BY JASON J. NICHOLS, OD, MPH, PHD
Preservatives and disinfectants are used in many household personal care and medical products, including general antiseptics, mouthwash and contact lens care products. Over the years, numerous reports have shown the effects of lens care solutions and potential ocular toxicities and hypersensitivities relative to their impact on the ocular surface. Early on, preservatives such as thim-erosal and benzalkonium chloride were associated with significant hypersensitivity responses in soft lens wearers. These would manifest themselves via ocular surface signs such as staining, severe hyperemia and patient symptoms.
More recently, we've been deluged by reports suggesting a link between polyhexamethylene biguanide (PHMB) and either silicone hydrogels or some FDA Group II lenses relative to an ocular surface response (primarily in terms of corneal staining). Evidence has surfaced from case reports in addition to observational studies and clinical trials. In fact, my last count showed at least 10 published clinical studies and 12 case reports and editorials suggesting some link between PHMB and corneal staining.
What's Going On?
In 1997, Jones et al suggested a mechanism associated with the observed staining phenomenon. In that report, the authors extensively discuss solution-preservative reactions, and suggest the mechanism of PHMB uptake on Group II lenses is due to the N-vinyl pyrolidone (NVP) lipid binding on contact lens surfaces (as PHMB works by binding to phospholipids associated with bacterial cell membranes). Later, it appears as though the first reported study to document the phenomenon in silicone hydrogel lenses was also by Jones and coworkers, 2002. Several case reports and editorials also appeared in Contact Lens Spectrum at that same time. Similar to the proposed mechanism associated with FDA Group II lenses, Jones et al here again implicated NVP and lipids as the medium for the binding of PHMB with balafilcon A.
Bausch & Lomb responded to these initial findings with a re-analysis of their large (n = 381) clinical trial showing what amounted to no effect in terms of increased ocular surface staining with Renu MultiPlus and balafilcon A (both B&L). Since that time, other independent and sponsored studies have addressed other silicone hydrogel materials and PHMB-containing care solutions in terms of ocular response impact. For example, other research implicates lotrafilcon A (Focus Night & Day, CIBA Vision), but to a lesser degree.
Compared to the ocular surface response, what isn't clear is PHMB's role in inducing ocular surface-related symptoms, both with and without clinical signs of staining. In my review of the studies evaluating the relation between PHMB and ocular surface staining, most appear to have assessed patient-reported outcomes in one form or another. However, few seem to have found a significant impact of the ocular surface response on patient symptoms.
Of course, we as practitioners are always concerned about any compromise of the ocular surface, but one might expect more symptoms. I wouldn't personally expect to see induced dryness per se, as I wouldn't expect a preservative to impact tear film stability or production, but more of a general irritation and lens discomfort related to the ocular surface.
As new preservatives enter the contact lens care solution market, we look forward to improving the lens care regimen for patients.
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #126.
Dr. Nichols is an assistant professor of optometry and vision science at The Ohio State University College of Optometry.