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It's the leading cause of irreversible blindness in Canada, and up to half of those affected don't know they have it.

Glaucoma is a debilitating eye disease that primarily strikes older Canadians and, if left untreated, often leads to permanent blindness.

Breakthrough medical advances in recent years, including improved screening procedures, surgery and laser treatment options, have helped make glaucoma a manageable disease, allowing patients to see well into old age.

But despite these advances and the ability of doctors to stop the disease's progression, a crucial obstacle is leading to serious and permanent vision loss in thousands of Canadians: Many are unaware they are at risk for the disease and do not realize they should receive regular screening.

About 250,000 Canadians have glaucoma, but only about 50 per cent of them are aware of it, according to the Canadian National Institute for the Blind.

Although timely screening is an important factor in the detection of nearly all diseases and serious medical conditions, it's particularly important in the detection of glaucoma. That's because by the time a patient experiences the tunnel vision, blurriness or loss of reading vision associated with the disease, it's already too late.

Glaucoma symptoms appear after a significant amount of irreversible nerve damage has already occurred.

"It's a silent disease that's asymptomatic in the early stages," said Catherine Birt, glaucoma specialist at Sunnybrook Health Sciences Centre and ophthalmology professor at the University of Toronto. "If caught early, it can be stopped."

Concerned health-care professionals around the world created the first-ever World Glaucoma Day, which occurs today.

"What we're trying to do is reduce the number of people who are walking around with glaucoma and don't know it," Dr. Birt said.

The Canadian Ophthalmological Society, the Canadian Glaucoma Society and the CNIB are jointly launching a month-long campaign designed to raise awareness of the disease and emphasize the importance of early screening and detection.

"If it's going to happen, it's going to happen, but if you diagnose it in the early stages, then you're much more likely to save the vision," said Hubert Drouin, executive director of the Canadian Ophthalmological Society.

Some ophthalmologists in Canada are offering special screening sessions for glaucoma today in an effort to raise awareness of the fact the disease strikes without warning.

Glaucoma occurs when increased pressure in the eye damages the optic nerve, resulting in a progressive loss of vision that, if untreated, usually leads to blindness. In Canada, glaucoma is the leading cause of irreversible blindness.

It is estimated that 4.5 million people around the world have glaucoma, according to the World Glaucoma Association and the World Glaucoma Patient Association. But that number is expected to rise to 11.2 million by 2020 as baby boomers reach old age.

Dr. Birt said it's not feasible to screen the entire population for glaucoma, particularly since young people are at a fairly low risk of contracting the disease.

Risk groups include people with a family history of glaucoma, people of African or Hispanic descent, people in poor health, those who are nearsighted and those with raised intraocular pressure.

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Under pressure

Glaucoma is a disease caused by too much pressure within the eye. It is one of the most common causes of blindness and affects 1 in 100 Canadians over age 40. Glaucoma has traditionally been treated by relieving pressure with medication or surgically draining excess fluid.

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The arrows represent the fluid that forms in the eye's posterior chamber and flows through the pupil, nourishing nearby tissue. With glaucoma, fluid exits too slowly. The buildup causes pressure inside the eye.

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The enzyme NOS-2, associated with elevated eye pressure, produces too much nitric oxide at the optic nerve head. The nitric oxide damages the optic nerve and impairs vision despite measures to relieve pressure.

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SOURCE: WASHINGTON UNIVERSITY SCHOOL OF MEDICINE