264
Fig. 1. The anterior chamber tube shunt to encircling band (ACTSEB) or Schocket procedure utilized the capsule of a scleral buckle to act as the bleb capsule for aqueous drainage. The tube portion shunted aqueous from the anterior chamber into the space under an encircling band, or portion thereof (modified Schocket) (A) (Dark grey arrow). A disadvantage of this technique was that the tube opening under the encircling band (B) (white arrow) was prone to occlusion by fibrous ingrowth. Adapted with permission from Shaarawy et al. (2008). Courtesy of Moorfields Eye Hospital.
Optics, Irvine, CA), the Ahmed Glaucoma Valve (New World Medical, Rancho Cucamonga, CA), and more recently the Molteno 3 (Molteno Ophthalmics Limited, Dunedin, New Zealand), are all still based on these concepts (Fig. 2).
Extensive experience with the Molteno implant also highlighted two impediments to achieving safe, predictable, physiological, long-term IOP levels. The first is difficulty in producing a physiological IOP in the early postoperative period, and the second is controlling long-term encapsulation.
Of the two more popular shunts at the time of writing, the Baerveldt Glaucoma Implant and the Ahmed Glaucoma Valve, arguably the Ahmed has substantially overcome the former problem and the Baerveldt, the latter.
Current shunts and factors affecting their function
All of the shunts mentioned above, which are currently available, follow three basic principles, i.e., they provide a permanent sclerostomy, they divert the aqueous to the equatorial rather than limbal subconjunctival space, and the end plate determines the surface area for aqueous absorption (Fig. 3).
They also have a similar luminal diameter and length after implantation and hence a similarly low natural resistance to aqueous flow. However, unlike the Molteno implant and the Baerveldt Glaucoma Implant, the Ahmed Glaucoma Valve (see below) contains an additional flow resistor, designed to reduce the incidence of early hypotony.
The main determinant of longer-term shunt function is the degree to which the plate encapsulates. All shunt end plates develop a surrounding capsule to some degree (Molteno, 1969a, b; Lloyd et al., 1996). In non-valved shunts, this is the main point of resistance to aqueous flow and therefore the major determinant of IOP in the longer term.
The factors influencing the degree of encapsulation are not well defined but include plate surface area, material, surface profile, flexibility, and the presence or absence of a flow resistor.
Shunt-related factors
Surface area
Although the surface area of the external plate is only one variable that might influence
265
Fig. 2. The Ahmed Glaucoma Valve (A) contains a valve mechanism on the upper surface, which must be primed (B) prior to insertion. The arrow shows balanced salt solution (BSS) emerging from the valve mechanism on priming. (C) The arrow shows infused BSS flowing across the upper surface of the Baerveldt 350 Glaucoma Implant and (D) demonstrates the large plate surface area in comparison with the globe, as well as the curved thin profile on implantation. Adapted with permission from Shaarawy et al. (2008). Courtesy of Moorfields Eye Hospital. (See Color Plate 19.2 in color plate section.)
encapsulation, and hence the major determinant of long-term IOP control, the importance of plate surface area has been well demonstrated in two randomized controlled trials (Heuer et al., 1992; Britt et al., 1999).
Heuer et al. (1992) randomized 132 aphakic or pseudophakic eyes to either a single-plate or double-plate implant. Neovascular glaucomas were excluded. The reported success rate, in terms of IOP control between 6 and 21 mmHg
(inclusive), was better at 2 years in the doubleplate group (71% vs. 46%). The mean percentage IOP reduction was 46733% for the double-plate implant versus 25743% for the single-plate implant and there was less hypertensive phase in the former. Both groups required glaucoma medications at 2 years (1.270.9%) versus (1.670.9%) for the single plate. However, the rate of complications such as choroidal hemorrhage, flat anterior chamber, corneal decompensation, and phthisis due
266
Fig. 3. This illustration shows a diffuse drainage bleb overlying a Baerveldt 350 implant. Shunts differ from trabeculectomies in that there is a permanent sclerostomy (the tube portion), equatorial drainage (arrows), rather than limbal drainage, avoiding limbal blebrelated problems, and the surface area for absorption can be determined by the size of the end plate.
to hypotony was more common in the double-plate group.
The influence of plate size was further investigated by Britt et al. (1999), who compared a 350-mm2 Baerveldt Glaucoma Implant (Advanced Medical Optics, Inc.) with a 500-mm2 plate in a randomized controlled trial and found that overall the success rate was actually lower with the larger plate size. The success rate at 5 years was 79% in the 350-mm2 group compared with 66% in the largeplate group, suggesting that there might be an optimal plate size above which no further increase in size is beneficial. There was no apparent difference in visual acuity, complications, and average IOPs at 5 years, although there was a trend toward more sequelae from hypotony in the 500-mm2 group.
