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Wing Li, Andrew D Graham, Meng C Lin; Ocular surface cooling corresponds to tear film thinning and breakup.. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2000. doi: https://doi.org/.
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To investigate the relationship between reduction in ocular surface temperature (OST) and tear film thinning and breakup.
Subjects were acclimated to the exam room for 10 minutes before measurements. Simultaneous imaging of fluorescein tear film break-up (FTBU) and OST was performed on the right eyes of 25 subjects. Subjects were asked to open their eyes and refrain from blinking for as long as they could during testing. OST was measured using an infrared thermographer (FLIR A655sc) and rates of cooling were analyzed using commercially available software. A novel method was developed to quantify the rate of FTBU formation using image-processing software (Vision Assistant 2012). The change in % dark areas in the fluorescein images over time was modeled as a function of the OST at the same time points.
During testing, 84% of the subjects were observed to have FTBU. In this group, the pattern of FTBU appeared to match the area of OST decrease in 86% of subjects (Figure 1), with OST decrease preceding FTBU formation by an average of 1.6 seconds. There was a statistically significant relationship between the increasing percent area of dark pixels and OST decrease over time in all but 1 subject (p<0.001, R2 = 0.76 - 0.99). In subjects whose time to first blink was less than 15 sec, this relationship exhibited a characteristic pattern suggesting non-linearity. Subjects who reported more severe end-of-day dryness had a greater increase in the percent area of dark pixels with decreasing OST.
The relationship between decreasing OST and tear film breakup has been long theorized but this is the first study to show a direct relationship. Based on the current understanding about the physical properties of the tear film, increased tear evaporation is the only factor that could explain this relationship. This is in agreement with theoretical models of the formation of tear film breakup spots in fluorescein images. Clinical assessment of evaporative dry eye is difficult due to the inability to measure tear evaporation directly, but our study suggests that measuring the rate of OST decrease could be a valid indirect assessment.
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