By the year 2020, about 2 million Canadians will have macular degeneration -- the leading cause of vision loss in people over aged 60.

Although there is no cure, there are treatments that can slow the disease's progression -- but only in those with certain genes. Those with different genetic make-ups can actually see their vision worsen on the treatments.

Now there is a new genetic test that could help determine who can benefit and who will not. But it's a test not without controversy.

Toronto-area senior Percy Ford-Smith discovered he was slowly losing his vision after he went in for a routine eye exam. As an avid model plane builder, he was shocked to learn he was in the early stages of AMD, or age-related macular degeneration.

"I had no symptoms whatsoever and it was rather alarming," he told CTV News.

AMD causes damage to the macula -- the light-sensitive cells at the centre of the retina at the back of the eye. The macula is responsible for our ability to see detail while reading, and for also the straight-ahead vision we need to drive or watch television.

If the disease is diagnosed early, blindness can be avoided, but many cases aren't diagnosed until the disease is already advanced.

Mississauga-based ophthamologist Dr. Fareed Ali says many with early-stage AMD don't realize they have it.

"Often the patient won't notice it and I will be the one to pick it up. The sooner you pick it up, the better the outcomes are going to be," he said.

The top risk factors for AMD are smoking, high blood pressure, being over age 60, and a family history of the disease.

Scientists believe AMD often runs in families because of genetics. And now a new genetic test could help doctors determine the risk of a patient's AMD progressing to vision loss, using cells not from the eye, but from a simple cheek swab.

The test, called Macula Risk PGx, is thought to be 90 per cent accurate at identifying which AMD patients will progress to vision loss, giving eye doctors a good idea of who needs to be monitored most closely.

"Generally, I see (AMD patients) every six months. (With this test), I can say, 'I need you in four months because you are at higher risk,' or I might say, 'I will see you in a year,' said Ali.

A companion test called Vita Risk can go even further by showing which patients would most benefit from commonly-prescribed vitamin supplements that have been shown to slow AMD degeneration.

Two decades ago, a groundbreaking study found that offering AMD patients zinc supplements and antioxidants can help slow the progression of their disease. Since then, the treatment has become standard.

But more recent research found that while many AMD patients will do well on the supplements, in as many as 15 per cent of patients, zinc might actually worsen their condition.

Ford-Smith took the test, which costs approximately $400 and it isn't covered by provincial health plans, and learned he was one of those who should avoid zinc.

"I was told I was taking the wrong vitamins and it was time to change," he said.

He now takes zinc-free supplements and his most recent test shows he still has his eyesight.

But the tests are not without controversy. Some have noted that the Macula Risk test cannot predict definitively who will develop vision loss from AMD and who will not.

They note that Macula Risk looks for variants in only two genes, when there are likely many genes involved in AMD. As well, there are environmental risk factors involved in AMD, such as smoking and hormone use, and how these factors interact with genetic is still unknown.

Critics also note that there is not enough evidence to suggest that frequent eye examinations lead to better outcomes.

Though some have contested the research suggesting that some patients should avoid zinc, Dr. Demetrios Vavvas, an associate professor of ophthalmology at Harvard Medical School, recently conducted an analysis that re-affirms the finding that some patients should avoid zinc, based on their genetics.

He says genetic tests are helping to move medicine away from a one-size-fit-all approach and closer to more precise and personalized treatments.

"We can tailor the treatment for this patient and say, 'You know what, this is more likely to help you, this is more likely to harm you,'" he told CTV News. 'That is the future of medicine: personalized medicine."

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip.