Purpose: Tear film thinning primarily occurs by two mechanisms: evaporation and tangential redistribution. The Tear Film Imager (TFI; AdOM, Israel) was used in healthy subjects to simultaneously measure multiple tear film parameters through normal blink cycles to bring a new perspective on the relative contributions of these mechanisms to tear film thinning. Methods: Tear film lipid layer thickness (LLT), muco-aqueous layer thickness (MALT), and inter-blink muco-aqueous layer thinning rate (MALTR) were measured with the TFI in 20 healthy subjects. Tear film evaporation rates were obtained from a PubMed literature review. Results: Tear film geometric mean (geometric standard deviation) measurements in healthy subjects were 52.7 (1.55) nm for LLT (n = 20), 3282 (1.3) nm for MALT (n = 20), and -63 (2.0) nm/s for MALTR (n = 17). Literature-based mean tear film evaporation rates in healthy subjects, measured using either closed chamber evaporimeters or ventilated chambers allowing air flow across the ocular surface, ranged from 4.07 to 39.3 nm/s (weighted mean = 11.3 nm/s). Conclusions: In healthy subjects, the inter-blink MALTR measured by the TFI is too rapid to be accounted for by reported rates of tear film evaporation. The MALTR in the early period after blinking appears to result mainly from tangential flow and redistribution of the muco-aqueous layer, predominantly driven by the Marangoni effect, with gravity and other forces playing a comparatively minor role. Increased understanding of the mechanisms of tear film thinning and break-up time in healthy subjects may potentially help guide the development of novel therapeutic options for dry eye.
Sun et al. (Mon,) studied this question.