Pressure dependency of transcleral flow

Outflow facility and extent of angle closure in a porcine model

PURPOSE

To establish the extent of anterior chamber angle circumference needed to maintain a physiological outflow facility (C). This could create a model to investigate focal outflow regulation.

METHODS

Twenty anterior segments of porcine eyes were assigned to five groups, each with a different degree of cyanoacrylate-mediated angle closure: 90° (n = 4), 180° (n = 4), 270° (n = 4), 360° (n = 4), and four unoccluded control eyes. The outflow facility was measured at baseline, 3, 12, 24, and 36 h after angle closure. Outflow patterns were evaluated with canalograms and the histomorphology was compared.

RESULTS

Baseline outflow facilities of the five groups were similar (F = 0.922, p = 0.477). Occlusion of 360° induced a significant decrease in facility from baseline at all time-points (p ≤ 0.023 at 3, 12, 24, and 36 h). However, no difference from baseline was found in any of the partially occluded (0-270°) groups (F ≥ 0.067, p ≥ 0.296 at 3, 12, 24, and 36 h). The canalograms confirmed the extent of occlusion with flow through the unblocked regions. Histology revealed no adverse effects of blockage on the TM or aqueous plexus in the unoccluded angle portions. The unoccluded TM appeared normal.

CONCLUSION

Cyanoacrylate-mediated angle occlusion created a reproducible angle closure model. Ninety degrees of unoccluded anterior chamber angle circumference was sufficient to maintain physiological outflow. This model may help understand how outflow can be regulated in healthy, nonglaucomatous TM.

Unconventional aqueous humor outflow: A review

Aqueous humor flows out of the eye primarily through the conventional outflow pathway that includes the trabecular meshwork and Schlemm's canal. However, a fraction of aqueous humor passes through an alternative or 'unconventional' route that includes the ciliary muscle, supraciliary and suprachoroidal spaces. From there, unconventional outflow may drain through two pathways: a uveoscleral pathway where aqueous drains across the sclera to be resorbed by orbital vessels, and a uveovortex pathway where aqueous humor enters the choroid to drain through the vortex veins. We review the anatomy, physiology and pharmacology of these pathways. We also discuss methods to determine unconventional outflow rate, including direct techniques that use radioactive or fluorescent tracers recovered from tissues in the unconventional pathway and indirect methods that estimate unconventional outflow based on total outflow over a range of pressures. Indirect methods are subject to a number of assumptions and generally give poor agreement with tracer measurements. We review the variety of animal models that have been used to study conventional and unconventional outflow. The mouse appears to be a promising model because it captures several aspects of conventional and unconventional outflow dynamics common to humans, although questions remain regarding the magnitude of unconventional outflow in mice. Finally, we review future directions. There is a clear need to develop improved methods for measuring unconventional outflow in both animals and humans.

Computed tomography of aqueous humour outflow pathways

The anterior chamber of adult Rhesus monkeys (Macaca mulatta) was perfused with a radio-opaque contrast medium (Amipaque; mol. wt 789 daltons) at controlled intraocular pressure (IOP), and its subsequent distribution examined by computed tomography. In the living monkey no contrast medium was detectable outside the anterior chamber, even after prolonged (8 hr) perfusion. However, if the animal was then killed, and the IOP maintained artificially at its previous level, opacity appeared immediately throughout the anterior segment. Subsequent scans showed the contrast medium to diffuse into the anterior orbital tissues, and to move posteriorward through the globe wall and extraocular muscles. None entered the vitreous body and very little progressed behind the point at which the extraocular muscles exited from Tenon's capsule. Elevation of IOP did not appear to accelerate this posterior diffusion. It is inferred that in vivo contrast medium passing into the posterior, non-conventional aqueous drainage pathway is cleared immediately by the circulating blood in the uvea and possibly also the extraocular muscle. Tenon's capsule may provide a barrier to further intra-orbital diffusion.