Exposure of the subconjunctival portion of an aqueous drainage implant (ADI) is one of several complications of such devices that are related to the hardware. This review discusses tube exposure that involves erosion of a portion of the tube through the conjunctival surface—that is, a segment of tube becomes externalized. Such an exposure may occur early postoperatively or late. Early exposure generally results from wound dehiscence, which is a topic outside the scope of this review. This review concerns late tube exposure that develops well after wound healing is visibly complete and when conjunctival inflammation is absent. Exposures such as these commonly occur 2 to 4 mm posterior to the limbus and do not involve the limbus, the posterior plate hardware, or the bleb.
I use the term ADI to mean devices such as the Molteno implant (several versions), Krupin valve, Hood valve, Ahmed valve (several versions), Baerveldt implant (several versions), and anterior chamber tube to scleral encircling band. These devices share the configuration of an anterior chamber tube with a long subconjunctival course to an equatorial plate and bleb. I do not use the term ADI to refer to short tubes, such as the ExPRESS shunt. Many synonyms for the term ADI can be found in the ophthalmic literary and vernacular languages, including tube shunt, aqueous shunt, aqueous tube shunt, seton, stent, glaucoma drainage device, glaucoma drainage implant, glaucoma drainage shunt, and just plain tube. Readers may encounter yet other synonyms elsewhere.
The goals of revision surgery for an exposed tube are (a) to resolve the exposure with good tissue coverage while (b) avoiding interference with the position or function of the implant, (c) avoiding fresh complications, and (d) minimizing the likelihood of future tube reexposure. The risk of endophthalmitis from an untreated exposed tube is documented, although there are no published data regarding the incidence of endophthalmitis associated with exposed tube. Observation alone is risky, and it is plausible to conjecture that the risk of endophthalmitis increases as the period of observation increases. Gedde et al1 recommended prophylactic surgical revision with patch graft in all cases of exposed tube.
This review will not treat the pathophysiology of tube exposure—or why some patients develop exposure whereas others do not. There is speculation, but surprisingly little published data, on the causes of tube exposure.
METHODS OF TUBE REVISION
To date, the published literature describing the surgical technique of revision for exposed anterior segment tubes has been limited. Gedde et al1 removed the implant in 3 of the 4 patients they reported on who developed endophthalmitis after tube exposure. In their remaining patient, they reported that the tube was removed from the anterior chamber and placed subconjunctivally. A scleral patch graft was placed over the peripheral corneal defect where the erosion had occurred, and conjunctiva was advanced to cover the scleral graft. However, the implant no longer functioned for aqueous drainage. The eye subsequently became phthisical.
Other authors have described either covering the tube with a patch graft of commercially prepared, cadaveric dura mater (Tutoplast Dura, Biodynamics, Tampa, FL) with no mention of method of conjunctival closure2,3 or covering the tube with a patch graft with mobilization of a conjunctival flap to cover the graft.3–5 The materials used for these latter grafts were full-thickness donor sclera,3,4 pericardium,3 fascia lata,3 and autologous scar tissue harvested as a collagenous membrane at the time of surgery.5
Several authors have employed repositioning of the tube as a part of the revision.3,4,6,7 Lama and Fechtner4 reported repositioning the tube in 1 patient before placing a full-thickness donor scleral graft and mobilizing a conjunctival flap. Billson et al6 described fashioning a half-thickness flap of existing sclera, under which to reposition the tube within the anterior chamber. This avoided the use of autologous graft material. Joos et al7 reported a series of 4 patients who underwent repositioning of the tube into the vitreous cavity after or concurrent with pars plana vitrectomy. Each of these patients required the repositioning because of recurrent tube exposure that had been initially treated, at least once, with a patch graft of pericardium or sclera but without tube repositioning. For the repositioning revision, a scleral patch was placed over both the closed former anterior chamber fistula and the tube, and the conjunctiva (as a fornix-based peritomy) was closed over the patch.
