Ординатура / Офтальмология / Английские материалы / Mechanisms of the Glaucomas_Shields, Tombran-Tink, Barnstable_2008

shunts is desirable. A 10-O prolene episcleral anchoring suture is necessary to preclude extrusion of the tube segment from the capsule. The tube segment, inserted into the capsule, should be side-perforated and have a sharp bevel. Continual irrigation with balanced salt solution during placement within the capsule will ensure it is not kinked or blocked during insertion.

ONEVERSUS TWO-QUADRANT SURGERY—SUPERIOR OR INFERIOR LOCATION

Many surgeons prefer single-quadrant surgery and opt for either the Ahmed singleplate (PMMA or silicone) or one of the available Baerveldt devices (250 or 350 mm2). The double-plate Molteno or the double-plate Ahmed require two-quadrant access with the inter-plate connecting tube placed either under or over the superior rectus muscle. Molteno’s preferred location of the primary tube of his double-plate shunt is in the superior nasal quadrant, reflected by the labeling as “right” or “left” implant. Historically, nasal tube location facilitated subsequent extracapsular large incision cataract surgery. Current small incision cataract procedures do not necessitate avoiding temporal tube placement. The preferred location of the relatively larger 350 Baerveldt is superior temporally as there is more space available there for a large bleb. The Molteno accessory plate has worked reliably as demonstrated by improved IOP control and definite bleb formation over both plates and a stair-step drop in IOP with addition of a second plate. The Ahmed double-plate device, requiring an in-surgery hook-up between plates, in the experience of some, has a higher rate of function failure of the accessory plate, probably because of fibrous occlusion of the inter-connecting tube ends that are not well-protected from enveloping fibrous tissue. Any of the single-plate devices can be added if necessary to another quadrant if IOP control is inadequate after the first single-quadrant installation.

Although sometimes necessary, inferior location of aqueous shunts, especially if the patient has good vision in both eyes, is relatively undesirable as the globe may be displaced superiorly with resulting vertical diplopia.

Current versions of the Baerveldt implant have fenestrated explants that permit rivet-like connections of fibrous tissue across the plate. This has limited the vaulting of the capsule and has probably decreased the incidence of postoperative strabismus. The fibrous pillars between the top and bottom capsules make excision of the device more difficult as they must be severed before the plate can be removed.

TUBE INSERTION AND METHODS OF TEMPORARY LIGATURE FOR NON-VALVED DEVICES

The site chosen for anterior chamber tube insertion should be cauterized to minimize bleeding along the insertion needle tract. The tube should be cut with a sharp bevel at a length estimated to approximate the pupil edge. Tube insertion into the anterior chamber should provide for at least 2 mm of clearance from the inner eye wall to prevent migration of endothelium over the tip and tube blockage. Insertion is usually easy through a 23-gauge disposable needle track. The needle should be carefully inspected under the operating microscope before use to detect tip damage. It is inserted

Fig. 2. Methods of temporary ligation of non-valved devices and repair strategies for short or lacerated tubes: (A) 10-O prolene tie in anterior chamber for laser lysis postoperatively; (B) Polygalactin (Vicryl®) simple tie (7-O or 8-O) (usually will spontaneously release in 4–6 weeks); (C) Single arm 5-O nylon segment included in polygalactin tie as spacer and left in sub-conjunctival space to be removed 3+ weeks after installation through cut-down. (D) 5-O nylon intra-luminal tube suture with polygalactin tie. When removed before wound closure the tube remains partially occluded—with unpredictable flow rate immediately. 5-O nylon is a monofilament suture that will easily slide out of tissue at any time after installation. (E) 3-O chromic intra-luminal suture occluding the tube (surrounding tie optional) brought out through Tenon’s and conjunctiva to rest in the fornix. When removed 3–5 weeks postoperatively the tube will function but often a shower of debris sometimes simulating infection will pass into the anterior chamber through the tube. (F) Rip-cord of 5-O nylon with both ends of a loop included in a polygalactin surround tie. The loop can be left in the sub-conjunctival space or externalized and removed at 3–5 weeks to open the tube to aqueous flow. Some of the externalized loops will be internalized beneath the conjunctiva over a week or more but can still be easily recovered through a small cut-down.

