Key messages

• The Ahmed valve probably achieves a lower decrease in intraocular pressure, might achieve less qualified success than the Baerveldt implant and probably increases the need for reoperation.
• The Ahmed valve might be equivalent to the Baerveldt implant in terms of changes in visual acuity.
• Regarding safety, the Ahmed valve might increase the development of choroidal effusion. However, it probably decreases the development of corneal edema, could decrease the development of narrow anterior chamber and it is not clear whether it increases the development of hypotonic maculopathy.

What is the evidence.
See evidence matrix  in Epistemonikos later

We found five systematic reviews [2],[3],[4],[5],[6] including 10 primary studies reported in 16 references [7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20], [21],[22] of which two corresponded to randomized trials, reported in eight references  [7],[8],[10],[11],[12],[14],[15],[16].
This table and the summary in general are based on the randomized trials  [10],[14], since the observational studies did not increase the certainty of the evidence or provide additional relevant information.

What types of patients were included*

Regarding type of glaucoma, one trial included patients with primary open-angle glaucoma, primary chronic closed-angle glaucoma, neovascular glaucoma and uveitic glaucoma [10] and one trial included patients with primary open-angle glaucoma, primary chronic closed-angle glaucoma, neovascular glaucoma, uveitic glaucoma, post-traumatic glaucoma, combined mechanism glaucoma, congenital glaucoma and penetrating keratoplasty associated glaucoma [14].
Both trials included patients with patients glaucoma refractory  to standard surgery or with a high risk of failure [10],[14]

What types of interventions were included*

Both trials compared the Ahmed FP7 valve versus the Baerveldt 350 mm2 implant  and placed them in the superotemporal quadrant  [10],[14].
Regarding the use of antimetabolites, mitomycin C was not used in any of the arms of the trials  [10],[14]. 

What types of outcomes
were measured

The trials measured multiple outcomes, which were grouped by the systematic reviews as follows:

• Average intraocular pressure  at the end of follow-up.
• Percentage reduction of postsurgical intraocular pressure.
• Absolute reduction of postsurgical intraocular pressure.
• Qualified success (achieving target intraocular pressure independent of the use of medical treatment).
• Complete success (achieving target intraocular pressure without the need of medical treatment).
• Complications (narrow anterior chamber, choroidal effusion, iritis, corneal edema, encapsulated bleb, tube obstruction, tube deviation, tube erosion, ocular motility alteration/ diplopia, hyphema, hypotonic maculopathy, endophthalmitis, corneal ulcer, dysesthesia, persistent diplopia, hypotony, malignant glaucoma, suprachoroidal hemorrhage, retinal detachment, cystic macular edema).
• Need for reoperation.
• Number of drugs used in the postoperative period.
• Decrease in visual acuity.
• Decrease in visual field.

The average follow-up of the trials was 33 months, with a range between 12 and 60 months.

To whom this evidence does and does not apply

• The evidence presented in this summary applies to glaucoma patients with surgical treatment indication, with failure or high risk  of trabeculectomy failure, which is the main indication for the use of the aqueous shunt in glaucoma.
• The patients included in the trials were adults. So, the results cannot be extrapolated to the pediatric population.

• The outcomes included in this summary are those considered critical for decision-making by the authors of this summary. In general, they coincide with the outcomes most frequently reported by the identified systematic reviews.
• The outcomes intraocular pressure reduction, qualified success and reoperation were chosen because they are  crucial for the success of the procedure. In addition, complications of surgery were chosen as safety parameters of the intervention. 

• The use of the Baerveldt implant probably achieves a greater decrease in intraocular pressure and a lower need for reoperation, and might achieve greater qualified success. In addition, it might decrease the development of choroidal effusion.
• On the other hand, the use of the Ahmed valve probably decreases the development of corneal edema and might decrease the development of narrow anterior chamber. However, it is important to keep in mind the limitation of the existing evidence.
• Even though the use of the Baerveldt implant seems to be more effective than the Ahmed valve, it is associated with an increase in complications. However, It is not possible to make an adequate balance between benefits and risks because of the existing uncertainty. 

• None of the systematics reviews considered an economic analysis within its outcomes. Therefore, it is difficult to estimate if the cost-benefit would be favorable to the use of the Baerveldt implant, considering that it would be more effective, but it would increase some of the complications when compared to the use of the Ahmed valve.
• If we consider the cost of each aqueous shunt, the Baerveldt implant is significantly less expensive than the Ahmed valve, thus attending to the conclusions of this work, it seems reasonable to consider the Baerveldt implant as more cost-effective.

• Faced with the evidence presented in this summary, most clinicians should favor the use of the Baerveldt implant because it would achieve a greater decrease in intraocular pressure in the medium and long term. 
• However, the Ahmed valve unlike the Baerveldt implant, is technically easier to install and allows an immediate decrease in intraocular pressure, which is why its use could be preferred by clinicians,  despite the favorable evidence for the latter. 
• The limited certainty of the evidence in some outcomes may also be a factor that leads to variations in decision-making, especially considering the larger clinical change perceived by patients is in visual acuity.

Differences between this summary and other sources

• The systematic reviews reached similar conclusions to those presented here. However, the reviews were cautious about these results due to the limitations of the primary studies and their risk of bias.
• The main guidelines recommend the use of aqueous shunts in patients who are refractory to standard surgery or at high risk of it [23],[25] and in patients with moderate or advanced glaucoma as an alternative to trabeculectomy [24],[25]. One clinical guideline concluded that the Baerveldt implant would have a higher efficacy but, considering its  increase in complications, especially in those related to hypotony, it did not recommend one aqueous shunt over the other [25]. Other two guidelines did not report differences between both aqueous shunts.

• The probability that future research changes the conclusions of this summary is high, due to the uncertainty on the existing evidence.
• No ongoing trials were found in the International Clinical Trials Registry Platform of the World Health Organization comparing the use of the Ahmed valve versus the Baerveldt implant.
• We did not identify ongoing systematic reviews registered in the International Prospective Register of Systematic Reviews (PROSPERO) answering the question of interest.