Glaucoma is a group of diseases in which the optic nerve is damaged in a characteristic fashion, incrementally reducing vision. Usually it takes years for large amounts of damage to occur, but sometimes this can happen in weeks to months. It is a leading cause of blindness worldwide, and might be the greatest cause of irreversible but preventable blindness. An estimated 4 million people in the United States have glaucoma, but only half of them realize it, according to the Glaucoma Research Foundation.
The optic nerve is like a cable that carries information from your eye to your brain; without it, the brain cannot process what the eye sees. Glaucoma’s damage to the optic nerve initially damages peripheral vision – the outside edges of the typical field of vision. By the time that the eye’s central vision is affected, glaucoma’s damage to the optic nerve is well-developed – and existing damage cannot be reversed. Most people experience glaucoma in both eyes, though it can start earlier in one eye.
Ophthalmologists cannot “cure” glaucoma’s root cause. Rather, they treat the disease and try to halt or, more realistically, slow its progression. The vast majority of people with glaucoma are not blinded by it at 15 years after diagnosis, UW Medicine ophthalmologist Philip Chen reported in 2004.
Treatment is best begun before noticeable peripheral vision damage occurs, so ophthalmologists encourage people to maintain a regular schedule of eye exams. At increased risk are African-Americans over age 40, people with a family history of glaucoma, and anyone over age 60 – particularly people of Hispanic ancestry. Everyone in these groups should seek a comprehensive eye screening every one to two years.
Most people with glaucoma have no symptoms. People don’t feel pressure building because it happens so gradually. Early loss of peripheral vision is also so gradual and mild that it is not readily detectable to the patient. Ultimately, glaucoma can severely reduce the eye’s peripheral vision, creating “tunnel vision."Figure 1: Normal Vision
Figure 2: Early stages of glaucoma showing beginnings of tunnel vision
The central vision of the eye is usually not affected until glaucoma is very advanced, but central vision can be lost to glaucoma.
Figure 3: Advanced glaucoma
People with closed-angle glaucoma can experience brow-aches above and pain in the affected eye, blurred vision, halos or rainbows around lights, and eye redness. These symptoms often occur in the evening or at night, in dim lighting. Usually only one eye is affected.
The eye constantly makes fluid, called aqueous humor, and drains it. The fluid nourishes the cornea and lens. The fluid circulation determines the eye pressure. The drain of the eye is called the trabecular meshwork, located in a 360-degree ring where the white sclera meets the colored iris and the cornea, in a part of the eye called the angle.
Often, glaucoma occurs when the fluid’s pressure rises above the normal range of 10-21 millimeters of mercury (mmHg, the unit of eye-pressure measurement, same as blood pressure). However many people develop glaucoma when the eye pressure is within the normal range; this is called Normal Tension Glaucoma.
About 70 percent of all glaucoma cases are described as “open-angle
.” This glaucoma type is idiopathic, meaning its cause is unclear; the optic nerve is damaged even though the angle and trabecular meshwork appear to function properly. Among patients with open-angle glaucoma, about half have elevated eye-fluid pressure, and half have normal pressure. Ophthalmologists do not know why glaucoma develops in these cases. Some patients can have identifiable causes of open-angle glaucoma, such as pseudoexfoliation or pigment dispersion.
About 15-20 percent of glaucoma cases are described as “closed-angle
.” This glaucoma is marked by an anatomic closure or narrowing of the angle’s trabecular meshwork. This causes fluid to back up, building pressure on the optic nerve.
About 10-15 percent of glaucoma cases are described as “secondary-cause
.” In these cases, an identifiable cause results in high eye pressure and optic nerve damage. Such causes include inflammatory, neovascular, steroid-response, and traumatic glaucomas, including glaucoma after eye surgeries of any kind.
Many people have high eye pressure –
– but never develop glaucoma. In fact, a National Eye Institute study showed that, over a five-year period, only 10 percent of people with high eye pressure developed glaucoma. Anatomic differences such as thicker corneas are markers for some people’s ability to better tolerate high eye pressure. Nevertheless, the condition is a strong risk factor for glaucoma, especially eye pressure greater than 30 mmHg. People with ocular hypertension should see their ophthalmologist regularly to ensure they are not developing glaucoma.
Risk factors for glaucoma include:
- Advanced age
- Elevated eye pressure
- African-American or Hispanic ancestry
- Family history of glaucoma
- History of trauma (direct blow) to the eye
Advanced age is an important risk factor. Population-based studies show that the prevalence of glaucoma rises substantially above age 60. People 60 and older, particularly those of Hispanic ancestry, should have eye exams every year or two, even if they have no trouble seeing.
Glaucoma is the leading cause of blindness in people of African-American ancestry, who are affected earlier (by as much as a decade, on average) and more severely by glaucoma than are Caucasians, in general.
Eye pressure is discussed in the preceding section.
A family history of glaucoma (especially among brothers, sisters, parents, or children) increases a person’s risk of glaucoma. Congenital glaucoma develops in infants and youths, but often is diagnosed in the child’s first year.
People who experience a direct blow to the eye are more prone to develop glaucoma, often years after the traumatic event.
Other risk factors with a lower correlation to glaucoma include diabetes mellitus, extreme nearsightedness, retinal vein occlusion, and chronic use of corticosteroid medications.
For most people, a glaucoma diagnosis comes during a routine eye exam – for a new eyeglasses prescription, for example. Annual eye exams, particularly for people over age 60, are important because glaucoma’s slow progression makes it very difficult for patients to recognize they are losing peripheral vision. By the time that the eye’s central vision is affected, glaucoma’s damage to the optic nerve is well-developed – and physicians cannot reverse the damage.
Ophthalmologists evaluate eye health with visual inspection and instruments. Computerized instruments called perimeters help physicians evaluate patients’ peripheral vision in both eyes in about 15 minutes. This testing allows ophthalmologists to see if a patient’s glaucoma damage is stable or changing. Photos of the optic nerve often are taken to record its appearance for future comparison. Another instrument measures the eye’s pressure. Physicians also examine the eye and optic nerve with a microscope called a slit lamp.
Patients with glaucoma can experience loss of peripheral vision, and some will experience loss of central vision. Patients who have had surgery for glaucoma often develop cataracts that can reduce vision or visual quality. Patients may encounter side effects from medications used to treat glaucoma. It is relatively common to need to change medications because of lack of effectiveness in lowering eye pressure, or side effects.
Most important are regular eye screenings by ophthalmologists, particularly for people with higher risk factors. Additionally, people who take medicinal eye drops for glaucoma should diligently adhere to their scheduled dosages.