beyond the bevel with a short-stroke push–pull motion to round the hole. Pars plan insertion similarly can be done with a 23-gauge needle tract. It is desirable to place pars plana tubes so that they are easily visualized through the pupil postoperatively. Either a curved forceps or a Fechtner inserter may be used to facilitate tube insertion. Too long a segment of tube in the anterior chamber may interfere with the visual axis. If after initial insertion, the tube appears too long inside the eye, it can be pulled partially out and left curved on the episcleral surface and tacked down with appropriately placed sutures. Inadvertently cutting a tube too short may necessitate replacing the entire device or alternatively lengthening the tube (see complications of shunts). If an eye containing an anterior chamber or vitreous tube subsequently suffers hypotony, the tube may migrate across the chamber or vitreous cavity with globe collapse, then spontaneously reposition correctly as IOP recovers.

A variety of methods have been described for temporarily ligating non-valved shunts (see Fig. 2). An alternative to temporary ligatures is to install non-valved devices, Baerveldt or double-plate Molteno in two stages, allowing weeks to months for encapsulation and resolution of inflammation before inserting the tube into the anterior chamber or vitreous cavity at a second surgery. So-called first-stage implants can sit for years before use if necessary, but do rarely erode necessitating surgical removal or repair even though non-functional. A first-stage installation may be done in association with other procedures such as penetrating keratoplasty or vitrectomy in anticipation of their subsequent need for IOP control. A two-stage insertion, with many weeks or months between placing the plate and inserting the tube, probably has the least risk of provoking hypotony and choroidal hemorrhage in fragile eyes. Viscoelastic installation before tube insertion during a second-stage procedure may facilitate gradual IOP reduction and increase safety in fragile eyes.

Shunts may be placed over a pre-existing silicone-rubber buckle element if space allows and even sewn to the buckle material. If in direct contact, the bleb that develops will often include the buckle element and/or the encircling band. The bulk of the device when installed over a pre-existing silicone buckle may complicate wound closure.

The primary tube on shunts is sufficiently long to permit curving it around perilimbal pre-existing blebs or areas of scaring that are inadvisable to dissect. A temporaly explanted tube can be inserted nasally or vice versa. For a pars plana insertion, it is wise to avoid the vicinity of a recent vitrectomy site as extra vascularity is often present in the eye wall increasing the chance for intraocular bleeding during insertion. Intraoperative visualization of the intra-vitreal tube after pars plana insertion through the pupil or indirect ophthalmoscopy is highly recommended to allow detection of intra-choroidal tube insertion, which surprisingly can occur without provoking choroidal hemorrhage.

CLINICAL INDICATIONS FOR SHUNTS

Evolving indications for aqueous shunts as a method of IOP control include contact lens dependence precluding a standard filtering bleb; epithelial ingrowth not amenable to surgical excision; congenital glaucomas not amenable or responsive to standard pediatric glaucoma surgery (goniotomy; trabeculotomy; trabeculectomy); surface disease (pemphigoid; Stevens–Johnson); trauma precluding standard surgery; chemical burns; ICE syndromes failing standard surgery; and uveitis-related glaucomas

(2,5,57). They also have application as an alternative to trabeculectomy or other forms of filtering surgery as the glaucoma part of combined cataract and glaucoma surgery (58). They may be employed as first-stage backup systems (Molteno or Baerveldt) following debridement and repair of leaking filters or in combination with corneal or retina-vitreous procedures. Aqueous shunts require substantial periocular space and may not be applicable to some congenital anomalies associated with contracted orbits or exceedingly small eyes.

