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J.I. Prydal; Changes in Roughness of the Ocular Surface with Blinking . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2467.
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Purpose: A smooth tear film surface is presumed to be important for sharp image formation and clear vision. Our aim was to examine the role of blinking in the maintenance of an optically smooth surface. Methods: A technique based on laser speckle interferometry was developed to measure roughness of the eye surface. Video recordings were used to follow dynamic changes with time and individual frames digitized for analysis. A measure of the variance of pixel intensity was used to quantify speckle. The apparatus was calibrated with surfaces of known Ra (the mean deviation of surface irregularities). Roughness was measured under normal conditions and after application of the mucolytic agent acetylcysteine (20%) for 5 minutes at 34°C. The effects of blinking were examined within 20 minutes of exposure to acetylcysteine. Results: Speckle was not evident in images from eyes under normal conditions indicating an Ra of less that 7.0 ± 5.9 nm (mean ± 95% confidence interval). Immediately after application of the mucolytic agent, surface roughness increased to an Ra of 55.3 ± 5.0 nm. It then gradually decreased reaching baseline values by 40 to 60 minutes. After blinks, motion of the tear film was evident and measurements could not be made until the fluid stabilised, 40 to 300 ms after the eye opened. At this time the Ra of the surface was 3.6 ± 1.1 nm greater than that preceding the blink. It gradually became smoother until 4 to 5 seconds after the eye opened at which time roughness was approximately 80% of that before the preceding the blink. It then remained approximately constant. Conclusions: Our technique was not sensitive enough to measure roughness of the eye surface under normal conditions, but was able to quantify increased roughness after the mucolytic action of acetylcysteine. Contrary to our expectations, blinking itself roughened the surface. The smoothing that occurred between blinks may be due to other mechanisms, possibly the effect of surface tension forces, absorption of fluid by mucus, or the spread of the molecular lipid layer over the surface.
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