Asian or African descent. Suspected causes include genetic
mutations and exposure to sunlight.
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