Ординатура / Офтальмология / Английские материалы / Visual Prosthesis and Ophthalmic Devices New Hope in Sight_Rizzo, Tombran-Tink, Barnstable_2007

glaucoma surgery, recently published studies that address current use of these agents, and new methods of wound modulation refer to Yoon and Singh, 2004 [83]).

ANIMAL MODELS

Increased utilization of animal models of glaucoma will become crucial for novel GDDs to be more critically evaluated in comparison with existing implants. For a comprehensive review of spontaneously occurring and experimental animal models of glaucoma refer to Gelatt et al. (4,86). Such animal models are useful when studying, not only the effects of new GDD designs, but also the effects of new drainage sites for the diversion of aqueous humor (69,87) and antimetabolites to reduce the occurrence of postoperative fibrosis.

FLOW CONTROL: NEW VALVE DESIGNS

The Ahmed Glaucoma Valve (New World Medical, Inc., Rancho Cucamonga, CA) was invented by Mateen Ahmed, and was approved by the US Food and Drug Administration in November 1993 (88). The Ahmed valve design offers a unidirectional, pressure-sensitive valve created of a silicone, elastomer membrane that resides in a trapezoidal chamber. The membranes of this valve have an opening IOP of 8 mmHg and a Venturi effect is created to assist with the flow of aqueous through the chamber (88). The Ahmed valve design has been reported to reduce the complications associated with overfiltration and subsequent hypotony in the immediate postoperative period compared with other drainage devices, although the long-term results are similar to other GDDs (89). As such, new Ahmed valve designs have subsequently been developed and studied, and different designs of Ahmed GDDs are available varying in size, shape, number of explant plates, and materials used (88).

Future studies continuing to investigate novel designs for GDD valve mechanisms, building on the significant information gained from current valve mechanisms may help to prevent acute and chronic hypotony associated with GDDs.

TISSUE BIOCOMPATIBILITY: ALTERNATIVE MATERIALS

Inflammation associated with the biomaterials used in the construction of GDDs, and their explant plates in particular may contrbute to bleb failure secondary to fibrosis (59). Consequently, choosing a biomaterial with limited potential to cause inflammation may improve the success rate of GDDs (59).

Experimental GDDs made of new polymers or polymer modifications to help reduce the risks of chronic inflammation and subconjunctival fibrosis have been described. The alternative materials used in these experimental studies with potential application in GDDs used the management of glaucoma in humans, include: GDDs constructed of expanded polytetrafluoroethylene (90); hydroxylapatite reservoir tube implants (91); and the potential to use the same material used to manufacture Acrosof, a new acrylic (polymethylmethacrylate), foldable intraocular lens implant, for producing explant plates of GDDs (59). In addition, successful development of a new group of bioinert materials synthesized from monomers based on phosphorylcholine appears promising regarding improving the biocompatibility of medical devices, such as GDDs (92). One study reported significantly less protein (fibrinogen and fibrin) and cellular (macrophages and scleral fibroblasts) adhesion to phosphorylcholine-coated polymethylmethacrylate discs compared with levels of adhesion to materials used in contemporary GDDs

186 Cullen

including uncoated phosphorylcholine-coated polymethylmethacrylate, PTFE, polypropylene, and silicone (92).

NEW DRAINAGE SITES: EPITHELIUM-LINED EXTRAORBITAL SITES

Postoperative fibrosis at the explant site may be prevented with GDDs, which divert aqueous humor to extraorbital sites with epithelial lining. Recipient sites in the periorbital region including the lacrimal drainage apparatus and paranasal sinuses have been implanted with GDDs in patients with keratoprosthesis and severe glaucoma (71,72). Based on studies conducted not only in humans (71,72), but also in dogs (69,70), additional studies investigating these and similar innovative surgical procedures for shunting aqueous humor to these epithelium-lined sites in different forms of glaucoma are warranted. In particular, because the explant plate placed subconjunctivally is the main site of the fibrosis, designing shunts for extraorbital diversion without explant plates, as is performed in dogs (69,70), may be beneficial.

CONCLUSIONS

In order to effectively and critically evaluate such modifications in GDDs, future research should be directed at prospective studies using animal models of glaucoma with systematic investigation of variables including biocompatible materials, novel drainage sites, and current and newer antimetabolites, and standard guidelines regarding success with shortand long-term follow-up.

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Table 2

Indications for Glaucoma Drainage Device Implantation

Neovascular glaucoma Failed trabeculectomy

Glaucoma following Retinal detachment surgery Glaucoma following previous ocular surgery with extensive

conjunctival scarring PKP with glaucoma Congenital glaucoma Uveitic glaucoma Traumatic glaucoma Silicone oil glaucoma

Irido-corneal endothelial syndrome Sturge–Weber syndrome

Glaucoma with chronic topical medication Aphakic glaucoma

increased the surface area of his GDD by introducing the double plate implant in 1981. Surgeons are now faced with many choices of both valved and nonvalved implants (Table 1) in the management of refractory glaucoma (Table 2). The following chapter would attempt to summarize the current knowledge on the efficacy and safety of these implants.

EFFICACY IN IOP CONTROL

In the literature, GDD surgical success is usually defined as IOP less than 22 mmHg, but more than 4 mmHg and these eyes should not become totally blind or undergo phthisis. However, comparisons between studies are difficult because they differ in terms of surgical indications, surgical techniques, postoperative management, criteria for defining success as well as duration of follow-up. Furthermore, the numbers are usually small and lacked the power to elucidate risk factors for failures. The success rates were impressive in most reports considering the fact that GDD are usually implanted in refractory glaucoma where conventional surgery would otherwise have failed. They were quoted in

the region of 58–98% for the Caucasian population (2–5). Long-term follow-up of Molteno tubes over a median of 44 mo had success rates of 57% (6).

