Ординатура / Офтальмология / Английские материалы / Mechanisms of the Glaucomas_Shields, Tombran-Tink, Barnstable_2008
.pdf694 |
Fechtner and Khouri |
We also highlight some of the potential complications and opportunities for further improvements.
THE MECHANISMS OF GLAUCOMA SURGERY
Trabeculectomy
Trabeculectomy is designed to divert aqueous from the anterior chamber to the subconjunctival space. One of the fundamental limitations of trabeculectomy is the selection of the subconjunctival space as the target for aqueous drainage. Normal aqueous drainage is not to the subconjunctival space, but rather flows from the anterior chamber through Schlemm’s canal to collector channels and back into the venous circulation.
The subconjunctival space has some initially appealing characteristics. The tissue pressure is approximately 0 mmHg providing a favorable gradient for outflow (3). The space is in direct proximity to the anterior chamber, separated only by sclera. It is readily surgically accessible from an external approach. It is also easily visualized by slit lamp biomicroscopy to allow postoperative manipulations (e.g., laser suture lysis).
The overwhelming disadvantage of the subconjunctival space is the exuberant wound-healing response of Tenon’s tissue. Although it is biologically quite a favorable adaptation for an ocular wound to promote a vigorous wound-healing response, it is a marked obstacle in glaucoma surgery for the subconjunctival space to elicit this same exuberant response. The success of glaucoma surgery depends upon aqueous flowing to the subconjunctival space, distributing within the space, and being reabsorbed. Excessive healing and scarring can prevent outflow at the level of the sclera or create encapsulation of the fluid in the area surrounding the ostium.
This wound-healing response defines the success of trabeculectomy. In the absence of a wound-healing response, a communication could be created between the anterior chamber and the subconjunctival space with a measurable and calibrated resistance to outflow and a defined opening and closing pressure. In fact, this is the condition in the operating room at the end of surgery before the initiation of wound healing; aqueous can freely exit the eye through the new drainage pathway restricted only by the external resistance of the flap. Then the wound-healing response starts and continues for many months. Fibroblastic and vascular proliferation in the postoperative period defines the effective tissue pressure of the filtering bleb. Although the subconjunctival space has a theoretical tissue pressure of zero, the actual resistance to outflow is a complex function of wound healing and scarring. The single greatest advance in trabeculectomy surgery in the past two decades has been the introduction and widespread adoption of anti-fibrosis agents, typically fluorouracil or mitomycin (4–7). However, the effect of anti-fibrosis therapy is nearly as unpredictable as the wound-healing response itself. Improved modulation of this response may be the single greatest area of opportunity for improved success in glaucoma surgery.
Non-Penetrating Filtering Procedures
A group of variations on trabeculectomy are the “non-penetrating” procedures such as deep sclerectomy and viscocanalostomy. The basic strategy for these operations
Mechanisms and Mechanics of Incisional Surgery for Glaucoma |
695 |
is to create a superficial scleral flap as in trabeculectomy and then remove the inner wall of Schlemm’s canal and the juxtacanalicular trabeculum. A second flap is dissected and removed exposing cut ends of Schlemm’s canal and leaving a thin membrane of trabecular meshwork instead of creating a full-thickness ostium as in trabeculectomy (8,9). The purpose is to bypass the site of highest resistance to outflow, namely the juxtacanalicular trabecular meshwork while avoiding penetration into the anterior chamber. Additional steps advocated by some include injecting viscoelastic into the cut ends of Schlemm’s canal (viscocanalostomy) and peeling the inner wall of Schlemm’s canal and placing a solid implant under the scleral flap (deep sclerectomy with implants). The use of an implant is to keep the created intrascleral space patent. Collagen, reticulated hyaluronic acid, and Hema implants have been used.
These procedures were developed in an attempt to prevent some of the complications of trabeculectomy by avoiding the full thickness entry into the eye. The mechanism for intraocular pressure (IOP) lowering may be the same as with trabeculectomy, that is aqueous diversion to the subconjunctival space. The retained trabecular meshwork acts as a porous membrane. Aqueous can be observed to slowly flow through it at the time of surgery. In close to half of cases, a subconjunctival filtering bleb appears that is usually described as more diffuse and shallower than a typical trabeculectomy bleb. Other mechanisms of aqueous resorption have been proposed (9). There may be some outflow through the thin remaining scleral bed of the deep sclerectomy into the suprachoroidal space. This space has a lower tissue pressure than the IOP, which would create an outflow gradient. It has also been hypothesized that the aqueous humor may reach the cut ends of Schlemm’s canal ostium from the scleral space and subsequently be drained in the aqueous veins. It is interesting that this was one of the original rationales for development of trabeculectomy.
