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S.A. Kasper, P.A. Scott, D.R. Overby, T.F. Freddo, H. Gong; Changes in IOP Alter the Pattern of Aqueous Humor Outflow in the Bovine Eye . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1359.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:To understand how intraocular pressure affects the pattern of aqueous humor outflow using fluorescent microspheres in bovine eyes. Methods: Freshly enucleated bovine eyes were perfused at three different pressures (7mmHg, n=8; 15mmHg, n=7; 30mmHg, n=7). A baseline outflow facility (C) was recorded for each eye, perfusing with Dulbecco's PBS containing 5.5 mM glucose (D–PBS) for 30 min. Fluorescent microspheres (0.5µm diameter, 0.002% v/v) in D–PBS were then exchanged (7 ml) into the anterior chamber and perfused (0.5 ml), while C was continually recorded. Each eye was then exchanged (7 ml) and perfusion–fixed with 0.5 ml of Karnovsky's fixative at the same pressure. Thin sections were cut radial and tangetial to the limbus in all four quadrants and counter–stained for laser confocal and light microscopy. The pattern of fluorescent tracer distribution was visualized in the trabecular meshwork, aqueous plexus and collector channel ostia. Results: C decreased with increasing IOP, consistent with previous reports. The average baseline C at 7mmHg was 2.3±0.3 µL/min/mmHg (mean±SEM), which was significantly larger than the C measured at either 15mmHg (1.2±0.1 µL/min/mmHg, p<0.007) or 30mmHg (1.3±0.2 µL/min/mmHg, p=0.024). At 7 mmHg a uniform distribution of microspheres was found along the inner wall of the aqueous plexus. In contrast, at 15mmHg and 30mmHg, there was a segmental pattern of microsphere distribution with a greater concentration of microspheres in the trabecular meshwork near collector channel ostia. This concentrated distribution was visually more dramatic at 30 mmHg than at 15 mmHg. In these areas it was clear that there was focal herniation of the inner wall and JCT into the collector channel ostia not seen in eyes perfused at 7 mmHg. Conclusions: The pattern of aqueous humor outflow becomes highly segmental at elevated IOP, tending to concentrate in the vicinity of collector channel ostia. More importantly, this change from uniform to segmental flow corresponds to a decrease in C that is likely related to the progressive herniation of the inner wall and JCT region into the collector channel ostia. These data suggest that elevation of IOP concentrates flow to segments of the inner wall closest to the collector channel ostia. This correlates with herniation of inner wall and JCT into the ostia, resulting in an attendant reduction in C that could perpetuate the elevation in IOP. Whether the observed shift from a uniform to a segmental pattern of flow is a distinguishing feature in glaucomatous eyes merits further study.
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