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Clinical Update: Glaucoma
TVT: A Study That Could Change Your Practice
By Miriam Karmel, Contributing Writer
 
 

Which is better? Trabeculectomy or drainage implants? Nobody knows. Thus far, glaucoma surgeons have had to rely on anecdotal evidence and case series to make an informed judgment about which is the better technique for eyes that have undergone previous cataract surgery or trabeculectomy.

This April, ophthalmologists moved a bit closer to the answer when the first year of the five-year Tube Versus Trabeculectomy Study (TVT) ended. Clinical trials involving glaucoma surgeries are rare, so a lot is riding on this multicenter randomized study. Preliminary findings, which won’t be ready until later this year, aren’t expected to yield a definitive answer. But researchers hope that in the final analysis, the trial will take the guesswork out of a clinical decision regarding two commonly performed glaucoma operations for a population of patients that one observer called “the bread and butter for glaucoma specialists.” 

“Sometimes we don’t know what’s better for our patients. There are a variety of situations where it’s unclear which is a better surgery,” said Steven J. Gedde, MD, a TVT study cochairman and associate professor of ophthalmology at the University of Miami.

“Whatever its results, the study will probably be a landmark,” said James D. Brandt, MD, a TVT principal investigator and professor of ophthalmology and director of glaucoma service at the University of California, Davis. 

Tubes or Trabeculotomy?

Trabeculotomy remains the procedure of choice. The TVT study is the first multicenter trial comparing trabeculotomy to glaucoma drainage implants.

While trabeculotomy remains the procedure of choice in many surgical scenarios, there’s no authoritative
evidence for the preference.The TVT Study, which started a year ago, is the first multicenter randomized trial comparing trabeculotomy to glaucoma drainage implants.



About the Study
The TVT study compares the safety and efficacy of placement of a Baerveldt glaucoma implant to trabeculectomy with mitomycin C (0.4 milligrams/milliliter for four minutes) in 212 patients who have undergone previous trabeculectomy, cataract extraction with intraocular lens implantation or both. The primary outcome measure is IOP, but researchers will pay close attention to complication rates. 

Patient population. The patients, aged 18 to 55, had inadequately controlled glaucoma with IOP from >18 mmHg to <40 mmHg on tolerated medical therapy. Eighty-one percent of the eyes had primary open-angle glaucoma. Others had chronic angle-closure glaucoma, pseudoexfoliative glaucoma, pigmentary and other types of glaucoma. Excluded from the study are several secondary glaucomas, including neovascular glaucoma, uveitic glaucoma and glaucoma associated with iridocorneal endothelial syndrome and with epithelial or fibrous downgrowth.

The implant. For the sake of standardization and statistical analysis, only one device—the  350-millimeter2 Baerveldt glaucoma implant—is being used. Its appeal, explained Dr. Gedde, is the surface area, which is significantly larger than other glaucoma drainage implants. Theoretically, it may lead to greater pressure reduction. And, as a single-plate device, it is easier to insert. The device is placed in the superotemporal quadrant, with a limbus-based or fornix-based flap, depending on the surgeon’s preference. All trabeculectomies in the study are performed superiorly, with a standard dosage of mitomycin C.

Looking Back
The history of these two commonly performed glaucoma operations—trabeculectomy and drainage implants —dates back to the late 1960s. In the intervening years, there has been no rigorous scientific study to guide surgeons in choosing one over the other. The Cairns-trabeculectomy was quickly adopted after it rivaled the success of full-thickness procedures, with fewer complications. The introduction of antimetabolites yielded even better results with trabeculectomy. But then as late-onset complications associated with antimetabolite use began to emerge, surgeons looked for alternatives. Glaucoma drainage implants became a leading option.

The lack of consensus regarding the best procedure for different groups of patients was underscored by two surveys. The first, a 1996 survey of both the American and Japanese glaucoma societies, conducted by Richard K. Parrish II, MD, and Philip P. Chen, MD, concluded that the majority of respondents preferred trabeculectomy with MMC.¹

Six years later, when Dr. Parrish readministered the survey to American glaucoma society members and compared results against the 1996 AGS data, trabeculectomy was still the preferred treatment in most scenarios, but use of glaucoma drainage implants increased from 7 percent to 20 percent in eyes with previous trabeculectomy and from 8 percent to 22 percent in eyes with previous extracapsular or intracapsular cataract surgery.2 Both increases are statistically significant.