Although non-penetrating procedures leave a thin membrane at the time of surgery, many patients will require subsequent YAG laser of this membrane in order to achieve sufficient IOP lowering (10). This is, in effect, a staged trabeculectomy. Some authors have reported substantial IOP lowering despite intended watertight closure of the scleral flap and no apparent bleb. They have suggested that an intrascleral aqueous reservoir is created and that aqueous is absorbed within sclera (11,12).
Angle Surgery
An alternative approach to incisional IOP lowering is to reduce the resistance to outflow of the trabecular meshwork and enhance the physiologic outflow through Schlemm’s canal. Chandler and Grant estimated that most of the resistance to outflow resides at the trabecular meshwork (13,14). In experimental models, they measured a decrease of 41% when the TM was surgically incised (15). This is the mechanism behind angle surgery. The basic mechanism of angle surgery is to incise trabecular meshwork and increase aqueous access to Schlemm’s canal. These procedures have been quite effective for pediatric glaucoma, although they are far less so in adult primary open-angle glaucoma. Renewed interest has sparked the development of new techniques aimed at trabecular meshwork and Schlemm’s canal, such as the Trabectome by iScience.
696 |
Fechtner and Khouri |
PREOPERATIVE PLANNING
Preoperative planning is an essential component of a glaucoma surgical procedure. The surgeon must evaluate the eye for any concurrent conditions that would either alter the intraoperative or the postoperative approach. Particular attention must be paid to the health of the conjunctiva and surrounding ocular tissues. Preoperative allergy or inflammation will alter the wound-healing response. The ocular surface must be intact. Dry eye syndrome, blepharitis, or other external diseases should be addressed before surgery. Any eye that has had previous surgery must be carefully evaluated to ascertain whether the anatomy has been distorted or altered requiring a corresponding adjustment in the surgical approach.
The most commonly accepted approach to glaucoma therapy is to use topical medications first and incisional surgery only if other measures fail to control IOP. Yet, there are several indicators that chronic topical medical therapy is pro-inflammatory. A significant increase in the number of macrophages, lymphocytes, and fibroblasts in the conjunctiva and Tenon’s capsule was seen in patients who received longterm glaucoma topical therapy (16–19). This may actually interfere with surgical success, but despite this information, medical therapy remains first line for most patients.
Because most patients facing glaucoma surgery have been previously treated with topical ocular medications, which can induce subconjunctival inflammation, hyperemia, or a conjunctival allergic reaction, if the eye appears red or irritated, it is often worth reducing or discontinuing the topical medication load before surgery. Some surgeons choose to initiate treatment with topical steroids to suppress inflammation before surgery. Although it has been clearly established that postoperative steroids are beneficial, it is less clear that pre-treatment of steroids will improve outcome (20–22).
Successful incisional filtering surgery results in the creation of a bleb. The bleb is the fluid-filled subconjunctival space and overlying conjunctiva. It assumes a dome or blister-like shape just posterior to the surgical limbus. This disruption of normal curvature of the surface of the eye can result in focal drying anterior to the bleb. Preoperatively, the surgeon should assess the eyelid margins for blepharitis and Meibomian gland dysfunction. Patients with dry eye should also be evaluated for production of the aqueous component of tears. Ocular surface disease problems should be addressed before glaucoma surgery.
Some of the other essential components of the preoperative evaluation include assessing the corneal endothelial health, gonioscopy to detect unexpected angle abnormalities, and assessment of the lens stability and zonular integrity. A special consideration is eyes with prior trauma or complicated cataract surgery with a violated posterior capsule. Vitreous occlusion of the ostium results in failure of trabeculectomy. Avoiding iridectomy in these eyes may prevent vitreous migration into the anterior chamber. Trabeculectomy is relatively contraindicated in eyes with inflammatory and neovascular glaucoma. The clinical examination must include careful biomicroscopy to identify these conditions.