None of the above authors advised simple conjunctival closure without a patch graft. A graft of collagenous human tissue seems to be necessary for reliable conjunctival healing. Conjunctiva does not reliably remain closed over immediately underlying synthetic materials such as plastics, silastic, silicone, or polypropylene. The graft tissues reported to have been used for initial ADI implantation and for revision procedures include autologous sclera,3,6,8 donor human sclera,1,3,4,7–11 donor human pericardium,3,11,12 donor human dura mater,2,3,11 autologous fascia lata,3,13,14 autologous scar tissue,5 and donor human clear cornea.15 Graft tissues are sterilized and preserved by various methods, which are beyond the scope of this review.
Additional unpublished methods of tube revision have been discussed in nonliterary forums. A few of these ideas include the use of autologous free conjunctival grafts to close conjunctival defects, donor amniotic membrane and autologous temporalis fascia grafts, and the “tacking” of the tube (with suture) to sclera posterior to its limbal entry to “flatten” the outward bowing of the tube at that critical location.
MY METHOD OF TUBE REVISION
I approach tube revision by opening the conjunctiva surrounding the exposed segment of tube, placing a donor patch graft over the tube in the scleral bed, and closing conjunctiva over the patch graft.
Anesthesia may be topical plus subconjunctival, peribulbar injection, or retrobulbar injection. For the first method, I use topical tetracaine 0.5% eye drops, plus a 1:1 mixture of lidocaine 2% without epinephrine and bupivacaine 0.75% injected into the subconjunctival space through a short 30-gauge needle so as to surround the exposed segment of tube. For peribulbar and retrobulbar injections, I use the 1:1 mixture of lidocaine 2% without epinephrine and bupivacaine 0.75% with added hyaluronidase at a concentration of 75 units/10 mL injected through a 23-gauge Atkinson needle.
After the operative eye is prepared and draped in a sterile manner, I inspect the conjunctiva using the operating microscope at low-to-moderate magnification (Fig. 1). Some eyes require no traction suture to obtain good exposure of the surgical quadrant. Other eyes do not offer ready exposure and require a traction suture. For these, I use a 6-0 polyglactin suture on a spatula needle placed half-thickness through the corneal stroma at the 12-o'clock position of the corneal periphery. This is pulled and clamped in a direction to expose the surgical quadrant.
The conjunctival incision is made perpendicular to the tube, at about the midpoint of the exposed segment, and 8 mm long. It is roughly parallel to the limbus, and it straddles the tube, 4 mm on either side. I generally use straight Vannas scissors and nontoothed forceps, such as the Fechtner ring forceps. Larger scissors are too large for the detail required of this dissection. The incision is carried down to the level of episclera, although this may be difficult to identify. In most patients, the subconjunctival space is scarred with adherent fibrovascular tissue, but there is usually little or no old patch graft material to be found over the tube, although graft material may persist on either side of it. Such material should be lifted with conjunctiva, when possible, to reduce the likelihood of cheese-wiring of the meandering suture. An anterior flap of conjunctiva is undermined in the episcleral plane until it is hinged at its limbal base (Fig. 2). I take care to dissect the flap off the tube itself. A similar posterior flap of conjunctiva is undermined 3 to 4 mm posteriorly, again with dissection of the tube from subconjunctival adhesions. A rounded, crescent-style blade is sometimes helpful for the undermining—it helps to maintain the planar dissection and cuts more easily than scissors. I take great care to avoid dissecting as far posteriorly as the equatorial plate and to avoid rupturing the fluid-filled bleb space. Also, I avoid severing or dislodging the tube throughout the dissection. Each end of the wound is then undermined 2 to 3 mm beyond the terminus, to provide a large enough episcleral pocket to accept the patch graft. Throughout the dissection, hemostasis of episcleral and subconjunctival bleeders is provided with 23-gauge bipolar cautery (Fig. 3). Cautery of conjunctival epithelium is strictly avoided. Finally, the “underpass” of conjunctival epithelium that had grown beneath the exposed tube is scraped and/or dissected off the episcleral surface and discarded to minimize the chance of formation of an epithelial inclusion cyst or worse—reexposure of the tube as buried epithelium seeks the surface.
