Glaucoma surgery is performed when a further reduction of IOP is needed despite maximum tolerated medical therapy and appropriate laser treatment. Trabeculectomy is generally the initial incisional surgical procedure for managing glaucoma.1,2 Eyes in which the procedure has failed, however, are at greater risk of failed subsequent filtering surgery.3-5 Ophthalmologists therefore perform wound modulation with antifibrotic agents such as mitomycin C (MMC) and 5-fluorouracil (5-FU), because the technique increases the success rate of trabeculectomy in eyes that have undergone previous ocular surgery.6-8 Although antifibrotic agents have increased the likelihood of IOP control following filtering surgery, they have also unfortunately heightened the risk of complications.6-8 Histological studies of MMC-treated blebs with complications have shown irregular surface epithelium and a largely avascular and acellular subepithelium of loosely arranged connective tissue.9-11 The prevalence of bleb-related infections, leakage, and dysesthesia associated with a perilimbal filtering bleb suggests the need for ophthalmologists to consider alternatives.
Glaucoma surgeons are often faced with the question of the best procedure to perform when the trabeculectomy has failed. Should they repeat the trabeculectomy or place a tube shunt? Practice patterns vary in this situation.1,2 In 1996, Chen and colleagues conducted an anonymous survey of members of the American Glaucoma Society and Japanese Glaucoma Society to evaluate their use of antifibrotic agents and tube shunts.1 The survey presented 10 clinical situations requiring glaucoma surgical intervention. The majority of respondents (59% to 83%) preferred trabeculectomy with MMC for the clinical scenarios involving prior ocular surgery, although many of those surveyed elected to use a tube shunt, trabeculectomy with 5-FU, or trabeculectomy without an antifibrotic agent. In 2002, Joshi et al readministered the same survey to members of the American Glaucoma Society.2 Respondents still favored trabeculectomy with MMC for most clinical situations, but the percentage using tube shunts had significantly increased. Although the use of tube shunts has grown as an option for the surgical management of glaucoma, there is a risk of complications with these devices. Motility-related disturbances and corneal decompensation are possible, as are occurrences of orbital complications.12-14
The Tube Versus Trabeculectomy (TVT) Study15,16 was designed to prospectively compare the safety and efficacy of nonvalved tube shunt surgery and trabeculectomy with MMC in order to provide information to assist surgeons' decision making in similar patient groups.
Methodology
The multicenter clinical trial enrolled subjects with uncontrolled glaucoma who had prior cataract extraction with IOL implantation and/or failed trabeculectomy. One hundred seven patients were randomized to receive a 350-mm2 Baerveldt glaucoma implant (Advanced Medical Optics, Inc., Irvine, CA) or to undergo trabeculectomy with MMC (0.4 mg/mL for 4 minutes).
A total of 212 patients were enrolled in the TVT Study between October 1999 and April 2004. Randomization assigned 107 patients to receive the Baerveldt glaucoma implant and 105 patients to undergo trabeculectomy with MMC. Patients' characteristics are noted in Table 1.
Results
Both surgical procedures effectively lowered IOP. Among patients who completed 1 year of follow-up visits, trabeculectomy with MMC produced a 49.6% reduction in IOP, and the tube shunt achieved a 49.9% decrease in IOP. Many glaucoma surgeons believe that low IOP levels cannot be achieved with tube shunts, but this opinion was not supported by the TVT Study's results, which found a mean IOP of 12.4 mm Hg in the tube group at 1 year. The IOPs at 1 day, 1 week, 1 month, and 3 months were statistically lower in the trabeculectomy versus the tube group, but there were no significant differences at 6 months and 1 year.
Patients in the tube group were more likely to achieve IOP control and avoid persistent hypotony or reoperation for glaucoma than those in the trabeculectomy group during the first year of follow-up. Failure was defined prospectively as inadequate IOP control, persistent hypotony (IOP < 5 mm Hg), reoperation for glaucoma, or a loss of light-perception vision. At 1 year, the cumulative probability of failure was 3.9% in the tube group and 14.8% in the trabeculectomy group. Inadequately controlled IOP was the most common reason for failure in both treatment arms.
Successful treatment was subdivided into complete and qualified successes, depending on whether or not patients required supplemental medical therapy. Although the qualified success rate was higher for the tube group, the trabeculectomy group had more complete successes. This finding was consistent with the observed greater need for glaucoma medications by the tube group compared with the trabeculectomy group at all time points during the first year of follow-up. Patients with tube shunts seemingly require more supplemental medical therapy than those who undergo trabeculectomy with MMC in order to achieve a desired level of pressure reduction.
The incidence of most postoperative complications was similar in both treatment groups. Dysesthesia and wound leaks occurred with greater frequency, however, in the trabeculectomy versus tube group. Although implanting a tube shunt delays the drainage of aqueous humor to an area remote from the limbus, trabeculectomy results in an immediate filtration of aqueous near the limbus with a greater tendency toward postoperative wound leaks. There was a trend toward a higher incidence of diplopia in the tube group and hyphema in the trabeculectomy group, but these differences did not reach statistical significance.
Both subject groups experienced a reduction in visual acuity during the first year of follow-up, but Snellen and Early Treatment Diabetic Retinopathy Study visual acuities were similar between treatment groups at 1 year. Patients who developed surgical complications suffered a greater loss of Snellen visual acuity than those without complications. The presence of any postoperative complication was associated with a higher risk of vision loss, and this risk increased as the number of complications grew. Most surgical complications were not associated with vision loss, but patients with persistent corneal edema and choroidal effusion were more likely to experience a decline in vision. Although it is expected that persistent corneal edema will reduce vision, choroidal effusions are generally considered benign sequelae of glaucoma surgery. Some complications that commonly produce a reduction in vision, such as hypotony maculopathy and cystoid macular edema, were not significantly associated with vision loss in the TVT Study's risk factor analyses. This finding may indicate the successful treatment of these conditions or inadequate power to identify them as risk factors because of their low rates of occurrence.
Conclusion
The results of the TVT Study suggest that ophthalmologists should consider expanding the role of tube shunts in the surgical management of glaucoma. In eyes with previous cataract extraction with IOL implantation and/or failed trabeculectomy, the TVT Study found that nonvalved tube shunt surgery and trabeculectomy with MMC produced similar reductions in IOP at 1 year. Trabeculectomy with MMC required less supplemental medical therapy, but tube shunts were more likely to maintain IOP control and avoid persistent hypotony or reoperation for glaucoma during the first year of follow-up.
The TVT Study does not demonstrate the clear superiority of one glaucoma operation over the other. When selecting a surgical procedure for patients with medically uncontrolled glaucoma, ophthalmologists must consider several factors, including the degree of IOP lowering desired, the availability of and the patient's ability to pay for medication, the patient's compliance with therapy, and the surgeon's skill and experience with both operations. The author agrees with the points raised in the editorial by Jamil and Mills17 that it will be important to follow the TVT Study's subjects over time to see if one procedure is indeed beneficial over the other in a chronic disease such as glaucoma.
Leon W. Herndon, MD, is Associate Professor of Ophthalmology at Duke University Eye Center in Durham, North Carolina. He is an investigator for the TVT Study and a contributor to one of its manuscripts. Dr. Herndon may be reached at (919) 684-6622; leon.herndon@duke.edu.
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