Otherwise, uncontrollable neovascular glaucoma (NVG) has been a frequent indication for placing an aqueous shunt ideally after completing pan retinal photocoagulation (PRP), because remission of iris or angle vessels within weeks often correlates with improvement of IOP, especially if the angle has not been closed (59,60). Most likely, blood flow in the abnormal iris and angle vessels just decreases below clinical recognition without actual atrophy. Placing an aqueous shunt in an eye with florid NVG is likely to result, as after trabeculectomy, in simply transferring the stimulus for abnormal vascularization, vascular endothelial growth factor (VEGF), into the filtering bleb with resulting high risk of bleb failure. Ahmed, Molteno, and Baerveldt implants have all been modestly successful in NVG short-term (61,62). Ahmed valves may be problematic because of valve obstruction by blood or fibrin if anterior chamber hemorrhage is present or occurs postoperatively.

The roles of new anti-VEGF drugs (Avastin®; Lucentis®; Macugen®) in NVG management and the subsequent need for and ideal timing of surgical intervention for NVG either with conventional surgery or with shunts remain to be clarified in clinical trials.

ANTIFIBROTICS (5-FU AND MITOMYCIN) AND AQUEOUS SHUNTS

Relatively few publications have addressed the use of either intraoperative or postoperative 5-FU with shunts (62–69). Only three randomized controlled trials have been published, wherein the main outcome issue was IOP control after MMC compared with no MMC with shunts (66,68,69). Two of the three published trials concluded there was no benefit to using MMC (see Table 2). A recent, somewhat flawed, trial reported from China however, did conclude that MMC was of benefit as an adjunct to a “gel” explant (Hunan shunt) and included novel ultrasound findings supporting the conclusion (68).

Experimental studies in rabbits have indicated only short-term benefit from intraoperative application of MMC but persisting benefit from continual release of 5-FU incorporated into fibrin plugs within the device (64,70).

COMPLICATIONS OF SHUNTS

The major short (surgery—5 years) to medium term (5–10 years) complications of aqueous shunt devices continue to include immediate hypotony after surgery; excessive capsule fibrosis and clinical failure; erosion of the tube or plate edge; and strabismus and rarely infection. Long-term complications may rarely include “over-filtration” as relative sick or elderly eyes undergo age-related decrease in aqueous production. Over-filtration can be addressed at any time after installation of silicone-rubber shunts

Fig. 4. Diagrams illustrating a modified Schocket shunt. (A) A segment of silicone-rubber tubing is inserted through a small incision to lie on top of the buckle element, most easily accomplished by inserting the tube into an 18-gauge needle passed through the capsule edge. By 3 weeks after buckle installation, a useable capsule will be present. The tube segment should be tacked to the episcleral surface between the limbus or pars plana insertion site and the buckle edge with 10-O prolene to prevent extrusion. Side perforation of the inserted tube segment is wise along with continual infusion of balanced salt during insertion into the buckle capsule to prevent kinking and obstruction. Usually 6–12 mm of tube can be inserted into the capsule before closure of the capsule incisions.

when in contact with the lens surface, and lead to diffuse or localized corneal edema sometimes geographically limited to the tube location. In eyes with pre-existing peripheral anterior synechia (PAS), especially those associated with rubeosis, advancing PAS may ratchet the tube anteriorly into contact with the endothelium.

Erosion of the tube through host sclera or a patch graft (see Fig. 1) will occur over weeks to months in some cases. Observation during follow-up exams of loss of conjunctival capillaries over the tube, usually 1–3 mm from the cornea-scleral junction,

is an indication of impending erosion through the surface. Re-patching can usually be easily accomplished before exposure of the tube by re-dissection from the posterior aspect of the patch and placement of a new patch, ideally donor-preserved sclera under intact conjunctiva. If the tube is exposed, often difficult extensive lateral dissection is necessary to achieve sufficient conjunctival mobility to re-cover a re-patched tube. Conjunctival grafts may be necessary. Amniotic membrane may be helpful to ensure healing in cases with sparse covering tissues. As long as the edges of the patch graft can be covered by conjunctiva, healing will usually occur by migration of conjunctival epithelium over the central defect.