Next, I cut a 6×6 mm square of donor acellular dermis graft [Alloderm Regenerative Tissue Matrix, Acellular Human Dermis (thickness=0.79 to 1.78 mm), Lifecell, Branchburg, NJ] with scissors (Fig. 4). This graft is laid on the episcleral bed over the exposed segment of tube and tucked beneath the anterior and posterior conjunctival flaps (Fig. 5). The graft should seem oversized for the space, so that there will be a gap of 2 to 3 mm between the conjunctival edges. No sutures are placed in the dermis graft. Rather, the graft will become “caged in” by the conjunctival closure. I close conjunctiva with a running horizontal mattress (meandering) suture of 7-0 polyglactin (Fig. 6). The suture should pull light tension only on the conjunctiva. There will remain the 2 to 3 mm gap over the dermis graft. An attempt at tight, complete conjunctival closure will result in wound dehiscence, from cheese-wiring of the suture through the tissue. Instead, the gap is allowed to heal (epithelialize) by secondary intention. At follow-up visits, epithelial healing may be monitored using topical fluorescein at the slit lamp, which easily differentiates epithelium (glistening, with little or no stain uptake) from bare graft collagen (stain uptake similar to corneal stroma).
The traction suture is removed from the cornea (if present). I inspect the anterior chamber to assure that the chamber depth and the tube position have not changed as a result of the procedure. Remember that no entry of any kind has been made into the anterior chamber with my method. After the lid speculum and drapes have been removed, a combination of antibiotic/corticosteroid ointment is applied, and the eye is protected with a rigid shield. Postoperative follow-up includes 1 day and 1-month visits.
RESULTS OF TUBE REVISION
Descriptions in the literature of the follow-up results of tube revision procedures have been limited. In the 1 patient for whom Gedde et al revised rather than removed the tube, the eye was reported as phthisical at 1-year follow-up. Brandt2 reported that at 8 months' follow-up, the dura mater patch graft seemed well tolerated, without surrounding or overlying conjunctival reaction. Lama and Fechtner4 reported follow-up for 1 case at 15 months and the other at 8 months. Both eyes demonstrated intact overlying conjunctiva and no evidence of thinning of the donor scleral patch graft. The autologous scar tissue patch used by Puustjärvi et al5 was reported to be a smooth and partly translucent area covering the former operated area with a well-healed conjunctival covering at 3 months. Billson et al6 described 3 pediatric patients in whom resiting a Molteno tube under a half-thickness scleral flap was performed. All 3 cases were simply described as “stable” 4 to 7 years postoperatively. The 4 patients of Joos et al7 who underwent posterior repositioning of a tube for recurrent erosion had no further recurrent erosions during follow-up, which was 1 to 42 months at most.
In my series of tube revisions with acellular dermis graft, there have been 30 cases over 4 years. The period of follow-up observation was 1 to 42 months, with a median period of 12 months. Twenty-nine of the procedures created good tissue coverage initially over the tube with secondary epithelialization of the graft. None had active infection preceding the revision, so culturing and preoperative treatment of infection were moot. There were no extrusions of the dermis graft. All tubes remained in stable position in the anterior chamber without loss of drainage function. Some of the grafts seemed to maintain their original thickness, whereas others seemed to thin partially with time. Complications are discussed in the next section.
COMPLICATIONS OF TUBE REVISION SURGERY
The literature lists some complications of revision surgery for tube exposure. Phthisis bulbi1 realistically followed from endophthalmitis, which was a sequela of tube exposure. Recurrent tube erosion,7 tube migration toward the corneal endothelium,3 and cystoid macular edema2 have been described as the result of revision surgery.
