Black-Spot Formation and Fluorescence Tear Break-up Time

Abstract

 

Purpose

 

Fluorescence tear break-up time (FBUT) is the most common diagnostic test for tear-film stability and ocular-surface health. Appearance of black spots and/or streaks is normally interpreted as tear-film rupture (i.e., breakup of the tear film down to the glycocalix). We investigate whether black spots are either deep depressions in the tear film or actual ruptures.

 

Methods

 

Fluorescence intensity is determined by image analysis of aqueous sodium fluorescein (NaF) solutions (Tan et al, AAO annual meeting, 2011) in a micron-deep PDMS-microfabricated channels. Based on the Beer-Lambert law and self-quenching theory (Arbeloa, J. Chem. Soc., Faraday Trans., 1981), we relate measured emitted fluorescence intensity to film thickness and NaF concentration. The theoretical correlation is combined with an evaporation-driven tear-film-instability model (Peng, ACS Colloid and Surface Science Symposium, 2012) to predict the dynamic color change of tear film during an FBUT test.

 

Results

 

We generate a color map for various film thickness and NaF concentrations that ascertains the thickness at which a NaF film turns black. Fig. 1 demonstrates that a tear film containing 5 x 10-3 g/mL of NaF that turns black at a thickness of 5 μm. Accordingly, our color map permits conversion of black-spot images obtained from a FBUT test into local film tear-film thickness. Instability theory predicts appearance of a black spot 25 s after the instillation of aqueous NaF where the tear film thickness reduces from 3.5 to 0.89 μm. Complete tear rupture does not occur until after 35 s. (See Fig. 2.)

 

Conclusions

 

Black spots in FBUT tests indicate local regions of the tear film with small but finite thicknesses, and not necessarily complete tear-film rupture. We provide a color map that, for the first time, allows measured fluorescence intensity in FBUT examinations to be converted into local tear-film thickness. This result is also useful to clarify the current discrepancy between FBUT and non-invasive tear break-up time (NIBUT) measurements and provides a new standardized procedure for FBUT examination with higher reproducibility.

 

 

Fig. 1. Comparison between fluorescence intensity and NaF concentration from a 5-ìm thick film (filled diamonds) and theory (line). Colors corresponding to intensity values are also shown.

 

 

Fig 2. Predicted dynamic of color change for black-spot formation.

 

Keywords: 486 cornea: tears/tear film/dry eye