Aqueous tube shunts

Published on 08/03/2015 by admin

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CHAPTER 39 Aqueous tube shunts

Introduction

Glaucoma drainage implants in use today were originally developed by Molteno over 40 years ago, as a single-plate paralimbal implant1. However, the paralimbal position resulted in significant complications; the drainage implants were redesigned so that the plate was made larger, was moved away from the limbus, and was connected to the anterior chamber by a long silicone tube. The physiological principles leading to this design were based on the concepts that a larger surface area would produce a larger drainage area and that the posteriorly positioned plate would result in fewer complications than the original paralimbal plate. Further modifications of Molteno’s implant saw the addition of a second plate, and use of a dual chamber, which restricts drainage initially to a small area anteriorly on the plate surface with later aqueous spread to the remaining plate surface.

Currently used implants are all long-tube implants, and the most common are single plate large area implants, which are either valved or unvalved. The valved implants include the Ahmed (New World Medical, Rancho Cucamonga, CA) and Krupin (Hood Laboratories, Pembroke, MA) implants. The non-valved implants are the Baerveldt (Advanced Medical Optics, Inc., Santa Ana, CA) and Molteno (IOP, Costa Mesa, CA) implants. Ahmed and Molteno implants are also available as double-plate devices; however, the trend is not to use these double-plate devices. Molteno has recently introduced a larger, thinner and more curved single plate, the Molteno3, which is available in two sizes, 175 mm2 and 230 mm2 (Fig. 39.1). The purpose of glaucoma implants is to create a large drainage bleb, whose permeability controls pressure. Knowledge about drainage bleb physiology should allow modifications that render greater efficiency.

Fundamental principles and physiology of implant blebs

The implant drains aqueous to the plate surface, creating a bleb by elevating the overlying tissue. Drainage efficiency depends on bleb permeability, which in turn depends on aqueous composition, bleb surface area, and tissue reaction. The concept that the bigger the bleb the better the control is only partly true. The original studies supporting ‘bigger is better’ compared single plate Molteno implants with double-plate models over only 12 months2. Subsequently pressure lowering effects of a double-plate Molteno implant were shown to be no more effective than a single plate3. Later studies comparing Baerveldt 350 mm2 with 500 mm2 implants showed the 350 mm implant was more effective in pressure reduction4. Thus, although larger plate size has a minimal effect to reduce pressure further, this effect does not increase exponentially; bleb efficiency does not improve exponentially above a certain size, which may be somewhere between 175 and 240 mm2. Double plate implants become unnecessary, leaving an upper quadrant available for possible future drainage surgery, and eliminating the more arduous and invasive form of implant surgery.

Glaucomatous aqueous contains pro-inflammatory cytokines5,6. Allowing this aqueous to reach the plate surface immediately after implant insertion, as with valved implants, produces a more intense hypertensive phase, and a less efficient bleb7,8. Preventing ‘glaucomatous aqueous’ from reaching the plate surface until IOP has been normalized results in a more efficient bleb7,8. A study has also shown that the lining of the bleb during the ‘hypertensive phase’ produces high levels of pro-inflammatory cytokines, resulting in greater bleb fibrosis9. During the hypertensive phase, the IOP can be decreased by tapping the bleb with a 30-gauge needle, thereby preventing further cytokine production10. Molteno et al.11 described the histopathology of capsules around plates in primary and secondary glaucomas. Without aqueous reaching the plate surface, the plates were encapsulated by a thin avascular collagen layer. The delayed release of aqueous from eyes with a pressure of 25–35 mmHg resulted in capsules with fewer fibrovascular than fibrodegenerative components. Immediate aqueous flow, as occurs in valved or non-ligatured implants, produced thick capsules composed of an outer fibrovascular layer and an inner fibrodegenerative layer of approximately equal thickness. Molteno concluded that without aqueous flow the capsule consists of a thin collagenous avascular layer. With immediate flow of aqueous to the plate surface, an inflammatory reaction occurred, resulting in a capsule containing collagenous and vascular components. After a period of time, a fibrodegenerative process develops in the deeper layers of the capsule, maintained by activation, migration, apoptosis, and production of death messengers by mesodermal cells. This fibrodegenerative process may depend on sufficient IOP for aqueous to displace interstitial fluid from the deeper layers of the capsule. The final thickness of the capsule depends on the timing of these opposing processes, which can be influenced by surgical technique and the use of anti-inflammatory medications.

Modification of the tissue overlying the plate, by supra-Tenons’ implantation of the implant, will be discussed in the section on surgical technique.