A number of additional potential complications, although not published, can be foreseen, and efforts should be made to minimize the risk of these. The likelihood of de novo infection as the result of revision surgery must be minimized with attention to sterile technique and the use of preoperative and postoperative topical antibiotics. Hypotony, with attendant choroidal effusion and/or suprachoroidal hemorrhage, can be avoided by not entering the anterior chamber and by avoiding dislodging the tube from its fistula. Of course, a dislodged tube can be replaced easily, but the intraocular pressure drops quickly to near 0 mm Hg. In this event, the anterior chamber should be reinflated with balanced salt solution after tube replacement, to restore moderate pressure. If the tube is inadvertently transected during dissection of the tissues, then it should be repaired using one of several tube splicing techniques. However, tube splicing is beyond the scope of this discussion. A needle puncture or small nick in the tube wall, absent transection, rarely warrants repair.
Wound dehiscence is a risk. Realistically, all wounds closed by my method are “dehisced,” because of the 2 to 3 mm intentional gap between the conjunctival edges. The risk is of cheese-wiring the suture through tissue, wider dehiscence, and even loss of the dermis graft. The most reliable hedge against this is to resist the impulse to tighten the polyglactin closure. A loosely meandering polyglactin suture that “cages in” the dermis graft is more likely to remain intact for weeks than a taut closure. When cheese-wiring and further dehiscence do occur, close observation is the management of choice. Most wounds will go on to completely epithelialize the dermis graft. In the unlikely event of displacement or loss of the dermis graft during postoperative follow-up, the revision procedure must be repeated with a fresh graft, or using one of the variations described in the literature.1–7
Hemorrhage is a potential complication, but rarely a serious problem. Most wounds ooze copiously during dissection, because the tissue is previously disturbed and fibrovascular. The surface bleeding is easily accessible with fine-point bipolar cautery. Postoperative surface bleeding is rare and may emanate from suture erosion through conjunctival vessels. Suprachoroidal hemorrhage may theoretically occur as a sequela of anterior chamber penetration with loss of intraocular pressure or of scleral perforation with a sharp instrument reaching the highly vascular ciliary body or choroid.
Potential complications may occur to the equatorial plate. Buttonhole defects created in the posterior conjunctiva could result in plate exposure, which can lead to complete extrusion in some cases. The fluid-filled bleb space may be breached, with sudden egress of aqueous humor and loss of intraocular pressure. In the event of a breached bleb, the opening of the fibrous bleb wall may be closed with polyglactin suture, avoiding incorporating epithelium. The intraocular pressure may be partially restored by injecting viscoelastic into the anterior chamber through a paracentesis incision. The risk of these complications may be minimized by careful, deliberate dissection.
Epithelial entrapment and epithelial inclusion cyst formation are potential complications. The risk of these may be minimized by, first, the decision to operate sooner rather than later in the course of the tube exposure (allowing for less extensive undergrowth of epithelium beneath the tube) and, second, by careful dissection, scraping, and cautery of the underpass of epithelium on the episclera. A rare potential complication of breaching the bleb space would be epithelialization, or “marsupialization,” of the posterior bleb lining.
The rate of complications for my series of tube revisions, over 4 years, is 5 of 30 patients. Four of them involved tube reexposure and 2 involved endophthalmitis. One patient developed a tube reexposure 6 months after successful revision with acellular dermis graft. He underwent a second revision procedure with acellular dermis graft, and had no further complications 6 months later. A second patient developed tube reexposure 20 months after a revision that had healed well. Because the eye was blind (no light perception), she opted to undergo removal of the tube 30 months after the revision rather than have another revision operation. The third patient developed what is better described as wound dehiscence, with tube exposure and graft retraction posteriorly 1 month after the revision. There was not complete extrusion of the graft. Because his eye had very poor vision, he opted for removal of the tube 3 months after the revision.
