contact lens materials
Preventing Infection Isn't All About Oxygen
BY JASON J. NICHOLS, OD, MPH, PHD
For many years, clinicians and scientists involved in the cornea and contact lens field have believed that increased oxygen transmissibility of contact lenses would dramatically reduce the rates of infection associated with contact lens wear. This was thought to be particularly true for overnight wear as prior epidemiological research indicated that overnight wear was one of the most significant risk factors for microbial keratitis during lens wear. It's also well established that reduced partial oxygen pressure at the cornea due to the presence of a contact lens leads to corneal swelling. Further supporting the idea of the importance of oxygen in the etiology of lens-related microbial keratitis was the clinical presentation of infections sometimes in the absence of frank corneal injury.
With some silicone hydrogel lenses available for extended wear or continuous wear up to 30 nights, it seems appropriate to revisit the issue between oxygen transmissibility and infection risk during contact lens wear.
Recent epidemiological data suggest that the rates of microbial keratitis in silicone hydrogel lens wearers may be similar to those among conventional lens wearers. We need to qualify this, however, in that many silicone hydrogel lens wearers are sometimes in a continuous wear modality, and the actual rates of infection due to lens wear are low. Yet, most would still agree that the problem is associated with morbidity and that highly permeable materials haven't necessarily resolved it.
The Eye's Defenses
This highlights important concepts that need further consideration relative to understanding the ocular surface and its immune response to infection that go beyond the traditional hypoxiainduced keratitis models. For instance, we've known for years that the tear film contains many proteins that are antimicrobial, and contemporary research is revealing significant numbers of newly identified proteins present in the tear film that probably play a role in host-defense.
Beyond tear film chemistry, it's important to consider other factors that may take part in hostdefense. For instance, more physical (rather than biochemical) mechanisms that could play such a role and should be considered in the response include tear film clearance, blinking and barriers (mucins). Tear film flow and clearance may be a critical component associated with lens-related microbial keratitis. Tear film clearance with hydrogel lenses is substantially reduced (compared to both clearance of the precorneal tear film and from behind a rigid lens); this may, in part, explain the continued susceptibility of patients to microbial keratitis even in highly permeable silicone hydrogel materials. Obviously the large mucins (particularly those forming the glycocalyx, which are bound to the surface) play a significant role in trapping and engulfing pathogens.
A Contact Lens Answer?
There's been some discussion of so-called combination products — hydrogel materials with antimicrobial agents to help facilitate safer contact lens wear. This idea, while not necessarily new, is probably not likely to come to fruition in the next several years. There's certainly concern about bacterial resistance, in addition to the complexities of incorporating the agent into the polymer. Further, additional regulatory issues might make these combination products a bit more costly and difficult to obtain FDA approval.
However, with careful scientific evaluation, they could prove to be a valuable addition in ultimately making contact lens wear safer than it is today. CLS
Dr. Nichols is an assistant professor of optometry and vision science at The Ohio State University College of Optometry.