In a systematic review of literature, Dr Donald Minckler reports on randomized, controlled trials of aqueous shunts and compares the success rates with those achieved by trabeculectomy.
The review also failed to demonstrate that any shunt among those currently in widespread use (Ahmed Glaucoma Valve, New World Medical Inc.; Molteno, Molteno Ophthalmic Ltd.; Baerveldt, Advanced Medical Optics) was superior to others currently in widespread use.
Donald S. Minckler, MD, MS, professor of ophthalmology and pathology at the University of California, Irvine, USA, conducted the literature search for Cochrane Review, studying results of 15 randomized and quasi-randomized controlled trials in the world's literature for aqueous shunts. The trials involved 1,153 participants with varied types and severity of glaucoma. He described his findings at the American Glaucoma Society subspecialty day lecture at the annual meeting of the American Academy of Ophthalmology, held in Las Vegas last year.
What was included in the study?
Dr Minckler's study included only randomized trials with all types of participants (age, gender and disease stage) and all comparative interventions, assuming that one arm of each trial included a shunt of any type. The literature search included multiple electronic databases, hand searching, screening for information from companies and manufacturers and any other published or unpublished trials.
The principal outcome measure was IOP at one year. Secondary outcome measures initially planned included visual fields, surgical complications, time after surgery to achieve a 15% decrease in IOP, success or failure of treatment defined in a standardized manner and whether a hypertensive phase occurred. None of the planned secondary outcomes could be determined across all studies because of inconsistencies of reporting.
Selection bias (concealment of allocation and methods of randomization) was assessed. Attrition bias (accounting for all patients during follow-up) was also assessed and, although generally similar in control and study groups, varied across studies. In such trials, it was not possible to mask surgeons or evaluators.
Implants go to trial
Dr Minckler found 15 randomized, controlled trials and combined two of them (Lloyd 1994 and Britt 1999), because they were serial studies of the same patients. A meta-analysis of two studies (Wilson et al. 2000 and Wilson et al. 2003) evaluating the Ahmed valve versus trabeculectomy was conducted, showing a statistically significant difference in IOP levels at one year, favouring trabeculectomy.
Two papers examined the effectiveness of implants with or without mitomycin-C. The paper by Costa et al. involved the Ahmed valve, whereas the earlier study by Cantor et al. used a Molteno implant. "From these, we can conclude that mitomycin-C really had no measurable benefit on aqueous shunts," Dr Minckler said.
Other studies looked at the question of surface area. A paper by Heuer et al. compared single- and double-plate Molteno valves, and a study by Britt et al. compared a 350 mm2 versus a 500 mm2 Baerveldt implant. In the former, Dr Minckler concluded that "bigger was better" in only one of three groups of patients: those whose outcome was considered a qualified success (final IOP not >21 mmHg in patients who were taking adjunctive anti-glaucoma medications).