The Ahmed Baerveldt Comparison (ABC) Study: Long-Term Results, Successes, Failures, and Complications

Over several decades, glaucoma surgical options for adult patients have progressed from various forms of full-thickness filtering surgery to the later additions of trabeculectomy, trabeculectomy with antifibrosing agents, glaucoma implants, and the more recent proliferation of small implantable glaucoma surgical devices. Among the available choices of glaucoma tube shunt implants, 2—the Ahmed and the Baerveldt Glaucoma Implants combined—have captured most of the United States market share. The use of glaucoma implants has been increasing with a corresponding reduction of the total number of trabeculectomies performed annually in the United States.

In the Tube Versus Trabeculectomy (TVT) Study, in which only the Baerveldt 350 mm 2 implant was used, intraocular pressure (IOP)-lowering ability was 1.8 mm Hg better in the trabeculectomy group than in the glaucoma implant group and the need for postoperative glaucoma medication was similar for both groups. The cumulative failure rate was significantly less in the glaucoma implant group (29.8%) compared to the trabeculectomy group (46.9%) and the rate of reoperation for uncontrolled glaucoma was 9% in the tube group and 29% in the trabeculectomy group. Early complications were more frequent in the trabeculectomy group (37%) than in the tube group (21%) and late complications developed in 36% of the trabeculectomy group and in 34% of the tube group. The TVT Study has helped clinicians to increase the use of glaucoma implant tube shunts for refractory glaucoma in the United States. However, after the TVT Study, trabeculectomy surgery with or without antifibrosing agents continues to play an important role in the surgical management of glaucoma for selected patients. Both procedures remain good choices for both glaucoma fellowship–trained ophthalmologists and some comprehensive ophthalmologists who perform trabeculectomies and/or glaucoma tube shunt surgery.

The more recent Ahmed Baerveldt Comparison (ABC) Study was a 16 clinical center, international, prospective, randomized, unmasked clinical trial comparing outcomes of the valved Ahmed FP7 and the nonvalved Baerveldt 101–350 glaucoma implants in 276 patients. In a previous report, Budenz and associates showed that at 5 years, the Baerveldt glaucoma implant patients had a 2.0 mm Hg lower mean IOP than the patients with the Ahmed implant and mean postoperative glaucoma medication use was less for the Baerveldt implant group than the Ahmed implant group. The 5-year cumulative failure rates were 44.7% in the Ahmed group and 39.4% in the Baerveldt group. Most failures (80%) in the Ahmed group were because of failure to adequately lower IOP and 20% were because of too low an IOP, explantation of the implant, or loss of light perception vision. However, in the Baerveldt group 47% of eyes were failures for 1 of those 3 reasons. Surgeons can expect better intraocular pressure control with the Baerveldt glaucoma implant, but with greater risk of achieving too low an intraocular pressure.

In this issue, Budenz and associates report long-term complications in the Ahmed and Baerveldt glaucoma implants. At 5 years, the cumulative rates of complications were 46.8% in the Ahmed implant group and 56.3% in the Baerveldt implant group. Serious complication rates were 15.9% and 24.7% for the Ahmed and Baerveldt glaucoma implants, respectively. Baerveldt implants were 7 times more likely to become obstructed than Ahmed implants and tube erosions were more likely in the Baerveldt (3%) than in the Ahmed (1%) glaucoma implants. Details about the reason(s) for tube obstructions were not provided in the manuscript. Rates of persistent diplopia (12%) and corneal edema (20%) were similar in both groups. Also in both study groups, some of the patients with postoperative corneal edema had preexisting corneal disease.

The ABC Study design incorporated preoperative and postoperative motility measurements for all study patients. The similar incidence of diplopia in the 2 study groups may reflect a few possible issues. There were reports of more frequent diplopia with the since-discontinued large 500 mm 2 Baerveldt glaucoma implant. Also, the addition of plate fenestrations may have helped to reduce the incidence of diplopia after the implantation of currently available Baerveldt glaucoma implants. Factors such as glaucoma implant bleb height and proximity to extraocular muscle insertion or superior oblique muscle tendon may play a role in causing diplopia in any patient with any chosen glaucoma implant.

