Pathology of ocular irritation with bleaching agents in the rabbit low-volume eye test

Despite differences in the processes leading to tissue damage, the ocular irritation response to various surfactants, two concentrations of an acid and an alkali, and an acetone, alcohol, aromatic amine, and aldehyde has been shown to depend on the extent of initial injury. The purpose of this study was to assess the extent to which this fundamental relationship exists for bleaching agents in the rabbit low-volume eye test. Ten μl of sodium perborate monohydrate (NaBO₃), sodium hypochlorite (NaOCl), 10% hydrogen peroxide (H₂O₂), and 15% H₂O₂ was applied directly to the cornea of the right eye of each rabbit. Macroscopic assessments for irritation were made 3 hours after dosing and periodically until 35 days. Light microscopic examinations were conducted on tissues obtained at 3 hr and on 1, 3, and 35 days. In vivo confocal microscopy (CM) and measurements of dead corneal epithelial cells and keratocytes at 3 hours and 1 day were used to characterize quantitatively initial corneal injury, while in vivo CM performed at 3 hours and 1, 3, 7, 14, and 35 days was used to characterize quantitatively the corneal changes over time. The changes with NaBO₃ and NaOCl were consistent with mild irritancy. For both, corneal injury was limited to the epithelium and superficial stroma. The changes with 10% H₂O₂ and 15% H₂O₂ were consistent with severe irritation. Both concentrations affected the epithelium and deep stroma, with 15% H₂O₂ also at times affecting the endothelium. However, unlike other irritants previously studied, with 10% H₂O₂ and 15% H₂O₂ there was an incongruity between the extent of epithelial and stromal injury, with stromal injury being more extensive than epithelial injury. A similar, although less dramatic, effect was observed with NaBO₃. Additionally, there was still significant keratocyte loss at 35 days with 10% H₂O₂ and 15% H₂O₂ even though the eyes at times were considered to be macroscopically normal. These observations highlight the need to include both epithelial and stromal components in an ex vivo or in vitro alternative assay. In conclusion, these results continue to support and extend our hypothesis that ocular irritation is principally defined by the extent of initial injury despite clear differences in the means by which irritants cause tissue damage. Importantly, we have identified unique differences in the ocular injury and responses occurring with bleaching agents that are important to consider in the development and validation of alternative ocular irritation tests to characterize a broad range of materials differing in type and irritancy.