Similar short-term results were echoed in Asian patients. Aung et al. (7) reported success in 73.5% and qualified success in 12% for Molteno and Baerveldt tubes in East Asians with average follow-up of 13.41 mo. While Lai et al. (8) had 73.8% success at 12 mo for Ahmed tubes in Hong Kong Chinese patients. Success rates of 82.83% at 24 mo were seen in Indian patients (9). Complete success experienced for Baerveldt was 83.3% and 66.7% for Ahmed implants and qualified success were 4.2 and 16.7%, respectively with a mean follow-up of 22.8 mo (10). As for the intermediate term, Seah et al. (11) had 54% complete success and 22% qualified success for Baerveldt GDD. Future randomized comparative trials with long-term follow-up may shed light on the impact of race on GDD outcomes.

Plate Area

Historically, it has been assumed that the plate size determined the size of the encapsulated bleb and therefore, has a positive influence on surgical success (12). With twice the surface area, the double-plate Molteno device achieved higher success rates and lower IOP than single-plate Molteno GDD (13). Although, most surgeons considered these findings as proof of theory, Camras (14) pointed to the fact that the success rates and IOP reduction had not been twice as good. Further evidence from Smith et al. (15) and Wilson et al. (16) showed that the smaller double-plate Molteno produced similar or lower IOP when compared with the Schocket encircling device. Studies comparing similar devices, i.e., the Baerveldt 250 mm2 with 350 mm2 in Asians (11) and 350 mm2 with 500 mm2 in Western population did not find significant differences in final IOP control (17). Evidently, IOP decrease is not proportional to the plate area used as the effective surface area may be quite different from the total surface area of the GDD endplate. For example, a significant proportion of the Baerveldt endplate is tucked under both the vertical and the horizontal recti muscles and the extension of bleb under these muscles has been demonstrated echogenically (18). However, the plate-muscle adhesions and pseudocapsule here may not be as permeable to aqueous, therefore, the resultant effective surface area may be significantly less.

Molteno described the best plate-size for the development of optimal bleb surface area. He mentioned that the circumferential distance between recti muscle delimits the size and shape of the episcleral plate. Plates larger than an ocular quadrant with the involvement of recti muscles may not necessarily increase functioning encapsulation. The posterior ciliary artery limits the posterior dimension. Finally, encapsulation within the orbit as opposed to being near the conjunctiva reduces permeability to aqueous humor (19).

Second Tube Implant

In the event of suboptimal IOP control or failure of surgery, Management options include shunt revision, additional tube implant, and supplemental cyclophotocoagulation. Burgoyne et al. examined a group of patients with second tube implant and found that corneal morbidity is a common complication, whereas other rates of complications were not higher than expected. Pressure control was achieved with fewer hypotensive agents

and stable visual acuity could be maintained (20). Shah et al. compared additional tube shunt with shunt revision and found more patients (62% vs 42%) achieved at least qualified success (no statistical significance). Moreover, the sample size was very small with overrepresentation of neovascular glaucoma in the shunt revision group (21).

Semchyshyn et al. treated a small group of tube failures with supplemental trans- scleral diode laser cyclophotocoagulation. Twenty-one eyes were treated and followed up during a mean period of 26.9 mo. The average IOP was significantly reduced from a preoperative level of 35.7 +/− 14.7 (SD) mmHg to a postoperative level of 13.6 +/− 7.1 (SD) mmHg. Therefore, in cases of tube failure, adjunctive trans-scleral diode cyclophotocoagulation treatment(s) is a viable option to lower IOP (22).

Comparison With Other Surgical Modalities

GDD do not offer any advantage in terms of improving IOP control or reducing complication rates when compared with trabeculectomy alone in uncomplicated glaucoma (23). However, for the Asian patients who have high risk of trabeculectomy failure (24), GDD may have more to offer as the primary procedure (7,10). Pars plana Baerveldt GDD surgery is superior to neodymium:YAG cyclophotocoagulation in terms of IOP control, less hypotony and with higher preservation of visual acuity. As compared with 5.6% in the Baerveldt group 26.3% of Nd:YAG failed at 6 mo. The neodymium:yttrium aluminum garnet (Nd:YAG) group lost light perception in 23.3% vs 5.6% in the Baerveldt group (25).

When Lima et al. compared endoscopic cyclophotocoagulation (ECP) with Ahmed GDD in the treatment of refractory glaucoma, efficacy was similar, but there were more complications with Ahmed GDD, specifically, choroidal detachment (Ahmed 17.64%, ECP 2.94%), shallow anterior chamber (Ahmed 17.64%, ECP 0.0%), and hyphema (Ahmed 14.7%, ECP 17.64%) (26). As such, ECP may be promising as a treatment modality for refractory glaucoma.

Augmentation With Mitomycin C

Antimitotic agents have not been used in any of the patients mainly because of the lack of evidence for their efficacy. The issue of using antimitotic agents to improve the outcome of GDD remained unresolved as the few comparative studies produced contradicting results. Kook et al. (27) reported increased success rates with Ahmed GDI without increasing complication rates whereas Costa et al. (28) did not observe any significant improvement in the IOP lowering effect. Meanwhile Irak et al. (29) reported no differences with the Baerveldt GDD in a noncomparative study. Likewise, Molteno GDD did not show significant differences in terms of survival rates, IOP control, and antiglaucoma medication use (30). Conversely, complications related to mitomycin C use, such as conjunctival melts (31), infections, and hypotony have been reported (32,33).

Combined Cataract and GDD Surgery

Combined phacoemulsification and lens implantation with GDD surgery has been met with good outcomes. Chung et al. (34) reported a series of 32 patients who underwent such surgery with both Ahmed and Baerveldt GDD, and they benefited from