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P. King-Smith, K.K. Nichols, E.J. Wood; Is Inferior Tear Film Thinner than Superior Tear film? . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2476.
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Purpose: In dry eye patients, inferior cornea typically shows more staining with rose bengal than superior cornea. Also, dry spot formation is more common in inferior than in superior cornea. Benedetto et al. (1984, Arch Ophthalmol 102, 410) provided fluorophotometric evidence that inferior tear film is thinner than superior, and this may account for both the preceding observations. We have attempted to confirm this thickness difference using interferometic measurement. Methods: Reflection spectra (562 to 1030 nm) were measured for two locations on the cornea separated vertically by 0.25 mm. Twenty such paired spectra were obtained from the right eyes of each of 30 females. Thirteen of these were identified as having dry eye disease (mean age 48 yrs), either from the College of Optometry records, or were recruited from patients previously enrolled in dry eye clinical trials. Seventeen normals (mean age 46 yrs) were recruited from faculty, staff, and students without report of dry eye, contact lens wear, or symptoms of eye irritation. Each measurement spot was nominally 28 x 17 µm, and an exposure of 0.5 s was taken about 2 s after a blink. Interference causes oscillations in the reflection spectrum which can be used to determine tear film thickness (King-Smith et al., 2000, IOVS 41, 3348). Results: On average, inferior thickness was (mean ± SE) 0.0417 ± 0.0131 µm thinner than superior thickness (P < 0.005). Remarkably, average thickness in dry eye, 2.97 ± 0.16 µm was not significantly different from normals, 3.05 ± 0.19 µm. Conclusions: Our results support the proposal that inferior tear thickness is less than superior. As proposed by Benedetto et al., this thickness difference may be due to the upward drift of the tear film after a blink, which carries tear fluid from inferior to superior cornea. The proposal that inferior tear film is thinner than superior could explain why inferior cornea has a greater prevalence of dry spot formation, and also shows more frequent staining in dry eye disorders. At this stage of these ongoing studies, we have not found a significant difference in tear thickness between dry eye and normals.
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