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M. M. Hom, J. R. Paugh; Pre-Lens Tear Thinning Time (PLTTT) and Relative Humidity. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5381.
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Relative humidity has definite effects on the tear film. The measurement of pre-lens tear thinning time (PLTTT) with specular reflection is a clinical method of quantifying tear film stability over the anterior contact lens surface. The relationship between relative humidity and PLTTT in a clinical setting was studied.
35 subjects were measured for PLTTT in a clinical setting. Specular reflection was performed on the pre-lens tear layer and the timed appearance of a colored interference fringe within the reflection under high magnification indicated PLTTT. 3 readings were taken and averaged for each eye over 5 visits (baseline, one, two, three and four-weeks). Minimum wear time was 1 hour. The patients were wearing methafilcon A, lotrafilcon B and galyfilcon A. Relative humidity was measured with a Digital hygrometer thermometer accurate to 0.9%. The probe was held on the temporal side of the patient near the slit lamp.
For relative humidity of 30-40%, the mean PLTTT was 8.08 seconds SD 3.3. For relative humidity of 40-50%, the mean PLTTT was 6.92 seconds SD 4.0. For relative humidity of 50-60%, the mean PLTTT was 5.95 seconds SD 2.8. For relative humidity of greater than 70%, the mean PLTTT was 4.85 seconds SD 3.8. There was statistical signficance between 30-40% and 40-50% relative humidity (p =0.03) and between 40-50% and 50-60% relative humidity (p =0.04). There was no significant difference between 50-60% and 70%+ relative humidity. (p =0.16). This was probably due to small sample size of the 70%+ group (n=4).
The mean PLTTT appears to decrease as relative humidity increases divided over the three intervals (30-40%, 40-50%, 50-60%). This finding is different than previous studies where relative humidity has been shown to increase tear film stability.
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