Experimental Laboratory Modeling of Choroidal Vasculature: A Study of the Dynamics of Intraoperative Choroidal Hemorrhage during Pars Plana Vitrectomy

Abstract

Objectives: Choroidal hemorrhages (CH) result from rupture of choroidal vessels leading to extravasation of blood into the suprachoroidal space. In this study, we aimed to understand the hemodynamics of CH by developing a purpose-built scale model of the choroidal vasculature and calculating stress levels in the model under different conditions.

Materials and methods: We modeled the choroidal vasculature using a rubber tube 10 cm in length and 1 cm in diameter that was wrapped with conductive thread to enable the measurement of stress at the walls of the tube. Stress levels across the tube were continuously measured under different systemic intravascular blood pressure levels (IVP), intraocular pressure (IOP) levels, and distortion.

Results: Stress values across the choroidal vessel model correlated negatively with IOP and positively with IVP and distortion. All correlations were statistically significant (p<0.05) and were stronger when the model was filled with expansile tamponade compared to non-expansile tamponades. Distortion showed the strongest correlation in terms of increasing stress across the model, while IVP showed stronger correlation compared to IOP. Raising IOP to counteract the stress in the model was effective when the stress in the model was secondary to increased IVP, but this approach was not effective when the stress in the model was caused by distortion.

Conclusion: Excessive distortion of the globe during surgical maneuvers could be the primary reason for the rarely observed intraoperative CH. Non-expansile ocular tamponade provides better support for the vascular bed against CH and should be the recommended choice of tamponade in patients with existing CH. Increasing IOP excessively is of limited effect in preventing CH in vessels that are under stress as a result of distorting surgical maneuvers.

Keywords: Choroid hemorrhage; choroid; suprachoroidal space; vitrectomy.

Figure 1

A diagram of the model. 1: a rubber tube 10 cm in length and 1 cm in diameter wrapped with conductive thread to enable the measurement of stress at the walls of the tube. 2: a hydraulic actuator used to apply various longitudinal strain on the tube to simulate ocular distortion during surgical maneuvers. 3: an infrared distance sensor to measure the strain applied on the tube. 4: a sealed container enclosing the model and pressurized to various levels to simulate variable intraocular pressures (IOP). The tests were performed with the container filled with air or water to simulate expansile and non-expansile tamponades, respectively. 5: a pump to pressurize the tube to various levels to simulate systemic intravascular blood pressure (IVP). 6: a pump to pressurize the container to various levels to simulate IOP. 7: a pump to pressurize the hydraulic actuator to various levels to simulate ocular distortion. 8: a pressure sensor to continuously monitor IVP. 9: a pressure sensor to continuously monitor IOP. 10: a microcontroller to control the pumps, process the signals from the sensors and send the information through a serial connection to a computer. 11: a laptop to save and process data from the microcontroller

Figure 2

The scale model of the choroidal vessel made of a rubber tube 10 cm in length and 1 cm in diameter equipped with stress sensors consisting primarily of conductive thread, fitted in an actuator and placed in a sealed container. The container is partially filled with water. The container and the tube were pressurized using a special pump system to simulate the intraocular pressure (IOP) and systemic blood pressure (IVP). The actuator was used to apply controlled longitudinal strain on the tube to simulate eye distortion during surgery. Stress levels in the tube were continuously monitored under various IOP, IVP, and strain levels

Figure 3

The results of phase 1 experiments when the model was filled with non-expansile tamponade (left column) and expansile tamponade (right column), showing the relationships between stress in the choroidal vasculature and intraocular pressure (top row), systemic intravascular pressure (middle row), and ocular distortion (bottom row)

Figure 4

The results of phase 2 experiments when the model was filled with non-expansile tamponade (left column) and expansile tamponade (right column), showing the role of increasing intraocular pressure in counteracting stress in the choroidal vasculature produced by high systemic intravascular blood pressure (top row) and produced by distortion (bottom row)