Newer Lens Materials Have Not Reduced Infection

Contrary to what most people and experts expect, neither the silicone hydrogel contact lenses that allow more oxygen into the eye nor its daily disposable counterparts have actually reduced the risk of dangerous eye infection, two new studies revealed in the October issue of Ophthalmology. Researchers also emphasized that whatever the type of lens you use, sleeping with them in is the biggest risk factor for a painful infection of the cornea called microbial keratitis.

John Dart, DM, a consultant ophthalmologist at Moorfields Eye Hospital in London, who is the lead author of one study and co-author of the other, said that these are the first well-designed studies to look at daily disposables and the newer silicone hydrogel lenses.

Introduced in 1999, the silicone hydrogel lenses were expected to improve oxygen transmission to the cornea, which has no blood supply of its own, to decrease infection risk. Daily disposables, on the other hand, were also thought to be protective against infection because they're not exposed to lens cases, which can be contaminated.

According to Dart, however, it's important to put the risk in perspective. "The risk of getting microbial keratitis is actually overall not large," he said. While it only affects about 1 in 2,000 contact lens wearers, the infection can cause vision loss, sometimes permanently.

In the first study, lead researcher Dart 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 from late 2003 to 2005. They found that 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. Dart said that while gas-permeable lenses are safer than any other type of lenses, they are not a popular choice among contact lens wearers, who tend to find them uncomfortable.

He also said that 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 "because the type of bugs you get are less nasty." In fact, none of the daily disposable lens wearers lost vision beyond 20/40, Dart said.

Reusable lenses, on the hand, must be disinfected and stored, because Dart said that "lens cases harbor nasty bacteria in some patients." He also found that some brands were associated with more infections than others.

The second study co-authored by Dart was led by Fiona Stapleton, PhD, of the University of New South Wales. It involved 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. Just like in the first study, Stapleton and her team found that new lens materials haven't reduced infection. Overnight use was the strongest risk factor for infection.

The study revealed 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.

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