discovering dry eye
Brushing Up On Tear Film Viscosity
BY BARBARA CAFFERY, OD, MS, FAAO
A clinician recently asked me what I thought about tear film viscosity and its influence on dry eye and contact lens success. My first thought was that I really didn't have a clue! But my next thought was that I should find out. So I proceeded to do some research. Here's what I found.
Getting a Basic Understanding
Taber's Dictionary defines viscosity as, "The state of being sticky or gummy" and "the resistance offered by a fluid to change of form or relative position of its particles due to attraction of molecules to each other." Water has a low viscosity while glycerol has a high viscosity.
Clinicians have a "feel" for viscosity when looking at the tear film. This is especially true when observing the flow of the tear film over a contact lens. We believe that sludgy, thick tears that coat a contact lens are highly viscous and are a detriment to contact lens comfort. We also believe that thin, runny tears may not remain on the surface of the eye long enough to protect the underlying epithelium and therefore may result in dry eye symptoms.
Figure 1. Particle movement in the meniscus helps determine
Clinically, we "measure" tear film viscosity using the slit lamp with lowest illumination (to avoid reflex tearing) to observe bubbles or particles in the tear film of the lower meniscus (Figure 1).
If the bubbles or particles travel quickly to the puncta, then the tears are likely low in viscosity. If the particles take a long time to traverse the lower lid margin to the puncta, then the tears are likely highly viscous. This requires clinical judgment and a good deal of looking at the normal flow before you can recognize abnormal flow.
Analyzing the Factors
Aside from how to measure the tear film, what determines tear film viscosity is poorly understood. Some researchers believe that the amount of soluble mucins created by the goblet cells and epithelial cells of the cornea and conjunctiva determine tear film viscosity. Others believe that the lipids that mix with the aqueous portion of the tears are the main contributing factor. It's likely that both play a role.
An Intricate Component
How important is tear viscosity to the environment of the ocular surface? Much of our body tissue is surrounded by liquids. From amniotic fluid to saliva and the tear film, liquids interact with the surrounding tissues in many ways. Each has an optimum composition, including the viscosity. When the tear fluid composition is too viscous or too watery, the underlying epithelial cells are likely affected in many metabolic ways. Also, the viscosity of the tears affects the shear force action of the fluid and changes the interaction of the lid with the tear film and the underlying epithelium.
There's so much that we don't know about the tear film. Viscosity is only one area that requires a great deal of further research to determine its true role in lens wear and dry eye disease. Perhaps patients who have thicker tears need a less viscous tear supplement or lens lubricant and those who have thinner, less viscous tears need a thicker supplement. The same may be applied to contact lens solutions.
Dr. Caffery has practiced optometry in Toronto, Canada, in a group setting dedicated to contact lens and tear film research since 1977.