The ABC study was designed to compare the outcomes and complications for two aqueous shunts commonly used for refractory glaucoma, the Ahmed glaucoma valve and the Baerveldt glaucoma implant. Both procedures lowered IOP and medication use significantly from baseline. In addition, the failure rates by predetermined criteria were similar for both implants. However, the Ahmed group had a higher rate of reoperation for glaucoma than the Baerveldt group. This is consistent with a greater efficacy of the Baerveldt implant, as indicated by slightly greater pressure reduction, and a tendency for greater glaucoma medication use by the Ahmed group. The average IOP after 1 year of follow-up was 2.2 mmHg lower in the Baerveldt group, a statistically significant difference. During the first postoperative month, IOP was lower in the Ahmed than the Baerveldt group, as might be expected due to the valve mechanism in the Ahmed implant. Surgeons had the option of using tube fenestrations in the Baerveldt group for early IOP pressure control, but this surgical maneuver has variable success.17
The patients in the Baerveldt group required twice as many postoperative interventions and experienced a third more serious postoperative complications, which were also statistically significant. While the Baerveldt implant provided slightly better IOP lowering at 1 year and less need for reoperation for elevated IOP, this improved success came at the price of more serious complications.
There have been numerous retrospective case series reporting the results of the use of a single model implant such as the Ahmed and Baerveldt in refractory glaucoma. Schwartz and colleagues recently published a review of the literature comparing results with different types of aqueous shunts.13
They point out that it is difficult to compare results from single model implant case series since each of these studies involve different groups of patients, surgeons, definitions of success, and follow-up times.13
There are four studies that have directly compared the results of the Ahmed and the Baerveldt implants in retrospective comparative case series.7–11
Tsai and colleagues published early- and intermediate-term results7, 10
in a group of patients who underwent either implantation of the older, AGV-S2 or the BGI (350 mm2
or 250 mm2
) by a single surgeon. The two groups differed significantly in age, preoperative IOP, and diagnosis. Similar to the current study, IOP was lower in the Ahmed group at the 1-day and 1-week postoperative visits and no significant difference in overall survival rates between the implants was found. IOP was the same in the two groups at one year, but after one year, the IOP in the Ahmed group got steadily higher and stayed higher out to four years of follow-up. Wang et al9
published a retrospective comparative case series of 41 Asian patients who underwent either an AGV-S2 or BGI 350 mm2
implant by a single surgeon. The patients were followed for an average of 23 months in both groups. The average age in the Ahmed group was 12 years older than the Baerveldt group. They found no statistically significant differences in survival rates or IOP at last follow-up in their small group of patients, although the survival rate was higher in the Baerveldt group (88%) compared to the Ahmed group (77%) and the final IOP was 2.5 mmHg lower in the Ahmed group. To attempt to compensate for the selection bias inherent in retrospective studies, Syed and colleagues8
performed a comparison of patients who received the AGV-S2 (n = 32) and the BGI 350 mm2
(n = 32) matching for age, glaucoma diagnosis, and preoperative IOP. A non-time adjusted survival comparison failed to find any difference in success and the average IOPs were similar in the two groups throughout the approximate 1-year of the study. Survival analysis, which takes into account length of follow-up and drop outs, was not performed. Goulet and associates11
performed a retrospective comparative case series in their institution of 59 patients who received an Ahmed S2 implant and 133 patients who received a Baerveldt 250 mm2
implant. Their study showed a higher success rate and lower IOP for patients who received the Baerveldt implant after an average follow-up of 20 – 23 months. So the current retrospective evidence is inconclusive as to which implant lowers IOP better, has higher success rate, and fewer complications. These studies all suffer from their retrospective design and selection bias of which patients received which implant, although one study8
matched on several potentially confounding variables. In addition, all of the aforementioned studies used the AGV S2 model, which has a polypropylene plate, rather than the AGV FP7 model, which has a silicone plate (as does the BGI 350 mm2
). There is evidence that the silicone material in the AGV FP7 model provides better IOP reduction than the AGV S2 model.18, 19
The strength of the randomized prospective trial design used in the current study is that selection bias is eliminated and confounding variables tend to be balanced in the two groups, making conclusions stronger. The comparability of the two treatment groups created by randomization was examined in the baseline paper and no significant differences were found. In addition, the multicentered study design with 25 different surgeons operating upon patients in 4 continents improves the generalizability of the results.
