Glaucoma drainage devices (GDDs) have become instrumental in the management of glaucoma. The efficacy of these devices compared with more traditional trabeculectomy in lowering IOP was first demonstrated in the Tube Versus Trabeculectomy (TVT) study. Although both procedures were associated with a similar level of IOP reduction and use of supplemental medical therapy over a 5-year follow-up period, the Baerveldt tube (Johnson & Johnson Vision) was more likely to avoid persistent hypotony, reoperation, and loss of light perception than trabeculectomy with mitomycin C.1
AT A GLANCE
- Glaucoma drainage devices are essential in the management of glaucoma.
- Tube exposure, a complication of glaucoma drainage device implantation, results from the erosion of the overlying patch graft and/or conjunctiva.
- The risk of tube exposure can be decreased by placing the tube superiorly as opposed to inferiorly. An exposed tube should be repaired immediately.
As with any surgical intervention, GDDs have a potentially wide complication profile, including injection, iritis, hypotony, strabismus, infection, and excess capsular fibrosis.2 One of the most serious complications, however, is exposure of the tube and/or plate that comprises the GDD (Figure 1). It is estimated to occur in approximately 2.5% to 8.9% of cases3 and can happen any time after implantation, often 1 to 2 years postoperatively.4 Tube exposure can lead to vision-threatening endophthalmitis,5 as the exposed tube can serve as a pathway for organisms to enter the eye from the ocular surface. This article presents five fundamentals that may assist in preventing and managing GDD exposure, should it occur.
1. RECOGNIZE THE CAUSES OF TUBE EXPOSURE
During the implantation of most GDDs, the native sclera or a patch graft material (such as donor cornea, Tenon capsule, dura mater, pericardium, etc.) is used to cover the anterior aspect of the device. This is followed by closure of the conjunctiva. Early tube exteriorization is often caused by a dehiscence of the suture securing the material that overlies the device.2 Late-onset tube extrusion, however, is likely related to an erosion of the overlying patch graft and/or conjunctiva. This degradation may be due to micro-movements of the tube with eyelid blinking or ocular movements.2 It may also be related to increased tension of the overlying conjunctiva and/or abnormal positioning of the tube.4
2. CONSIDER PATIENT RISK FACTORS
A number of potential risk factors for tube exposure have been proposed. In a retrospective study, Netland et al4 found the proportion of patients with intraocular inflammation prior to tube exposure to be higher than in control patients, which may suggest an underlying immune process as a causative factor. In a retrospective review of 1,073 tube implants, Muir et al6 found female patients to be at higher risk of tube extrusion than male patients. The authors theorized that the smaller orbital dimensions of women may lead to increased friction between the GDD and ocular tissues, leading to tube exposure.6 This theory, however, remains controversial and has not been duplicated in most other studies.
Prior ocular surgeries,7,8 neovascular glaucoma,7 increased number of preoperative glaucoma medications,8 and diabetes8 have also been cited as potential risk factors for tube erosion; however, other similar studies did not find the same associations.4,6,9 The relationship between age or race and tube exposure also remains debatable.4,6,7,8
3. PRIORITIZE PLACEMENT OVER MATERIAL
There does not appear to be a significant correlation between the type of GDD utilized and the risk of tube extrusion.4,6,10 On the other hand, there remains some controversy as to whether tube exposure is caused by the melting of certain types of patch grafts. Advocates of this theory suggest that these grafts can thin and dissolve due to poor integration with the host tissue and a lack of vascular infiltration.2
However, most studies comparing patch graft material types do not show that one material is necessarily more prone to disintegrating than another.10 Levinson et al9 found that, although the choice of patch graft material approached statistical significance (9.2% risk of exposure with cornea, 7.9% risk with pericardium, and 0.5% risk with sclera), it did not reach it (P = .072).9 The authors ultimately posited that corneal patch grafts appeared to fail at an increased rate because the implants were often placed inferiorly (see below).9
4. IMPLANT SUPERIORLY
Overall, GDDs implanted inferiorly are more likely to become exposed than those implanted superiorly.11 This is likely related to the fact that there is less superficial tissue for implant coverage inferiorly due to shorter inferior fornices. This, in turn, leads to increased tissue tension at the sutured incisions and wound dehiscence.11 Further, the tear film pools inferiorly and harbors environmental organisms. Thus, patients with inferior device exposure are believed to be at increased risk of endophthalmitis as there is a more direct conduit for bacteria to pass into the eye from the tear film.9
5. REPAIR EXPOSED TUBES IMMEDIATELY
Exposed tubes should be repaired immediately. Typically, repair involves dissection of the eroded conjunctiva (Figure 2A), followed by placement of a dual layer of coverage—first with a sutured patch graft to cover the exposed tube (Figure 2B) and second with a conjunctival autograft (Figure 2C).12 Surgeons should consider switching the type of patch graft material used during the repair in case an immunologic component contributed to the initial patch graft failure.13 Free conjunctival grafts or rotation flaps, double-layer amniotic membranes, and buccal membrane transplants may also be used for superficial coverage in the event of conjunctival scarring.13
Figure 2. Step-by-step repair of an exposed Ahmed Glaucoma Valve (New World Medical). First, the eroded conjunctiva is dissected (A). A scleral patch graft is then used to cover the exposed tube (B), followed by superficial conjunctival coverage using a conjunctival autograft (C).
