Oct. 3, 2008 — Neither the newer contact lenses that allow more oxygen into the eye nor daily disposable lenses have reduced the risk of a dangerous eye infection as hoped, according to two new studies.

Whatever the type of lens, sleeping with them in is the biggest risk factor for a painful infection of the cornea called microbial keratitis, the researchers also find.

“If you wear any of these lenses overnight, you have five times the risk of infection,” says John Dart, DM, a consultant ophthalmologist at Moorfields Eye Hospital in London. He is the lead author of one study and co-author of the other.

“These are the first well-designed studies to look at daily disposables and the newer silicone hydrogel lenses,” Dart tells WebMD. The silicone hydrogel lenses were introduced in 1999 in the hope that by improving oxygen transmission to the cornea, which has no blood supply of its own, it would decrease infection risk, he says. Daily disposables, introduced in 1999, were also thought to be protective against infection because they’re not exposed to lens cases, which can be contaminated.

But neither of the studies, published in the October issue of Ophthalmology, found that to be true, Dart says.

However, Dart says, it’s important to put the risk in perspective. “The risk of getting microbial keratitis is actually overall not large,” he says. It affects about 1 in 2,000 contact lens wearers. But it can cause vision loss, sometimes permanently.

Contact Lenses & Infection Risk: The U.K. Study

In the study led by Dart, the researchers evaluated 367 contact lens wearers with microbial keratitis, 1,069 hospital patients who wore contact lenses but had no contact lens-related disorders, and 639 contact lens wearers in the general population.

The hospital patients answered a questionnaire and the control patients in the general population were interviewed by telephone from late 2003 to 2005.

Daily disposable wearers had 1.5 times higher risk of microbial keratitis than those who wore soft lenses that were replaced every one to four weeks, and those who wore rigid gas-permeable lenses had the least risk of infection.

”Gas-permeable lenses are safer than any other type of lenses,” Dart says, although he adds that they are not a popular choice among contact lens wearers, who tend to find them uncomfortable.

Even though the daily disposable wearers had more risk of infection than those who wore reusable soft lenses, vision loss from the infection was less likely to occur in the daily disposable lens wearers. None of the daily disposable lens wearers lost vision beyond 20/40, he says.

“It’s safer to use a daily disposable [than a reusable],” Dart says, “because the type of bugs you get are less nasty.” Reusable lenses must be disinfected and stored, and “lens cases harbor nasty bacteria in some patients,” he says.

Some brands were associated with more infections than others, Dart found, but he says most of the contact lenses he studied have probably been redesigned since the study was done.

Contact Lenses & Infection Risk: The Australian Study

In the second study, a team led by Fiona Stapleton, PhD, of the University of New South Wales, interviewed 285 contact lens wearers who had microbial keratitis and 1,798 lens wearers without the infection.

After looking at the type of lenses worn, wearing patterns, and other factors, they estimated the annual incidence for the infection.

They also found that new lens materials haven’t reduced infection. Overnight use was the strongest risk factor for infection, just as in the U.K. study.

For instance, they estimate that microbial keratitis occurred in 1.2 per 10,000 of those who wore daily-wear rigid gas-permeable lenses but in 25.4 per 10,000 of those who wore silicone hydrogel lenses overnight.

Other factors that increased the risk of infection included smoking, buying lenses over the Internet, wearing lenses beyond the recommended time spans, and improper hand cleaning before handling lenses.

Contact Lenses & Infection: Study Interpretations

Finding that the newer lens materials don’t reduce infection risk is disappointing, Dart writes.

Although experts thought that the lack of oxygen getting to the cornea was a factor in infections, the findings suggest that other factors may be more important, he says.

The lenses may reduce the turnover of skin cells on the front of the eye, for instance, Dart says, boosting infection risk.

The studies were funded from a variety of sources, including CIBA Vision USA, which makes a variety of contact lenses. One of the supporting organizations, The Vision Cooperative Research Center, receives a royalty on the sale of silicone hydrogel lenses.

Contact Lenses & Infection: Second Opinion

The findings about the risk of overnight wear come as no surprise to Thomas Steinemann, MD, a spokesman for the American Academy of Ophthalmology and professor of ophthalmology at Case Western Reserve University in Cleveland.

“That confirms what we have always known for years,” he says. “Sleeping in your lenses is not a good thing, even if they are continuous wear. It is a risk factor, probably the risk factor, for microbial keratitis-associated vision loss.”

“Don’t sleep in your contact lenses, ever,” he says. “That includes taking naps in them.”

Also important, he says, is to pay attention to lens hygiene. “Follow your eye care professional’s and manufacturer’s advice about cleaning,” he says.