More than 4 million Americans and about 70 million people worldwide have glaucoma, according to the Glaucoma Research Foundation. About half of those, the organization reports, won't know it until it's too late.
The damage occurs slowly and painlessly - typically over 20 or 30 years - and therefore many people don't realize they have the disease until irreversible damage has occurred, said Daniel Good, an assistant professor of ophthalmology at Texas Tech's Health Sciences Center.
While people who get routine eye exams - typically those who have prescription glasses - are screened for the disease, others are unlikely to know if their sight is diminishing.
"It's not like they wake up one day and suddenly have symptoms - they adapt to it day by day," Good said. By the time people realize they're losing the sight, the lost peripheral vision can be serious enough for them to be classified as legally blind.
Glaucoma is a group of diseases that produce increased fluid pressure in the eye. That pressure causes damage to the optic nerve, which carries images from the eye to the brain. According to the World Health
Organization, it's the second leading cause of blindness worldwide.
Since the damage, which usually begins in the periphery and moves inward, can't be undone, eye doctors can only slow further loss. But if diagnosed early, ophthalmologists can control progression of the disease before it becomes debilitating, Good said.
The Foundation of the American Academy of Ophthalmology is offering free eye exams for those who are uninsured and considered at risk. (For eligibility, call 1-800-391-3937.)
The Community Health Center of Lubbock also offers free glaucoma screenings, Terry Nelson, an ophthalmologist for the clinic, said.
"We found the folks at risk for glaucoma are those who haven't been able to get their eyes checked because they couldn't afford it," Nelson said. "A grant used to pay for (the screenings), but we kept offering them because it was a pretty good way to get people to come into the office."
The center also offers complete vision tests, priced according to a person's ability to pay, Nelson said.
Eye doctors diagnose the disease by checking eye pressure - the puff-of-air test - and looking at the optic nerve, Good said. Non-invasive tests to measure the thickness of the optic nerve and the visual field are used to confirm the diagnosis, he said.
People age 40 to 60 without other eye problems should be screened every two years and those older than 60 should be screened annually, Good said. Although glaucoma may affect people of any age, it is more common in adults who are approaching their senior years, according to the American Academy of Ophthalmology. African-Americans, Hispanics and people with diabetes are also at increased risk, the academy reports.
Eye drops that lower the pressure on the optic nerve are the most common treatment, Good said.
"If we can't control the pressure or somebody doesn't tolerate the drops, there are laser options and then surgical options," he added.