The fourth patient developed reexposure of her tube 6 months after the revision procedure. However, the graft and epithelial covering were healed and intact—the new exposure was posterior to the acellular dermis graft and close to the plate. This was treated with another revision and a second acellular dermis graft posterior to the first. This failed to epithelialize completely and 4 months later she developed endophthalmitis requiring a vitreous tap, intravitreal antibiotics, and removal of the tube. The fifth patient developed endophthalmitis 2 months after the revision procedure. By that time, the graft was well epithelialized and the conjunctival surface was free of aqueous leaks. He was treated successfully with vitrectomy and intravitreal antibiotics and his tube remained well-covered 29 months later. Because he had no signs of infection at the time of the revision, I conjecture that the infection, however slow to become evident, was introduced at the time of the revision procedure or the early postoperative period before complete epithelial healing.
DISCUSSION: KEY CONSIDERATIONS
Exposure of the tube of an ADI within a few millimeters of the limbus is not an infrequent long-term complication. Tube exposure that occurs within the first month of the original device implantation is recognized as a surgical wound dehiscence, and treated as such. This review discusses late tube exposure, typically years after the original surgery, and typically with a quiet eye. Most patients are unaware of a 1 to 2 mm tube exposure without infection. In fact, beyond a year after successful ADI implantation, tube exposure may be the most common complication. However, there are no data in the literature on rates of long-term (>1 y) complications of tubes exclusive of short-term (<1 y) complications. One study of the long-term outcomes of Ahmed valve implantation found that 3 of 78 eyes required revision for tube exposure.16 Although all eyes were observed for a minimum 3 years of follow-up, no mention was made of the timing (long-term or short-term) of the tube exposures.
I recommend that the surgeon, and subsequent nonsurgeon eye care personnel involved in the patient's care, bear in mind the following key considerations:
- Each patient must be informed of his/her commitment to lifelong follow-up. After successful ADI surgery, the risk of complications of the hardware is ever present. Because of the lifelong risk of tube exposure and endophthalmitis, complacency and neglect could be very costly. Regular eye examinations for life are required. Also, he/she should be advised to avoid all eye rubbing—even though no data in the literature associate chronic eye rubbing with tube exposure.
- Examine the ADI hardware at every visit. The surgeon, and subsequent nonsurgeon eye care personnel, must develop this habit. Tube position in the anterior chamber and its limbal crossing should be noted. Then the upper eyelid should be lifted while the patient gazes downward. At the slit lamp biomicroscope, the tube is inspected from limbus to plate, and the anterior edge of the plate and bleb (if possible) are inspected. Exposed hardware should be easy to identify. There is a rim of slight injection of the surrounding conjunctival vessels, and slight mucus or oil accumulation on the tube wall itself when exposure is present.
- I recommend periodic eye examinations at intervals no longer than 12 months, for the life of the patient. One could argue that every 6 months is a more appropriate interval to detect early exposures and to minimize the chance of endophthalmitis.
- Prophylactic antibiotics while awaiting surgery are of no proven value. Theoretically, they may be detrimental by selecting bacteria resistant to the antibiotic(s) used. There are no published data on reducing the incidence of endophthalmitis associated with tube exposure by using prophylactic antibiotics. In fact, there are few data on antibiotics for uninfected leaking trabeculectomy blebs. Wilensky17 reported that 2 of 12 patients with bleb leaks after trabeculectomy developed endophthalmitis. Because both had received prophylactic antibiotic therapy before endophthalmitis developed, Wilensky stated that “therefore, it would appear that such treatment does not afford major protection against the development of intraocular infection.” Prophylactic preoperative antibiotics are typically started 30 to 60 minutes before surgery.
- An exposed tube in a prephthisical or phthisical eye (hand motions vision or worse) should simply be removed. Revision with a patch graft is too big an operation for an eye with no salvageable vision. Furthermore, the risk of later reexposure increases if the globe shrinks. In such a case, I recommend removing the tube from its limbal fistula, closing the fistula tightly with 7-0 polyglactin suture, and stretching the tube so as to transect it as closely as possible to the plate. The plate (if unexposed) need not be removed, as that again would be too big an operation for such an eye.
With attention to these considerations and to good intraoperative technique, one can achieve good tissue coverage of the tube while maintaining its drainage function.
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