Aqueous Shunts for the Treatment of Glaucoma

Jeffrey A. Tice, MD



In This Article


Multiple randomized trials have evaluated the efficacy of aqueous shunts for lowering IOP in patients who have failed maximal medical management. None of the trials directly compare the use of aqueous shunts to a sham procedure, so it is difficult to quantify the benefit of aqueous shunts compared to continued medical therapy. However, it would be unethical to not treat patients with inadequately controlled glaucoma when another effective treatment, trabeculectomy, is available. The TVT study convincingly demonstrated the equivalence of the Baerveldt 350 shunt to trabeculectomy in patients who had undergone prior surgery for cataracts or glaucoma. The data for other shunts is less robust, but the ABC trial suggests that the Ahmed shunt is roughly equivalent to the Baerveldt shunt and the two trials comparing the Ex-PRESS shunt to trabeculectomy also suggest equivalence.

TA Criterion 4: The Technology Must be as Beneficial as Any Established Alternatives. Surgical trabeculectomy with either mitomycin C or 5 flurouracil is the current established alterative to which other surgical procedures, like aqueous shunts, are usually compared. The risk of infection (endophthalmitis) following trabeculectomy with anti-fibrotic agents is approximately 1% per year[99] and severe central vision loss is reported in up to 6% of surgically treated eyes.[100] The Tube versus Trabeculectomy study directly addresses the question of whether aqueous tube shunts are as beneficial as trabeculectomy in a population of patients with glaucoma who had previously had ocular surgery. Through three years, the patients who were randomized to the aqueous tube shunt (Baerveldt 350) had fewer complications and fewer failures, while maintaining equivalent control of IOP. Patients in the shunt group had a higher incidence of diplopia (5%) and strabismus (10%). The study was adequately powered and had very little loss to follow-up, but neither the patients nor the investigators were blinded. The study will continue to report outcomes through five years, but the overall conclusions are unlikely to change. Observational studies through ten years of follow-up have not identified a significant increase in the risk for failure of aqueous shunts,[47,52,55] though the efficacy of both aqueous shunts and trabeculectomy continues to wane with time.

The TVT study study investigated one of the most highly studied aqueous shunts, the Baerveldt 350 mm2 shunt. It is unclear whether other shunts are equally effective. However, the ABC study[96] compared the Ahmed shunt to the Baerveldt 350 shunt and reported similar outcomes at one year. The Ahmed shunt had slightly fewer complications, but controlled IOP slightly less effectively. It will be important to watch the long-term results of this study. For now, the evidence supports rough equivalence of the two devices, though the balance of benefits and harms differ and may influence the choice of which device to use based on individual patient characteristics.

The data for the Ex-PRESS shunt are less robust. There are two single surgeon randomized trials that reported equivalent outcomes to trabeculectomy for up to five years with lower complication rates, but the quality of the reporting in the trials and some methodological concerns decrease confidence in the results.[88–90] The total number of patients randomized in the two Ex-PRESS trials is less than half the number in the TVT trial. However, the low complication rate and in the context of the results from the TVT and ABC studies, the Ex-PRESS shunt appears to be equivalent to the existing standard. A randomized trial directly comparing the Ex-PRESS shunt to one of the more commonly used aqueous shunts, like the Baerveldt 350, would better define the balance of benefits and harms with the Ex-PRESS shunt and help surgeons individualize treatment decisions for their patients.

TA Criterion 5: The Improvement Must be Attainable Outside of the Investigational Setting. Surgery to implant aqueous shunts is technically difficult and has an important learning curve. However, the procedure has been successfully performed in multiple centers by multiple surgeons in multi-center trials, like the TVT Study[82] and the ABC study,[96] and have been used in clinical practice for more than twenty years. The results demonstrated in those two randomized trials should be attainable outside the investigational setting. The one exception is the Ex-PRESS shunt. The two trials demonstrating equal or better outcomes compared to trabeculectomy both were single surgeon studies. The results in those trials may reflect the individual surgeon's expertise and may not be attainable in other settings. However, the device was designed to be easier to implant than other aqueous shunts and was performed with similar complication rates as trabeculectomy by residents in a training program.[67]