research review

New Gold Standard References for Contact Lens-Related MK

BY LORETTA B. SZCZOTKA-FLYNN, OD, MS, FAAO

Our new editor Dr. Jason Nichols has requested this new Research Review column for 2009 to highlight the most recent and relevant contact lens research in the world. I have the privilege of writing the first installment of this series.

The two papers that first and foremost come to mind are those published in October 2008 by Dr. Fiona Stapleton and John Dart and their colleagues. These two papers establish the new gold standard references for incidence of and risk factors associated with microbial keratitis (MK) with daily and extended wear of modern contact lens types. These two new papers deserve much attention as they continue to substantiate the classically cited 1989 references from Schein and Poggio in the New England Journal of Medicine that established the incidence of MK with extended wear at about 20 per 10,000 extended wear users (or about 1 per 500 extended wear users).

In fact, I feel these two papers are so relevant to modern day contact lens practice that in my December The Contact Lens Exam column titled "Don't Let Early Studies Sway You From Using New Technology," I also highlighted one aspect of their message. This column will provide more detail and the relevant clinical aspects of their findings.

Examining Incidence of and Risk Factors for Contact Lens-Related MK

The two papers titled "The Incidence of Contact Lens–Related Microbial Keratitis in Australia" and "Risk Factors for Microbial Keratitis with Contemporary Contact Lenses. A Case-Control Study" both published in the same issue of Ophthalmology as companion studies. Stapleton's Australian population-based surveillance study and Dart's case-control study in London share disease definition, severity classification, contact lens wearing schedules definition and hygiene scoring systems to allow for direct comparison of results.

The Australian study identified 285 cases of MK and established the number of wearers of different lens types in the community through a national telephone survey of 35,914 individuals. Table 1 lists the crude incidence rates for MK by lens type. Of clinical relevance is that overnight use continues to be the main risk factor for corneal infection. The risk of MK with regular overnight use remains at about 1 in 500 wearers despite the availability of silicone hydrogel materials. (Although the silicone hydrogel rate is higher than that of traditional soft hydrogel materials, the difference is not statistically significant.)

The study also assessed the crude incidence of severe MK (as opposed to all forms of MK) by lens type (as seen in the third column of Table 1). As expected, the incidence of "severe MK" is lower than all forms of MK for every soft lens type. However, with the relative change in incidence across lens types it becomes apparent that pure daily wear use of daily disposable lenses has the lowest risk of severe MK of all lenses and modalities including daily wear GP use. In fact, they further provided data on the incidence of MK with >2 lines of vision loss (data not shown), and daily disposables worn for pure daily wear or occasional overnight wear (as well as GP daily wear) had zero incidence.

The London researchers identified 367 cases of contact lens-related MK at Moorfields Eye Hospital and 1,708 controls. They similarly concluded that compared with planned replacement soft lenses and adjusted for wear schedule, the relative risk of MK was no different for silicone hydrogel lenses. Although they found a surprisingly increased risk of MK with daily disposable lenses, the risk of vision loss was less than that for planned replacement soft contact lens wearers, which corroborates Stapleton's findings that daily disposable lens use has the lowest risk for severe disease.

Table 2 outlines the risk factors identified in both studies classified as to whether the factor is potentially modifiable. Many of the modifiable risk factors have been previously identified (smoking, poor compliance and overnight lens use); however, the Australian study is the first to show a more than four-fold increased risk of MK if a user purchases lenses from Internet or mail-order sources.

Putting it All Together

Has extended contact lens wear seen its last chance for successful penetration into the contact lens market? As the modality with the greatest risk of corneal infection, we must continue to prescribe extend wear contact lenses with caution. Closely monitor the sub-groups known to be at highest risk in the first few months of wear and clearly educate them of a five-to-six-fold greater risk of MK with overnight use of soft lenses regardless of lens type. Moreover, you should pursue extended wear only if you have a solid practitioner-patient relationship with that patient.

What is readily apparent from the above findings is that when patients fail to comprehend the role of compliance or "shop" lenses for perceived better convenience or price, they also place their eyes at greater risk.

These two new references will likely remain the gold standard until the next generation of lens materials, such as those that feature antimicrobial surfaces, make their way into the market and become widely disseminated for extended wear. CLS

For references please visit www.clspectrum.com/references.asp and click on document #158.

Dr. Szczotka-Flynn is an associate professor at the Case Western Reserve University Dept. of Ophthalmology & Visual Sciences and is director of the Contact Lens Service at University Hospitals Case Medical Center. She has received research funding from CIBA, Vistakon, Alcon and CooperVision.