The distal portion of the conventional outflow pathway is responsible for nearly 50% of outflow resistance in low-pressure perfused eyes. As perfusion pressure increases, the SC and CCs still contribute 30% of total outflow resistance through the conventional outflow pathway.
12 Even with the reported importance of the distal outflow pathway, no studies have compared the anatomic changes in SC and CCs at various pressures between normal and POAG eyes. This region has undergone minimal examination due to technical limitations in imaging the conventional outflow pathway as a whole. Using 3D micro-CT, we have compared the anatomic changes in SC and CCs in normal and POAG eyes using low (10 mm Hg) and high (20 mm Hg) perfusion pressures. In both normal and POAG eyes, SC and CCs were anatomically responsive to increases in perfusion pressure. In response to high pressure in normal eyes, SC volume and CC orifice area decreased while CC orifice diameter remained relatively constant. High pressure resulted in normal eyes having a 26.6% increase in observable open CCs. In POAG eyes, SC volume, CC diameter, and CC orifice area were decreased when pressure was elevated from 10 to 20 mm Hg. In contrast to findings in normal eyes, the number of open CCs in POAG eyes did not change between 10 and 20 mm Hg. Partial and total occlusions were present in normal eyes, but a greater number of total CC occlusions were observed in POAG eyes at high pressure.
Previous studies have reported 24 to 31 CCs in immersed eyes and 25 to 31 in low-pressure perfused specimens.
18,25 Our study found similar results in normal eyes perfused at 10 mm Hg with an average of 28.3 CCs per eye. The number of CCs increased to 37.3 in the contralateral eyes perfused at 20 mm Hg. While it is possible that distension of the tissue due to high pressure may have enlarged channels, making them easier to identify, we do not believe this was the case since CC orifice diameter at 20 mm Hg was nearly identical to that measured at 10 mm Hg.
While distension may have a small role in providing a means to image additional CCs at higher pressure, an alternative hypothesis is that some CCs at 10 mm Hg are inactive, obscured by flaps or outer wall undulations. As pressure increases (20 mm Hg), these flaps or outer wall undulations retract, opening up additional CCs, enabling increased fluid flow. Collector channels with lip-like and sieve-like structures at CC orifices have been reported.
31,35 An anatomic study done on CC orifice structure identified an additional type of complex orifice flap with fan-like structures that attached to both the inner wall and the outer wall (Bentley M, et al.
IOVS 2012;53:ARVO E-Abstract 3234). This CC flap could be envisioned to move with the inner wall and outer wall, producing an open CC orifice under high pressure and, conversely, closing the CC orifice under low pressure. Change in outer wall undulations to a smooth profile at high pressure has also been reported.
36 Under this scenario, CCs may be part of a pressure-induced compensatory mechanism within the distal portion of the conventional outflow pathway. Although overall SC volume was reduced at high pressure, areas of open SC volume were generally associated with CCs. The reduced SC volume at high pressure is similar to observations in a study by Van Buskirk and Grant,
37 who found only a partial collapse of SC using perfusion at 20 mm Hg. This compensatory mechanism may serve to alleviate stress during acute rises in pressure but may become limited in chronic pressure elevation or increases in pressure past a certain threshold level. Experiments that can visualize CCs in real time are required to determine if some CCs are inactive due to flap closures but can respond to fluid flow under high pressure. Whether the change observed in CCs is a passive response to pressure or whether an active mechanosensitive response is required to activate the compensatory CCs will need to be determined.
Another observation from this study is that POAG eyes perfused at 10 mm Hg have SC dimensions similar to those of normal eyes perfused at 20 mm Hg. Normal eyes perfused at 20 mm Hg (6.7 μm
3) and POAG eyes perfused at 10 mm Hg (7.0 μm
3) had nearly identical SC volume and similar CC orifice area (8825.2 and 8049.2 μm
2). The percent of total occlusions was also similar between normal eyes perfused at 20 mm Hg (6.3%) and POAG eyes at 10 mm Hg (7.2%). This may indicate that occlusions observed in aging eyes are not reversible but are a permanent change. With aging, changes in the inner wall and outer wall during short bursts of high pressure, for example, during normal head movement and sneezing,
38 may occur with resistance due to apposition, fibrosis, and occlusion. The similarity of 10 mm Hg perfused pressure POAG eyes with high perfusion pressure normal eyes suggests that POAG eyes may have lost their ability to compensate for increased pressure through use of CCs from the reserve pool. Remodeling of the extracellular matrix in the inner and outer wall may reduce SC and CC elasticity, making this area less pliable and stiffer. Increased stiffness and extracellular matrix deposition have been reported in the trabecular meshwork in POAG eyes.
39,40 The loss of outer wall elasticity may explain the same number of nonoccluded CCs found in POAG eyes at 10 and 20 mm Hg, suggesting an inability of POAG eyes to compensate for pressure-induced changes. It is interesting to note that all three POAG patients were on various ocular hypotensive medications, yet none of the POAG eyes had SC and CC dimensions similar to those of normal eyes at low pressure.
Total occluded CCs were observed in normal and POAG eyes at both 10 and 20 mm Hg. While less than 10% of CCs had total occlusions in normal eyes at 10 and 20 mm Hg, total occlusions in POAG eyes were present in 24.8% of CCs at 20 mm Hg. The increased number of total occlusions in POAG eyes observed at higher perfusion pressures indicates that many of the CCs may be rendered partially or totally nonfunctional in POAG. Distension of the JCT into CC orifices has been observed under experimentally induced high pressure.
20,21 In normal eyes, distention of the inner wall may interfere only sporadically with CCs due to a large SC volume and reduced inner and outer wall juxtaposition. In POAG eyes, SC volume is reduced under high pressure, providing increased likelihood of apposition between inner and outer walls. This could lead to increase adhesion areas under elevated pressure, minimizing fluid flow and eventually resulting in totally occluded CCs. The occluding material may be a local production of extracellular material from aging SC endothelial cells, JCT cells, or CC endothelial cells. Characterization of the composition of the occlusive material may serve to identify its origin and provide an opportunity to interfere with its production and minimize its occlusive potential.
In summary, this study suggests that decreased outflow facility in POAG eyes may be due to a decrease in SC area and an increase in total occlusions resulting in a reduced number of open CCs available for fluid movement. Results also indicate a loss of adaptation in SC and CCs in POAG eyes to counteract an increase in pressure. While this study assessed nearly 200 CCs, it is limited due to the small number of individual eye pairs (n = 3 normal and n = 3 POAG). Additionally, while all normal eyes were from individuals in their 60s, two pairs of the POAG eyes were from individuals in their 80s. This leaves open the possibility that some of the differences we observed were due to age-related changes. Future SC and CC three-dimensional studies in additional human eyes will help in further validating the anatomic changes in normal and POAG eyes under pressure, and to elucidate the function and contribution of the distal outflow pathway to outflow facility.