Giant vacuole survival time and implications for aqueous humor outflow

A quantitative study was conducted to estimate the survival time of giant vacuoles of the endothelial wall of Schlemm's canal once perfusion pressure has dropped to zero. Vacuoles are known to be sensitive to intraocular pressure and could serve as real-time markers of aqueous pressure in the juxtacanalicular tissue if their survival time is short once pressure is lowered.

Six pairs of healthy human eyes were perfused with phosphate-buffered 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 to 120 minutes after perfusion had been stopped. Light microscopy was used to determine the number of giant vacuoles on the inner and outer walls of Schlemm's canal.

A 70% decrease in giant vacuole counts was found 3 minutes after discontinuation of saline perfusion when compared with perfusion-fixed fellow 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 minutes at zero pressure, vacuole counts were 12% of those in the perfusion-fixed eyes (88% decrease; P = 0.003). Giant vacuoles occurred on the inner and outer walls of the canal, but were more numerous on the inner wall.

Most giant vacuoles have a short survival time, less than 3 minutes, once perfusion pressure decreases to zero. This suggests that they can respond rapidly to intraocular pressure changes and thus can serve as markers of aqueous pressure differences across the inner and outer walls of Schlemm's canal.