Effects of preservative-free artificial tear solutions on corneal epithelial structure and function
J. L. Ubels, M. D. McCartney, W. K. Lantz, J. Beaird, A. Dayalan and H. F. Edelhauser
Department of Ophthalmology, University of Pittsburgh, Pa.
OBJECTIVES: 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. METHODS: 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. RESULTS: 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.
CONCLUSIONS: 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.
