TVT STUDY

Glaucoma Surgery: One-Year Results of the Tube vs. Trabeculectomy (TVT) Study

An investigator shares early study results comparing the Baerveldt glaucoma implant to trabeculectomy with mitomycin C.

By Steven J. Gedde, M.D., and the TVT Study Group

Trabeculectomy and tube shunt (or glaucoma drainage implant) surgery are the two most common incisional procedures used to manage medically uncontrolled glaucoma. However, if you're a new ophthalmologist, it's important to know that in the last decade, glaucoma specialists have started to shift from trabeculectomy toward tube shunt surgery — and this is a trend that may influence your treatment decisions about patients with glaucoma.^1,2

We probably can attribute this change to increasing concern among ophthalmologists about late complications associated with trabeculectomy, especially when an adjunctive antifibrotic agent is used. In particular, the prevalence of late-on set bleb leaks and bleb infections has prompted many surgeons to consider tube shunts as an alternative. Although tube shunts appear to have a lower infection rate, they have their own unique set of complications. Therefore, in some clinical situations, you may be unsure of the best surgical approach to use in treating certain patients, such as those who've had previous ocular surgery.

To shed additional light on these issues, several glaucoma specialists at academic centers designed the Tube vs. Trabeculectomy (TVT) study. This multicenter, randomized clinical trial compares the safety and efficacy of these procedures in patients who've had previous ocular surgery. Read on to learn about our findings at 1 year.

Trabeculectomy vs. Tube Shunts

During trabeculectomy and tube shunt surgery, we construct a new route for aqueous humor outflow, bypassing the pathologic resistance present in the natural outflow system in patients with glaucoma. With trabeculectomy, this is accomplished by creating a scleral fistula that allows aqueous to flow into the subconjunctival space close to the limbus. In glaucoma drainage implant surgery, a tube shunts aqueous humor to an explant or plate attached to the sclera in the equatorial region of the globe, which promotes formation of a capsule around it. When aqueous flows through a shunt, it pools in the space between the plate and the nonadherent capsule, passes through the capsule by passive diffusion and is absorbed by periocular capillaries.

The TVT study compared the safety and efficacy of trabeculectomy and the tube shunt in patients who've had previous ocular surgery.

The bleb formed after tube shunt surgery is very different in character from a trabeculectomy bleb. The tube shunt bleb is thicker and remote from the limbus, in contrast to the thin-walled, perilimbal bleb seen after trabeculectomy.

Therefore, it appears there's less risk of infection with tube shunts, which I believe is one of the major advantages of using a tube instead of a trabeculectomy.

Study Design

The TVT study is a randomized, clinical trial performed at 17 centers in 212 eyes of 212 patients, including 107 in the tube group and 105 in the trabeculectomy group. Eligible patients were 18 to 85 years old and had previous trabeculectomy and/or cataract extraction with IOL implantation and uncontrolled glaucoma with IOPs ranging from 18 mm Hg to 40 mm Hg while using maximum tolerated medical therapy.^3–5

Patients were randomized to receive either the 350 mm² Baerveldt glaucoma implant (AMO, Santa Ana, Calif.) in the superotemporal quadrant or trabeculectomy with mitomycin C.³ After surgery, follow-up data were collected at 1 day, 1 week, 1 month, 3 months, 6 months, 1 year, 18 months and then every year until 5 years.

The procedure was considered a failure if the patient's pressure was greater than 21 mm Hg or not reduced by 20% below baseline on two consecutive visits after 3 months; if pressure was less than or equal to 5 mm Hg on two consecutive visits after 3 months; if the patient required additional surgery for glaucoma; or if the patient lost light perception vision.³

An Early Glimpse

The study had excellent retention, with 97% of follow-up visits completed during the first year of the study.⁴

Both groups had significant reduction in IOPs and almost identical pressure levels at 1 year, but there was a significantly greater need for adjunctive medical therapy in the tube group.⁴ Patients in the tube group required an average of 1.3 medications, whereas the trabeculectomy group required an average of approximately 0.5 medications.

A large number of patients in the study had postoperative complications, but most were transient and self-limited. Thirty-four percent of patients in the tube group and 57% of those in the trabeculectomy group had postoperative complications.⁵ Serious complications — resulting in a loss of two or more lines of vision, a repeat operation to treat complications, or both — occurred in 17% of patients in the tube group and 27% of patients in the trabeculectomy group, a difference that was not statistically significant.

At 1 year, tube shunts had a higher success rate than trabeculectomy, with a 13.5% failure rate in the trabeculectomy group and 3.9% in the tube group.⁴

Some glaucoma specialists believe that once a tube shunt works, it tends to function well over the long term and seems less prone to late failure. However, we're looking forward to seeing more long-term data from this 5-year study. Keep in mind that during this study we used a 350 mm² Baerveldt implant in a select type of patient, so we can't necessarily apply these results to other implants or other patient groups.

Experience With the Implant

A number of tube shunts are commercially available. They're similarly designed, consisting of a tube connected to a plate. However, to prevent hypotony, some implants include a valve device that limits flow through the tube to the plate if pressure becomes too low. In my experience, when you're using nonvalved implants, like the Baerveldt implant, it's important to temporarily restrict flow through the device until a capsule forms around the plate to prevent hypotony in the early period after surgery.

I favor the Baerveldt implant mainly because the surface area of the plate is significantly larger than that of other single-plate devices currently available, and there's good evidence to suggest that the degree of pressure reduction observed after glaucoma drainage implant surgery is proportional to the surface area of the plate of the implant.⁶ Therefore, using implants with larger surface area plates results in greater pressure reduction compared with smaller plates. Some double-plate devices offer a surface area comparable to the Baerveldt; however, they require surgery in two quadrants for implantation.

Looking Ahead

It will be interesting to see how the results of the TVT study influence practice patterns in the upcoming years. The use of tube shunts already has been expanding. Medicare data reveal that over the last decade, the number of trabeculectomies being performed has been declining gradually, whereas during that same time period, the number of tube shunts being implanted has increased approximately threefold.

In addition, a survey of members of the American Glaucoma Society in 1995 presented 10 different clinical scenarios and asked glaucoma surgeons what operation they'd choose to manage these hypothetical patients. When the same survey was redistributed to members in 2002, an increasing percentage of glaucoma specialists were using tube shunts for these different clinical scenarios and the percentage of trabeculectomies had declined.

We're looking forward to additional results with longer follow-up, but the early results don't demonstrate a clear superiority of one operation over the other.

Consider Your Patient, Yourself

In addition to weighing these results as you treat your glaucoma patients, you'll need to consider a variety of factors when selecting a surgical procedure, such as your comfort and experience with each surgical procedure, as well as your patients' tolerance and compliance with medication regimens. However, preliminary results from the TVT Study show a clear role for tube shunt surgery in the surgical treatment of patients who've had prior ocular surgery. nMD