The use of shunts in eyes containing silicone oil remains controversial, but they may offer a desirable alternative to cyclodestruction in some cases. Placing the device tube inferiorly, preferably inferior nasally, may decrease the amount of oil escaping into the shunt bleb.

PEDIATRIC GLAUCOMAS

Shunts have been successfully applied in otherwise difficult-to-manage pediatric glaucomas (72,73). Complications of shunts in pediatric patients may result from unique risk factors, especially erosion through thin host tissues and even perforation of the cornea. Epithelial ingrowth can occur if an anterior chamber tube becomes exposed especially at the corneal–scleral junction. Rarely massive ingrowth along tubes has virtually filled the anterior chamber. Placing the tube too close to the eye wall in a young child may lead to tube-withdrawal and obstruction as the eye grows.

The placement of the explant is an especially important issue in children, although also potentially problematic in adults with small eyes. The surgeon needs to be aware of the proximity of the posterior edge of the device to the optic nerve.

CLINICAL FAILURE AND ITS MANAGEMENT

Aqueous shunt failure (inadequate control of IOP) can be managed in many cases by re-starting topical anti-glaucoma medications and advancing them as necessary for the individual patient. In some case series, approximately two-thirds of shunts require adjunctive topical medications to achieve clinically acceptable IOPs, usually in the mid-upper teens (9,40). Systemic carbonic anhydrase inhibitors may also be effective for long periods if well tolerated by the patient. In rare instances, when IOP drops below desirable levels, removal of topical or systemic medications may restore acceptable IOPs.

Surgical removal of a failed shunt and re-insertion of a new similar device or an alternative device has also been successful. Revision (excision) of the capsule over a failed device has been successful according to some reports but generally provides only transient improvement in IOP. Benefit of topical application of MMC during revisions has not been proven of long-term benefit. Placing an additional shunt may be preferable to revision of the existing shunt (74). In the rare instance when a previously well-functioning tube migrates out and becomes obstructed at the eye wall or is closed by iris in children or adults, a new shunt may be placed within a pre-existing capsule or a new, longer tube slid down the pre-existing tunnel into the anterior chamber even

years after the original installation. Tubes will easily move within their sheath at any time after installation. The sheath around the tube is a non-cellular membrane that develops around the tube in sub-conjunctival tissues shortly after installation.

RANDOMIZED TRIALS INVOLVING AQUEOUS SHUNTS

Several important issues related to aqueous shunts have been addressed in published randomized trials (see Table 2) (9,45,46,66–69,75–86). Results from the ongoing Tube versus Trabeculectomy Study are challenging prior conclusions, which were based primarily on retrospective case series, that shunts do not provide IOP levels as low as trabeculectomy with MMC (86).

NEW CONCEPTS AND INNOVATIONS

Clinical studies of aqueous shunts draining into peri-orbital sinuses or the lacrimal system are ongoing (87). The C-tube described by Wilcox et al. is in an ongoing clinical pilot study (35).

A gold seton from Italy, with laser adjustable flow rates, is currently in trial (SOLX gold micro-shunt: http://www.solx.com).

The 2006 ARVO presentations included numerous reports on innovative materials and new concepts for aqueous shunts. A novel new device in theory adjustable according to individual need for aqueous outflow, includes a valved tube shunt to the conjunctival surface has not yet undergone clinical trials (Camras CB et al., Valved tube shunt from the anterior chamber to the external ocular surface for use in refractory glaucoma. IOVS ARVO Suppl. 1992).

SUMMARY

During this last decade, a plethora of case series and a few randomized trials have appeared describing clinical outcomes and complications of aqueous shunts. The next few years will undoubtedly include several pilot clinical studies on new devices, randomized trials comparing existing devices, and further publications on new randomized studies comparing existing shunts and standard surgery (84). The literature on this subject, as for many subjects in glaucoma, desperately needs more standardization with regard to terminology, data capture, follow-up, reporting of complications, and statistical analysis. It seems highly probably that these devices will continue to evolve and their clinical performance improve as new less biologically reactive materials become available and new concepts are further explored.

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