Preoperative assessment

A number of factors need to be considered when evaluating a patient for a tube shunt. Of prime importance is the underlying cause for the high IOP. Other factors include the anatomy of the eye and orbit, the ethnicity of the patient, the availability of surgical sites, other coexisting pathologies such as corneal disease or cataracts, and finally the type of implant that may be best suited for the problem at hand.

The anatomy of the eye and orbit may determine the implant type, and where it can be placed. A very small orbit may preclude the insertion of any implant. The optimal positioning of different implants has been evaluated using cadaver eyes13, although this study did not include the newer larger single plate Molteno3 implant. The Molteno implant could be placed most posteriorly from the limbus without impinging on the optic nerve, and the Ahmed implant was least amenable to posterior placement. Postmortem findings in an eye with an Ahmed implant led the authors to suggest more anterior placement of the device if used in the supero-nasal quadrant, to avoid possible optic nerve damage14. The size of the orbit needs to be evaluated, as postoperative ocular mobility problems within a small orbit may occur due to bleb size. Implants such as the Baerveldt implant, which gives a lower bleb profile, may be more appropriate in such a case.

Previous surgical procedures may preclude the use of superior quadrants, and an inferior quadrant may need to be used. As this is more likely to result in mobility problems, and possibly vertical diplopia, the choice of an implant with a lower bleb profile may be necessary. Inferior quadrant surgery, consequently, is better suited for poorly sighted eyes or one-eyed patients, as the chance of diplopia is lessened or eliminated. Superior placement of an implant even in the presence of an existing plate is possible if the implant is placed in a supra-Tenons’ location, avoiding the underlying implant.

The pressure lowering effects of five implants (Molteno single and double plate, Ahmed, Baerveldt, and Krupin implants) was determined to range between 51 and 62%, with no significant differences in either percentage change in IOP or the overall surgical success based on the size of the end plate15. As discussed, larger plate sizes have not been shown to produce significantly lower IOP and a plate size somewhere between 175 mm2 and 250 mm2 may give the maximum pressure lowering potential. This allows the use of single plate implants, which are far easier to insert than double plates, as well as requiring only a single quadrant.of the eye. Achieving immediate postoperative pressure lowering without overt hypotony may seem possible only with a valved implant, but this can be achieved also by occluding the silicone tube in non-valved implants and creating slits in the tubes to control pressure prior to removing the stent16.

The preoperative assessment needs to consider the fibrotic potential of the eye, which is likely to be greater in eyes with previous failed filtration procedures, young patients, and black patients. The use of systemic anti-fibrosis medications, as suggested by Molteno17, or surgical modification such as supra-Tenons implant insertion may be an option in these eyes18.

The presence of a cataract, or corneal opacity, may necessitate a combined cataract extraction and/or corneal transplantation1921. If there is vitreous in the anterior chamber, more likely in aphakia, a vitrectomy (anterior or posterior) may be required to prevent blockage of the tube by vitreous. Placing the tube in the posterior chamber necessitates a total vitrectomy.

Surgical technique

The preferred site for implant placement is the superior temporal or nasal quadrant. All single plate implants available today can fit into either quadrant, although bulkier implants may be associated with motility problems if placed in the superonasal quadrant.

The conjunctival flap may be limbal or fornix based, with most surgeons preferring fornix-based flaps. The conjunctiva is elevated off the underlying sclera with an injection of balanced salt solution (Fig. 39.2). The initial limbal incision is made with a Bard-Parker blade, and continued with spring scissors, for approximately 12 mm. Vertical relieving incisions are then made approximately 5 mm in length parallel to the superior rectus and/or lateral rectus muscles depending on quadrant use. The two edges of the cut limbal conjunctiva are marked with identifying sutures allowing for accurate replacement of the conjunctiva on completion of the surgery (Fig. 39.3). At this stage, if total subconjunctival placement of the implant is the intention, then further posterior dissection of the conjunctiva and Tenons’ capsule off the sclera is continued using a spring scissors, until a pocket approximately 12 mm deep has been created for placement of the implant. The implant may also be placed into a supra-Tenons pocket18. After the initial limbal incision and relieving incisions have been made, a 7-0 Vicryl suture is placed into Tenon’s capsule below the conjunctiva, and Tenon’s capsule is pulled forward, facilitating the dissection of the overlying conjunctiva off Tenon’s capsule below it (see Fig. 39.3). A plane of dissection is then found between Tenon’s and conjunctiva, and dissection in this plane is carried out creating a pocket between the conjunctiva and the underlying Tenon’s into which the implant will be placed (Fig. 39.4

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