What Is Glaucoma?
Glaucoma is a sight-threatening disorder marked by an increase in intraocular pressure (IOP)—the pressure within the eye—that can ultimately damage the optic nerve. Because the condition does not cause noticeable symptoms until the later stages, most 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 dysfunction of the 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. Because the drainage system does not function as it should, more aqueous humor forms than is usually removed and IOP increases.
There are two main categories of glaucoma: open angle and closed angle. Open-angle glaucoma, which accounts for 90% of all cases, is a slow, progressive disease. By contrast, closed-angle glaucoma is relatively uncommon and can be 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.
Who Gets Glaucoma?
Worldwide, there are an estimated 65 million cases of glaucoma. There are 3 million cases in the United States, but only one-half of those have been diagnosed. About 2% of people between the ages of 40 and 50, and 8% of those over 70, have elevated intraocular pressure in one or both eyes.
There are 120,000 people in the United States who are blind as a result of glaucoma, which accounts for 9 to 12% of all cases of blindness. It is the second leading cause of permanent vision loss and the leading cause of preventable blindness. Open-angle glaucoma accounts for 19% of blindness in African Americans and 6% in Caucasians. Glaucoma is 6 to 8 times more common in African Americans than Caucasians, and they are more likely to become blind from it.
Asians and Eskimos have a higher prevalence of closed-angle glaucoma than other ethnic and racial groups. This type also is more common in women, the elderly, people with farsightedness (hyperopia), and those with a family history of the condition.
- Open-angle glaucoma: gradual loss of peripheral vision, marked by blind spots. Symptoms only develop at the later stages of the disease, which is why screening exams are advised. The damage is often not detectable on visual field tests (perimetry tests) until as much as 40% of the optic nerve fibers are destroyed. Eventually, the fibers needed for central vision may be lost as well.
- Closed-angle glaucoma: acute attacks involving severe eye pain, nausea and vomiting, blurred vision, and the appearance of rainbow-colored halos around lights. Attacks typically do not occur in both eyes at once, but after an initial attack, there is a 40 to 80% chance that a similar attack will occur in the other eye within five to 10 years if no treatment is provided.
- 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 of the drainage system 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 hours.
What If You Do Nothing?
Glaucoma will worsen if not treated, and may lead to blindness. Early detection and treatment can help prevent or limit vision loss.
Diagnosis of glaucoma requires a thorough eye exam performed by an ophthalmologist or an optometrist. The exam includes measurement of eye pressure, viewing of the optic nerve, and assessment of optic nerve function, using the following tests.
- Tonometry: use of an instrument to apply slight pressure upon the eyeball to measure IOP.
- Ophthalmoscopy: 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.
- Gonioscopy: a gonioscopy lens is placed on the cornea, which allows inspection of the drainage of aqueous fluid to determine if open-angle or closed-angle glaucoma is present.
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:
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% on average, but significant systemic side effects and occasional serious complications (such as kidney stones, depression, diarrhea, and blood abnormalities) limit their use.
Periodic ophthalmologic examinations are essential throughout drug therapy. Following your medication regimen exactly as your doctor prescribes also is essential.
Surgery successfully lowers IOP 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% 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 hours. Making a hole in the iris (iridotomy), usually with a laser, 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.
Topical prostaglandins (bimatoprost, latanoprost, tafluprost, travoprost) are the most frequently used drugs for glaucoma. They reduce IOP by improving drainage of aqueous humor.
Beta-blockers (timolol, levobunolol) lower IOP by reducing the production of aqueous humor. Possible side effects include slowed heart rate, lowered blood pressure, reduced libido, anxiety, nausea, and breathing difficulties.
Adrenergic agonists (brimonidine, iopidine) can increase the drainage of aqueous humor but work primarily by decreasing its production. Possible adverse effects include burning in the eyes, enlarged pupils, and allergic reactions; their use in small children is not advised because of a potential to depress the nervous system in children.
Carbonic anhydrase inhibitors (dorzolamide, brinzolamide) lower IOP by decreasing aqueous humor production. Common side effects include stinging, burning, and other eye discomfort.
Miotics (pilocarpine) are drugs that constrict the pupil and increase the outflow of aqueous humor. Side effects include headache (or eyebrow ache), nearsightedness, and reduced night vision.
- 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.
When To Call Your 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.
Reviewed by Daniel E. Bustos, M.D., M.S., Private Practice specializing in Comprehensive Ophthalmology in Nashville, TN. Review provided by VeriMed Healthcare Network.