Postoperative pressure regulation in glaucoma shunt surgery - Focal tube constriction is not the answer

Purpose: To evaluate, in a laboratory setting, the accuracy and the clinical use of focal tube constriction as a means to regulate intraocular pressure after glaucoma drainage implant surgery. Materials and Methods: A silicone tube identical to the one used in the Baerveldt and other glaucoma drainage implants was connected to a syringe-pump delivering a continuous flow of demineralized water at a rate of 2.5 microliters per minute. Focal constriction of the tube was obtained by a ring made of a shape-memory metal alloy designed for this purpose. After complete occlusion of the tube lumen by crimping the ring with calibrated pliers, the ring was opened in a stepwise manner by heating it with an argon laser beam. In a second experiment the tube lumen was constricted by placing the tube between the jaws of a micrometer. Results: In both experiments a stepwise lowering of the pressure could be obtained. The resulting pressure levels, however, lacked consistency and predictability to such an extent that clinical application is not feasible. Conclusion: It is not possible to regulate pressure in a reliable and predictable way merely by constricting the tube lumen of glaucoma drainage implants.