Dogma vs Data: Glaucoma Drainage Implants

When and which kind?


The glaucoma drainage implant (GDI) has been used by ophthalmologists for about 50 years. The basic concept involves a tube draining aqueous from the anterior chamber into a plate reservoir. Since the introduction of the Molteno device, GDIs have increased in popularity for the treatment of glaucoma.1 Since then, other types of drainage implants have been introduced into the market. The devices can be generally split into 2 categories: non–flow-restricted GDIs and flow-restricted GDIs.

Currently, glaucoma providers in the United States have access to the Molteno drainage implant (non-flow-restricted; Molteno Ophthalmic Limited), Baerveldt glaucoma implant (BGI, non-flow-restricted; Johnson & Johnson Vision), and Ahmed implants (flow-restricted Ahmed glaucoma valve [AGV] and non-flow-restricted Ahmed Clearpath; New World Medical).

Over the last 2 decades, glaucoma surgeon practice patterns have changed. Surveys of members of the American Glaucoma Society (AGS) showed that 93% of respondents would proceed with a second trabeculectomy with antifibrotics after a failed trabeculectomy vs 7% who would choose a glaucoma drainage device in 1996. In 2016, only 20% of respondents would choose a repeat trabeculectomy while 71% would move forward with a GDI.2 These survey results have been confirmed by Medicare claims data. Although the number of glaucoma procedures performed annually remained stable between 1994 and 2012, the number of trabeculectomies performed decreased by 77%, while the number of GDIs increased by 410% over the same time period.3

While GDIs were initially considered secondary surgery, several studies evaluated the role of GDIs as primary surgery for glaucoma. A prospective, randomized study showed that postoperative intraocular pressure (IOP) was comparable after trabeculectomy with or without mitomycin C and AGV in eyes without prior ocular surgery through 3 to 4 years. In this comparative study, complications and reoperation rates were similar between the 2 groups.4 The Tube vs Trabeculectomy study was a prospective, multicenter, randomized study that compared the outcomes of trabeculectomy with mitomycin C and Baerveldt GDI. At 5 years, the overall success rate was higher in the tube group with decreased complications and lower rates of reoperation than in the trabeculectomy group.5 These studies have broadened the clinical use of GDIs for glaucoma treatment. In the AGS practice preferences survey, GDI was favored over trabeculectomy in 7 out of 8 clinical settings in 2016 and in none of the same clinical settings presented in 1996.2

Choices of Implants: Studies

The BGI and AGV have also been compared in different settings. The Ahmed Baerveldt Comparison (ABC) study was a prospective, multicenter, randomized clinical trial comparing the outcomes of the Ahmed FP7 Glaucoma Valve with the Baerveldt 101-350 in refractory glaucoma or after intraocular surgery. At 5 years, eyes that underwent implantation with the Baerveldt device had lower IOP (12.7±4.5 mmHg vs 14.7±4.4 mmHg, P=.012). Sight-threatening complications (persistent hypotony, explantation of implant, or loss of light perception) accounted for double the failures in the BGI group compared with the AGV group (47% BGI vs 20% AGV).6

The Ahmed Versus Baerveldt (AVB) study was also a prospective, randomized, multicentered study. Patients with failed trabeculectomy or at high risk for failure were randomized to have a Baerveldt 350 or Ahmed FP-7 implanted. The 5-year follow-up data showed that the Baerveldt group had lower postoperative IOP (13.6±5.0 mmHg vs 16.6±5.9 mmHg, P=.001), while the Baerveldt group had a small risk of hypotony that was not observed in the Ahmed group.7


Various types of dogma are encountered among clinicians, often concerning when to use GDIs or what type to use. We have a number of patients who have been treated with a nonvalved implant in one eye and a valved implant in the other eye. Some of these were told by other clinicians that they should achieve lower IOP than the eye with the nonvalved implant, and they happily point out each visit when the opposite is found. Much of this is the result of individual variability in the postoperative results, which reminds us that individual outcomes often vary from average results in clinical research studies. It is likely, however, that experience and expertise with the specific implant are important factors in outcomes.

