Understanding the mechanism of unconventional drainage from the eye #

Jennifer Tweedy, Mariia Dvoriashyna, Jessica Crawshaw, Darryl Overby, Rodolfo Repetto, Paul Roberts, Tamsin Spelman, Peter Stewart, Alexander Foss

11:30 Monday in 4Q56.

Part of the Physiological flows and transport session.

Abstract #

Aqueous humour is a fluid that circulates in the anterior part of the eye to maintain intraocular pressure (IOP) and nourish the tissues. Its outflow is via two main routes: the conventional outflow and the uveo-scleral or unconventional outflow. In uveo-scleral flow the aqueous humour bypasses the conventional route, seeping posteriorly through the tissues of the eye and eventually crossing the sclera and leaving the eye. The uveo-scleral outflow accounts for a significant fraction of the total flow, and moreover represents a potential route for delivering drugs to the posterior part of the retina to treat conditions such as age-related macular degeneration. Despite this, the mechanisms driving this flow have not yet been clearly elucidated. The choroid is a dense layer of blood vessels between the retina and the sclera, and in this research, we develop a mathematical model to test whether the uveo-scleral outflow can be explained as primarily a flow through the choroidal tissue driven by the IOP forcing fluid posteriorly through the tissue. The flow is additionally affected by the exchange of fluid with blood vessels via Starling forces.

We model fluid and albumin flow within the choroidal tissue, using albumin as a single tracer in the fluid to capture the effects of Starling forces. Choroidal tissue is modelled as axisymmetric shell of porous material; fluid and albumin exchange across blood vessel walls within the choroid and across the sclera (and out of the eye) is governed by the Kedem-Katchalsky equations describing transport through a thin membrane. Flow into the choroid from the inner surface (retinal pigment epithelium) is assumed to be prescribed, with no albumin transport. The narrowness of the choroidal layer allows the equations to be simplified to two second-order coupled ordinary differential equations for fluid and albumin transport, which may readily be solved using a numerical scheme. Furthermore, we allow for the presence of a potential layer that opens between the choroid and the sclera, depending on the pressure.

Solving the model yields the flow and albumin concentration profiles as a function of position within the choroid, as well as allowing the exchange with the blood vessels to be found. The flow exhibits only weak dependency on the IOP, in agreement with experimental observations.