Early changes in optic disc compliance and surface position in experimental glaucoma.

PURPOSE: To detect changes in the compliance and baseline position (position at the baseline time point of a compliance test) of the monkey optic disc after the onset of chronic experimental glaucoma.
METHODS: Sixty-six compliance tests were performed on 26 eyes of 13 monkeys. Longitudinal Study. In seven normal monkeys, compliance tests were performed three times in one eye (study eye) and once in the contralateral eye. In the study eye of five of these monkeys, chronic experimental glaucoma was then induced and compliance tests were performed at some or all of the following postglaucoma testing intervals: 1 to 2 weeks, 3 to 4 weeks, 5 to 8 weeks, 9 to 12 weeks, 13 to 18 weeks, and more than 18 weeks after the onset of elevated intraocular pressure (IOP). In the study eye of the remaining two monkeys, the optic nerve was transected, and compliance was tested at 5, 9, and 13 weeks after transection. An analysis of variance (ANOVA) was performed to detect an increase (hypercompliance) or decrease (rigidity) in the compliance of the glaucomatous eyes at each testing interval. A second ANOVA was performed to detect the onset of chronic posterior deformation of the baseline position of each disc. Cross-Sectional Study. In six additional monkeys with pre-existing experimental glaucoma, the glaucomatous study eye was compliance tested at one of the postglaucoma testing intervals used in the longitudinal study. The contralateral normal eye was compliance tested once. These data were then added to the data from the five longitudinally studied monkeys at the appropriate preglaucoma and postglaucoma testing intervals. A third ANOVA was done to compare the compliance of the expanded group of glaucomatous eyes at each postintervention testing interval with the compliance of the 13 normal contralateral eyes.
RESULTS: Compliance. In the longitudinally (Pr > F = 0.0005) and cross-sectionally (Pr > F = 0.0001) studied glaucomatous eyes, optic disc compliance increased significantly by 1 to 2 weeks and then returned to a level statistically indistinguishable from normal within 13 to 18 weeks after the onset of glaucoma. In the transection eyes, the optic discs were significantly less compliant (more rigid) at 5 and 9 weeks after transection compared with the discs in either the normal or the glaucomatous eyes (Pr > F < 0.05). Baseline Optic Disc Position. Chronic posterior deformation of the disc was detected in one of three eyes tested 1 to 2 weeks and three of four eyes tested 3 to 4 weeks after the onset of glaucoma (Pr > F < 0.05). Chronic posterior deformation was not detected in the discs of either of the transection eyes at any of the post-transection testing intervals.
CONCLUSION: Changes in optic disc compliance and surface position were detected by digitized image analysis within 2 to 4 weeks of the onset of experimental glaucoma in the monkey eye. These findings are unlikely to be due to axon loss alone, because they did not occur in optic nerve transection eyes (which constitute a model of axon loss in which intraocular pressures remain normal). The results suggest that IOP-related damage to the load-bearing connective tissues of the optic nerve head may occur early in the course of experimental glaucoma.