Ординатура / Офтальмология / Английские материалы / Mechanisms of the Glaucomas_Shields, Tombran-Tink, Barnstable_2008
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Fig. 8. In a phakic eye, the paracentesis is directed toward the 6 o’clock meridian over the iris.
it to slowly decompress the eye over several minutes. This may reduce the risk of decompression retinopathy or suprachoroidal hemorrhage (25).
Scleral Flap
There is nearly as much variety in the creation and dimensions of the scleral flap as there are surgeons who perform glaucoma surgery. However, some general principles apply. The scleral flap is a partial thickness flap, hinged anteriorly. It can be square, triangular, trapezoidal, rectangular, or even semicircular. A typical dimension might be a 3 × 3 mm square or trapezoid (wider at the limbus). Typically, the flap is 50–75% scleral thickness (see Fig. 9). The important consideration is the relationship between the scleral flap and the underlying ostium. In the immediate postoperative period, it
Fig. 9. Dissection of a partial thickness scleral flap.
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is this relationship that determines the resistance to outflow. The lateral dimension of the ostium must be slightly smaller than that of the scleral flap and the suture tension must be great enough to create some resistance.
The mechanics of creating a scleral flap involves making an initial groove, and then performing a dissection within a single plane of scleral fibers. The relatively orderly organization of the scleral wall permits creation of a uniform flap thickness. The availability of blades set at a predetermined depth (e.g., micrometer diamond blade) allows the surgeon the convenience of precisely establishing the initial depth. Most commonly, the three sides of the flap are outlined with a groove and then a sharp curved blade is used to dissect the flap free from its bed in a single scleral plane.
One problem that can be encountered during creation of the scleral flap is cutting an intra-scleral vessel along the flap margin. Bipolar cautery must be used sparingly as it creates thermal coagulation of tissue with distortion of the relationship between the flap and the bed. This can interfere with proper flap closure and may lead to overfilteration.
A flap that is too thin is difficult to work with and may not provide sufficient resistance to outflow. It may be fragile and tear during dissection or closure. Handling a thin flap with toothed forceps may create holes that allow too vigorous outflow. In a later step of creating the ostium, the surgeon must hold the flap forward over the cornea, so the scleral flap must have some substance to it.
A flap that is too thick can also lead to difficulties. It invites inadvertent premature entry into the anterior chamber. The radius of curvature of the globe is greater than that of the cornea. Thus, the surgeon must change the plane of dissection at the surgical limbus to achieve the desired anterior entry. If the chamber is entered prematurely, simply establish a new more superficial plane of dissection and continue into clear cornea. Sometimes, the inadvertent entry point can be used as the posterior edge of the ostium. A thick flap and deep dissection may also cause exposure of the choroid. As long as this is not accompanied by bleeding, it is not a serious problem. Again, the plane of dissection can be reestablished for successful completion on the flap.
A properly designed flap and ostium combination allows suturing of the flap back into the bed of the sclera to provide proper flow resistance. With the use of intraoperative and postoperative anti-fibrosis agents, most surgeons prefer to tie the flap relatively tightly in anticipation of releasing sutures later. This planned over-resistance with subsequent titration by release of resistance has avoided complications that were previously associated with leaving the flap too loose. Before the introduction of antifibrosis agents, a loose flap with over filtration, a flat anterior chamber, and choroidal effusions were the common early postoperative complications of trabeculectomy. Such complications are uncommon with current trabeculectomy techniques.
In conjunction with better wound-healing modulation, the relationship between flap and ostium could be better defined. As an example, if we could achieve ideal conjunctival wound healing with little or no subconjunctival fibrovascular proliferation, then the IOP at the end of surgery would closely approximate the final IOP after healing. One could imagine using a calibrated manometer or other pressure measurement technique in the operative room to set the flap/ostium resistance relationship to achieve a pre-
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established IOP. In reality, we are always likely to be dealing with some undesired wound-healing response with current surgical techniques.