Mechanisms and Mechanics of Incisional Surgery for Glaucoma |
697 |
ANESTHESIA
Glaucoma surgery is most commonly performed with local anesthesia. Only in rare cases would general anesthesia be considered. The approach to anesthesia must balance safety, comfort, and ease of surgery. Peribulbar anesthesia has the advantage of improved safety compared with retrobulbar anesthesia. However, the distortion of Tenon’s tissue by the volume of injected anesthetic can interfere with the surgical approach and with watertight wound closure. We do not favor this type of anesthesia. A low-volume retrobulbar injection can provide excellent anesthesia without distorting the anatomy but carries a slightly greater risk of retrobulbar hemorrhage and injury to the globe.
For the cooperative patient, a useful technique is topical anesthesia, supplemented with sub-Tenon infusion of additional local anesthetic once the conjunctiva and Tenon layer are open. Topical tetracaine 1/2% and lidocaine jelly 2% are applied to the globe before the surgical prep. Lidocaine 1% solution is irrigated posteriorly with a blunt tip cannula after the surgical incision is made, taking care not to overinflate the sub-Tenon’s space.
Sufficient anesthesia can be achieved by combining topical anesthesia for the initial incision followed by periodic sub-Tenon’s injections. This carries the least risk of unexpected hemorrhage and allows patient cooperation by avoiding significant akinesia. It is not a suitable technique for the highly anxious patient or one with limited understanding or communication.
The remaining portions of this chapter will focus on the mechanics of aqueous diversion as illustrated by trabeculectomy and non-penetrating deep sclerectomy.
MECHANICS OF TRABECULECTOMY
Conjunctival Incision
The design of the conjunctival incision is intimately related to the closure of the incision. It is somewhat paradoxical that trabeculectomy requires complete and rapid healing of the external conjunctival incision to provide a watertight closure while simultaneously requiring incomplete healing of the scleral incision and subconjunctival tissues to allow for aqueous egress from the anterior chamber and distribution within the sub conjunctival space. In a puzzling lexicon, trabeculectomy is described as “limbusbased” or “fornix-based.” This refers to the hinge of the conjunctival flap. A limbusbased flap has an incision in the fornix, whereas a fornix-based flap has the incision at the limbus. These terms have persistently caused confusion, and the following discussion will use the alternative terms “limbal incision” and “fornix incision” that are descriptive of the surgical maneuver rather than the result.
A fornix incision was favored, particularly around the time of the introduction of the use of 5-FU. It allows a more secure watertight closure, but exposure is more difficult, particularly without an assistant. A fornix incision should be placed far posteriorly, about 9–10 mm behind the limbus. This is posterior to the insertion of the superior rectus muscle, so care must be taken not to injure that muscle inadvertently. An initial incision should be made through conjunctiva and Tenon’s until bare sclera can
698 |
Fechtner and Khouri |
be identified. Then the wound can be extended circumferentially to allow retraction forward and exposure of the limbus. Care must be taken when dissecting Tenon’s from sclera anteriorly to avoid a button hole of the conjunctiva. Any button hole must be identified and closed, particularly with the use of antifibrosis agents.
A limbal incision provides more convenient exposure, but it is more difficult to achieve a watertight closure. This approach involves disinserting the conjunctiva to make a small limbal peritomy large enough for the creation and closure of the scleral flap. Although it is a subtle point surgically, the initial grasping and incision of conjunctiva can create either a radial or a circumferential opening. Grasping the conjunctiva radially creates a radial incision. A small radial incision at one end of the peritomy improves access and exposure to the sclera for creation of the flap. However, additional suturing is required to close the radial incision. Grasping and lifting the conjunctiva parallel to the limbus allows for a circumferential entry wound (see Figs 1 and 2). A circumferential limbal incision is easier to close but makes exposure more challenging, particularly when placing sutures along the posterior edge of the scleral flap. Further issues regarding the incision will be discussed in the section on conjunctival closure.
The complications and challenges with the conjunctival incision in primary trabeculectomy are few, although an eye that has had previous conjunctival surgery can pose many challenges related to underlying scarring. The surgeon must take care to avoid areas of previous scarring. Handling of delicate tissue such as conjunctiva should be done with atraumatic forceps to avoid the unwanted creation of perforations. Many such forceps have been designed. The choice of which to use is one of surgeon’s preference. Toothed forceps are probably inappropriate for handling the conjunctiva particularly in the setting of the anti-fibrosis therapy. Once the conjunctiva has been opened, tissues can be handled by grasping Tenon’s from underneath, which reduces the risk of an inadvertent buttonhole. The remaining discussion in this chapter assumes
Fig. 1. Grasping the conjunctiva parallel to the limbus and lifting allows a circumferential entry wound.
