Hyperosmolarity II: Diagnosis and Treatment of Dry Eye Disease

Hyperosmolarity II: Diagnosis and Treatment of Dry Eye Disease

Hyperosmolarity II: Diagnosis and Treatment of Dry Eye Disease

Gary N. Foulks, MD, FACS

Hyperosmolarity is a feature of both aqueous deficient and evaporative dry eye. Therefore, measurement of tear film osmolarity could become a very important diagnostic tool. Unfortunately, measuring osmolarity currently requires sophisticated equipment that is largely confined to research settings. However, an osmometer suitable for clinical use is in development, so clinicians may soon be able to include tear film osmolarity measurement in their routine ocular surface testing.

Once providers have the tools they need to measure tear film osmolarity, the first question will be: what osmolarity value can serve as the point cutoff point between normal and hyperosmolar states. According to a metaanalysis by Tomlinson, the best referent value for diagnosing dry eye disease is 316 mosm/mL, which is probably a good starting point for diagnosis.1 Of course, hyperosmolarity values and dry eye disease both exist on a continuum, so additional research will be needed to determine precisely how various degrees of hyperosmolarity correlate with dry eye signs and symptoms.

Treating Hyperosmolarity

Even though we can’t yet measure osmolarity in every patient, we know from a wealth of research that patients with dry eye disease have hyperosmolar tears, and several therapies have been developed that target this alteration in tear film chemistry. Because hyperosmolarity results from an imbalance between the solute and liquid components of the tear film, available therapies take one of two approaches.

One straightforward approach is to dilute the solutes by increasing the volume of tear liquid. Two commercially available hypoosmotic artificial tears take this approach. Lipid-containing tears can also reduce tear film osmolarity by refreshing the lipid layer in order to reduce tear fluid evaporation, thus preserving a larger tear fluid volume. Similarly, secretagogues can be expected to reduce hyperos­molarity by stimulating production of tear fluid, although no secretagogue is currently FDA-approved for the treatment of dry eye disease.

Another approach to treating hyperosmolarity is to adjust solute concentrations within ocular surface cells. Termed “osmoprotection,” this strategy involves providing ocular surface cells with compatible solutes to help balance the hyperosmolarity of the tear film. In theory, these solutes will be absorbed into ocular surface cells to increase the osmolarity of the intercellular fluid. By lowering the osmotic gradient between the intracellular fluid and the extracellular environment, these solutes may reduce damage caused by ocular surface cell dehydration.

Reference

1. Tomlinson A, Khanal S, Ramaesh K, et al. Tear film osmolarity: determination of a referent for dry eye diagnosis. Invest Ophthalmol Vis Sci. 2006 Oct;47(10):4309-4315.

Gary N. Foulks, MD, FACS, is the Arthur and Virginia Keeney professor of ophthalmology, University of Louisville, Louisville, KY, and is editor-in-chief of The Ocular Surface.