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Editors Selection IGR 16-4

Surgical treatment: Aqueous drainage devices

Comment by Franz Grehn on:

59586 Five-Year Treatment Outcomes in the Ahmed Baerveldt Comparison Study, Budenz DL; Barton K; Gedde SJ et al., Ophthalmology, 2015; 122: 308-316

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This paper reports the five-year results of a randomized prospective study comparing the outcome of Ahmed Glaucoma Valve Implant (AGV) versus the 350 mm² Baerveldt Glaucoma Implant (BGI). Primary outcome was failure rate as defined by the study protocol (inadequate IOP control without re-operation, reoperation, explantation, persistent hypotony, loss of light perception). Secondary outcome measures were IOP, visual acuity, glaucoma medications, and complications. The protocol was designed according to the 'Guidelines on Design and Reporting of Surgical Trials' of the World Glaucoma Association.

Out of 276 patients (eyes) entering the study at beginning, 174 (87 in each group) completed the five-year follow-up. Similar rates of surgical success were observed with both implants. In general, the BGI produced greater IOP reduction and a lower rate of glaucoma reoperation than the AGV, but the BGI was associated with twice as many failures because of safety issues.

The detailed results of this study allow further insight in the differences of outcomes of the two implants.

The Kaplan Meier success survival rates were virtually identical during the first four years with a non-significant difference at five years (44.7% in the AGV group and 39.4% in the BGI group). The IOP of the AGV group was reduced from 29.6 ± 10.1 mmHg at baseline to 14.7 ± 4.4 mmHg, and the IOP of the BGI from 28.3 ± 9.3 mmHg at baseline to 12.7 ± 4.5 mmHg at the five-year follow-up visit, respectively. This difference was significant even if adjusted for the lower baseline IOP of the BGI. The AGV patients had more inadequately controlled IOP or reoperations as compared to the BGI group, counting as failures (AGV 80% of all failures vs. BGI 53% of all failures). However, the BGI had more sight-threatening complications ‒ 25 (47% of failures) vs. 11 (20% of failures). One possible bias discussed in the paper was lower failing IOP before re-operation in the AGV group (20.0 vs. 23.0 mmHg). This might have increased the number of re-operations (counted as failures) in the AGV group. In the AGV group 20.8% compared with 8.6% in the BGI group underwent reoperation for glaucoma during five years of follow-up.

The number of complete successes at five years was nine (8%) in the AGV group compared with 14 (14%) in the BGI group (P = 0.27). This demonstrates a general need of additional medication in tube implants and outlines the very low rates of complete success. In trabeculectomy, even after five years of follow-up, the complete success rate is considered considerably higher by most surgeons.

The cumulative probability of qualified (with or without medications) success, defined as IOP ≤ 17 mmHg and reduced IOP by at least 20%, was 39.6% in the AGV group and 53.9% in the BGI group (P = 0.048) at five years.

There was a significant decrease in Snellen VA in both treatment groups during the five years of follow-up. Approximately 43% of subjects lost two or more lines of Snellen VA. There was no significant difference in logMAR Snellen VA between the two groups at five years. In neovascular glaucoma, at five years, the cumulative proportion of patients who progressed to NLP in the AGV group was 28.3% compared with 51.1% in the BGI group (P = 0.030). The authors also report that compared with the three-year study results, there were no additional subjects in the AGV group who lost two or more lines of vision, but there were eight additional subjects in the BGI group who lost two or more lines of vision.

In summary, the BGI seems more effective in providing long-term IOP control than the AGV implant (2 mmHg lower). However, the BGI had a higher number of inadequately low IOPs and also a considerable number of phthisis cases. Therefore, averaging the IOP of all cases may have shifted the mean value towards lower IOPs in the BGI group. However, with the WGA success criteria that define a bimodal IOP cut-off, the BGI still provides higher rates of IOP control.

Both the ABC and AVB Studies observed significantly greater longterm IOP reduction and less need for glaucoma medical therapy with the BGI compared with the AGV, with similar success rates after three years of follow-up

The major challenge of the comparison is the balance between IOP results and safety issues. Although the AGV had more reoperations and needed more additional medication, which may have a major impact on quality of life of the patient, the BGI was more challenged with severe safety issues such as phthisis, loss of light perception, hypotony, and implant explantation. Safety differences may be more relevant for complictated cases such as neovascular glaucoma (which would favor AGV), whereas in less complicated cases the safety aspects are less frequent and IOP control becomes more important (which would favor BGI).

One important aspect is the postoperative hypertensive phase in ligated BGI tubes that only open after approximately six weeks, whereas the AGV will better control the IOP in the early postoperative phase. Therefore, the BGI may have advantages when the risk profile of an individual case is lower.

The present study does not explicitly report corneal endothelial counts, which would be a surrogate of corneal impact. However, in Table 5 of the paper, three of 36 cases (8%) of AGV and ten of 38 (26%) cases of BGI were reported to have more than two Snellen lines visual deterioration due to corneal problems of various reasons.

The general results of the ABC study are also in accordance with the AVB study, the other comparative tube implant study. Both the ABC and AVB Studies observed significantly greater long-term IOP reduction and less need for glaucoma medical therapy with the BGI compared with the AGV, with similar success rates after three years of follow-up.

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