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Reduction of the Available Area for Aqueous Humor Outflow and Increase in Meshwork Herniations into Collector Channels Following Acute IOP Elevation in Bovine Eyes

¹From the New England College of Optometry, Boston, Massachusetts; ²Department of Ophthalmology, Boston University School of Medicine, Boston, Massachusetts; ³Department of Ophthalmology, Huashan Hospital of Fudan University, Shanghai, People’s Republic of China; ⁴Department of Biomedical Engineering, Tulane University, New Orleans, Louisiana; and ⁵University of Waterloo School of Optometry, Waterloo, Ontario, Canada.

PURPOSE. To understand how hydrodynamic and morphologic changes in the aqueous humor outflow pathway contribute to decreased aqueous humor outflow facility after acute elevation of intraocular pressure (IOP) in bovine eyes.

METHODS. Enucleated bovine eyes were perfused at 1 of 4 different pressures (7, 15, 30, 45 mm Hg) while outflow facility was continuously recorded. Dulbecco PBS + 5.5 mM glucose containing fluorescent microspheres (0.5 µm, 0.002% vol/vol) was perfused to outline aqueous outflow patterns, followed by perfusion-fixation. Confocal images were taken along the inner wall (IW) of the aqueous plexus (AP) in radial and frontal sections. Percentage effective filtration length (PEFL; IW length exhibiting tracer labeling/total length of IW) was measured. Herniations of IW into collector channel (CC) ostia were examined and graded for each eye by light microscopy.

RESULTS. Increasing IOP from 7 to 45 mm Hg coincided with a twofold decrease in outflow facility (P < 0.0001), a 33% to 57% decrease in PEFL with tracer confined more to the vicinity of CC ostia, progressive collapse of the AP, and increasing percentage of CC ostia exhibiting herniations (from 15.6% ± 6.5% at 7 mm Hg to 95% ± 2.3% at 30 mm Hg [P < 10^–4], reaching 100% at 45 mm Hg).

CONCLUSIONS. Decreasing outflow facility during acute IOP elevation coincides with a reduction in available area for aqueous humor outflow and the confinement of outflow to the vicinity of CC ostia. These hydrodynamic changes are likely driven by morphologic changes associated with AP collapse and herniation of IW of AP into CC ostia.