Microscopic changes with acetic acid and sodium hydroxide in the rabbit low-volume eye test

Differences in ocular irritancy have been hypothesized to reflect differences in the extent of initial injury. Although differences in the processes leading to tissue damage may exist, extent of injury is believed to be the principal factor determining final outcome of ocular irritation. Previous studies characterizing the pathology of surfactant-induced ocular irritation support this premise. The purpose of this study was to begin to determine the applicability of this premise in terms of nonsurfactants; we planned to accomplish this by assessing the ocular irritancy of different concentrations of an acid and an alkali. Ten microliters of 3 or 10% acetic acid (C2H4O2) or 2 or 8% sodium hydroxide (NaOH) were directly applied to the cornea of the right eye of each test rabbit. Untreated left eyes served as the controls. Eyes and eyelids were macroscopically examined for signs of irritation beginning 3 hours after dosing and periodically until recovery or day 35. Eyes and eyelids from animals in each group were collected for microscopic examination after 3 hours and on days 1, 3, and 35. The macroscopic and microscopic changes were consistent with slight (3% C2H4O2), mild (2% NaOH, 10% C2H4O2), and severe (8% NaOH) irritancy. The spectra of changes were similar to those previously reported for surfactants of differing types and irritancies. As with surfactants, as the extent of initial injury increased, the intensity and duration of the subsequent responses increased. These results indicate that our hypothesis also applies to nonsurfactants. The results also support our belief that the initial extent of injury associated with ocular irritation may be used to predict the subsequent responses and final outcome. Finally, our results further indicate that such an approach may be applicable to the development of alternative assays that are based on either injury to ex vivo eyes or injury to an in vitro corneal equivalent system.