the contact lens exam

Association or Causation? The Corneal Staining Debate

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

An underutilized technique in the contact lens examination process is the use of a yellow fluorescein enhancement filter (Kodak Wratten No. 12) over the objective lens in combination with the cobalt blue filter to visualize corneal staining. Make this a part of everyday practice to closely document epithelial disruption and to monitor patients for any sequelae of soft lens wear.

The Significance of Staining

Two recent articles show an association between corneal staining and subsequent corneal inflammatory events. In a retrospective analysis of several nonrandomized trials in Australia, subjects exhibiting solution toxicity staining during soft lens daily wear had a four times greater risk of experiencing an inflammatory event than did non-affected controls. In a study that I published, corneal staining posed a seven-fold higher risk for the subsequent development of an inflammatory event among 30-night silicone hydrogel continuous wear patients.

However, the two studies to date have determined only an initial association between corneal staining and corneal infiltrates, which doesn't imply causation.

Making the Leap

In 1965, Sir Austin Bradford Hill wrote a telling article describing nine factors that assist us in making the leap from association to causation. These include strength (the risk is so large that we can easily rule out other factors), consistency (the results have been replicated by different researchers and under different conditions), specificity (the exposure is associated with a very specific disease as opposed to a wide range of diseases), temporality (the exposure preceded the disease), biological gradient (increasing exposures are associated with increasing risks of disease), plausibility (a credible scientific mechanism can explain the association), coherence (the association is consistent with the natural history of the disease), experimental evidence (a physical intervention shows results consistent with the association) and analogy (there's a similar result that we can draw a relationship to). To date, only a few of these factors have been satisfied.

Figure 1. Without a yellow barrier filter we notice no appreciable staining.

Figure 2. Same patient with a yellow Wratten No. 12 filter in place.

Neither study was initially designed to evaluate corneal staining as a risk factor for infiltrates. In fact, one of the studies didn't require a Wratten filter to evaluate staining. Neither controlled for time of day, bacterial contamination of lenses or pattern of stain, and neither assessed location of staining in relation to the infiltrative event. These potential confounders can skew the results of the analyses.

One thing is certain: more research is needed in this area. Until then, examine the cornea using a barrier filter. It will allow you to visualize subtle corneal staining that you may otherwise miss (Figures 1 and 2). CLS

For references, please visit www.clspectrum.com/references.asp and click on document #141.

Dr. Szczotka-Flynn is an associate professor at Case Western Reserve University Dept. of Ophthalmology and is director of the Contact Lens Service at University Hospitals of Cleveland.