In Vitro Simulation of Flow Characteristics of Valved Glaucoma Drainage Devices After Fibrous Capsule Formation

Abstract

Abstract: : Purpose: To evaluate for an ongoing contribution of the valve to the total resistance of a valved glaucoma drainage device (VGDD) after a fibrous capsule forms around the external plate. Methods: The set-up consisted of a syringe pump connected to a glass standpipe via a 3-way stopcock. The third outlet was connected by a 27 gauge needle to an Ahmed^TM Glaucoma Valve (AGV), which was water sealed inside a glass test tube. A 21 gauge needle from the tube then connected to three Anopore^TM filters (Structure Probe, Inc.) in series, which simulated the in vivo fibrous capsule. These Anopore filters are nanoporous membranes with pore dimensions similar to the in vivo fibrous capsule. They have a filtration area of 0.8 cm² when in their holders. Tests were performed at room temperature using filtered saline at a flow rate of 45 µl/min. After initial testing with functional AGVs, retesting was done with the valves disabled (done by cutting away the distal valve membrane and cover). Results: Each test was done three times. The three Anopore filters in series were found to generate a pressure of 8.24 ± 0.30 (SD) mmHg. The following table summarizes our findings in testing the AGVs by themselves and in series with the Anopore filters, both before and after disabling the valves: Disabling the AGV when in a series configuration with the simulated fibrous capsule decreased the total pressure by about 9 mmHg, essentially the pressure generated by the AGV itself. Conclusions: This study suggests that the resistance across the valve in a VGDD continues to significantly contribute to the pressure even after fibrous capsule formation around the subconjunctival plate. These results have potentially important therapeutic implications.  

Keywords: intraocular pressure • clinical laboratory testing