Abstract
Purpose :
To determine if the inner diameter of a tube-shunt device for IOP lowering in glaucoma has a significant impact on outflow, and thus IOP.
Methods :
A series of benchtop experiments were conducted using tubes with same dimensions as commercial tube-shunt products: Ahmed Clear Path (305µm); Ahmed Glaucoma Valve (305µm); Baerveldt Glaucoma Implant (305µm); PAUL (127µm). For the valved device (Ahmed Glaucoma Valve), the flow regulator was removed to isolate the effect of tube inner diameter on IOP. All tubes were tested at the commercially available length of 16mm.
Theoretical (expected) IOP was calculated using the Hagen Poiseuille Equation based on each tube’s dimensions: 305µm tube (0.02mmHg); 127µm tube (0.6mmHg). Water was then run through each tube at a fixed flow rate of 2.75 µL/min to determine the steady-state IOP. This mimics the early postoperative period before a capsule has formed to provide flow resistance.
Results :
The observed benchtop IOPs were as follows: 0.03mmHg for the 305µm tubes and 0.6mmHg for the 127µm tube. A small rise in IOP (from 0.02mmHg to 0.6mmHg) was derived by reducing the tube’s inner diameter from 305µm to 127µm.
Conclusions :
Hypotony occurs when IOP falls to 5-6mmHg or below, which means that a theoretical IOP of at least 5mm Hg is required for a tube to be considered restrictive to prevent hypotony. None of the tubes tested in these experiments maintained IOP above this threshold. Thus, none of these devices would be expected to prevent early postoperative hypotony based on tube diameter alone.
These laboratory findings are published clinical studies: Ahmed Clear Path 7.1%; Ahmed Glaucoma Valve 5.7%; Baerveldt Glaucoma Implant 8.3%; PAUL Glaucoma Implant 9.5%.
All 4 tube-shunt devices are associated with some incidences of early postoperative hypotony (~6-10%). These rates appear independent of tube inner diameter, although the Ahmed Glaucoma Valve shows the lowest rate of hypotony likely due to a benefit from the presence of a flow-restriction valve.
Both benchtop testing and clinical data, show that none of the devices evaluated show flow restrictive properties that would prevent hypotony, including the device with the smallest diameter tube (PAUL Glaucoma Implant). Instead, early postoperative hypotony is likely attributable to other factors than device selection.
This is a 2021 ARVO Annual Meeting abstract.