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A high risk of graft failure exists after placement of a glaucoma drainage device in the anterior chamber. Strategies to improve outcomes include placing the device farther away from the cornea, use of a scleral tunnel technique for tube insertion, and performing Descemet's stripping endothelial keratoplasty instead of penetrating keratoplasty in eyes with a pre-existing glaucoma drainage device.
Washington, DC-A high risk of graft failure after anterior chamber placement of a glaucoma drainage device has prompted the development of various alternative techniques to improve graft survival, said Leon W. Herndon, MD, at the annual meeting of the American Glaucoma Society.
"Various studies have reported graft failure rates ranging from 20% to 50% after glaucoma drainage device placement in the anterior chamber," noted Dr. Herndon, associate professor of ophthalmology, Duke Eye Center, Duke University, Durham, NC. "Several theories have been proposed to explain the mechanism, including mechanical endothelial damage from contact between the tube and the endothelium during eye rubbing, movement, or blinking; tube-to-endothelium contact during surgery; or eddy currents of cells flowing in and out of the tube that may increase inflammation in the anterior chamber."
In an effort to reduce contact between the tube and the cornea, surgeons have used alternate sites for glaucoma drainage device insertion.
Reviewing some published studies, Dr. Herndon noted that a recent paper by Tello et al. in the British Journal of Ophthalmology reported on eight eyes that had placement of a glaucoma drainage device in the posterior chamber sulcus. No intraoperative complications occurred, and during 18 months of follow-up, good IOP control was maintained, and corneal decompensation did not develop.
In a retrospective analysis of 40 eyes with a glaucoma drainage device placed after penetrating keratoplasty, however, Alvarenga et al. reported that only 60% of grafts remained clear after 1 year, and the graft clarity rate decreased to 26% at 2 years. IOP control was achieved in only 74% and 63% of eyes at 1 and 2 years, respectively.
"Control groups included eyes without glaucoma that had undergone penetrating keratoplasty [PKP] and eyes with medically treated glaucoma, and they achieved much better graft survival rates," said Dr. Herndon.
Sidoti et al. reported on 34 consecutive patients who underwent pars plana insertion of a glaucoma drainage device before, concurrent with, or after PKP. Complete success, defined as IOP control with graft clarity, was achieved in only 63% of eyes after 1 year, and that rate fell to 33% at 2 years. In addition, a relatively high (44%) rate of posterior segment complications was seen.
Scleral tunnel technique
When placing the glaucoma drainage device in the anterior chamber, a scleral tunnel technique for tube insertion may be preferred over limbal placement. In his technique, Dr. Herndon starts the tunneling about 3 to 4 mm posterior to the limbus using a 22-gauge needle. He advances the needle until it reaches the anterior chamber, as manifested by aqueous flow at the bevel, and then abruptly redirects it down toward the iris. Thereafter, he continues the procedure using standard techniques.
"With limbal placement, there may be scleral patch graft retraction, leaving only a thin layer of conjunctiva covering the tube. In contrast, the scleral tunnel technique affords better tissue reinforcement that can reduce the risk of tube erosion," Dr. Herndon said. "Limbal placement also helps to establish more posterior placement to decrease the likelihood of peripheral tube to cornea contact."
Varying graft technique
In eyes with a pre-existing glaucoma drainage device that require corneal transplantation, graft survival may be improved with the use of Descemet's stripping endothelial keratoplasty (DSEK) instead of PKP. A crucial element for success in DSEK is to maintain apposition of donor tissue to recipient stroma in the immediate postoperative period.
"This can be challenging after glaucoma surgery, but the patients are kept supine for 60 to 90 minutes postop to maintain the air tamponade," Dr. Herndon said.
In a paper in press, Dr. Herndon and colleagues describe techniques for combining glaucoma drainage device placement and DSEK.