dry eye dx and tx
Lipids and Proteins in Dry Eye
By William Townsend, OD, FAAO
Rumpakis (2010) showed that discomfort is the most common reason for discontinuation of contact lens wear, and discomfort is frequently associated with dry eye. By improving patient comfort, we may be able to reduce the number of patients who drop out of contact lens wear.
Deposition and Dry Eye
Silicone hydrogel (SiHy) materials offer increased corneal oxygen levels and may improve patient comfort over traditional hydrogels (THy). But surface deposits, the most common lens complication (Suvajack, 2008), are highly influenced by the nature of the material and the composition of a wearer's tear film. Proteins make up the bulk of deposits on THy lenses (Jones, 2003).
SiHy lenses now comprise more than 50 percent of all lenses prescribed in the United States (Nichols, 2011). They bind smaller amounts of protein, but their inherently hydrophobic surfaces may encourage lipid deposition. Manufacturers of SiHy lenses have reduced the inherent lens surface hydrophobicity through surface treatments and changes to material chemistry. Despite these interventions, lipid deposits can still be problematic in some wearers.
A contact lens divides the tear film into a precorneal tear film (PCTF) and a prelens tear film (PLTF). Nichols et al (2005) found that the PLTF is half the thickness of the PCTF and thins more rapidly than the PCTF. PLTF thinning may accelerate the deposition of lipids and other tear components on the lens surface. Contact lenses also increase evaporation from the ocular surface, and though lens water content does not affect the rate of increase, older lenses cause a significant increase in evaporation (Guillon, 2008; Mathers, 2004). This is particularly important in individuals who already have dry eyes and whose evaporation rates, tear osmolarity and surface temperature are elevated.
It is thought that the lipid layer of the tears is composed of an outer nonpolar phase, functioning as an air-tear film interface that retards the transmission of water vapor, and a polar surfactant phase that adheres to the underlying aqueous-mucin phase. It is very effective in retarding aqueous phase evaporation. Meibomian gland disease can decrease polar lipids (McCulley, 2003), which can result in destabilization of the lipid tear layer, decreased tear breakup time, increased aqueous-mucin phase evaporation, and dry eye symptoms.
How can we manage contact lens dry eye resulting from protein and lipid deposits? We must recognize that no-rub is no option. The cleaning efficacy of lens care solutions is significantly enhanced by rubbing to remove deposits (Sindt, 2007). In recalcitrant cases, consider an alcohol-based cleaner in conjunction with rubbing to reduce lipid deposition (Sindt, 2007).
Despite their advantages, SiHy lenses may not be the best option for every patient. For some, the negative impact of hydrophobic characteristics and lipid deposition may outweigh the increased oxygen permeability. Finding the best material for a given individual may require several visits and multiple trial lenses. The good news is that technology, and even new care solutions, may one day make this process simpler.
At the Swedish Contact Lens Association Meeting, I heard Gunilla Runstrom, a graduate student at Ashton University, UK, describe research by Brian Tighe, herself, and coworkers at the Biomaterials & Biomolecular Sciences Research Group. They used ELISA, Lab-on-a-chip and other technologies to identify lens material compatibility with specific lipid and protein profiles. Such technologies may eventually help us manage lipid and protein-based conditions and, in the not-too-distant future, may allow us to pair specific lens materials and care solutions with patients' tear chemistry profiles. CLS
For references, please visit www.clspectrum.com/references.asp and click on document SE2011.
Dr. Townsend practices in Canyon, Texas and is an adjunct faculty member at UHCO. He has also received research funds from Alcon. E-mail him at firstname.lastname@example.org.