A new method for determining corneal epithelial barrier to fluorescein in humans

To derive the value of corneal epithelial barrier to fluorescein in humans from experiments in which the fluorophore in instilled in a single drop.

A commercial scanning fluorophotometer, the Fluorotron Master, was used to scan through an anterior segment. It could not resolve the tear film from the cornea, but in the early stages of measurement, the tear component predominated. After 20 minutes, the remaining fluorescein was washed out of conjunctival sac, and the amount that penetrated into the cornea was estimated. Because the resolution was not sufficient to estimate the concentrations, the total masses in the tear film and in the cornea derived from the area under the profile were used to calculate the epithelial permeability. A value for the tear film thickness had to be assumed.

The technique led to reproducible values for epithelial permeability-a range of 2:1 in a subject. High concentrations of fluorescein were required to achieve a sufficient penetration into the cornea, and this led to error in estimating the tear film concentration of the dye. A method is presented that corrects for this effect. For 17 normal subjects, the corrected permeability was 0.15 nm/second, and the tear turnover was 0.15/minute ( +/- 0.016, standard error.)

This technique is convenient and yields two useful physiological parameters from a single clinical procedure. The permeability values are considerably higher than those found by previous workers, and the source of the discrepancy is discussed.