Ostium
The defining step of a trabeculectomy might be considered the creation of the ostium. Underneath the scleral flap, the surgeon now creates a communication with the anterior chamber (see Fig. 10). With the evolution of the surgical procedure, the name “trabeculectomy” has become a misnomer. We now position the ostium more anteriorly and actually remove tissue from the peripheral cornea. This is more accurately a lamellar keratectomy under a scleral flap.
Surgeons have variously used either blades, scissors, or punches to create the ostium. There is no clear ideal relationship between the ostium and the scleral flap, but as mentioned above, most surgeons create an ostium that is smaller than the horizontal diameter of the flap. The position of the ostium is determined by external landmarks. By placing the ostium in peripheral cornea, we reduce complications of bleeding and can sometimes avoid the need for an iridectomy. Any bleeding from the posterior lip of the ostium should be controlled to avoid hyphema or clot under the flap. The Kelly Descemet’s or Crozafon-De Laage punches are typical instruments used for creation of the ostium. There is opportunity for innovation in instrumentation for this step of the procedure. Investigations have been performed using various lasers to ablate the tissue. Shunt devices have been designed that can be placed through a small incision, thus eliminating the need to remove tissue (e.g., ExPress™ glaucoma mini shunt).
If the challenges of the wound-healing response could be addressed to provide predictable or modifiable wound healing, there might be greater need to address the relationship between the ostium and the flap. For example, if there were no wound healing in the subconjunctival space, then all the external resistance would be defined
Fig. 10. A Kelly punch is used to create an ostium under the flap.
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by this relationship. Improved wound-healing modulation may allow us to explore this portion of the procedure in the future.
Iridectomy
Iridectomy serves two purposes in the trabeculectomy procedure. Historically, pupillary block and aqueous misdirection were reduced through performing an iridectomy. In practical terms, the most important function of the iridectomy is to remove the iris that could occlude the internal ostium. It is an evolutionary adaptation that the iris is well suited to plug small wounds in the cornea or proximate eye wall. When peripheral iris plugs the ostium, the trabeculectomy cannot function. The iridectomy should be small and peripheral. The two most significant potential complications are bleeding from the iris or from the ciliary processes, and optical adverse effects from the iridectomy. Some patients notice a ghost image from having a second optical pathway.
Iridectomy is performed by grasping the peripheral iris with forceps, pulling a small portion of the iris out through the ostium, and excising that piece with scissors (see Fig. 11). The instrumentation for this portion of the surgery has not changed for decades. Although lasers have been developed to perform iridectomy in the office, it has been considered either surgically or economically impractical to adapt laser technology for this simple step of the procedure. Some surgeons now believe that the iridectomy creates additional inflammation and potentially compromises the success rate of trabeculectomy. They will avoid iridectomy unless the iris prolapses at the time of surgery. Another potential complication of iridectomy occurs in eyes that have had previous surgery. If the lens capsule or lens zonules are not intact, vitreous can prolapse into the space behind the iris. The iris is then the only structure keeping the vitreous from plugging the ostium. Vitreous has an even greater propensity to
Fig. 11. A peripheral iridectomy is performed by grasping the peripheral iris with forceps, pulling a small portion of the iris out through the ostium, and removing that piece with scissors.
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plug small wounds than does iris. When vitreous presents through the iridectomy, the surgeon must perform a vitrectomy that increases the risk of the procedure. It would be a useful advance for surgeons to know in such eyes whether vitreous is likely to present. Alternatively, it could be helpful to have a reliable method for avoiding iridectomy in eyes that had previous surgery.
Flap Closure and Testing Flow
Flap closure should provide for adequate resistance to outflow in the immediate postoperative period to avoid hypotony or a shallow chamber. As wound healing progresses in the subconjunctival space, planned laser suture lysis or releasable sutures can reduce resistance provided by the flap to increase aqueous outflow. Although a single suture can be enough to close the flap intraoperatively, this provides no margin for titration of resistance. Three or more sutures (depending on flap shape, size, and relationship to the ostium) can provide the ability to titrate resistance by releasing one suture at a time. Although there are a variety of geometries used for these sutures, a few general principles apply. A completely unsutured flap may slip in its bed causing induced against-the-rule astigmatism. A flap that has one or two sutures still in place after planned suture lysis is less likely to slip. If there is no flow on the operating table, there will be no flow postoperatively. If no flow is observed, the reason must be identified and corrected. The common causes are an occluded ostium (iris, vitreous, or blood) or too much flap resistance. We prefer to place the one or two sutures that we plan to leave in place and then test flow by injecting BSS through the paracentesis and observing flow around the flap (see Fig. 12). Once flow is confirmed, we place the additional sutures as needed that we plan to release by later laser suture lysis with a Ritch or Hoskins lens.
