discovering dry eye
Does Water Content Affect Contact Lens-Related Dry Eye?
BY JASON J. NICHOLS, OD, MS, MPH
All clinicians realize that important differences exist among water contents of hydrogel polymers. For example, traditional hydrogel materials show a somewhat linear relationship between water content and oxygen transmissibility. This, however, isn't true for silicone hydrogel materials, which show relatively higher oxygen transmissibilities at lower water contents.
The resultant water content of a hydrogel lens is a product of the proportions of the number of hydrophilic sites and cross-linking within the polymer. The number of hydrophilic sites within the polymer gives rise to "bound" water, while the polymer-polymer interactions give rise to "free" water within the matrix. Both bound and free water are concentrated in "pores," which are about 1nm to 3nm wide and contain about 10 water molecules (the diameter of a molecule of water is about 3Å).
Lens Dehydration and Dry Eye
Darcy's Law describes water movement through a hydrogel material, which occurs through bulk flow in which the water molecules move together as part of a liquid body.
Most experts believe that changes in hydrogel water content under normal wearing conditions are surface-related. In-vivo hydrogel lens water content may vary with initial lens water content, lens thickness, pre-lens tear breakup, ocular surface temperature, osmolarity, pH, humidity, wearing schedules, material chemistry, blinking abnormalities and cleaning regimens.
In turn, contact lens dehydration may affect contact lens parameters and fitting characteris tics such as oxygen permeability, power, base curve and diameter. This may result in ocular surface desiccation and issues with lens comfort during wear.
However, clinicians don't completely understand the relation between lens-related dry eye and hydrogel dehydration. For instance, it's unclear whether the surface properties of a hydrogel lens are related to patient symptoms, or if the bulk properties of the material determine comfort.
A few hydrogel polymers, including balafilcon A (PureVision, Bausch and Lomb), lotrafilcon A (Focus Night & Day, CIBA Vision), galyfilcon A (Acuvue Advance, Vistakon), omafilcon A (Proclear Compatibles, CooperVision) and hioxifilcon A (Extreme H2O, Hydrogel Vision Corp.), resist surface dehydration. Some studies have found that the Proclear lens (which has an FDA indication for improved comfort relating to dryness) improves comfort during wear, while others haven't. Similarly, some initial studies show improved comfort with the lotrafilcon A material, which may be related to dehydration resistance. This issue of hydrogel dehydration and comfort needs further study.
Another point worth discussing is how contact lens solutions relate to contact lens dehydration and symptoms. If lens dehydration and resulting reduced lens thickness are related to dryness and discomfort symptoms, then some lens care solutions may help reduce these outcomes through the use of wetting or viscous agents. In this regard, lens solutions may also improve the pre- and post-lens tear film thickness, which may reduce corneal staining.
Continuing the Search
Clinicians should further study these issues to understand what causes contact lens-related dry eye. Understanding the role of water content, dehydration and contact lens care solutions in relation to lens-related dry eye may ultimately help us keep our patients in contact lenses.
Dr. Nichols is a senior research associate at The Ohio State University College of Optometry.