Contact lenses are a good alternative to eyeglasses, but are not entirely risk free. Even with today's improved designs, a small but significant number of contact lens wearers develop painful microbial infections of the cornea (microbial keratitis, or MK) that can cause vision loss.
The largest combined study of this problem to date was conducted by Australian and United Kingdom researchers, who were especially interested in whether the new silicone hydrogel lenses would reduce infection risk as a result of their improved oxygen permeability. The cornea, the clear front covering of the eye that helps focus light, needs oxygen to remain healthy. The studies looked at use patterns for several types of contact lenses in relation to infection factors, the incidence of MK-induced vision loss, and risk levels associated with specific lens types. Eye specialists and contact lens wearers will be able to make better choices based on these new findings.
Both studies agreed on key points: new lens types, including silicone hydrogel "soft" lenses and daily disposables (DD), did not reduce the risk of MK for wearers. Disease incidence fell within the ranges reported in earlier soft and rigid lens studies: 2.7 to 6.4 cases per 10,000 users per year for daily wear and 9.3 to 20.9 cases per 10,000 users per year for overnight wear. The researchers now think that other factors---such as tear film stagnation, corneal changes beneath the lenses, and slowed turnover of corneal surface cells—may be more important than oxygen permeability in MK development.
Overnight wear of any contact lens, including "extended use" types, was found to be the strongest risk factor for permanent vision loss, which occurred in about 13 percent of MK cases in these studies. DD wearers who developed MK had less severe infections and were least likely to sustain permanent vision loss. DDs do not require lens cleaning or lens cases, where infective agents can grow and thrive.
The Australian study linked use of any contact lens type for less than six months with increased MK risk. Earlier research showed that most new wearers' eyes adapt to lens use over several months: at first the corneal surface thins and sheds cells; later these cells are replaced, the rate of shedding declines, and the ability of bacteria to bind to shed cells also decreases.
"Users of overnight wear contact lenses have an increased risk for developing MK during their early wear experience," says Fiona Stapleton, PhD, University of New South Wales, who led the Australian study. Eye specialists should advise patients accordingly, she adds.
The Australian prospective population survey was conducted from October 2003 to September 2004 with a total of 285 study subjects and 1,798 controls. The U.K. case-controlled study was led by J.K.G. Dart, DM, Moorfields Eye Hospital, and conducted from December 2003 through December 2005 with a total of 367 study subjects and 1,069 controls. The two studies were designed to complement each other and shared key parameters so that results could be directly compared.
Smoking, purchasing lenses via the internet, and higher socio-economic status were additional risk factors in the Australian study. The UK study found risk levels varied significantly depending on the lens brand; a better understanding of how specific brand characteristics contribute to infection is needed so that safer lenses can be developed. Other risk factors in the UK study included improper hand cleaning, younger wearer age, use beyond recommended time spans, and male gender. The differences in risk results between the studies are probably due to prescription and use patterns, the authors say.
This research was published in the October 2008 issue of Ophthalmology, the journal of the American Academy of Ophthalmology.
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