Glaucoma is a sight-threatening disorder marked by an increase in intraocular pressure (IOP)—the pressure within the eyeball—that is too high for the optic nerve to tolerate.

Because the condition does not cause symptoms in its early stages, half of the three million Americans who have glaucoma don’t know it. Between five and 10 million others are at increased risk because of elevated IOP that hasn’t yet caused optic nerve damage.

Ophthalmologists suspect that the most common form of glaucoma occurs because of a partial blockage of a spongy network of tissue called the trabecular meshwork, which forms a drainage system for the aqueous humor—the clear fluid inside the eyeball that provides nutrients and carries away waste from the lens and cornea of the eye (which have no blood supply). The blockage traps aqueous humor (though how this occurs isn’t clear), and as more aqueous humor forms than is removed, IOP increases.

There are two forms of glaucoma: open angle and closed angle. Open-angle glaucoma, which accounts for 90 percent of all cases, is a slow, progressive disease. By contrast, closed-angle glaucoma is relatively uncommon and is characterized by rapid and extreme elevations of IOP, often causing acute symptoms such as severe eye pain and rapid blurring of vision.

Both types of glaucoma may lead to blindness by damaging the optic nerve; however, early detection and treatment can usually control IOP and prevent severe vision loss. Most prevalent among those over age 40, glaucoma is more common in African Americans or when there is a positive family history.

Symptoms of Glaucoma

• Open-angle glaucoma: gradual loss of peripheral vision, marked by blind spots. Symptoms only develop at the later stages of the disease. Screening exams are advised.

• Closed-angle glaucoma: acute attacks involving severe eye pain, nausea and vomiting, blurred vision, and the appearance of rainbow-colored halos around lights.

What Causes Glaucoma?

• The primary cause of open-angle glaucoma appears to be an excessive buildup of aqueous humor, increasing IOP. When IOP remains elevated or continues to rise, fibers in the optic nerve are compressed and destroyed, leading to a gradual loss of vision over a period of years. (In some people, however, even a normal IOP level can contribute to optic nerve damage.)

• A family history of visual-field loss due to glaucoma increases the risk of optic nerve damage. Evidence also suggests that cardiovascular disease, diabetes, and myopia (nearsightedness) increase the risk of nerve damage from glaucoma.

• The use of inhaled steroids (commonly used to treat asthma) or nasal sprays with steroids appears to increase the risk of open-angle glaucoma.

• Closed-angle glaucoma is caused by a sudden blockage near the iris that prevents aqueous humor from reaching the trabecular meshwork. The blockage results in the rapid onset of extremely high IOP that may cause severe, permanent vision loss within a day or two.

Prevention of Glaucoma

• In people with an elevated IOP but no apparent damage from glaucoma, early detection and treatment may delay or prevent the onset of the disease or damage from the disease. However, not everyone who develops glaucoma has an elevated IOP. Therefore, it’s important for people after a certain age to be properly screened by undergoing thorough eye examinations (see When To Call A Doctor).

Diagnosis of Glaucoma

Glaucoma is diagnosed by an eye examination that emphasizes eye pressure measurement, viewing of the optic nerve, and assessment of optic nerve function:

• Tonometry (use of an instrument to apply slight pressure upon the eyeball to measure IOP).

• Ophthalmoscopy (in which the pupils are dilated with eye drops so that the optic nerve may be magnified and examined with an ophthalmoscope, a lighted viewing instrument).

• Perimetry (a visual-field test that determines the extent of peripheral vision).

How To Treat Glaucoma

• Glaucoma is a chronic disorder that cannot be cured. Open-angle glaucoma can often be treated safely and effectively by medical or surgical measures, though lifelong therapy is almost always necessary. Medications are nearly always used first. However, initial surgery is becoming a more common option.

• Eye drops are the most frequent medical treatment for glaucoma because they have fewer overall side effects than oral medications. Five types of eye drops are currently used: topical prostaglandins (latanoprost), beta-blockers (like timolol or levobunolol), adrenergic agonists (such as epinephrine or dipivefrin), carbonic anhydrase inhibitors (dorzolamide), and miotics (such as pilocarpine). Beta-blockers diminish the production of aqueous humor. Latanoprost, the newest type of glaucoma eyedrop, reduces IOP by increasing the outflow of aqueous humor. Because it works differently than other glaucoma drugs, it can be used along with a second medication to reduce IOP further. Dorzolamide drops, previously available only as tablets, decrease production of aqueous humor. Miotics (drugs that constrict the pupil) and adrenergic agonists increase the outflow of aqueous humor.

• Carbonic anhydrase inhibitor pills are generally used when optic nerve damage continues or seems highly likely despite maximal topical treatment. These oral medications initially lower IOP by 20 to 30 percent on average, but significant systemic side effects and occasional serious complications (such as kidney stones, depression, diarrhea, and blood abnormalities) can limit their use.

• Periodic ophthalmologic examinations are essential throughout medical therapy.

• Surgery is successful more often than medical treatment. However, it carries a greater risk of complications, including the development of cataracts, and repeat procedures may be required. About 10 percent of those with open-angle glaucoma require surgery, when drug therapy fails or when the patient has a medical condition (such as hypertension or severe heart or lung disease) that precludes maximal drug therapy. The two most common surgical procedures—laser trabecular surgery and filtration surgery—reduce ocular pressure by opening a passage for aqueous humor.

• Unless high IOP is relieved promptly during an acute attack of closed-angle glaucoma, blindness can occur within a day or two. Making a hole in the iris (iridotomy) creates a drainage path for the aqueous humor. Iridotomy in the other eye is generally recommended owing to the high likelihood that it will be involved in a future acute attack. The procedure can usually be performed with a laser.

When To Call A Doctor

• Contact an ophthalmologist right away if you develop symptoms of acute closed-angle glaucoma.

• See an ophthalmologist for a complete eye exam every two years after age 50. (Those of African descent should start having such exams at age 40.) People with a family history of glaucoma should also have periodic exams.

For more information on glaucoma and related vision and eye care conditions, click on this link -- Johns Hopkins Health Alerts: Vision and Eye Care

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