Uveo-scleral flow: understanding the mechanism of unconventional drainage

Abstract

Purpose : Uveo-scleral flow accounts for a significant fraction of total aqueous outflow; more in the case of glaucoma patients receiving the commonest class of drugs (prostaglandin F analogues). Moreover, the uveo-scleral pathway 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. In this research, we develop a mathematical model to test whether the outflow can be explained by flow through the choroidal tissue driven by the intraocular pressure (IOP) forcing fluid posteriorly through the tissue and outward through the sclera. The flow is additionally affected by the exchange of fluid with blood vessels via Starling forces.

Methods : The model set up is shown in Fig. 1: choroidal tissue is an axisymmetric shell modelled as a porous medium; fluid and albumin exchange across both blood vessel walls and the sclera is governed by the Kedem-Katchalsky equations describing transport through a thin membrane. Flow into the choroid from the RPE 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 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 suprachoroidal layer that opens between the choroid and the sclera, whose height varies linearly with pressure, also modelled as a porous medium.

Results : 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. We find uveoscleral flow to be about 10-20% of the total outflow. The flow also exhibits only weak dependency on the IOP, in agreement with the observations of Bill (1966), Fig. 2.

Conclusions : We have developed a model to demonstrate that it is possible to understand the uveo-scleral flow as being driven by the IOP and Starling forces. In future work we will use the model to investigate the distribution of drugs injected into the anterior chamber to the posterior of the eye via the uveo-scleral route.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.

 

Model set up

 

Graph of uveo-scleral flow against IOP for different values of the permeability of the suprachoroidal space K_p, where K_c is permeability of the choroid