AbstractPurposeThis study investigates the effect of one versus two fenestrations on both fluid egress and opening pressure from a non-valved glaucoma implant.MethodsIn this laboratory study, we used anin vitroclosed system comprised of ligated silicone tubing connected to a fluid reservoir and manometer to simulate the tubing found in a Baerveldt glaucoma drainage implant. Fenestrations were created using an 8-0 Vicryl TG140-8 suture needle. Main outcome measures included volume of fluid egress and fenestration opening pressures, which were measured via micropipette and increasing pressure until fluid egress was observed.ResultsNo significant difference was observed in fluid egress between tubing with one versus two fenestrations at pressures≤40 mmHg. At 50 mmHg, a statistically significant difference was observed in fluid egress between tubing with one versus two fenestrations (P<0.05). The first fenestration opened at 10.5±3.77 mmHg and the second fenestration opened at 28.83±5.09 mmHg (average±standard deviation).ConclusionOurin vitrofindings suggest there may exist a critical pressure>40 mmHg at which the second fenestration starts to play a significant role in fluid drainage. There may be no difference in the amount of fluid egress and effect on intraocular pressure between one or two tube fenestrations when preoperative intraocular pressure is≤40 mmHg.