Purpose: It is hypothesized that evaporation is the major mechanism of tear film thinning between blinks. This can be demonstrated through the ‘self-quenching’ property of fluorescein. At low concentrations, fluorescent efficiency is independent of concentration, while at high concentrations, fluorescent efficiency falls rapidly. If tear film break up is due to evaporation, then at high concentrations, the measured fluorescein intensity will decay as the fluorescein becomes more concentrated within the tear film. The purpose of this study is to determine if tear film thinning corresponds with changes in the fluorescent intensity of fluorescein in the tears.
Methods: Thirty subjects were recruited for this study (34.3 ± 13.1 years of age, 57% female). At baseline, 1µL of 2% fluorescein was placed in the right eye of each subject. Two concurrent 20 second open-eye spectral interferometer recordings, capable of measuring tear film thickness and fluorescein intensity, were obtained. Five minutes after the initial 2% drop instillation, 1 µL of 10% fluorescein was placed in the right eye of each subject. Two concurrent interferometry measurements were obtained. The fluorescein decay rate was compared to the tear film decay rate using nonparametric statistical analyses.
Results: The mean fluorescent decay rate for the first trial of 10% fluorescein was 3.09% per second, while the second trial of 2% fluorescein decay rate was 0.50% per second. The difference between these two decay rates was significant (p < 0.0005, Wilcoxon Sign Rank). The difference in mean thinning rates for the first trial of the 10% fluorescein (1.83% per second) and second trial of the 2% fluorescein (1.58% per second) was not significant (p = 0.76, Wilcoxon Sign Rank).
Conclusion: The significant difference in fluorescein decay observed between the 2% and 10% fluorescein trials provides further evidence that evaporation is the major mechanism contributing to tear film thinning between blinks.