The message of these 2 reports is that both glaucoma implant devices substantially reduce IOP from baseline values. However, with the Baerveldt glaucoma implant, success rates were higher and patients achieved both a statistically significant lower IOP ( P = .015) and a trend toward statistical significance ( P = .28) for reduced need for glaucoma medication compared to the Ahmed implant. Complication rates and the risk of hypotony were greater for the Baerveldt implant than for the Ahmed implant. The larger size of the Baerveldt implant over the Ahmed implant probably explains the greater intraocular pressure reduction and trend for the need for less postoperative glaucoma medication in the Baerveldt implant eyes. Perhaps the Baerveldt implant could be preferred by some surgeons over the Ahmed implant in patients with more advanced glaucomatous optic nerve damage and glaucomatous visual loss. Once again, surgeons have 2 good choices in using either the Ahmed or Baerveldt glaucoma implants for their patients.

No well-designed clinical trial can cover all possible clinical situations and variations. There are questions not addressed in these 2 important reports and in other reports. How would success and complication rates of the Ahmed and Baerveldt glaucoma implants compare with other devices such as the ExPress Shunt and i-Stent? FDA-approved indications can differ for the newer assorted glaucoma surgical devices. What is the best management for the patient with uncontrolled glaucoma and visually significant cataract? Are glaucoma surgical patients better off when their surgeon uses the Ahmed or Baerveldt glaucoma implant with which he or she has more or exclusive experience? A surgeon’s choice of glaucoma implant could depend, in part, on clinical training experiences. The Baerveldt glaucoma implant requires some form of temporary tube stenting or ligation. Surgical variations used by some surgeons include placing slits in the Baerveldt tube or combining the Baerveldt glaucoma implant surgery with an “orphan” trabeculectomy.

Should either or both glaucoma implants be equally preferred in pediatric glaucoma patients? Is a smaller implant such as the Ahmed FP-8 glaucoma implant better for small (early postnatal or nanoophthalmic) eyes? Is a valved (Ahmed) or stented nonvalved (Baerveldt) preferred for eyes with aphakia, for an anterior chamber intraocular lens implant, or after pars plana vitrectomy? With partially or subtotally removed vitreous, these eyes are at a greater risk for choroidal hemorrhage during transient postoperative episodes of hyoptony after any glaucoma surgical procedure. Could a valved glaucoma implant be safer in these eyes? Would the incidence of new postoperative corneal edema vary depending on whether the tube is placed in the anterior chamber in front of the iris, behind the iris, or into the vitreal cavity through the pars plana? What is the preferred surgical option for the patient with 1 failed glaucoma implant because of inadequately controlled intraocular pressure? Should a second glaucoma implant be placed, or should a cyclodestructive procedure be offered?

There are other new challenges facing surgeons performing glaucoma implant surgery. The Center for Medicare and Medicaid Services bundled reimbursement for glaucoma implant surgery with a scleral reinforcing graft into 1 payment in January 2015. This has caused some glaucoma implant surgeons to abandon using a scleral reinforcement graft in favor of passing the tube through a scleral tunnel. Could there be any increased risk of tube exposure in these eyes? If a graft is used, is any one type of scleral reinforcement graft (cornea, sclera, pericardium) preferred and less likely to erode over time after glaucoma implant surgery?

The TVT and ABC studies were well-conducted, prospective multicenter clinical trials that have helped quantify the long-term success rates and mild and serious complication rates of our glaucoma surgical procedures and will continue to help ophthalmologists reshape our surgical armamentarium and improve the quality of care that we provide to our glaucoma patients. However, surgical choices will have to be made with the individual glaucoma patient in mind. Glaucoma diagnosis, severity, level of visual acuity, presence or absence of preexisting conjunctival scarring, lens status, history of vitrectomy, and presence of other ocular comorbidities can sway the surgeon to decide in favor of one procedure over another.

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Jan 6, 2017 | Posted by in OPHTHALMOLOGY | Comments Off on The Ahmed Baerveldt Comparison (ABC) Study: Long-Term Results, Successes, Failures, and Complications

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