Four new studies investigate the relationship between the tear film, contact lenses and lens care solutions.
By Louise Sclafani, OD, FAAO
Research continues to enrich our understanding of how contact lenses and lens care solutions affect the eye. The studies reported here are good examples of how basic science helps improve the way we care for our patients.
TEAR FILM LAYERS
In the first study, Jason J. Nichols, OD, MPH, PhD, FAAO, uses interferometry to examine the effect of contact lenses on tear film stability. His results are important because they remind us that contact lenses essentially split the tear film in half, which has consequences for visual quality and comfort. In one sense, contact lenses provide a protective barrier that preserves the postlens tear film, but they also create a prelens tear film that's vulnerable to evaporation. His finding that the prelens tear film thins during silicone hydrogel contact lens wear almost two times faster after a blink than the precorneal tear film in non-contact lens eyes reminds us that forces other than evaporation can contribute to contact lens dewetting. These data also suggest that lid irritation due to prelens tear film thinning, and not the cornea, may be the primary source of contact lens discomfort.
In the last part of his paper, Dr. Nichols evaluated the effect of hydroxypropyl methylcellulose (HPMC)-containing solutions on tear film thickness and stability. HPMC has been known to thicken the tear film layer, so his conclusion that the prelens tear film was thinner in patients who wore the same type of contact lenses but used a non-HPMC solution makes sense. Dr. Nichols' results were statistically significant, but we need to see more correlation between tear film thickness and symptoms of lens wear discomfort before his findings can be applied to a clinical situation.
TEAR FILM PROTEIN PROFILES
Franz H. Grus, MD, PhD, used a new device to evaluate tear film protein profiles. I was impressed that SELDI ProteinChip technology easily distinguished between the tears of normal patients and those of patients with dry eye, as well as between contact lens wearers and non-contact lens wearers. Knowing that proline-rich proteins are downregulated in patients with dry eye may lead us to new protein-based therapies.
I was surprised to learn that proline-rich proteins were also downregulated in the tears of contact lens wearers, and that Dr. Grus was able to change the tear film protein film profiles of contact lens wearers so drastically by switching them to a different multipurpose solution. These findings reinforce the idea that lens care solutions actively affect tear film protein profiles and may be an important factor in comfort and wearing success. I would like to see a longer study (8 to 12 weeks) investigate whether dry eye and contact lens wear affect other inflammatory markers in tears.
CORNEAL BARRIER FUNCTION
Glaucoma specialists have used flurometry to measure fluid uptake on intraocular structures and have seen variations in the diabetic population. It is interesting that Meng Lin, OD, PhD, FAAO, has applied this technique to assessing the effect of contact lenses and lens care solutions on corneal permeability.
At what point does permeability cause damage? Dr. Lin's finding that 43% of silicone hydrogel lenses and 57% of conventional soft lenses significantly increased corneal epithelial permeability after only 1 night of overnight wear shows how vulnerable eyes are to infection during contact lens wear. Increased permeability with contact lens wear reminds us we should take every possible measure to reduce this risk, including choosing lens solutions that don't disrupt the cornea's barrier function. If Dr. Lin found a 27% difference in corneal permeability between Opti-Free Express and Complete MoisturePlus multipurpose solutions, others may have even more dramatic effects on barrier function.
The tendency of contact lenses to increase corneal permeability may not always be detrimental. We can take advantage of this effect to administer topical medications that usually are blocked by the corneal barrier. Soaking bandage contact lenses in medications may improve penetration rates and help more of the active agent reach the target tissues.
I'd like to see future studies investigate if solutions affect corneal permeability differently during silicone and non-silicone hydrogel lens wear.
DRY EYE SEVERITY
In her contribution, Glenda Secor, OD, FAAO, discussed
the relationship between dry eye and contact lens wear. I agree that silicone hydrogel
beneficial for patients who have higher demands or wearing restrictions, but the true value of her study is her use of the Ocular Surface Disease Index (OSDI) to
quantify the severity of dry eye in contact lens wearers.
Like other researchers reporting in this supplement, Dr. Secor found that lens care solutions affect wearing comfort. After using Complete MoisturePlus multipurpose solution for 30 days, 82.6% of the participants reported improvement in their dry eye symptoms. Dr. Secor could measure this improvement because OSDI scores are good indicators of dry eye severity, a finding that suggests this objective 12-question survey should be used to evaluate clinical dry eye more often.
While reviewing Dr. Secor's results, I wondered why OSDI scores became higher between day 7 (8.0) and day 30 (11.5). I also would like to know if patients using a specific pre-study solution were happier with Complete MoisturePlus multipurpose solution at the end of the study than patients using other pre-study solutions.
Dr. Secor proposes a follow-up study with a control solution to determine if the results of the current study are influenced by the placebo effect. I would like to see future studies include a larger sample population and comparisons of interactions between solutions and different lens materials.