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Adam Winkeler, Carolyn Begley, Richard Braun, Robert Welch; Do Tear Film Thinning Rates Vary Locally?. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4355.
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© ARVO (1962-2015); The Authors (2016-present)
The purpose of this study was to determine whether tear thinning rates differ within and outside areas of tear film break-up (TBU) using fluorescein (FL) imaging corroborated by a second imaging method, retroillumination (RI).
Mydriacyl (1.0%) and proparicaine (0.5%) were instilled into the eyes of 6 subjects, followed by 2μl of 2% FL. Subjects kept one eye open as long as possible while a modified slit-lamp biomicroscope simultaneously imaged the tear film by FL and RI. Two areas of the tear film, superior and inferior to the pupil, were systematically selected for analysis. TFT rates inside and outside of areas of TBU were calculated over time at each location using the slope of a linear least squares fit of the square root of FL intensity to account for FL quenching (Nichols et al, 20120). Surface profiles obtained from FL intensity were correlated with integrated RI intensity for corroboration of changes in slope. TFT estimated in µm/min by assuming a 3µm starting tear film thickness.
FL intensity decreased in all subjects over time, reaching a minimum within areas of TBU, suggesting full thickness breaks. The TFT rates for 2 subjects with very stable tears and minor to no TBU were 0.0063±0.0018 units/sec (Mean ± SD). The other 4 subjects developed variable and sometimes extensive TBU with an overall average TFT rate of 0.0775±0.0055 units/sec, almost 10x faster than subjects with stable tears. A subject with immediate TBU after the blink had an overall average TFT rate of 0.1477±0.0424 units/sec. The average TFT inside areas of TBU was 0.1038±0.0444 units/sec compared to 0.0576±0.0565 units/sec just outside and adjacent to TBU. This corresponded to estimated thinning rates of 2.913±1.327 µm/min inside TBU areas and 1.150±1.481 µm/min in adjacent areas. The local tear film surface profiles calculated from FL intensity in areas of TBU showed increasing correlation with changes in integrated RI image intensity as TBU developed.
TFT varied markedly within and outside of areas of TBU, whereas TFT showed slow even declines in a stable tear film. Our results show that spatially local differences in TFT within areas of TBU persist long after visible upward movement of the tear film ceased. Our results suggest that, if evaporation is the main mechanism accounting for TFT, it may vary locally across the tear film during TBU.
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