Aqueous Shunts: Choice of Implant

Surface Area

Although the plate surface area ( Table 110-1 ) is only one variable that influences encapsulation, it has been well-demonstrated to be a major determinant of long-term IOP control in two randomized, controlled trials.

Heuer et al. randomized 132 aphakic or pseudophakic eyes with non-neovascular glaucoma to either a single- or double-plate Molteno implant (Molteno Ophthalmics Limited, Dunedin, New Zealand) and reported better success in terms of IOP control (<22 mmHg) in the double-plate group at 2 years (71% vs. 46%). The mean percentage IOP reduction was 46 ± 33% for the double-plate group vs. 25 ± 43% for single-plate, and there was less hypertensive phase in the former. Both groups required glaucoma medications at 2 years (1.2 ± 0.9 vs. 1.6 ± 0.9, respectively). However, the rate of complications such as choroidal hemorrhage, flat anterior chamber, corneal decompensation, and phthisis due to hypotony was higher in the double-plate group.

A later randomized trial by Britt et al., who compared a 350 mm ² Baerveldt implant (Advanced Medical Optics, Inc., Irvine, California, USA) with a 500 mm ² plate, found the overall success rate to be slightly lower with the larger plate, suggesting that 500 mm ² is too large for optimal IOP control. The success rate at 5 years was 79% in the 350 mm ² group compared with 66% in the 500 mm ² group. There was no significant difference in visual acuity, complications, and average IOP at 5 years, although there was a trend to more sequelae from hypotony in the 500 mm ² group.

In a small series, Molteno found similar results when comparing one-, two-, and four-plate implants. The IOP control with two plates was significantly better than with one. The IOP control with four plates was marginally better again, but at the cost of early hypotony in all of the three cases reported. A retrospective study by Seah et al. comparing 70 Baerveldt 350 mm ² implants with 54 Baerveldt 250 mm ² implants in Asian eyes found little difference in IOP reduction between the two groups after a mean follow-up of 33 months.

Plate surface area is one of the easiest implant-related factors to modify, given that implants of several different sizes are available. From these data, a plate size of 250 to 350 mm ² appears to afford the best compromise between encapsulation and hypotony for the average eye. However, as the indications for shunts include a variety of secondary, developmental, and difficult primary glaucomas, there is a wide individual variation in response. Many surgeons prefer shunts with smaller plate sizes, accepting a higher long-term pressure as a safer option than risking a higher likelihood of hypotony, in order to achieve better IOP control.

Plate Material

There is some evidence that the material from which the plate is manufactured may influence encapsulation. Ayyala et al. compared polypropylene with silicone plates implanted subconjunctivally in rabbits and reported more inflammation with polypropylene, and more with rigid than flexible 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 encapsulation, the observed effect may not be exclusively due to the type of material or surface area. Choritz et al. demonstrated markedly different surface topography among the most commonly implanted shunts. The Ahmed Glaucoma Valve (New World Medical, Rancho Cucamonga, California, USA) FP7 and S2 plates had greater surface roughness than the Baerveldt 101-350 and Molteno implants on confocal microscopy. Cell adhesion after 72 hours in tissue culture was higher on those with higher surface roughness.

Valved Versus Nonvalved

One of the most important features of a shunt is the presence or absence of a flow restrictor ( valved or nonvalved ). Although the flow restrictors in the former group have not been shown to actually function as valves , the name has nevertheless stuck.

Valved devices, in theory, allow only unidirectional flow with a minimum opening pressure, whereas nonvalved devices are passive, incapable of influencing flow.

The Ahmed Glaucoma Valve is an example of the former, whereas the Molteno Implant and Baerveldt Glaucoma Implants are examples of the latter. These implants have a similar lumen diameter (approximately 300 µm). Without a valve, this diameter of tube offers virtually no resistance to flow and can drain the anterior chamber completely of aqueous relatively quickly.

This does not occur in the early postoperative period with valved implants because the integral flow restrictor prevents hypotony in most cases. With the Ahmed, the implant must be primed with a fluid such as balanced salt solution (BSS) in order to separate and wet the valve leaflets.

Nonvalved shunts do not contain a flow restrictor and must be occluded effectively by the surgeon at the time of implantation, to avoid severe hypotony. A number of techniques have been described to prevent early hypotony with nonvalved aqueous shunts. The most commonly used at the time of writing is external ligation with an absorbable ligature such as 7/0 polyglactin 910 (Vicryl, Ethicon, Johnson & Johnson International, Brussels, Belgium). No method has yet been described that will permit aqueous flow to be successfully titrated to a clinically safe level with a ligature. The purpose of ligation is therefore to occlude the tube completely. Failure to achieve complete occlusion may result in severe hypotony.

Successful ligation often results in a high IOP and, to counteract this, many surgeons 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. Even if sufficient encapsulation has developed, the 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 occlusion techniques have been described using an intraluminal occlusion suture in conjunction with a ligature so the rapidity of the drop in pressure is blunted. A commonly used intraluminal suture is a 3/0 braided nylon (Supramid, S. Jackson Inc., Alexandria, Virginia, USA) as described by Sherwood and Smith. This has been described to provide some resistance when used in isolation.