Quantifying Minor Irritancy to the Human Corneal Surface

Purpose. To assess the response of human corneas to minor irritancy using cell counts from contact lens cytology. Methods. Three potential irritants were investigated: 0.01% benzalkonium chloride (BAC), surface exposure produced by voluntarily holding the eye open (EXP), and irrigation of the corneal surface (IRR). One of the two eyes was randomly selected to be tested and the other eye served as a control. Following the treatment corneal surface cells were collected from soft contact lenses with four insertions and removals. Cells were counted using fluorescent staining of acridine orange and Hoechst. Ten normal human subjects took part in each experiment. Results. Nucleated cell counts increased significantly (p<0.05) with both BAC and EXP, but not with IRR (p>0.05). Cell structures without nuclei were always present (cell ghosts), and they were counted separately. These structures showed green fluorescent staining from acridine orange, but no nuclear staining with Hoechst. The number of cell ghosts increased with EXP (p<0.05), but not with BAC or IRR (p>0.05). No correlation was found between the number of cell ghosts and nucleated cells in the same eye. Conclusions. Both BAC and exposure increased nucleated cell counts. The finding that BAC affects nucleated cells and cell ghosts differently suggests separate mechanisms for the two pathways of cell shedding. The noninvasive technique of cell collection and cell counting may be useful for assessing minor irritancy to the corneal surface of human eyes.