Other Glaucoma Implants

Introduction

The ultimate goal of glaucoma surgical treatment is to perform a fistulizing procedure communicating the aqueous humor in the anterior segment with the sub-Tenon's space or, as an alternative, with the suprachoroidal space or conventional outflow pathways. These procedures tend to lead to a more substantial intraocular pressure (IOP) reduction and prevention of deleterious IOP fluctuation than currently available medical and laser treatments. Traditionally, eyes with uncontrolled IOP under medical or laser therapy – or which present a level of glaucomatous damage unlikely to be stabilized with these modalities of therapy – first undergo trabeculectomy with antifibrotic agents, followed by glaucoma drainage devices (GDD), and finally cyclodestructive procedures. The use of GDDs is recommended in cases of failure of trabeculectomy with antifibrotic agents, extensive superior conjunctival scarring, neovascular glaucoma, or in situations in which trabeculectomy is unlikely to succeed. In comparison with trabeculectomy as the initial surgical procedure in patients with primary glaucomas, the probability of IOP control at 21 mmHg or less following trabeculectomy at 1, 2, 5, 10, 15, and 20 years was 95, 93, 89, 82, 74, and 68%, respectively, whereas eyes that underwent Molteno implants had success probabilities of 98, 97, 96, 96, 91, and 91%, respectively, which was statistically significant favoring the implant.

The present chapter will focus on GDDs other than Ahmed, Baerveldt, and Molteno implants, which also produce a scleral fistula that shunts aqueous humor to an episcleral plate located in the equatorial region of the globe. These implants can be alternatively called ‘longer tubes’ to differentiate from newer systems such as the ExPRESS, iStent, and supra-choroidal shunts, which are discussed in detail elsewhere in this book (see Section 18).

In the United States, recent data have shown an increase in the utilization of GDDs and a decline in the use of trabeculectomy in the past decades. Despite many different types of implants, GDDs can be classified primarily into valved or non-valved, depending on the presence of an aqueous flow restriction mechanism. A large variety of valved and non-valved implants have been developed and tried. The American Academy of Ophthalmology has published an evidence-based summary of commercially available GDDs addressing consensus and controversies regarding these different devices. Nevertheless, most glaucoma specialists would agree that new implants need better biocompatible materials, better pharmacological sub-conjunctival fibrosis control, accurate flow resistance control, and internal drainage shunt. These needs are not completely fulfilled yet with commercially available devices and warrant more research.

OptiMed, Schocket, and the new Susanna implant are presented and discussed below. These devices are placed and fixed into the episclera, draining aqueous from the anterior segment to the sub-Tenon's space, with different designs when compared to the Ahmed or Baerveldt devices.

OptiMed Implant

The OptiMed implant or OptiMed Glaucoma Pressure Regulator (OGPR; Optimed, Inc., Int., Santa Barbara, CA) is a valved implant made up of a silicone tube with a polymethyl methacrylate (PMMA) matrix plate. The inner diameter of the silicon tube is 0.38 mm and the outer diameter is 0.76 mm. The implant has a 5 mm polymethyl methacrylate tube inserted into a silicone base with the dimensions of 1 × 2 × 3 mm. The non-adjustable flow resistance mechanism of this device is positioned within the rectangular box situated at the end of the tube within the plate. The base contains 180–200 microtubules through which aqueous percolates through to the subconjunctival space. The OptiMed implant's ‘flow-restricting’ unit of multiple microtubules provides a pressure gradient governed by Poiseuille's formula ( Fig. 114-1 ). The aqueous outflow through the tube to the plate occurs when the IOP exceeds 10 mmHg. Capillary action draws fluid through the matrix as the IOP increases.

The three available models of this implant vary according to the length of the capillary microtubules. The external portion of this device does not lie as posteriorly as other shunt implants. Eisenberg and colleagues compared in vitro flow properties of this device with others and observed improved outflow facility when compared to the Ahmed and Krupin implants. The OptiMed device did not reveal an opening pressure, and had a mean facility of flow of 7.08 µL/min/mmHg for high-flow and 6.20 µL/min/mmHg for low-flow. Given its facility of flow compared to other implants, the OptiMed device was classified as a very high facility device, which was comparable with the Baerveldt implant. However, it did not reveal a consistent valve behavior. The OptiMed implant displayed resistance and pressure response to various flow conditions similar to those of a cannula or flow resistor, which allowed lumen resistance to remain relatively stable and IOP to increase linearly with flow.

