COMPARISON OF TEAR FILM THICKNESS IN 3 SPECIES DETERMINED BY THE GLASS-FIBER METHOD AND CONFOCAL MICROSCOPY

Using optical methods to measure tear film thickness we previously found that values were substantially larger than earlier estimates by mechanical and chemical techniques. The predominant component was found to be mucus, not aqueous fluid. This article describes the results of using confocal microscopy to re-examine an earlier method of measuring thickness, the glass fibre technique. Confocal microscopy enabled visual identification of tear and epithelial surfaces and accurate measurement of the separation between them, giving the full film thickness. Consecutive measurements were made by each method in eyes of 22 animals of three species. Measurements using fibres in rabbits gave results of 7 mum, replicating earlier findings. However, the fibre technique gave the same result in all species. Values were not correlated with measurements by microscopy, which ranged from 10 to 14 mum. We propose that fibres were too flexible to be pressed through all mucus layers and contact epithelium, and as a result this method underestimated film thickness.