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J.M. Tiffany, S.S. Lachowicz; Evaporation From the Cornea and Sclera . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4401.
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Purpose: Different areas of cornea and sclera are exposed in upgaze and downgaze. Many activities, such as computer screen use, can lead to drying and discomfort through exposure. Little is known about evaporation from the tear film over the sclera. We measured total evaporation from known areas and attempted to calculate the corneal and scleral contributions. Methods: A closed goggle incorporating the ServaMed probe in relative humidity (RH) mode was used, and recorded the time taken for RH to rise from 30 to 50%. Readings were taken with the eye closed (to establish the evaporative contribution from skin), and in 15° upgaze and 15° downgaze. The right eyes of 11 adult volunteers (7M, 4F, mean age 27y) were measured. Photographs were taken of the eye in both upgaze and downgaze and 3–dimensional areas and scleral/corneal ratios (S/C) calculated using the Ocularea® program. Scleral curvatures were determined from slit–beam photographs just outside the limbus, using the Marcher Scheimpflug camera. A single–subject evaporation reading was also made where the eye looked straight ahead or to the side, so that S/C changed twofold but the total exposed area remained the same. Results: Total evaporative rates were calculated after correcting for the contribution from surrounding skin. Mean values for S/C were 1.4 (up) and 0.7 (down) and the range of total areas was 1.6–2.7 cm2 (up) and 0.9–1.9 cm2 (down). Evaporation rates varied between subjects, reflecting individual differences in normal eye opening, but the rate in upgaze was significantly greater than that in downgaze (mean ± SD, 4.0 ± 1.9 vs. 2.7 ± 1.4 x 10–7 g.cm–2. s–1 respectively, p ? 0.001). Individual rates for cornea and sclera were found by solving simultaneous equations, but in some cases this gave an apparent negative corneal rate. Nevertheless, calculated mean ± SD were 5.9 ± 2.0 x 10–7 (scleral) and 1.4 ± 2.8 x 10–7 (corneal). In the single–subject case, both exposed areas were 1.7 cm2 but S/C for axial and side view was 1.6 and 0.8 respectively; calculated evaporation rates were 2.3 x 10–7 (scleral) and 0.5 x 10–7 (corneal). Conclusions: The rate of evaporation appears to be greater from the sclera than from the cornea. This may in upgaze be due to extension and thinning of the tear film lipid layer, giving less control of evaporation, but this seems less likely in side–gaze. The nature or integrity of the tear film over the sclera are still undetermined.
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