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Editors Selection IGR 16-4

Anatomical Structures: Proximal Aqueous Outflow Pathways

Comment by Kaweh Mansouri & Tun Wang Chang on:

75567 Estimating outflow facility through pressure dependent pathways of the human eye, Smith DW; Gardiner BS, PLoS ONE, 2017; 12: e0188769

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In clinical practice, we do not know exactly how the total pressure dependent outflow changes with intraocular pressure. In this extensive paper, the authors developed and tested a new theory for pressure-dependent pathways of the eye based on the differential rates of change of IOP and the membrane reference pressure with respect to changing IOP. Prompted by the so-called equilibrium defect between the aqueous production and outflow, they explored the theory that outflow also occurs through the retinal pigmented epithelium (RPE) into the choroid. Researchers have postulated some of the theory behind complexity of the outflow system from the trabecular meshwork to the episcleral veins.

The authors developed a new theoretical model for the analysis of pressure dependent outflow from the eye and subsequently applied the equation to the animal eye, enucleated eye and in vivo human eyes. The theory assumes that the pressure dependent outflow is primarily due to pressure dependent changes in the driving pressure and more precise as compared to Goldman's equation and Grant's equation.

Based on their model, the authors propose three important parameters that determine pressure dependent outflow which are: (1) the hydraulic conductance for the whole eye; (2) the exponential decay constant; and (3) no-flow reference IOP. The model fits well for human subjects with IOP up to about 40 mmHg. The no-flow IOP is from zero up to 3.0 mmHg. They find that pressure- dependent outflows are twice as large as fluorometric estimates for aqueous outflow. The reason for this discrepancy is postulated to be outflow via the RPE, possibly increased by age-related disease processes. This paper serves as an important milestone for future research and clinical applications of glaucoma.

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