Choroidal Venous Pressure Is Passively Controlled by Vortex Vein Collapse: A Theoretical Model

Purpose: Animal experiments have shown that choroidal venous pressure is tightly linked to the IOP. It has been hypothesied that the choroidal pressure is passively controlled by partial collapse of the vortex veins. This work aimed to test the feasibility of this hypothesis using mechanical modeling. Methods: We developed a theoretical model of venous outflow from the choroid as a Starling resistor. Each vortex vein was modeled as a collapsible tube formed from two segments arranged in series: one segment within the choroid, subject to an external pressure equal to the IOP, and another segment through the sclera subject to an external pressure, which is linearly decreasing from the IOP to the orbital tissue pressure. For a given choroidal blood flow, this model predicts the choroidal venous pressure and vortex vein cross-sectional area along its length. This model was validated against in vivo rabbit experiments, where the flow rate through the vortex vein was measured as the outlet venous pressure was adjusted. Results: In agreement with previous experiments, for constant choroidal blood flow, an increase in the IOP generates an almost equal increase in the blood pressure in the choroidal veins. Furthermore, the blood flow rate is insensitive to the outlet venous pressure provided it remains below the IOP. This model shows that partial collapse of the vortex veins explains both observations. Conclusions: This model provides a mechanical description of how choroidal venous pressure is passively controlled by vortex vein collapse. The model also shows that vortex vein collapse is expected within the sclera.