The surgeon must assess the thickness of Tenon’s layer for postoperative planning. A very thick Tenon’s layer may make laser suture lysis difficult or impossible. After flap sutures are placed, the surgeon should check their visibility. If the scleral flap sutures are not visible through the conjunctiva and Tenon’s at the end of the procedure, the
Fig. 12. Filtration is tested after forming the anterior chamber by injecting balanced salt solution through the paracentesis, drying the surface around the scleral flap, and checking for flow.
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surgeon has two options: to use releasable sutures or to perform a partial Tenectomy. The Blumenthal lens, a relatively new addition to suture lysis lenses, allows improved visibility of flap sutures in eyes with thick or encapsulated blebs. This may facilitate performing difficult laser suture lysis (26).
Conjunctival and Tenon’s Closure
The closure for conjunctiva and Tenon’s layers remains a topic often discussed by glaucoma surgeons, usually in the context of dissatisfaction with a current technique and inquiry if a better one exists.
As mentioned in the section on Conjunctival Incision, a fornix incision is relatively easy to close. Conjunctiva and Tenon’s can be incorporated in a single layer running suture closure with an absorbable or non-absorbable suture.
The limbal incision is more difficult to close and a variety of techniques exist, all variations on a few themes. In truth, there is probably no technique that does not leak a little, at least in some patients. The basic principles of closure of the limbal incision are to return the conjunctiva to its original position or advance it slightly onto the cornea without distorting it and without a frank leak. This can be achieved with a combination of interrupted suture techniques (radial and/or mattress sutures) or with a running suture technique. With an interrupted suture technique, most commonly the sutures are placed at the nasal and temporal extends of the wound to advance the conjunctiva back to the limbus. If there is enough Tenon’s, this can also be incorporated to create a more secure closure. Once the conjunctiva is reapproximated, a horizontal mattress suture (or more radial sutures) is used to close the free edge(s) of conjunctiva (see Fig. 13). A running suture should be anterior to the limbus to allow secure anchorage in the cornea (see Fig. 14). In this closure, the apparent length of the conjunctival edge may seem greater than the limbal edge. The surgeon must be careful to place the sutures evenly to avoid bunching conjunctival under a suture with a resultant gap.
Fig. 13. Closure of conjunctiva and tenon’s layer can be achieved with interrupted sutures.
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Fig. 14. Continuous “running” suture for closure of a limbal incision.
The limbal incision offers convenience and superior exposure. The clear disadvantage is the difficulty in achieving an immediately watertight closure. This remains an opportunity for improvement.
Wound-Healing Response
Trabeculectomy could be a predictably successful procedure for lowering IOP if it were not for the normal biological response of wound healing. A healed trabeculectomy is a failed trabeculectomy; if the scleral wound heals completely, there is no aqueous outflow and no reduction of IOP. Considering the competing priorities for watertight conjunctival healing versus incomplete or absent scleral and episcleral healing, there is no obvious solution to this dilemma. Surgery incites wound healing. The use of antifibrotic agents to improve the success of glaucoma surgery has become common practice. The benefits provided by these agents are in part offset by complications such as late or profound hypotony and bleb-related infections from fragile blebs. New approaches are discussed in chapters 38 and 39.