Dr. Gedde says the latter shift may reflect concern over late-onset complications from trabeculectomy.

Looking Forward
Because of those late-onset complications, the glaucoma community will need to wait for the complete study results, said Donald L. Budenz, MD, MPH, a TVT co-investigator and associate professor of ophthalmology, epidemiology and public health at the University of Miami. “It may be four or five years until we get a real handle on which operation is better in the long run for people.”  

When the study’s findings do come in, physicians should be able to translate them easily into clinical practice because the study population mirrors patients seen in an everyday practice.  Researchers call this “generalizability.” TVT subjects at the 17 trial centers were recruited in the course of routine clinical visits. They are being subjected to routine care and workup. The study protocol “very accurately reflects how patients are taken care of in the real world,” Dr. Budenz said. 

Expectations. Though it’s too soon for results, Dr. Budenz predicts that the primary outcome—pressure control—will be similar in both the trabeculectomy and glaucoma drainage implant arms of the study, “because once we have a trabeculectomy or tube functioning, we can get a pressure where we want it.” Lower pressures can be achieved with trabeculectomy, with- out medication, he added, “but tube implant results are good, if you expect to add medications.”

The more interesting question that the study addresses is: which group has the fewest complications? “Even though our primary outcome is IOP, that’s probably going to be equivalent in the two groups,” Dr. Budenz said. “It will come down to other factors, such as complication rates and how many medications people are on.”

Hope for Tubes?
Dr. Brandt predicts that in the first analysis, patients randomized to the trabeculectomy arm of the TVT study “will look very good at one year.” But it will take at least five years for the study to yield meaningful results, and then, “things are likely to get interesting.”

What may happen, he explained, is that the study will defuse the current bias against drainage implants in eyes with previous cataract surgery. “Part of our bias against tubes has been that they have been used as a surgery of last resort. They get used generally in very sick eyes that have had multiple previous operations.” 

But one of the unique aspects of the TVT study, he said, is that it allowed patients to enter the study on the basis of previous cataract surgery by whatever technique. Until now, he explained,  the literature has supported the idea that cataract surgery is a risk factor for filtering surgery. But that notion is based on scleral tunnel or extracapsular procedures, in which the conjunctiva is violated.

Over the last 10 years, however, more and more surgeons are performing corneal procedures that don’t violate the conjunctiva. Since the lens is removed through the cornea, “these are essentially virgin eyes,” he said.

About a dozen of the TVT patients enrolled in Dr. Brandt’s trial fit that profile and are doing extraordinarily well, he said, leading him to speculate that an otherwise virgin eye with a tube may do as well as an eye that undergoes a trabeculectomy.

“This is going to be the most interesting subpopulation to look at in the TVT study,” explained Dr. Brandt. “My prediction is that this group will do extremely well. Then we’ll have to look at whether tubes are being used too late. They may be a pretty good option for a first surgery.”

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1 J Glaucoma 1997;6(3):192–196.
2 J Glaucoma 2005;14(2):172–174.


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The TVT study was funded with an unrestricted grant from Pfizer, the previous manufacturer of the Baerveldt implant.



Pros and Cons

While there is still no consensus about which of the two commonly performed glaucoma operations is better for patients, most glaucoma specialists agree on the pros and cons of each technique. Hopefully, the TVT study will establish some definitive guideposts for surgeons. In the meantime, here are some observations from glaucoma experts regarding the two techniques.

 

TrabeculectomyDrainage Implants

Pros

  • Highly successful operation
  • Typically achieves low IOP from day one, without medication
  • Relatively predictable
  • Straightforward technique

Cons

  • May overshoot and cause hypotony
  • Increased risk of late-onset (often
    years later) bleb leaks and infections
  • Poorer prognosis when there is
    conjunctival scarring from previous surgery


Pros

  • Highly successful operation
  • Risk of late infection is quite low
  • IOP-lowering effect may be longer lasting
  • Straightforward technique
  • May be implanted in eye with scarred conjunctiva from previous surgery

Cons

  • Unique complications, including diplopia, tube erosion and corneal edema, if tube touches cornea
  • May not lower pressure as well as
    trabeculectomy
  • May need supplemental medical therapy to achieve desired pressure
  • Nonvalved implants don’t provide lower pressure until several weeks
    postoperatively

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