Techniques that involve partial-thickness scleral flaps, scleral tunnels, or a combination of both are now being utilized during initial GDD implantation (Figure 3) and as a method of repairing exposed tubes. Ollila et al14 reported a 0% erosion rate after initial GDD implantation with a scleral tunnel. Similarly, Lee et al15 noted a 0% re-erosion rate when exposed tubes were repaired with a split-thickness hinged scleral flap. These techniques are believed to be superior to the use of patch graft materials because the host sclera can act as a vascular bed that releases growth factors and enhances the viability of the tissues overlying the tube.15 Moreover, the tubing is embedded in the patient's own sclera, which tamponades it against the globe and prevents micromotions that may ultimately contribute to tube exposure.15
GDDs have become essential in the management of glaucoma. Tube exposure, however, is a potentially vision-threatening complication of GDDs that results from the erosion of the overlying patch graft and/or conjunctiva. The risk of exposure can be decreased by placing the tube superiorly as opposed to inferiorly. Any exposed GDD should be repaired immediately, and the use of a different type of patch graft material and/or a scleral tunnel or flap should be considered.
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2. Oana S, Vila J. Tube exposure repair. J Curr Glaucoma Pract. 2012;6(3):139-142.
3. Wishart PK, Choudhary A, Wong D. Ahmed glaucoma valves in refractory glaucoma: a 7-year audit. Br J Ophthalmol. 2010;94(9):1174-1179.
4. Netland P, Chaku M, Ishida K, Rhee D. Risk factors for tube exposure as a late complication of glaucoma drainage implant surgery. Clin Ophthalmol. 2016;10:547-553.
5. Gedde SJ, Scott IU, Tabandeh H, et al. Late endophthalmitis associated with glaucoma drainage implants. Ophthalmology. 2001;108(7):1323-1327.
6. Muir KW, Lim A, Stinnett S, Kuo A, Tseng H, Walsh MM. Risk factors for exposure of glaucoma drainage devices: a retrospective observational study. BMJ Open. 2014;4(5):e004560.
7. Koval MS, El Sayyad FF, Bell NP, et al. Risk factors for tube shunt exposure: a matched case-control study. J Ophthalmol. 2013;2013:196215.
8. Huddleston SM, Feldman RM, Budenz DL, et al. Aqueous shunt exposure: a retrospective review of repair outcome. J Glaucoma. 2013;22(6):433-438.
9. Levinson JD, Giangiacomo AL, Beck AD, et al. Glaucoma drainage devices: risk of exposure and infection. Am J Ophthalmol. 2015;160(3):516-521.
10. Smith MF, Doyle JW, Ticrney JW. A comparison of glaucoma drainage implant tube coverage. J Glaucoma. 2002;11(2):143-147.
11. Pakravan M, Yazdani S, Shahabi C, Yaseri M. Superior versus inferior Ahmed glaucoma valve implantation. Ophthalmology. 2009;116(2):208-213.
12. Einan-Lifshitz A, Belkin A, Mathew D, et al. Repair of exposed ahmed glaucoma valve tubes: long-term outcomes. J Glaucoma. 2018;27(6):532-536.
13. Lind JT, Shute TS, Sheybani A. Patch graft materials for glaucoma tube implants. Curr Opin Ophthalmol. 2017;28(2):194-198.
14. Ollila M, Falck A, Airaksinen PJ. Placing the Molteno implant in a long scleral tunnel to prevent postoperative tube exposure. Acta Ophthalmol Scand. 2005;83(3):302-305.
15. Lee ES, Kang SY, Kim NR, et al. Split-thickness hinged scleral flap in the management of exposed tubing of a glaucoma drainage device. J Glaucoma. 2011;20(5):319-321.