A second hard look at Glaucoma

You still see well with the glasses you got several years ago, and you may be tempted to go yet another year without an eye exam. But if you’re 60 and don’t have regular checkups, you may be risking your sight.

One of the most serious eye disorders is glaucoma, a stealth disease that robs vision but has no noticeable symptoms in its early stages. In fact, it is estimated that half the number of people who have glaucoma aren’t aware that they have it. That may be because there are no signs and symptoms of the most common form of glaucoma until you notice loss of your peripheral vision. By then, vision loss is usually significant and may be irreversible.

Early detection through regular eye exams, followed by prompt treatment, is the only safeguard against this potentially blinding eye disease. The take-home message for this week therefore is this: Get your eyes checked regularly!

What is Glaucoma?

Glaucoma isn’t a single disease. The common feature of these diseases is pressure-induced damage to the optic nerve, the bundle of nerve fibers that connects the retina to the brain. Damage to the optic nerve interferes with the transmission of electrical signals from the eyes to the brain and can result in profound loss of vision.

A certain amount of internal eye pressure is needed for the eyeball to hold its shape and function properly. The space behind your lens (vitreous cavity) is filled with a clear gel called vitreous fluid. The space in front of the lens (anterior chamber) is filled with a more watery fluid called aqueous humor.

Aqueous humor is continuously produced and circulated through the anterior chamber before draining out of the eye. This fluid nourishes the front part of the eye and removes debris. Aqueous humor drains from the eye through a spongy tissue (trabecular meshwork), and then through a duct called Schlemm’s canal, into your bloodstream (see diagram on Page D-1).

A healthy eye produces aqueous humor at the same rate that it drains fluid, thus maintaining normal pressure. However, with glaucoma, aqueous humor doesn’t drain properly and pressure within your eye builds up. Glaucoma is usually present in both eyes, but one may be affected more than the other.

Who is at risk?

Ophthalmologists have identified several risk factors for glaucoma:

• Age. Everyone over age 60 is considered to be at risk.

• Heredity.  Glaucoma tends to run in families.

• Race. Africans over age 40 have an increased risk of glaucoma, and some Asians have an increased risk of angle closure glaucoma.

Diabetes, a history of eye injury, hypertension, long-term steroid use, and high near-sightedness may also increase the risk of glaucoma, as can anatomic abnormalities in the eye.

Tests For Glaucoma

Routine eye exams are the key to detecting glaucoma before significant vision loss occurs. The most important screening is examination of the optic nerve for signs of damage. This is done with an ophthalmoscope, an instrument that allows your doctor to look directly through your pupil to the back of your eye. Screening for glaucoma also involves indirectly measuring the pressure inside your eyeball (tonometry). It’s recommended that you have a glaucoma screening:

• Every two to four years between 40 and 65.

• Every one to two years after age 65.

•Your doctor may recommend more frequent screenings — or that screenings begin at an earlier age — if you have risk factors for glaucoma. If screening indicates glaucoma, your doctor will likely test for other signs of glaucoma or potential glaucoma development. These tests may include the following:

• Intraocular pressure measurement. This test requires you to place your chin on the chin rest of a machine called a slit lamp. The doctor will use a light to align the machine to your eye and then will shoot a small burst of air into the open eye. The machine will calculate the amount of intraocular pressure in your eye based on the amount of resistance to the puff air.

• Dilated eye exam.  Drops are placed into your eyes that will cause your pupils to become larger. You generally will need to wait 20-30 minutes for the drops to work. Once your pupils are dilated, your doctor will examine the internal structures of your eye for abnormalities. The effects of the drops tend to last several hours, so you’ll need sunglasses afterwards (dilation causes increased sensitivity to light). Your doctor may recommend that you don’t drive until after the drops have worn off.

• Visual field testing.  Because glaucoma can cause blind spots, your doctors may want to perform a test to examine your peripheral vision, which will require you to respond to visual stimuli that are presented to different parts of your visual field.

Types Of Glaucoma

Primary open-angle glaucoma is the most common form, accounting for about 90 percent of glaucoma. It occurs if your trabecular meshwork becomes clogged. There’s no cure for primary open angle glaucoma, but vision loss can usually be prevented or slowed by lowering the pressure in your eye.

Angle-closure glaucoma occurs when the drainage channel is completely blocked by the iris, and the fluid can’t even flow into the trabecular meshwork. If the blockage occurs suddenly, a person can experience severe pain, blurred vision, haloes around lights, and sudden vision loss. This is a medical emergency and requires immediate treatment. More often, however, angle closure glaucoma develops gradually.

Another form of glaucoma is called normal-tension glaucoma. In this form, the optic nerve sustains damage even though the person’s eye pressure is normal (21 mm Hg or less). It’s unclear why this happens, but it may be that “normal” eye pressure is too high for some individuals and needs to be lowered further. Other possible contributing factors include a period of high pressure that has subsided, large daily fluctuations in pressure or a circumstance in which blood pressure dropped very low, as in a heart attack.

Treatment Options

Medication, most often eye drops, usually can bring eye pressure under control. These drugs include beta blockers like levobunolol, alpha-receptor agonists like brimonidine, and prostaglandin analogs like latanoprost. Eye drops typically need to be used several times during a day, and some may cause stinging or burning in the eye or trigger an allergic reaction that causes itching around the eyes. The drops must be used consistently in order to prevent further damage to the eye.

If eye drops alone don’t lower your eye pressure to a desired range, your doctor may also prescribe an oral medication for additional, usually short-term control. The most common oral medications used are carbonic anhydrase inhibitors. These include acetazolamide and methazolamide.

Surgery is usually performed only after eye drops or oral medications have failed. One method uses a laser to help open up your trabecular meshwork so that aqueous humor drains more easily from your eye. This procedure can be done in the doctor’s office.

Other non-laser surgeries may involve making a drainage passage, or implanting a small drainage tube to facilitate aqueous humor drainage. These are usually out-patient procedures requiring medications to relax your body and numb your eyes. After the procedure, antibiotics and anti-inflammatory eye drops may be used for a time to help prevent infection and scarring.

Catch It Early

Damage caused by glaucoma can’t be reversed. But once detected, vision loss from glaucoma can often be slowed or stopped. That’s why it’s important to catch it before significant vision loss occurs.