WHY DOES DRY EYE AFFECT INFERIOR CORNEA MORE THAN SUPERIOR CORNEA?

Title WHY DOES DRY EYE AFFECT INFERIOR CORNEA MORE THAN SUPERIOR CORNEA?
Author, Co-Author P. Ewen King-Smith, Kelly Nichols, J. Erin Wood
Topic
Year
2002
Day
Sunday
Program Number
Room
Room 1 SP
Affiliation
Abstract 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) have 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 were measured for two locations on the cornea separated vertically by 0.25 mm. Twenty such paired spectra were obtained from each of 12 subjects (11 female, mean age 46, of whom 5, all female, had dry eye). Each measurement spot was nominally 28 x 17 um, 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: The tear film thickness at the inferior location was significantly less than at the superior location in 5 of 12 subjects (3 of 7 normals, 2 of 5 dry eye, all P < 0.02). No subject showed a significantly greater thickness of the inferior tear film. On average, inferior thickness was 0.072 um ± 0.027 SE thinner than superior thickness (P < 0.05). Remarkably, average thickness in dry eye, 3.12 um ± 0.84 SD was not significantly different from normals, 3.38 um ± 0.92 SD.

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. So far, we have not found a significant difference in tear thickness between dry eye and normals.

ADDITIONAL COMMENTS: Supported by Ohio Lions ERF.
Affiliation of Co-Authors The Ohio State University, College of Optometry, The Ohio State University, College of Optometry
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