discovering dry eye
Are All Lipids the Same?
BY JASON J. NICHOLS, OD, MS, MPH
The tear film is classically described as a trilaminar structure with an inner mucus layer, water aqueous layer and a superficial lipid layer. The functioning of each is important in maintaining ocular surface health and in preventing dry eye symptoms. Dry eye disease is typically either aqueous deficient or evaporative in nature.
The primary role of the lipid layer of the tear film is to prevent evaporation, and if alterations to this layer occur, then evaporative dry eye disease occurs. However, some lipids also aid in spreading of the tears, which might be particularly important for the pre-lens tear film, which is probably lacking in mucus.
Looking at Lipid Interactions
The lipid layer of the tear film prevents evaporation of the aqueous tears, so it's important to develop a better clinical understanding of the impact of a contact lens on this layer. This impact includes outcomes such as thinning of the lipid layer (associated with various materials), instability of the lipid layer and evaporation of the aqueous component of the pre-lens tear film as it relates to lipid layer alteration and dry eye symptoms.
These potential outcomes warrant the need for a better understanding of the lipid layer of the tear film and its interaction with contact lenses of various material characteristics.
Understanding the Basics
In addition to understanding some of the clinical questions related to contact lens wear, the lipid layer and dryness symptoms, it's also important that we gain a better understanding of the basic components of these lipids and their potential changes in contact lens wearers. The principal components of normal human meibomian lipid are nonpolar lipids (wax and sterol esters and triglycerides), which make up about 85 percent of the lipid layer. Polar lipids including cerebrosides, ceramides and phospholipids make up the other 15 percent of the lipids of the tear film.
Experts believe that the lipid layer is composed of two phases including an outer nonpolar (hydrophobic) layer and an inner polar (hydrophilic) layer. Polar molecules have both positive and negative charges and are therefore attracted to water molecules, which are also polar.
The primary purpose of the nonpolar layer is to prevent evaporation, and it acts as a barrier to water in this regard. Experts think that the inner polar layer acts as a surfactant, allowing the nonpolar lipid layer to spread evenly over the aqueous phase of the tear film.
Hitting the Books
There is little in the clinical literature about the impact of a contact lens on the specific lipid components of the tear film. Clearly, lipids deposit on both hydrogel and GP contact lenses, which are lipophilic by nature. Polar lipids would presumably bind particularly well with ionic hydrogel materials, and studies have shown that FDA Group IV contact lenses are prone to lipid deposits, as these contact lenses are both ionic and high in water content.
However, other studies have suggested that nonionic polymers and higher water content can increase lipid deposition as well. The common denominator seems to be the water content of a lens. We might predict that polar lipid binding to a hydrogel surface would result in an uneven spreading or beading of the nonpolar lipid phase of the tear film over the surface of a contact lens. This, in turn, would lead to evaporation of the pre-lens tear film, dehydration of the contact lens and patient symptoms of dryness.
Getting to the Bottom of It
Understanding the specific lipid component associated with deposition to various contact lens materials may be the key to understanding the etiology of contact lens-related dry eye. We'll just have to see where further research takes us.
Dr. Nichols is a senior research associate at The Ohio State University College of Optometry.