discovering dry eye
Using the Tear Film to Predict Lens-Related Discomfort
BY JASON J. NICHOLS, OD, MS, MPH
The National Eye Institute and Industry 1995 report summarizing contemporary issues in dry eye classified contact lens-related dry eye as an evaporative form of the disease. Interestingly, however, the role of evaporation of the pre-lens tear film hasn't been well studied, as only a few investigators have the equipment to measure evaporation.
The Effects of Evaporation
Hamano et al (1981) conducted one of the first studies on tear film evaporation during lens wear. They found that the evaporation rate during contact lens wear was about 1.45µm/minute -- about 30 percent higher than evaporation from the precorneal tear film. The findings also showed increases in the evaporation rate the longer a patient wore a lens and also for higher water content contact lenses.
A study by Cedarstaff and Tomlinson (1983) found that the presence of a contact lens significantly increases evaporation of the tear film, but that the initial water content of the lens wasn't a factor related to evaporation rate. Another study by Martin (1985) showed, conversely, that evaporation from a higher water content lens is greater than for lower water content lenses.
These few studies provide initial evidence that tear film evaporation occurs more significantly during contact lens wear. However, we still don't know why evaporation is greater for the pre-lens tear film than for the precorneal tear film, and if evaporation causes the dryness and discomfort symptoms that many of our patients experience.
Wettability and Comfort
Lens wettability (the surface's ability to support a stable tear film) has been somewhat ignored, in part, because of the evaporative classification of contact lens-related dry eye. In general, wettability is the liquid's ability to spread over the surface of a solid. We need to study the relative contribution of both evaporation and wettability to better understand tear film and contact lens interactions.
One of the functions of the tear film is to lubricate the ocular surface to provide comfort and a smooth refractive surface. Experts believe that both the polar lipids and mucins of the tear film reduce the surface tension of the aqueous tears and allow them to spread readily over the ocular surface.
When you place a contact lens on the eye, it too must remain "wet" to ensure a good refractive surface and lubrication of the lens and ocular surface. The hydrogel's ability to remain wet throughout the day could also impact comfort. Studies support this by showing significant increase in the intensity of dryness symptoms over the course of a day's wear, which we might correlate with an increase in hydrophobicity and decrease in wetting of the lens.
The process of in-vivo wetting of contact lenses isn't fully understood and many factors might be associated with in-vivo hydrogel wetting -- a few include the general tear film composition, material water content and ionicity, and the use of a contact lens care solution. In addition, technology for in-vivo measures of wettability isn't widely available.
Applying What We Know
Although we need to resolve many issues relative to contact lens-related dry eye symptoms, we still can take some steps to help our patients. These include using multipurpose solutions (which contain surfactants that aid in wetting), lubricating the lens with rewetting drops, changing materials or removing the lens in a dry, non-humid environment (such as during air travel).
Our hope is that advances in material and lens technology will eliminate some of these inconvenient, short-term solutions.
Dr. Nichols is a research scientist at The Ohio State University College of Optometry.