Glaucoma valve: anterior versus posterior placement

January 15, 2009

Whether opting for an anterior or posterior approach when placing a glaucoma valve, both procedures appear to have similar success in lowering IOP and similar complication profiles, according to the results of a recent study. Nevertheless, some experts believe that the anterior approach may be more advantageous.

Key Points

Most experts seem to be undecided as to whether it is more advantageous to place the valve into the anterior or posterior chamber, because both approaches have relatively balanced pros and cons.

"Though both procedures have a similar efficacy in lowering IOP, the placement of the valve shunt in the anterior chamber has certain advantages that may render this to be the preferred approach," said Peter J.G. Maris Jr., MD, clinical assistant professor, Department of Ophthalmology, Columbia University College of Physicians and Surgeons, New York City.

Results showed that both anterior and posterior approaches using the valve achieved equal postoperative success. Compared with preoperative values, the posterior approach achieved a 44.6% reduction in IOP compared with 51% reduction in the anterior approach, at last follow-up. Also, the postoperative complications requiring surgical revision of the valve were similar in both study groups.

"One of the major advantages of the pars plana approach is seen in patients with corneal transplants because the tube is further away from the corneal endothelium," Dr. Maris said. "Eyes that have peripheral anterior synechiae can also benefit from the pars plana procedure. In these cases, there is not much of an anterior chamber, leaving little to no room to insert the tube at the limbus."

Vitreous incarceration, on the other hand, is one of the drawbacks associated with the posterior approach, according to Dr. Maris. Here, a vitreous wick enters the pars plana tube of the device and clogs it, requiring a trip back to the operating room. This complication is obviously not an issue if the tube is placed into the anterior chamber. However, this study did not demonstrate many postoperative complications associated with placement of the pars plana tube.

"Another disadvantage for the pars plana approach is that all the patients have to be pseudophakic, meaning they all had to have undergone cataract surgery, whereas patients who receive anteriorly placed valves can still have their crystalline lenses. Furthermore, the posterior chamber approach requires a lot of surgery, invariably leading to a more traumatized eye-an eye that now has more of a surgical ocular history, which is not necessarily the case in patients who undergo the anterior procedure," Dr. Maris said.

There is an ongoing debate as to which surgical procedure is better for patients with refractory glaucoma. A combined PPV and pars plana insertion of a glaucoma drainage device is not a new procedure and has been around for many years. Also, in eyes undergoing penetrating keratoplasty, the placement of glaucoma drainage devices through the pars plana following complete PPV can theoretically minimize trauma to the donor cornea endothelium, resulting in longer graft survival.

Nevertheless, according to Dr. Maris, there is a theory that eyes that have undergone a vitrectomy (for whatever reason) sustain oxidative damage to the trabecular meshwork, and though it could take several years, these eyes may have an increased likelihood of developing glaucoma.

"The superfluous removal of the vitreous body can induce oxidative stress within the eye and predispose it to aqueous outflow resistance and the future development of glaucoma that would not occur in an otherwise non-vitrectomized eye," Dr. Maris said.

Dr. Maris said that both techniques are good and are proven to achieve similar outcomes. Making the "correct" choice of procedure mainly depends on the circumstances of the patient. However if a pars plana insertion is chosen, the surgeon will likely need a multidisciplinary approach, involving a retina specialist, a glaucoma specialist, and sometimes a corneal specialist.

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