Tubes or trabeculectomy to prevent glaucoma: That is the question
Tubes have been gaining in popularity most likely due to more predictable postop course
Shunt implantation and trabeculectomy are important procedures in the battle to prevent blindness from glaucoma, and tubes are steadily gaining in popularity possibly because of the more predictable postoperative course.
Milwaukee—Trabeculectomy and shunt implantation are important procedures in the battle to prevent blindness from glaucoma, as both treatment modalities are important parts of the armamentarium, according to Dale Heuer, MD. Nevertheless, tubes are steadily gaining in popularity over time possibly because of the more predictable postoperative course.
Historically, aqueous shunts had been indicated for eyes at high risk of failure despite trabeculectomy and use of mitomycin C (MMC) and/or 5-fluorouracil. The eyes included those that had already failed a filtering surgery in which MMC was used and those with a neovascular or inflammatory glaucoma, Dr. Heuer noted.
“In these settings, the patients with poor prognoses had improved success rates,” said Dr. Heuer, professor and chairman of ophthalmology, Medical College of Wisconsin, and director, Froedtert and the Medical College of Wisconsin Eye Institute, Milwaukee. “The intraocular pressures (IOPs) were in the mid- to high-teens and patients frequently needed supplemental ocular hypotensive medications. However, the surgeries were not complication-free.”
Tube vs. Trabeculectomy
The Tube vs. Trabeculectomy (TVT) Study was a randomized clinical trial design to determine which procedure served patients better. It compared the safety and efficacy of the Baervelt 350-mm implant—a non-valved aqueous shunt, with the safety and efficacy of trabeculectomy with MMC in patients who might have undergone a previous trabeculectomy and/or clear cornea phacoemulsification.
The shunt group and the trabeculectomy group were well matched and the patients had moderately severe visual field loss (average mean defect, ~ –16 decibels).
Failure in the study was defined as an IOP of 21 mmHg or higher and less than 20% IOP reduction.
Dr. Heuer reported that the failure rates at 5 years in the trabeculectomy group and the tube group were 47% and 30%, respectively, with the difference between the groups reaching significance (p = 0.002).
“The relative risk of failure was more than double in the trabeculectomy group,” he said.
When considering the results with a lower IOP target of more than 17 mmHg, the risk of failure was still double with trabeculectomy. Even with a target of more than 14 mmHg, there was a significantly better outcome with the shunt, he recounted.
Dr. Heuer also noted the “surprising” finding that both procedures achieved comparable IOP lowering over time.
Patients who underwent a trabeculectomy initially required fewer medications compared with the tube group, but the difference equalized by years 3 and 5 of the study.
Trabeculectomy was associated with significantly higher rates of early wound leaks compared with tubes (11% vs. 1%, p = 0.004), late bleb leaks (6% vs. 0%, p = 0.014), and dysesthesia (8% vs. 1%, p = 0.018). The procedures did not differ in the rates of reoperations for complications or the percentage of serious complications. A major disappointment, however, was that 43% and 46%, respectively, of patients lost two lines or more of Snellen visual acuity.
Looking to the future, Dr. Heuer emphasized, the factors that need determining if the 5-year findings will be maintained are:
· If the medication burden will continue to increase in one or both groups.
· If late bleb or tube complications will have a greater impact on visual function or quality of life.
The complications that are specific to tube implantation are diplopia, tube erosion, and corneal edema.
A finding of the TVT Study was that notable percentages of patients had motility problems preoperatively, such as ocular deviations, 29% and 26%, respectively. Postoperatively, persistent diplopia developed in 5% of shunt patients and acquired or worsened ocular deviations were seen in 10% of shunt patients.
However, while most diplopia is transient, Dr. Heuer suggested that avoiding superonasal implantation and carefully identifying rectus muscles and implanting the devices could decrease the diplopia rates.
Tube erosion and patch melting can occur after shunt implantation. While different tissues (dura, pericardium, and sclera) have been used, all are associated with some erosion.
“The best prospective data we have are from the TVT Study, which showed a 5% rate of erosion over 5 years,” Dr. Heuer said.
Corneal tissue—which is now more readily and economically available—is gaining in popularity. Its advantages are better cosmesis and ability to laser tube ligation sutures postoperatively.
Although unconfirmed, there was less likelihood of tube erosion through corneal tissue than pericardium in one recent comparative study. Its disadvantages are that corneal tissue requires three 10-minute fluid exchanges during surgery and the tissue from some sources requires intraoperative thinning.
Erosion may also be reduced by inserting the tube 2 to 3 mm from the limbus, inserting the tube close to the 12 o’clock position, and inserting through the pars plana.
The TVT Study reported that corneal edema developed in 16% of all patients, with almost equal amounts in both groups.
Dr. Heuer advised inserting the tube posteriorly just anterior and parallel to the iris, inserting the tube into the pars plana, or inserting the tube into the posterior sulcus could reduce that corneal edema.
However, Dr. Heuer said, the jury is still out on which procedure is better, as shunts are definitely being used more frequently, “dramatically so,” based on Medicare data, with a concurrent decline in the number of trabeculectomies performed.