Rheology of Tear Film Lipid Layer Spread in Normal and Aqueous Tear-Deficient Dry Eyes

PURPOSE. To analyze the relationship between tear volume and tear film lipid layer (TFLL) spread. METHODS. Twenty-nine eyes from 22 subjects, including normal eyes and eyes with aqueous tear-deficient dry eye, were enrolled in this study. In all eyes, the radius of curvature (R: mm) of the central lower tear meniscus was measured with a video-meniscometer, and interference images from the TFLL were recorded with a video-interferometer. Interference images were captured as still images every 0.05 second, and the relationship between the acquisition time for each image after a blink and the averaged heights of the spreading TFLL in the upstroke of the blink were calculated. RESULTS. In all cases, the time-dependent changes in TFLL spread could be described by the expression H(t) - H(0) = rho[1 - exp(- t/lambda)], where H(t) is the averaged height in millimeters at time t, H(0) is the averaged height at t = 0, rho is a constant, t is time in seconds, and lambda is the characteristic time in seconds. A statistically significant correlation was found between those changes and the initial upward velocity of the spreading TFLL [H'(0) = dH(0)/dt] and R (r = 0.573; P = 0.003). CONCLUSIONS. This study demonstrated that the time-dependent changes of TFLL spread are compatible with the Voigt model of viscoelasticity and that the initial velocity of TFLL spread after a blink decreased in proportion to the decrease of tear volume. There is potential interest in using this parameter to diagnose and evaluate the severity of aqueous tear deficiency. (Invest Ophthalmol Vis Sci. 2008; 49: 5319-5324) DOI: 10.1167/iovs.07-1407