OTC Ophthalmics
Complexities of the Tear Film
Research has revealed a great deal about the tear film. The model of the tear film as a three-layer barrier that provides a smooth refracting surface, preventing evaporation is still essentially correct. But the components interact in many interesting ways, and each layer is tremendously more complex than once thought. We are still learning about the tear film and its many functions.
We do know that tear film lipids, the oils secreted by the meibomian glands and found primarily on the outer layer, are critical. Dry eye patients with a compromised lipid layer can have four times the aqueous evaporation rates found in healthy eyes. Lipids help the tear film maintain its structure. They also enter the middle aqueous layer and interact with other tear film components such as mucin proteins.
But our understanding of the lipid layer is evolving further; the tear film is thought to contain up to 50 different kinds of lipids. And this highly heterogeneous collection of lipids also contains cholesterol, cholesterol esters, and waxes.
Polarity in the Lipid Layer
An important theory involving lipid polarity helps explain how the lipid layer functions. A sheet of very small chemically polar lipids, which have a hydrophobic end and a hydrophilic end, is positioned with the hydrophilic end in the aqueous layer of the tear film. The thicker, non-polar lipids align in a uniform, outside layer exposed to the air.
The polar lipids serve as a surfactant between the aqueous phase and a larger, non-polar lipid layer. Once thought to be a homogeneous mixture, the lipid layer holds various types of lipids that work together to perform its duties.
Lipid Restoration as "First Line Therapy"
Knowing the structure of the lipid layer helps us understand how to restore the lipid layer. Lipid restoration is increasingly viewed as a "first line therapy" because its functions are so vital:
- Preventing evaporation of moisture from the aqueous layer
- Providing lubrication that makes the tear film more "spreadable" and decreasing friction
- Promoting a smooth, clear refracting surface for good vision
Of course, identifying and addressing the underlying cause of tear insufficiency such as systemic medications, eyelid disorders or hormonal changes is ideal. But restoring the lipid layer can help manage and control dry eye symptoms.
