Tear Film Osmolarity Revisited
dry eye dx and tx
Tear Film Osmolarity Revisited
By William Townsend, OD, FAAO
With many disease states, the crucial step in management is making the correct diagnosis; once that happens, treatment is often pretty straightforward. This is often the case in dry eye disease, but diagnosis of dry eye can be very confusing, challenging, and frustrating.
McMonnies questionnaire and the OSDI are two of the more common subjective tools for evaluating dry eye. Objective tests such as the Schirmer test, tear film breakup time, and fluorescein, lissamine green, and rose bengal staining are also important tools in evaluating ocular surface disease. Expression and transillumination of meibomian glands may reveal meibomian gland dysfunction as a cause of or contributor to dry eye disease. All of these tests provide information about the existence and degree of ocular surface disease, but unfortunately the data is sometimes conflicting and inconclusive.
What Test Works Best
In a large, prospective, multi-site clinical study Sullivan et al (2010) assessed tests that practitioners commonly use in diagnosing dry eye in addition to tear osmolarity. Schirmer's test showed the poorest correlation to disease severity followed by meibomian gland grading, corneal staining, and conjunctival staining. Tear film osmolarity showed the best correlation with severity of dry eye.
The relationship between dry eye states and tear film osmolarity has been studied for decades. Gilbard et al demonstrated that hyperosmolarity is related to keratitis sicca as early as 1978. He and another group of researchers demonstrated that ablation of meibomian gland orifices in rabbits also caused increased tear film osmolarity. In 1981, Holly and Lamberts established the positive effects of treating dry eye with hypotonic drops. Begley et al (2009) reported a definite association between elevated osmolarity and tear film instability.
Osmolarity measures the number of osmoles of solute particles per unit volume of solution. When tears become hypertonic, cells on the cornea lose cellular water and, as a result, begin to absorb solutes such as sodium and potassium from the environment. Traditionally, evaluation of tear film osmolarity has been measured by freezing point depression. Temperature of the freezing point is proportional to the total number of dissolved particles in the solution. Tears are frozen, then slowly heated and observed with a stereomicroscope; the temperature at which they melt is then recorded. This technique requires special instrumentation and is time consuming, but Khanal et al (2008) concluded that, "tear film osmolarity is the single best single test for the diagnosis of dry eye."
Easy, Accurate—in Your Hands
The value of elevated tear film osmolarity in dry eye disease has been recognized for decades, but outside of institutional or research settings, this technology has not been readily accessible. The recent introduction of the TearLab osmometer (TearLab Corp.) has made it possible for clinicians to measure osmolarity using electrical impedance to evaluate electrical conductivity of tears. Tomlinson et al (2010) compared osmolarity values obtained with traditional freezing point depression to those found using the TearLab instrument and found clinically significant correlation between values obtained with the two systems.
As we encounter more individuals who have ocular surface disease, we need an accurate and efficient method for detecting it and managing its therapy. Tear film osmolarity may prove to be the best means of achieving this end. The TearLab system is currently being evaluated in several studies, including one by myself. In the near future, reports from ongoing clinical trials with this instrument will provide us with information regarding its value in ocular surface disease. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #179.
Dr. Townsend practices in Canyon, Texas and is an adjunct faculty member at UHCO. He has received research funding from Alcon. You can reach him at email@example.com.
Contact Lens Spectrum, Issue: October 2010