Visual acuity decreased by 2 or more lines of Snellen visual acuity in approximately 32% of patients overall and was not different between the groups. Among patients with complications, 18% of patients in the AGV group and 27% of patients in the BGI group by one year of follow-up. Snellen VA was similar between treatment groups at 1 year and no significant differences in the rates and reasons for vision loss were present in the two groups. These rates of visual acuity loss are high but consistent with those found in the TVT study groups at one year.5
Most of the causes of loss of two or more Snellen lines were related to cataract, age related macular degeneration and glaucoma, which was also found in the TVT study.5,6
It is unclear whether the vision loss was more associated with surgical complications or the underlying severity of disease in the group of patients studied. Of note, all 8 patients who lost light perception vision were in the neovascular glaucoma stratum and there was a higher prevalence of surgical complications in this group as well.
Many surgical complications were reported in the ABC Study but most were transient and did not require intervention. A similar high rate of complications was reported in the TVT Study at one year.5
More patients in the BGI group experienced early postoperative complications than the AGV group in the ABC Study and the complication rate between groups was similar for late complications. However, all surgical complications are not equal in severity, and the rate of serious complications associated with reoperation and/or vision loss was higher in the BGI group.
The specific design features of the Ahmed and Baerveldt implants may explain some differences in clinical outcomes. The Ahmed implant has a restrictive “valve” device designed to prevent hypotony. This is particularly important in the immediate postoperative period before a capsule forms around the end plate, which restricts flow later. The Baerveldt implant does not have a flow restrictor, and hypotony with its resultant complications is much more common20
if flow is not restricted by the surgeon using a suture ligature,13, 21
which either dissolves or is removed once the end plate encapsulates to limit flow. It is therefore not surprising that in the current study, IOP was higher in the Baerveldt group in the period before flow was established 4 to 6 weeks postoperatively. Although there were only two cases of failure due to persistent hypotony by one year, both cases occurred in the BGI group. The second important design difference between the two implants is the size of the drainage plate, which are 184 mm2
for the Ahmed glaucoma valve and 350 mm2
for the Baerveldt implant. Several studies have shown that lower mean IOP can be achieved with larger implant plate sizes of the same general design. Heuer and colleagues12
performed a prospective randomized trial of the single- (134 mm2
) versus double-plate (268 mm2
) Molteno implant and found both higher success rates and lower IOPs with the larger implant. There were also more postoperative complications associated with the double plate Molteno implant. This same group performed a study randomizing 73 patients to the 350 mm2
and the 500 mm2
Baerveldt implant and found no difference in success or IOP lowering22
at 18 months. A subsequent longer term analysis of the same patients found the 350 mm2
implant to have a slightly higher success rate than the 500 mm2
In the current study, the Baerveldt implant, with a larger surface area, provided slightly lower IOPs at 1-year and fewer failures due to inadequate IOP control. Subsequent reports with longer follow-up of the current patients will provide additional information on the relative efficacy of these two implants on long-term IOP control.
One of the potential limitations in the current study is surgical experience with the two treatment arms of the study. Although one would hope that randomization would distribute patients receiving a particular implant to surgeons of differing experiences equally, we were concerned about this as a source of bias. Investigators were required to submit an estimate of the number of surgical procedures that they had performed using each type of implant. If a surgeon had performed fewer than five surgeries using a particular implant, they were required to submit a videotape of themselves performing the procedure to make sure proper surgical techniques were followed. In addition, an analysis of success and complications dividing surgeons into those who had performed fewer than 20 versus 20 or more of a particular procedure was performed. This analysis showed that experience (≥20 prior cases) with a particular implant reduced the risk of failure in the multivariate analysis but not in the univariate analysis. There was no relationship between the total number of complications and surgical experience with a particular implant. It does not appear that relative inexperience with implantation of either the Ahmed or Baerveldt implant affected success rates or complications.
In summary, the ABC study found greater IOP reduction with the Baerveldt Glaucoma Implant after 1 year of follow-up, but fewer early and serious complications were observed with the Ahmed Glaucoma Valve. The efficacy of glaucoma procedures in reducing IOP must be evaluated in light of the adverse events associated with their use. Therefore, this study does not demonstrate clear superiority of one implant over the other. Also, with a significance level set at 0.05, there is always a one in twenty chance that any statistically significant results found in this study could have occurred by chance alone. Additional follow-up is needed to fully evaluate the risk-benefit ratio of both devices in the surgical management of refractory glaucomas. The ABC study is designed to continue follow-up of participants out to five years.