In a smaller, nonrandomized clinical series, Molteno found similar results when comparing one-, two-, and four-plate implants. The IOP control with two plates was significantly better than that with one plate. The IOP control with four plates was marginally better again, but at
the cost of early hypotony in all cases (Molteno, 1981).
A retrospective study by Seah et al. (2003) comparing 70 Baerveldt 350-mm2 implants with 54 Baerveldt 250-mm2 implants in Asian eyes found very little difference in IOP reduction between the two groups after a mean follow-up of 33 months.
It seems that shunt size represents a trade-off between smaller plate size and higher long-term pressures, or large plate size and better long-term IOP control, but a higher risk of sequelae from hypotony.
Although plate surface area is only one of a number of implant-related factors that may influence long-term IOP control, it is one of the easiest to modify, given that there are implants of several different sizes on the market. Although there will be individual variation in the response, it seems that while 250–350 mm2 appears to afford good pressure control, with the Baerveldt implant, plate sizes greater than 350 mm2 are excessive and afford no extra benefit.
Plate material
There is some evidence that the material from which the shunt end plate is manufactured may influence the degree of reaction around the implant and hence the degree of encapsulation. Two studies (Ayyala et al., 1999, 2000) compared the influence of polypropylene with silicone end plates implanted subconjunctivally in rabbits and reported more inflammation with polypropylene than with silicone and more inflammation with rigid than flexible end plates. However, as these plates differ in other factors such as shape, profile, surface texture, contact area with adjacent tissues, flexibility, and micro-motion, all of which might influence the degree of encapsulation, the observed effect may not be exclusively due to the type of plate material or the surface area alone (Lim et al., 1998).
Valved versus non-valved
One of the most important features of an aqueous shunt from the clinician’s perspective is the presence or absence of a fixed flow-restrictor, i.e., valved or non-valved. Although, strictly speaking, the flow-restrictors in the former group have not been shown to act as valves, the term has nevertheless entered common parlance (Prata et al., 1995; Lee, 1998).
Valved devices have been defined as those that allow only unidirectional flow with a minimum opening pressure, whereas non-valved devices are passive, incapable of influencing either anterograde or retrograde flow (modified from Lieberman and Ewing, 1990). The Ahmed Glaucoma Valve (New World Medical) is an example of the former, whereas the Molteno (Molteno Ophthalmics Limited) and Baerveldt shunts are examples of the latter. These implants have a similar lumen diameter (approximately 300 mm). The flow potential of the non-valved Molteno and Baerveldt shunts is sufficient to far exceed the rate of aqueous production, or, in other words, drain the anterior chamber completely of aqueous in a very short period of time.
In valved shunts, this does not happen in the early postoperative period because of the presence
267
of the flow-restrictor, which prevents hypotony in most, but not all, cases (Syed et al., 2004; Law et al., 2005). With the Ahmed Glaucoma Valve, the implant must be primed with a fluid such as balanced salt solution in order to separate and wet the valve leaflets.
In non-valved shunts, the surgeon must physically stent or ligate the shunt tube at the time of implantation to avoid unrestricted flow and the severe sequelae of hypotony that may result.
A number of techniques have been described to prevent early hypotony with non-valved aqueous shunts. The most commonly used technique at the time of writing is external ligation with an absorbable ligature such as 7/0 Vicryl (Ethicon, Johnson & Johnson International, Brussels, Belgium). It is impossible to adjust flow to a clinically safe level with a ligature and therefore ligation needs to completely occlude the tube portion of the implant. Failure to completely occlude the implant will result in severe hypotony. However, to counteract the high IOP that often results from successful ligation, many surgeons will additionally fenestrate the tube proximal to the ligature (Sherwood slit). A further disadvantage of external ligation is sudden decompression, usually 5–6 weeks after surgery when the ligature absorbs (Fig. 4). Even if sufficient encapsulation has developed, the initial precipitous drop in pressure in eyes with larger implants, such as the Baerveldt 350, may be sufficient to cause a choroidal hemorrhage in a predisposed individual.
Alternative stenting techniques have been described in conjunction with a ligature so that the rapidity of the drop in pressure is blunted. A stent used regularly by one of the authors (K.B.) is the 3/0 braided nylon stent (Supramid, S. Jackson Inc., Alexandria, VA) as described by Sherwood and Smith (1993).
Commercially available devices
The current most widely available shunts include the Ahmed, Baerveldt, and Molteno.
1.The Ahmed Glaucoma Valve is manufactured with a flexible silicone plate (FP7) or a rigid polypropylene plate (S2) of similar surface