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BACKGROUND: An enlarged exposed ocular surface area is known to degrade tear film stability. Little is known of how tear film stability alters with vertical gaze and the change of palpebral fissure height. METHODS: Tear film break-up time (TBUT; fluorescent dye) and palpebral fissure height were measured in three vertical gaze positions (primary position and 20 degrees up and 20 degrees down) on 23 healthy volunteers. The effect of gaze direction on TBUT was analysed by ANOVA. RESULTS: Gaze had a significant influence on tear film stability F(2,44)=27.6, p<0.001. In up-gaze with an enlarged palpebral fissure height (11.1 +/- 1.4 mm), TBUT was degraded (17.3 +/- 12.7 seconds). In down-gaze and decreased palpebral fissure height (6.7 +/- 1.2 mm) TBUT was extended (43.5 +/- 35.4 seconds). CONCLUSION: Ocular tear film stability alters with vertical gaze. Our hypothesis is that the change of the palpebral fissure height and the exposed ocular surface area alters the thickness of the lipid layer and with it tear film evaporation.
Pansell et al. (2007) studied this question.