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T. Fjield, M. Johnson, H. Heitzmann; Mathematical Model of Unconventional Aqueous Humor Outflow. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1600.
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© ARVO (1962-2015); The Authors (2016-present)
Previous mathematical models have analyzed the conventional aqueous outflow. The present work extends mathematical modeling to examine the unconventional outflow pathway.
A systems model has been created that includes conventional outflow as a resistor, and models the unconventional outflow pathway in some detail. The ciliary muscle provides a resistance into the suprachoroidal space, and the resistance of the space itself is modeled as flow between two concentric spheres. Flow leaving the suprachoroidal space is either adsorbed osmotically by the choroid, or passes across the sclera. The former is modeled using Starling's equation while the latter with Darcy's law. Inputs to the model are provided by literature values for the choroidal filtration coefficient, choriocapillaris pressure, hydraulic permeability of the sclera, suprachoroidal space pressure, and other parameters.
If the scleral hydraulic conductivity is relatively high, e.g. 0.33 µL/min/mmHg (Jackson et al.), then uveoscleral flow (Fu), IOP, and suprachoroidal space pressure (Pss) are sensitive primarily to the ciliary muscle resistance (Fig. 1A). On the other hand, if the scleral hydraulic conductivity is negligibly small then Fu, IOP, and Pss are sensitive to the oncotic pressure gradient (Fig. 1B), ciliary muscle resistance, and filtration coefficient. Additional calculations describe the effects of glaucoma therapies such as prostaglandin analogs and surgical procedures.
A numerical model of aqueous humor outflow has been created integrating the parameters of uveoscleral flow. The model can be used to compute the effects of the oncotic pressure gradient, filtration coefficient, and ciliary muscle resistance in an effort to better understand the mechanisms of action underlying glaucoma drugs and devices.
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