A case-control study of ulcerative keratitis in soft contact lens (CL) users compared three groups of daily wear or extended wear patients: (a) 86 patients with ulcerative keratitis, (b) 61 hospital-based controls, and (c) 410 population-based controls. Overnight lens use, whether regularly with extended wear lenses or occasionally with daily wear lenses, emerged as the preeminent risk factor for ulcerative keratitis. It increased risk by 10 to 15 times in users of extended wear lenses and by 9 times in users of daily wear lenses when these groups were compared with subjects engaged in strict daily wear of daily wear lenses. Even when actual overnight wear was not taken into account, the relative risk was four times greater in users of extended wear lenses than in users of daily wear lenses. The study found a marginal association (p = 0.056) between lens care frequency in general and ulcerative keratitis in comparing ulcerative keratitis cases with population controls. Of the individual hygiene-related measures, evidence of a protective effect was strongest for cleaning the lens case. Smokers showed a threefold greater risk than nonsmokers, regardless of the type of lens worn. An incidence study found the rate of ulcerative keratitis to be approximately 1 in 2,500 daily wear lens users and 1 in 500 extended wear lens users per year. This finding was statistically consistent with the risk ratio noted in the case-control study. Extrapolations suggest that 4,000 of the 9 million U.S. users of daily wear soft lenses develop ulcerative keratitis annually. Among the 4 million U.S. extended wear soft lens users, 8,000 may do so per year.
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