the Ahmed versus Baerveldt Comparative (ABC) Study, a randomized prospective controlled trial has recently completed recruitment. Tsai et al. (2006) reported 118 eyes in a nonrandomized comparison of 70 eyes implanted with Baerveldt shunts (20 Baerveldt 250 and 50 Baerveldt 350 implants) and 48 eyes implanted with Ahmed shunts (S2). At 48 months after surgery, 75 eyes (30 Ahmed, 45 Baerveldt) followed up for 48 months, survival in terms of IOP control and complications was virtually identical in the two groups at 62% for the Ahmed group and 64% for the Baerveldt group (p ¼ 0.843).

The two study groups are not directly comparable, the authors reporting a case selection bias in that patients implanted with the Ahmed were more likely to have glaucoma associated with ocular inflammation (20.8% vs. 4.3%, p ¼ 0.005) and had higher preoperative IOP values (38.5 mmHg vs. 34.6 mmHg, p ¼ 0.032).

The types and causes of failure differed in the two groups in that the Baerveldt group were more likely to fail from hypotony-related complications, and the Ahmed group were more likely to require additional glaucoma medications.

Of note, bleb encapsulation was observed more frequently in the Ahmed group (60% of cases) compared with 27% in the Baerveldt group, even though no overall difference in outcome was noted at 4 years.

It has been suggested that aqueous flow in the early postoperative period might expose subconjunctival tissues to greater levels of inflammatory cytokines than is the case with ligated shunts, where aqueous flow does not occur until 5–6 weeks after surgery. This is an alternative explanation for the higher observed rates of bleb encapsulation with the Ahmed Glaucoma Valve than the plate material, profile, and other factors described above.

A smaller study by Wang et al. (2004) reported a higher success rate for the Baerveldt group at 42 months. In this retrospective study of 23 Baerveldt 350 implants and 18 Ahmed (S2) polypropylene Glaucoma Valves, after a mean of approximately 23 months follow-up in each group, the success rate for Baerveldt shunts was 83% versus 67% for the Ahmed. There did appear to be a difference in

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demographics between the two groups in this study. The mean age at implantation was 48 years for the Baerveldt group and 60 years for the Ahmed group. This was of borderline significance (p ¼ 0.07), despite relatively low study numbers, and possibly does represent a real difference in success between the two shunts.

A case–control study that was published the same year found no significant difference in pressure control, although at 1 year, the Ahmed eyes had a trend toward higher IOP than those with the Baerveldt (Syed et al., 2004). In this particular study, the 1-year outcomes of 32 Baerveldt implants (cases) versus 32 Ahmed implants (controls — matched for age, race, gender, glaucoma subtype, ocular surgical history, preoperative IOP, and surgeon) reported 66%

of devastating complications in this study (9% in each group). The hypotony rate at 37.5% (Baerveldt) and 34.4% (Ahmed) is poor for both study groups. 45.5% versus 29% of patients lost more than one line of Snellen acuity (p ¼ 0.37), but this is comparable to other studies and may also reflect ongoing disease. The authors point out that, in this study, patient selection might also account for some of these results.

Patient and ocular factors

There are many patient-related factors that might influence the type of implant and implantation position.

Severity of glaucoma damage

While early pressure control remains less than completely predictable after aqueous shunt implantation, there will remain arguments for and against using the different available shunts in patients with more advanced glaucoma. If it were possible to guarantee a stable IOP, in a safe range, early in the postoperative period, then the ideal shunt in this type of patient would be the shunt most likely to achieve a long-term pressure in the

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lower part of the normal range, ideally with less dependence on ocular hypotensive medications. The limited available evidence would suggest that the Baerveldt 350 is most likely to achieve this.

Nevertheless, if the eye must sustain a high IOP for several weeks followed by a sudden decompression at an unpredictable time, then there is a high potential cost in achieving a slightly better long-term IOP.

Most of the problems associated with early pressure control can be mitigated to some degree by adjustment of surgical technique. Early hypotony with the Ahmed can be avoided by leaving thicker viscoelastics, such as Healon GV or Healon 5 (Advanced Medical Optics, Irvine, CA), in the anterior chamber at the time of surgery. Early pressure control with the Baerveldt is more difficult to achieve. However, there are methods that can be used to avoid the scenario described above. When complete ligation is used, the tube can also be stented with a 3/0 nylon suture (Supramid, S. Jackson Inc.) in order to reduce the severity of the sudden pressure drop that occurs at the time the ligature opens (Sherwood and Smith, 1993). The early postoperative high pressure can be avoided to some degree with a fenestration (Sherwood and Smith, 1993) or simultaneous orphan trabeculectomy (Budenz et al., 2002). An approach favored by some is to ligate the tube within the anterior chamber with a polypropylene ligature (Budenz et al., 2002). This has the advantage that the ligature can be easily visualized and lasered to reduce the pressure at a preplanned point in time, in theory, more predictable than external ligation with an absorbable ligature. However, a fenestration will be ineffective in this situation and a simultaneous orphan trabeculectomy probably offers a better method of preventing high IOP in the early postoperative period when this type of ligation is used (Budenz et al., 2002).