Another dogma held around the world is that clinicians often believe that they need adjunctive mitomycin C to optimize results of GDI surgery, using a similar intraoperative dosing as used for trabeculectomy surgery. While this may be true for these clinicians’ unique patients using their regimen for dosing mitomycin C, it has been shown in randomized clinical trials that there is little effect of this approach on the outcomes of GDI surgery.8 Positive results have been reported for use of antifibrosis drugs with GDIs, but not for standard dosing used historically with trabeculectomy.

Is the Dogma the Data?

A useful approach to identifying clinician preferences is to track utilization data. Clinicians do express their preferences in the choices they make for drainage implant surgery devices. Currently, the majority of clinicians in the United States use valved GDIs compared with nonvalved implants, and internationally, a large majority of GDIs utilized are valved devices (Bilal Khan, New World Medical, personal communication). The utilization of specific implants does not appear to be reflected in clinical trial results. Perhaps our clinical trials are not addressing some of the variables influential to clinicians, such as ease of implantation or postoperative care. Perhaps variables that strongly influence clinicians are not weighted heavily enough. For example, many clinicians have an aversion to experiences of vision-threatening complications, while they may accept a small and clinically unimportant difference in postoperative IOP. Clinicians also make every effort to avoid other less “vision-threatening” problems that may be difficult to manage and dissatisfying in patients, such as diplopia or hypotony. It is possible that clinical trials are not adequately measuring variables that are important to clinicians.


Clinicians can achieve excellent results with both valved and nonvalved implants. Clinician experience and expertise with a specific implant may be important in achieving successful outcomes with either type of implant. While evaluating dogma and choices, it is helpful to remain evidence based but open minded. This approach is commonly used in the rapidly evolving field of minimally invasive glaucoma surgery, and it is also helpful in clinical use of more established GDIs. GP


  1. Ashburn FS, Netland PA. The evolution of glaucoma drainage implants. J Ophthalmic Vis Res. 2018;13(4):498-500.
  2. Vinod K, Gedde SJ, Feuer WJ, et al. Practice preferences for glaucoma surgery: a survey of the American Glaucoma Society. J Glaucoma. 2017;26(8):687-693.
  3. Arora KS, Robin AL, Corcoran KJ, Corcoran SL, Ramulu PY. Use of various glaucoma surgeries and procedures in Medicare beneficiaries from 1994 to 2012. Ophthalmology. 2015;122(8):1615-1624.
  4. Wilson MR, Mendis U, Paliwal A, Haynatzka V. Long-term follow-up of primary glaucoma surgery with Ahmed glaucoma valve implant versus trabeculectomy. Am J Ophthalmol. 2003;136(3):464-470.
  5. Gedde SJ, Schiffman JC, Feuer WJ, Herndon LW, Brandt JD, Budenz DL; Tube Versus Trabeculectomy Study Group. Treatment outcomes in the tube versus trabeculectomy (TVT) study after five years of follow-up. Am J Ophthalmol. 2012;153(5):789-803.
  6. Budenz DL, Barton K, Gedde SJ, et al; Ahmed Baerveldt Comparison Study Group. Five-year treatment outcomes in the Ahmed Baerveldt comparison study. Ophthalmology. 2015;122(2):308-316.
  7. Christakis PG, Kalenak JW, Tsai JC, et al. The Ahmed versus Baerveldt study: five-year treatment outcomes. Ophthalmology. 2016;123(10):2093-2102.
  8. Costa VP, Azuara-Blanco A, Netland PA, Lesk MR, Arcieri ES. Efficacy and safety of adjunctive mitomycin C during Ahmed Glaucoma Valve implantation: a prospective randomized clinical trial. Ophthalmology. 2004;111(6):1071-1076.