Postoperative pressure regulation in glaucoma shunt surgery: focal tube constriction is not the answer

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.

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.

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.

It is not possible to regulate pressure in a reliable and predictable way merely by constricting the tube lumen of glaucoma drainage implants.