Lab study supports longitudinal venting slit modification to Baerveldt tube shunt

While a resident in the Department of Ophthalmology at Wake Forest University Baptist Medical Center, Winston-Salem, NC, Dr. Gilbert and Brent Bond, MD, assistant professor of ophthalmology, performed a study in an in vitro model to assess the effects of various techniques for avoiding early postoperative hypotony. They compared results achieved with placement of the venting slit against two other modifications: partial occlusion of the tube with a 3-0 suture (Supramid, S. Jackson Inc.) and total occlusion of the tube lumen followed by perforation with a 27-gauge needle. Only the venting slit method was associated with an acceptable IOP.

Clinical study needed

Variability exists among surgeons with regard to the modification technique for preventing early postoperative hypotony, he said. In the absence of solid data to support any particular technique, selection of the various options is based primarily on surgeon preference.

To gain better insight into the issue, Drs. Gilbert and Bond devised a testing apparatus to measure intraluminal pressure while Baerveldt tube shunts mounted on cadaver eyes were infused with balanced salt solution at a physiological flow rate (2 μl/min). The shunts were positioned 10 mm posterior to the limbus and secured with a 10-0 nylon suture. To occlude the shunts, a 4-0 nylon intraluminal stent suture was placed and ligated watertight with a 6-0 suture (Vicryl, Ethicon).

Each of the three tube modifications was studied in three eyes. The mean (range) IOP results were 2 mm Hg (1.6 to 2.8) for the 3-0 suture, 5.9 mm Hg (4.6 to 8.2) for the 27-gauge perforation, and 14.8 mm Hg (12.6 to 17.3) for the 1.2-mm venting slit.

In a previous experiment using the same model, Dr. Gilbert and colleagues compared the effects of varying the length of the venting slit on intraluminal IOP. In that study, they investigated three different length slits: 0.5, 1, and 1.5 mm.

"Based on the results from that investigation, we felt the best length was somewhere between 1 and 1.5 mm, so we chose a 1.2-mm venting slit for the follow-up study comparing the different techniques," he said.

Long term IOP control

Over the longer term, IOP control is regulated by the device itself, Dr. Gilbert noted, because the occlusion suture is reabsorbed and the tube shunt provides the outflow resistance once the plate becomes encapsulated.

"Some modification is needed to bridge the gap between when the device is first implanted and the few weeks it takes for suture reabsorption," he said.