The OptiMed is relatively recent in origin but not routinely used today. The last published paper dates from 1999 which included a description of seven cases of refractory glaucoma. Postoperative success – defined as an IOP of less than 22 mmHg with or without medication without additional filtering surgery – was obtained in 4 out of 7 eyes after a mean follow-up period of 37 months (range 16–39 months). Early complications were hyphema (1 patient, 14.3%), blockage of intracameral portion of the tube (1 patient, 14.3%), hypertony (1 patient, 14.3%). Late complications were: external conjunctival bleb failure (2 patients, 28.6%), and blockage of intracameral portion of tube by fibrovascular tissue (1 patient, 14.3%).

Schocket Implant

The Schocket is a non-valved device that employs an encircling silicone band – similar to that used in scleral buckling retinopexy surgery – as the explant or plate. The 30-mm-long silastic tube (0.3 mm inner and a 0.64 mm outer diameter) is sutured into the grooved portion of a silicone band. Smaller-diameter silastic tubing may help reduce the occurrence of hypotony and conjunctival erosion although tube ligature is recommended.

A 360° #20 or #220 silicone strip is used as the external explant. Total surface area of a #20 device is approximately 300 mm ² and may vary according to the length of the band. During surgery, the silastic tube is placed through the previously created 1.0–1.5 mm notch at the wall of the silicone band into the groove. The total length of silastic tube within the groove is approximately 15 mm. The tube is fixed into the center of the groove using two or three 9/0 interrupted nylon sutures.

In eyes with prior scleral buckling procedure, the scleral band becomes encapsulated similarly to GDDs. In such patients, the fibrous capsule can be incised into the space between the plastic encircling element and the capsule. The tubing is then trimmed and its anterior portion inserted into the anterior chamber, thus simulating a Molteno two-stage procedure. Sidoti et al. reported on 13 such patients who were followed-up for a mean of 21.7 ± 14.1 months. Successful IOP control (6–21 mmHg) with or without glaucoma medications was achieved in 11 (85%) patients. One or more surgical revisions relating to tube obstruction were required in five eyes. Three patients had hypotony (IOP less than 6 mmHg) on the first postoperative day.

In another study, researchers attempted to compare the efficacy of different tube shunts. Smith et al. compared the double-plate Molteno with a Schocket procedure. With at least 6 months of follow-up, the mean IOP was 14.4 mmHg with the Molteno and 15.1 mmHg with the Schocket device. The mean number of required medications was 0.95 in the Molteno group and 0.43 in the Schocket group. Despite results favoring the Shocket device, these comparisons did not reach statistical significance. In a series of 42 eyes in patients with refractory glaucoma who underwent a Schocket procedure with a mean follow-up of 17.5 months, Spiegel et al. observed success rates of 81% defined as either an IOP less than 21 and greater than 5 mmHg, with a pressure reduction of at least 10%, or less with no medication; or a pressure reduction of at least 30%, with no change of medication. Of interest, 40% of all cases required revision. In four cases, the shunt had to be removed, three cases due to hypotony and one due to conjunctival erosion. In a randomized clinical trial comparing the Schocket shunt with a larger plate surface area and the double-plate Molteno implant, Wilson et al. showed that the Molteno implant produced a statistically lower IOP at 6 months, although postoperative visual acuity, glaucoma medications, and complications were statistically similar.

Sherwood at al. reviewed consecutively 55 eyes of 50 patients who underwent the Schocket procedure. A minimum follow-up of one year (mean, 18 months) was achieved for 30 eyes of 30 patients. For these 30 eyes, the success rate (as defined by an IOP of 25 mmHg or less, with or without medication) was 80%, with an IOP of 21 mmHg or less at 12 months achieved in 68%. The complications encountered in the overall study of 55 eyes are discussed, and six main causes of failure are identified. Causes of failure and complications include exposure of the silicone tube from erosion of the overlying conjunctiva; blockage of the proximal orifice of the tube; corneal decompensation secondary to tube touch; extrascleral tube compression proximal to the gutter; blockage of the distal orifice of the tube beneath the gutter; and insufficient aqueous absorption from the encapsulated area surrounding the 360 degrees encircling gutter. Revision surgery was required in 21 eyes (38%).