Mechanisms and Mechanics of Incisional Surgery for Glaucoma |
699 |
Fig. 2. Wescott scissors are used for circumferential conjunctival peritomy.
that the incision has been placed at the limbus. The surgical variant with a fornix incision differs only in the opening and closure of conjunctiva and Tenon’s layers.
Tenon’s Dissection
Although the conjunctival insertion at the limbus is discrete and easy to dissect, the Tenon’s insertion tends to be broader. It is a surgical misconception that conjunctiva and Tenon’s can be cleanly disinserted in a single layer. The Tenon’s insertion is broad and usually begins a millimeter or two behind the conjunctival insertion (see Fig. 3). In other words, there is a small extent where sclera is covered only by conjunctiva. With a limbal incision, it is best to perform a conjunctival peritomy and then disinsert Tenon’s layer separately.
Fig. 3. Tenon layer insertion is visible upon conjunctival disinsertion.
700 |
Fechtner and Khouri |
Tenon’s can be cleanly disinserted by lifting the conjunctiva to put Tenon’s under tension and making an incision through the full thickness using scissors to dissect posteriorly. The tips of the scissor should be perpendicular to the limbus. The full thickness of the Tenon’s insertion must be incised until sclera can be visualized posteriorly (see Fig. 4). Then the closed scissors blades can be passed through this opening and swept forward to either side, bluntly dissecting Tenon’s forward to its insertion. Tenon’s can then be cleanly dissected with the scissors in a single maneuver.
Posteriorly, any adhesions must be lysed to allow the placement of sponges carrying the anti-fibrosis agent. Traditionally, this was performed by inserting blunt tipped Wescott scissors and spreading them. Other surgical tools have been devised for this step. The Blumenthal dissector is an example of an incremental but helpful improvement. This instrument is a small disk with blunt edges on a straight rod that can be inserted through a small peritomy and create a relatively large area of dissection with much less risk of tearing the conjunctiva or creating radial extension of the conjunctival incision (see Figs 5 and 6). Blunt posterior dissection also reduces the risk of inadvertent injury of the superior rectus muscle with resultant bleeding or motility disturbances. The posterior dissection needs to be only large enough to accommodate the sponges soaked with 5 fluorouracil (5-FU) or mitomycin C (MMC). We no longer believe that a wide dissection is necessary.
The most common complications of this portion of the operation are uncontrolled posterior bleeding that cannot be easily reached with the bipolar cautery, or injury to the superior rectus muscle. Uncontrolled bleeding sometimes requires the creation of an extended radial relaxing incision in order to gain access to the bleeding vessel.
Hemostasis
Meticulous hemostasis is considered essential during trabeculectomy. Uncontrolled bleeding obscures the view intraoperatively. Postoperative bleeding under the scleral flap can cause flap adhesion and failure of aqueous to drain. Uncontrolled bleeding in the subconjunctival space will introduce wound-healing promoters and provide
Fig. 4. Tenon’s insertion is incised until the sclera can be visualized posteriorly.
Mechanisms and Mechanics of Incisional Surgery for Glaucoma |
701 |
Fig. 5. The Blumenthal dissector is an instrument with a small disk on a straight rod that can be inserted through a small peritomy and used to dissect a relatively large area with less risk of tearing the conjunctiva or creating radial extension of the conjunctival incision.
a scaffold for undesired wound-healing response. Blood also obscures the view preventing timely laser suture lysis.
On occasion, posterior bleeding is encountered intraoperatively. This can be controlled with a bipolar cautery tip. A 23-gauge blunt tipped bipolar cautery tip is a very useful tool for controlling bleeding during trabeculectomy. Current bipolar cautery instruments may deliver a coagulative current to overlying tissues with inadvertent contact. Most notably this may lead to coagulation and contracture of the conjunctiva complicating watertight closure. It might be advantageous to have a bipolar cautery
Fig. 6. The Blumenthal dissector showing under the conjunctiva during dissection.
702 |
Fechtner and Khouri |
tip that would allow the delivery of energy to the sclera while protecting overlying structures from thermal coagulative damage.
Application of Anti-fibrosis Agent
The use of 5-FU and MMC has revolutionized trabeculectomy surgery. Most trabeculectomies are now performed with one of these adjuncts (4). Yet, dose–response curves have not been established and the common treatment parameters were arrived at empirically. A few general principles can guide the use of these agents intraoperatively.
