Area and depth of surfactant-induced corneal injury predicts extent of subsequent ocular responses

PURPOSE. To correlate area and depth of initial corneal injury induced by s urfactants of differing type and irritant properties with corneal responses and outcome in the same animals over time by using in vivo confocal micros copy (CIM). METHODS. Six groups Of Six adult rabbits were treated with anionic, cationi c, and nonionic surfactants that caused different levels of ocular irritati on. Test materials included slight irritants: 5% sodium lauryl sulfate (SLS ), polyoxyethylene glycol monoalkyl ether (POE), and 5% 3-isotridecploxypro pyl-bis(polyoxyethylene) ammonium chloride (ITDOP); mild irritants: 5% 3-de cyloxypropyl-bis(polyoxyethylene) amine (DOP) and sodium linear alkylbenzen e sulfonate (LAS); and a moderate irritant: a proprietary detergent (DTRGT) . Ten microliters surfactant were directly applied to the cornea of one eye of each rabbit. Ten untreated rabbits served as control subjects. Area and depth of initial injury was determined by using in vivo CM to measure epit helial thickness, epithelial cell size, corneal thickness, and depth of str omal injury in four corneal regions at 3 hours and at day 1. Area and depth of corneal responses to injury were evaluated at various times from days 3 through 35 by macroscopic grading and quantitative confocal microscopy thr ough-focusing (CMTF). RESULTS. In vivo CM revealed corneal injury with slight irritants to be res tricted to the epithelium, whereas the mild and moderate irritants caused c omplete epithelial cell loss with increasing anterior stromal damage: DOP < WS < DTRGT. With the slight ocular irritants there was little or no change in corneal thickness or the CMTF intensity profiles. Three hours after tre atment, mild and moderate ocular irritants caused a significant increase in corneal thickness, which peaked at day 1 with DOP (483.3 +/- 80.1 mu m) an d LAS (572.3 +/- 60.0 mu m) and day 3 with DTRGT (601.4 +/- 68.7 mu m); ret urning to normal (similar to control values) by day 7 with DOP and day 35 w ith LAS and DTRGT. The CMTF intensity profiles also showed significant elev ation over that in the anterior stroma, which peaked at day 1 with DOP (14, 608 +/- 4,306 U [U is defined as micrometers X pixel intensity]) and day 3 with LAS and DTRGT(18,471 +/- 6,581 U and 22,424 +/- 3,704 U, respectively) and returned toward normal by day 7 with DOP and day 14 with LAS and DTRGT . Elevated CMTF profiles principally reflected the presence of hyperreflect ive, punctate keratocytes and inflammatory cells at days 1 and 3 and the pr esence of activated keratocytes at day 7. There was a significant correlati on between the elevated CMTF intensity profile and the corresponding macros copic total score in each eye (r = 0.839; P < 0.001). More important, there was a significant correlation between area and depth of initial stromal in jury measured at day 1, regardless of ocular irritant and the stromal respo nse measured by the area under the CMTF intensity profile curve in each cor nea (r = 0.87; P < 0.0005). A significant correlation between the area and depth of injury and the area under the corneal thickness curve was also obs erved in each cornea (r = 0.75; P < 0.0005). CONCLUSIONS. In individual animals, the extent of initial stromal injury co rrelated with the magnitude of the corneal responses, measured by the chang e in corneal thickness and the CMTF depth intensity profile. These findings further support the hypothesis that area and depth of injury are the princ ipal factors determining the early responses and eventual repair processes after accidental eye irritation. They also support the proposed use of area and depth of acute injury as a mechanistic correlate to ocular irritation in the development and validation of potential in vitro ocular irritation t ests.