Since the publication of large-scale population based studies to estimate the risks of contact lens-related microbial keratitis, the lens types and modes of wear have changed considerably, with the introduction of silicone hydrogel contact lenses for continuous wear of up to 30 nights and daily disposable contact lenses. It is time to consider epidemiologic studies to estimate the absolute and relative risk of microbial keratitis associated with these new exposures. This article summarizes current knowledge of the estimates and determinants of risk associated with re-useable hydrogel lenses and discusses study design considerations for future studies.
The absolute risk of microbial keratitis to the wearer has been remarkably stable since 1989. Risk factors consistently associated with increased risk have included overnight wear, the duration of continuous overnight wear, lower socioeconomic class, smoking, and lens hygiene practice, specifically in daily wear lenses. Men seem to be at a slightly higher risk compared to women. Factors such as the lens age, duration of wear or wear history, and time since the last aftercare visit do not seem to modulate risk. Outstanding questions include whether the risk has changed over time, whether the risk is modified with daily disposable or silicone hydrogel wear, whether the risk is different for 6 or 30 nights of continuous wear, or whether the severity of the disease or spectrum of causative organisms has been modified with new lens types.
Different study designs have been used to derive risk data in contact lens wear, including cohort, surveillance, and case-control designs. An appreciation of the advantages and limitations of each of these approaches will assist in the planning of future studies to address these outstanding questions.