MECHANICS OF NON-PENETRATING DEEP SCLERECTOMY
Deep sclerectomy techniques involve several modifications, yet share many of the surgical mechanics previously described for trabeculectomy. Exposure and conjunctival incision are performed in a similar fashion. A limbus or a fornix incision can be adopted according to the surgeon’s preference. A limbal incision however tends to offer better scleral exposure. It is helpful to thoroughly remove Tenon’s capsule residues from sclera to facilitate dissection of the larger scleral flap needed for this procedure. In patients at risk for failure, anti-metabolites on a sponge may be placed between the sclera and the Tenon’s capsule. The remaining sections will focus on the necessary modifications that are specific for deep sclerectomy.
Superficial and Deep Scleral Flaps
A relatively large partial thickness scleral flap, hinged anteriorly and measuring about 5 × 5 mm, is dissected. To be able later to access Schlemm’s canal, the flap has to be dissected 1–1.5 mm into clear cornea. Another deep sclerokeratectomy is
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performed by dissecting a deeper, smaller (4 × 4 mm) second scleral flap. The edges of the scleral incisions can be made using a diamond blade. The deep flap is smaller than the superficial flap, leaving a step of sclera on the three sides. Upon dissection of the deep scleral flap, the remaining scleral layer bed should be as thin as possible (almost 50–100 μm).
Unroofing Schlemm’s Canal
The deeper flap is dissected anteriorly to unroof Schlemm’s canal. Schlemm’s canal is typically located anterior to the scleral spur (where scleral fibers are regularly oriented parallel to the limbus). In some patients, Schlemm’s canal localization is more difficult and dissection should be carried out slowly. Oozing of aqueous may be noted once very thin tissue is left. Once Schlemm’s canal is opened, the dissection is advanced more anteriorly into cornea. This step has to be performed very gently because of risk of perforation and entry into the anterior chamber, which would lead to conversion into full-thickness penetrating surgery. If anterior chamber shallowing occurs with iris prolapse, a viscoelastic is used to reform the chamber, a surgical iridectomy is performed, and the procedure is converted into a trabeculectomy. When inadvertent anterior chamber entry occurs, the superficial flap needs to be tightly sutured to prevent overfiltration postoperatively. Because of the large surface area of the superficial flap, more sutures may be needed for secure closure compared with the smaller trabeculectomy flap.
Trabeculo-Descemet Dissection and Implant Insertion
Once the anterior dissection is completed, the deep scleral flap is cut across the anterior edge and removed. Diffuse, slow percolation of aqueous through the remaining trabeculo-descemet membrane is usually noted at this stage. Using a deep sclerectomy forceps, the juxtacanalicular trabeculum and Schlemm’s endothelium are then removed. One means of avoiding collapse of the superficial flap over the trabeculo-descement membrane and remaining scleral layer is by the insertion of an implant in the scleral bed. The implant can be secured with a suture. Collagen and reticulated hyaluronic acid implants are resorbable in a few months.
Flap and Conjunctival Closure
The superficial scleral flap is closed with 10–0 nylon sutures. Flap closure must be tight, particularly with the use of an implant to prevent displacement. The conjunctiva and Tenon’s layer are closed in a similar fashion to trabeculectomy closure.
SUMMARY
Surgery for glaucoma is aimed at lowering the IOP. At present, we have no medical, laser, or incision technique to restore normal physiology to the eye when aqueous outflow is compromised. Instead, the most common incisional approach for lowering IOP in adults is to divert aqueous to the subconjunctival space. This remains a problematic mechanistic approach. The results are not predictable, and there is a high failure rate of trabeculectomy in the several years following surgery because of
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wound healing and tissue remodeling. Even in successful surgery, the patient may have discomfort from the bleb and have a lifetime risk of bleb-related infection. The shortand long-term complications such as hypotony or infection can result in profound loss of vision. The non-penetrating filtration surgery techniques, such as deep sclerectomy and viscocanalostomy may offer different mechanisms of filtration by targeting the resistance at the level of Schlemm’s canal. However, the presence of blebs in many of these procedures suggests aqueous diversion to the subconjunctival space is at least partly the mechanism. Other approaches to successful surgical control of IOP that are being explored include more targeted modulation of the wound-healing response, devices to assist with aqueous outflow, angle surgery approaches for adults, or diversion of aqueous to another target site such as Schlemm’s canal or the surprachoroidal space.
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