Giant vacuole survival time and implications for aqueous humor outflow

Purpose: A quantitative study was conducted to estimate the survival time o f giant vacuoles of the endothelial wall of Schlemm's canal once perfusion pressure has dropped to zero. Vacuoles are known to be sensitive to intraoc ular pressure and could serve as real-time markers of aqueous pressure in t he juxtacanalicular tissue if their survival time is short once pressure is lowered. Methods: Six pairs of healthy human eyes were perfused with phosphate-buffe red saline. One eye was then fixed by perfusion, whereas the fellow eye was dissected into wedges and fixed by immersion at intervals ranging from 3 t o 120 minutes after perfusion had been stopped. Light microscopy was used t o determine the number of giant vacuoles on the inner and outer walls of Sc hlemm's canal. Results: A 70% decrease in giant vacuole counts was found 3 minutes after d iscontinuation of saline perfusion when compared with per-fusion-fixed fell ow eyes (5.9 +/- 4.0 vacuoles per histologic section versus 19.1 +/- 9.0; P = 0.02). Vacuole counts continued to decrease with time so that by 120 min utes at zero pressure, vacuole counts were 12% of those in the perfusion-fi xed eyes (88% decrease; P = 0.003). Giant vacuoles occurred on the inner an d outer walls of the canal, but were more numerous on the inner wall, Conclusions: Most giant vacuoles have a short survival time, less than 3 mi nutes, once perfusion pressure decreases to zero. This suggests that they c an respond rapidly to intraocular pressure changes and thus can serve as ma rkers of aqueous pressure differences across the inner and outer walls of S chlemm's canal.