Effects of preservative-free artificial tear solutions on corneal epithelial structure and function

To test the efficacy of a bicarbonate-containing artificial physiologic tear solution (solution PT) in providing an environment in which the damaged corneal epithelium can recover its normal barrier function and to compare this solution with other available artificial tears. Also, to investigate the effects on the corneal mucin layer and epithelial ultrastructure.

The corneal epithelial permeability of anesthetized rabbits was increased by exposure to 0.1% benzalkonium chloride. The corneas were then exposed to solution PT, with or without bicarbonate, or one of four commercially available artificial tear solutions for 1.5 hours, followed by a 5-minute exposure to 5(6)-carboxyfluorescein. Frozen sections of the corneas were examined by fluorescence microscopy. The fluorescence intensity (FI) of the epithelium was measured by image analysis. Undamaged corneas exposed to tear solutions were examined by transmission electron microscopy after fixation of the mucin layer with cetylpyridinium chloride.

The FI of corneas damaged by benzalkonium chloride was increased threefold above those of undamaged controls. Damaged corneas treated with either of two commercial isotonic tear solutions partially recovered their barrier function, but the FI did not reach control levels. Corneas treated with hypotonic solutions containing ethylenediaminetetraacetic acid (EDTA) did not recover. In contrast, the FI of corneas treated with solution PT returned to control levels. This effect was lost in the absence of bicarbonate. Solution PT and the two isotonic solutions maintained normal corneal ultrastructure and mucin layer. Lack of bicarbonate in solution PT resulted in focal damage to superficial epithelial cells, whereas the EDTA-containing solutions destroyed the first two cell layers and reduced the mucin thickness.

Bicarbonate-containing solution PT is superior to the other tear solutions tested in promoting recovery of the damaged corneal epithelial barrier and maintaining normal ultrastructure. The presence of bicarbonate appears to be essential to this process.