Antifibrosis agents can be applied either before or after creation of the scleral flap. Application before creation of the flap reduces the likelihood of the agent inadvertently entering the anterior chamber, with potential for corneal toxicity. Administration under the flap has been suggested to have the advantage of less exposure of the overlying Tenon’s and perhaps better bleb morphology. In practice, both approaches are used and both work.
Five-Fluorouracil is used at the commercially provided concentration of 50 mg/ml and is usually applied for 5 min. MMC is usually diluted to 0.2–0.4 mg/ml. We believe it is best to use a standard protocol for a consistent concentration of agent to avoid errors of dilution. Although many surgeons will increase duration of exposure based on some clinical impression of risk factors for failure, there is little evidence to guide our use of MMC. Even with low-dose application regimens of MMC, the concentrations in the inner side of the sclera were found to rapidly increase beyond the limits of the therapeutic range. Owing to its fast diffusion, it seemed that prolonged application time may lead to higher ciliary body concentrations (23). It is better to use too little and have a trabeculectomy fail than to use too much and have chronic hypotony.
A variety of absorbent materials are used to deliver 5-FU and MMC including filter paper, cut surgical spears, cornea protectors (see Fig. 7) and instrument wipe sponges. These approaches are all plagued by the certainty that the actual amount of drug delivered to tissue remains highly unpredictable (24). The surgeon should take care to protect the cut edge of conjunctiva and the limbus from exposure to antifibrosis agents
Fig. 7. Mitomycin C application intraoperatively.
Mechanisms and Mechanics of Incisional Surgery for Glaucoma |
703 |
as these tissues must heal well to avoid a leaking wound (wound healing is discussed below). After removing the delivery material, the globe should be rinsed with BSS to dilute away as much 5-FU or MMC as possible.
If anti-fibrosis therapy could be administered in a reliable and controlled fashion over a broad area with minimal posterior dissection, this would probably improve the procedure. Modulation of the wound-healing response remains the greatest nearterm opportunity to improve contemporary incisional glaucoma surgery. Development of new wound-modulating agents may augment early filtration success by enhancing fibroblastic inhibition beyond what is achieved with current regimens. Combining agents with different mechanisms of action may result in non-overlapping toxicities.
Improved methods of surveillance of the evolving morphologic features of the filtering bleb will provide clues regarding the status of wound healing and may provide the clinician with an opportunity for earlier intervention and bleb rescue. This is analogous to surveillance of cancer, for example, where markers indicate early cancer relapse before the frank clinical manifestations of relapse. Similarly, a bleb exhibiting signs of early failure before IOP elevation might be rescued with early intervention.
Long-term antifibrosis therapy may improve long-term bleb survival rates as wound maturation is not a static but rather a continuous process. Long-term antifibroblastic therapy may include chronic intermittent antiproliferative drugs, growth factor inhibitors, or gene therapy. The latter may be realized through activation of suppressor genes or through inhibition of promoters of fibroblast DNA or RNA synthesis.
Paracentesis
We consider a paracentesis to be an essential step for intra-operative and postoperative management of glaucoma surgery. In the operating room, the paracentesis allows the surgeon to refill the anterior chamber with fluid and to evaluate the outflow from around the trabeculectomy flap. The paracentesis is also invaluable in managing postoperative complications of shallow or flat anterior chamber. It can be difficult or risky to try to perform a paracentesis postoperatively in a soft eye with a shallow or flat chamber. The paracentesis is a very simple part of surgery. Special blades have been designed to perform the paracentesis, but any small blade will suffice. There are a few technical fine points that can make the paracentesis easier to perform. Place the external corneal wound in a temporal location that will be accessible at the slit lamp. It is easiest to direct the paracentesis toward the deepest portion of the anterior chamber, i.e., the pupil. But, in a phakic eye that is not advisable because a cannula inserted to reform a flat chamber would be directed toward the lens capsule. In a phakic eye, direct the paracentesis toward the 6 o’clock meridian over the iris (see Fig. 8). When making a paracentesis with a triangular blade, e.g., 22º, rotate the blade toward the cutting edge when you withdraw it to enlarge the internal opening without enlarging the external opening. A rectangular paracentesis is easier to use than a triangular one with a small internal opening. Scleral flap dissection is easier with the globe firm, so we prefer to perform the paracentesis after the flap dissection. Alternatively, it could be performed earlier in the procedure but the chamber should be reinflated with balanced salt solution (BSS) to maintain firm globe pressure. One exception is when operating on an eye with very high IOP. In such cases, perform the paracentesis early and use