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C. G. Begley, E. Salvo, Z. Wu, N. Himebaugh; Sensory Response to Tear Instability. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4243.
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© ARVO (1962-2015); The Authors (2016-present)
Previous investigations in our laboratory have used sensory descriptors of tear breakup as evidence to support the hypothesis that local fluctuations in tear hyperosmolarity occur during tear instability. The purpose of this study was to link sensory descriptors to the spatial progression of tear breakup and thinning in subjects with a range of dry eye (DE) symptoms to elucidate events during periods of tear instability.
Nine DE and 11 control (C) subjects filled out the Dry Eye Questionnaire (DEQ), 2µl of 2% fluorescein were instilled, and subjects kept one eye open as long as possible (maximum blink interval or MBI), while turning a knob indicating the level of discomfort (0-10 scale). Subjects rated irritation, stinging, burning, pricking and cooling on visual analogue scales (VAS) and noted the order the sensations occurred. Breakup and thinning of the fluorescent tear film were quantified by changes in pixel intensity over time (MATLAB program).
MBIs averaged 40.5sec (range 7.1 to 173.3sec). Subjects with long MBIs (>40sec) showed gradual thinning of the tear film and ranked VAS cooling significantly higher (4.6±2.5) and stinging lower (3.3±2.1) than subjects with short MBIs (<20sec), who showed rapid and distinct tear breakup and ranked cooling lower (1.7±1.0) and stinging higher (7.3±1.2) (t-test, p<0.05). The MBI was negatively correlated with DEQ score, stinging, burning, and the slope of tear breakup and discomfort over time (r= -0.47, -0.75, -0.53 -0.53, -0.75, respectively) and positively correlated with cooling (r= 0.50) (Pearson’s, p<0.05). All trials showed a decrease in tear film fluorescence over time.
The decrease in tear film fluorescence over time in the interblink interval is best explained by the phenomenon of fluorescent dye quenching as the tear film thins. Sensory descriptors strongly suggest that slow thinning of a stable tear film is associated with cooling (probably due to evaporation), whereas rapid tear breakup is sensed as intense stinging/burning and occurs in more symptomatic DE subjects. These results support the hypothesis that rapid increases in hypersomolarity occur in the unstable DE tear film.
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