In eyes that are likely to be especially vulnerable to a sudden drop in IOP, a traditional two-stage implantation technique can be used. Two-stage implantation involves an operation to position and secure the end plate of the implant without implanting the tube portion in the anterior chamber. A second procedure is then performed around 6 weeks later to place the tube portion

inside the eye. At that point in time, the end plate will have encapsulated so that implantation of the tube will permit aqueous flow sufficient to control the IOP, but not enough to cause severe hypotony.

A technique that one of the authors (K.B.) has used successfully is to introduce Sherwood’s 3/0 nylon Supramid, the entire length of the tube. When the tube plus stent are introduced into the anterior chamber via an entry site in the sclera that is tighter than normal (25 gauge rather than 23), the tube is squeezed where it passes through the sclera. Squeezing the tube around the outside of the stent suture increases the resistance to aqueous flow through the tube to a sufficient degree to prevent early hypotony in most cases without the need to ligate the tube.

When performing this technique, it is essential to test aqueous flow by looking for fluorescein dilution over the end plate portion of the shunt after the tube portion is in the eye. The reason for testing aqueous flow is that manufacturing variability in the suture diameter (200–250 mm) influences resistance to flow. In some cases, aqueous flow will be observed despite this stenting technique, and in such cases the tube must also be ligated. In other cases, resistance will be high and there will be no need to introduce the stent along the entire length of the tube. It is therefore worth examining aqueous flow with fluorescein where the tube exits the end plate in order to ascertain if there is aqueous flow with the stent in place. Ideally, the stent should be introduced into the tube just far enough to prevent any aqueous flow (Fig. 5).

Tolerance of topical ocular hypotensive medications

Clearly a patient who has difficulty with topical medications may have difficulty with any shunt as there is a significant chance that topical medications will still be required. Nevertheless, the Baerveldt implant seems to result in a lower need for topical medications than the Ahmed valve in the longer term.

Aqueous hyposecretion

Aqueous hyposecretion is difficult to quantify and is therefore often deduced from clinical

observations. Eyes with extensive ischemia, such as proliferative diabetic retinopathy or ocular ischemic syndrome; eyes that have had extensive cyclodestruction, e.g., two to three treatment episodes of transscleral diode laser cyclophotocoagulation; or eyes with severe chronic uveitis, especially those associated with juvenile idiopathic arthritis, are at greatest risk of hypotony, due to a relative overdrainage from a normally functioning implant.

Previous ocular surgery

Eyes that have undergone previous conjunctival surgery, especially scleral buckling or vitrectomy with silicone oil, and also strabismus surgery and prior trabeculectomy have a higher risk of scarring and plate encapsulation than other eyes and also may be more difficult to implant.

In general, it is better to implant superotemporal where feasible. Erosion rates are higher in the inferior quadrants and there is a higher risk of diplopia when implanting in the superonasal quadrant. While it may be possible to implant low-profile implants such as the Baerveldt 250 superonasally without induced strabismus, the Baerveldt 350 is more likely to interfere with muscle function, and the Ahmed produces a

high-profile bleb that might induce diplopia by displacing the globe slightly. An earlier model of the Baerveldt 350 caused a high rate of diplopia (Smith et al., 1993), but this has been reported less frequently since the introduction of fenestrations through the end plate, permitting fibrous tissue ingrowth through the plate and thereby lower blebs.

In eyes with extensive previous surgery, it may not be possible to implant the wings of the Baerveldt 350 under the muscles. However, if the end plate can be secured tightly to sclera via the fixation holes, it is not essential to implant under the muscles (Fig. 6).

Scleral thinning

Eyes with significant scleral thinning are particularly unsuitable for trabeculectomy surgery, but usually can be implanted safely with a shunt, assuming certain precautions are taken. There are no aspects of implanting an eye with scleral thinning that might alter the authors’ choice of shunt. However, it is important, if possible, to secure the plate at a point where there is adequate sclera for the fixation sutures and to enter the