A New, Specular Reflection-Based, Precorneal Tear Film Stability Measurement Technique in a Rabbit Model: Viscoelastic Increases Tear Film Stability

Citation: Nankivil D, Gonzalez A, Arrieta E, et al. A new, specular reflection-based, precorneal tear film stability measurement technique on a rabbit model: viscoelastic increases tear film stability. Invest Ophthalmol Vis Sci. 2014;55:4158-4163. DOI:10. 1167/iovs.14-14157 PURPOSE. To develop a safe, noninvasive, noncontact, continuous in vivo method to measure the dehydration rate of the precorneal tear film and to compare the effectiveness of a viscoelastic agent in maintaining the precorneal tear film to that of a balanced salt solution. METHODS. Software was designed to analyze the corneal reflection produced by the operating microscope's coaxial illumination. The software characterized the shape of the reflection, which became distorted as the precorneal tear film evaporated; characterization was accomplished by fitting an ellipse to the reflection and measuring its projected surface area. Balanced salt solution Plus (BSSþ) and a 2% hydroxypropylmethylcellulose viscoelastic were used as the test agents. The tear film evaporation rate was characterized and compared over a period of 20 minutes in 20 eyes from 10 New Zealand white rabbits. RESULTS. The ellipse axes ratio and surface area were found to decrease initially after each application of either viscoelastic or BSSþ and then to increase linearly as the tear film began to evaporate (P < 0.001) for eyes treated with BSSþ only. Eyes treated with BSSþ required 7.5 6 2.7 applications to maintain sufficient corneal hydration during the 20-minute test period, whereas eyes treated with viscoelastic required 1.4 6 0.5 applications. The rates of evaporation differed significantly (P < 0.043) between viscoelastic and BSSþ. CONCLUSIONS. The shape and surface area of the corneal reflection are strongly correlated with the state of the tear film. Rabbits' corneas treated with viscoelastic remained hydrated significantly longer than corneas treated with BSSþ. Keywords: balanced salt solution, corneal hydration, evaporation, precorneal tear film stability, viscoelastic W olff 1,2 first described the composition of precorneal tear film as a three-layered structure: a thin superficial lipid layer, an underlying thicker aqueous layer, and an innermost mucin layer. In the current description of tear film, instead of a boundary between the aqueous and mucin layer, there is a mucin gel and a concentration gradient in depth, with high concentrations of mucin near the epithelial cell surface and high concentrations of aqueous near the lipid layer. 6 During intraocular surgical procedures, a transparent cornea is essential in order to allow the surgeon good visualization of the ocular structures located behind the cornea, in the anterior chamber or in the posterior segment of the eye. Thus, irrigation by wetting agents (e.g., balanced salt solution [BSS] or BSS with glutathione [BSS Plus {BSSþ}]), performed by the surgeon or assistant, is required throughout the surgery to maintain corneal hydration and transparency. Several environmental and patient-related factors may influence the ocular surface and therefore, loss of corneal clarity during anterior segment as well as vitreoretinal surgery. These factors include: use of anesthetic and topical mydriatic agents, long surgery (e.g., vitreoretinal surgery), high temperature and low humidity in the operating room, intensity of the microscope illumination, and patients with a history of dry eye syndrome. 10 Furthermore, Emre et al. The aims of this study were to develop a safe, noninvasive, real-time, and noncontact approach for monitoring tear film stability in vivo and to compare the effectiveness of a viscoelastic fluid in maintaining a smooth ocular surface to that of BSSþ. This study developed an objective, quantitativ