anterior chamber in an area where there is not excessive scleral thinning.

Patient cooperation for and tolerance of potential slit-lamp interventions

There are other factors that might also influence the choice of implant. In an eye where a patient is unlikely to tolerate postoperative slit-lamp interventions, the individual surgeon will have personal preferences that will influence the choice. This might be to use an Ahmed shunt, leaving viscoelastic in the eye at the time of surgery, or to use a Baerveldt, ligated but partially stented, to prevent a sudden pressure drop at the time of ligature absorption. Although it is difficult to generalize on the course of action required in these situations, it is essential that the surgeon considers the possibility of adapting his or her technique in order to be prepared should the situation arise.

Future challenges

Until recently, aqueous shunt have been reserved for complicated glaucomas that are unresponsive to both medical and other forms of surgical therapy. In such cases, the potential benefits in

terms of IOP control have outweighed the risks in terms of unpredictability of early IOP control. However, improving surgical outcomes from aqueous shunts, as demonstrated in the tube versus trabeculectomy (TVT) study (Gedde et al., 2005, 2007a, b), in combination with persistent concern over the long-term risks of mitomycin trabeculectomy, have resulted in renewed interest in the use of aqueous shunts as the primary surgical intervention in primary open-angle glaucoma.

While there is some evidence of comparability in terms of safety and efficacy with trabeculectomy (Gedde et al., 2005, 2007a, b), and also some evidence that shunts may even be relatively safe when used as primary surgery for primary glaucoma (Wilson et al., 2003), there are some additional concerns that should be addressed if aqueous shunts are to be used as a primary alternative to trabeculectomy for primary glaucomas.

These include predictability of early IOP control, cataract formation, and the long-term effect on the cornea.

Predictability

Despite improvements in technique, a remaining concern with aqueous shunt implantation is the predictability in early IOP control, and that shunts

do not achieve low IOPs in the longer term. While early predictability is less of an issue with the Ahmed Glaucoma Valve, randomized controlled trials do suggest that IOP control in the lower teens can be achieved with certain implants, specifically the Baerveldt 101-350, albeit that most patients will still require some degree of glaucoma medication. Future improvements in shunt predictability will necessitate the development of implants with some features of both of the above, i.e., a valved implant that also possesses a thin, low-profile, flexible plate of large surface area and good biocompatibility.

Cataract formation

Acceleration of cataract formation (Lichter et al., 2001) and reduced bleb function after cataract extraction (Chen et al., 1998) are well-known consequences of trabeculectomy. As aqueous shunt implantation does not involve a peripheral iridectomy, and may be performed without alteration of anterior chamber depth at the time of surgery, it is possible that aqueous shunt implantation may result in a lower rate of cataract development than trabeculectomy. As most comparative trials, such as the TVT study (Gedde et al., 2005, 2007a, b), involved patients that were largely pseudophakic, it has not been possible to study this adequately. Similarly, the influence of cataract surgery on aqueous shunt function has not been documented.

The long-term effect on the cornea

The long-term influence of aqueous shunts on the cornea (Topouzis et al., 1999) and penetrating keratoplasties (Sherwood et al., 1993; Ayyala, 2000; Arroyave et al., 2001; Kwon et al., 2001; Al Torbak, 2003; Alvarenga et al., 2004) has long been the subject of debate. Promising results reported after 1-year follow-up in the TVT study (Gedde et al., 2007a, b), and following a prior study of primary shunt implantation in primary glaucomas (Wilson et al., 2003), have encouraged some investigators to consider implantation of shunts as the primary surgical treatment for primary glaucomas. The cited benefits of shunt

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implantation over trabeculectomy with mitomycin C relate to the absence of limbal drainage and hence very low risks of infection or dysaesthesia.

The concept of primary shunt implantation for primary glaucomas is becoming more feasible, especially as the predictability of IOP control will probably improve as shunts continue to evolve and as implantation techniques become more finely tuned. The likely remaining barrier to shunt implantation, as a primary surgical procedure for the treatment of primary glaucomas, is likely to be concern about the long-term effect on the corneal endothelium.

There has been greater focus in the literature on the effect of aqueous shunts on penetrating keratoplasties than on their effect on the normal cornea. However, the best of these are retrospective and employ case–control methodology. This is partly because any prospective study would have a low likely rate of recruitment, necessitating a prolonged recruitment period, but also because of the difficulties in devising a valid control group.

From the available literature, the overall rate of successful IOP control in post-keratoplasty glaucoma appears to be better in the published literature than after mitomycin C trabeculectomy (Ayyala, 2000). However, while the rate of graft failure may be higher in eyes with aqueous shunts, it can be seen in the same literature review by Ayyala (2000) that the average follow-up in aqueous shunt studies has been longer than in mitomycin C trabeculectomy studies.

There are a number of possible reasons why the rate of corneal graft failure might be elevated in eyes with aqueous shunts. Clearly those requiring aqueous shunts might have worse IOP control to start with. As glaucoma itself is a major risk factor for graft failure (The Australian Corneal Graft Registry, 1993), case selection alone might account for the observed risk of graft failure. Mechanical damage to corneal endothelium from the tube portion of the shunt is a likely cause of corneal endothelial damage. Although Kirkness hypothesized a further potential cause, i.e., aqueous shunts might elevate the risk of corneal graft failure because they permit a tidal flow of inflammatory cells from the bleb in and out of the eye, thereby compromising immune privilege (Kirkness et al.,

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1988), it seems that neither the influence of mechanical factors has been studied sufficiently to be able to exclude these as a cause nor the influence of case selection fully accounted for.

While the presence of tube-corneal touch is often documented (Topouzis et al., 1999), and in one study, by Al Torbak (2003), physical contact between shunt and endothelium was documented as a significant cause of graft failure (Al Torbak, 2003), studies reporting endothelial cell counts after aqueous shunts are rare (McDermott et al., 1993).

The answer to the question as to whether aqueous shunt implantation in an eye with healthy corneal endothelium, or a healthy penetrating keratoplasty, or, ideally, the shunt implanted before keratoplasty — in order to achieve stable IOP control at the time of keratoplasty, and positioned so that there is no possibility of endothelial contact — does or does not compromise the long-term prognosis for graft clarity has not been adequately investigated.

On available evidence, it is quite possible that shunts positioned behind Schwalbe’s line so that

the shunt entering the eye is not in contact with corneal endothelium, and sufficiently short so that there is no possibility of tube movement, e.g., on eye-rubbing, causing corneal endothelial-tube contact, may not compromise corneal endothelial function.

There are many situations, such as eyes with extensive peripheral anterior synechiae or small eyes with shallow anterior chambers, where it may not be possible to reliably implant a shunt in the anterior chamber far enough from corneal endothelium to prevent long-term damage.

It therefore remains to be proven whether aqueous shunts can reliably be placed in the anterior chamber in a position that can be guaranteed not to compromise corneal endothelium, e.g., behind Schwalbe’s line. Traditionally, it has been difficult to visualize the position of the shunt anterior chamber entry site in eyes with corneal edema. The intraocular course of a shunt can now be visualized in such cases with anterior segment optical coherence tomography (AS-OCT) (Fig. 7; Sarodia et al., 2007). It is likely with better