Fig. 4.1
Rational and presumed mechanism of action of the viscocanalostomy procedure. The viscocanalostomy aims to restore the physiological drainage pathway for the aqueous through the Schlemm Canal and the venous collectors; filtration into the sub-Tenon space is excluded. This mechanism is possible with the creation of an intra-scleral space, called a lake, into which the aqueous from the anterior chamber (AC) drains (seeps) through a specifically-created fine membrane: the Descemet’s window, constructed of the anterior layers of the sclerocorneal trabeculate and the Descemet membrane. The aqueous is collected in the lake and from here passes directly to the cut ends of the Schlemm Canal; the canal’s lumen has already been incannulated and dilated with an injection of viscoelastic. The aqueous drains from the canal into the collector vessels and finally into the episcleral venous network. The aqueous collected in the lake may also drain into the underlying choroid, increasing the uveo-scleral outflow pathway
The viscocanalostomy has two advantages over the trabeculectomy:
- 1.
The elimination of the filtering bleb eliminates all of the problems associated with it, primarily the failure caused by the conjunctival, Tenonian and scleral scarring processes;
- 2.
The fact that the AC has not been opened reduces the possibility of hypotonia, hypothalamy/athalamy, inflammation and cataract in the postoperative period.
The moderate popularity viscocanalostomy has achieved in recent years has meant that many surgeons have started using this technique, even in combination with phacoemulsification .
The literature available on viscocanalostomy, and the literature on the deep sclerectomy, was initially limited, though it has increased in recent years. Encouraging data have been reported with inferior success percentage rates compared to trabeculectomy in the majority of cases. A careful analysis of the literature has shown that non-penetrating procedures (viscocanalostomy, deep sclerotomy, canaloplasty) consent good control of the IOP in the early postoperative period but they are also associated with a high percentage of late failures. Regarding the comparison of trabeculectomy with non-penetrating glaucoma surgery, the literature reports contradictory results. While some authors have recently underlined how the use of the non-penetrating techniques (with or without phacoemulsification) have produced results that are comparable in terms of a reduction in the IOP, others have highlighted greater efficacy of the trabeculectomy with or without MM-C compared to the non-penetrating techniques. Recent literature has reported a better safety profile of the non-penetrating techniques compared to trabeculectomy, while a superiority in terms of tonometric reduction has not been observed. In conclusion, thanks to the better safety profile and further improvements to the surgical technique, with or without phacoemulsification, these types of non-penetrating techniques are currently a valid surgical option, particularly in those cases where the target pressure is not excessively low.
Fig. 4.2
Preparation of the scleral bed . Surgical technique : the surgeon exposes the eye bulb sufficiently using a traction suture or a Merocel sponge tip as illustrated for the trabeculectomy (see Chap. 3, Figs. 3.1 and 3.2). The surgical procedure begins at 12 o’clock with the preparation of a conjunctival flap with fornix based, 8–9 mm wide (similar but slightly wider than the flap of the trabeculectomy) and includes the Tenon capsule. The surgical limbus must be identified as this is an important surgical landmark (red lines). To keep the surgical field relatively free from blood, the surgeon may request the continuous help of an assistant or he can place small fragments of sponge on the sclera and replace them frequently. The surgeon must reduce to a minimum the diathermy of the episcleral vessels; these must be protected as they are essential for the drainage of the aqueous humor. The main vessels must be identified and not damaged to prevent excessive bleeding. If diathermy is inevitable, it will be necessary to use reduced values and coagulate the individual vessels one-by-one. As an alternative to parsimonious cauterization, good hemostasis can be achieved by the topical application of ornipressin (L-ornithine 8-vasopressin, marketed with the name “POR & Ferring”, Sandoz, Switzerland); this molecule is a vasoconstrictor free from any adrenergic action. When applied topically, it provokes a local ischemia that lasts for about 2 h. As an alternative, some surgeons apply a few drops of standard adrenalin
Fig. 4.3
Superficial scleral flap . When the surgeon has identified a suitable area, possibly lying between two venous collectors, he will create the superficial scleral flap that extends into clear cornea for about 1 mm.The flap can be created in a variety of shapes and sizes. Generally-speaking, it has a parabolic shape and the dimensions can vary from between 5 × 5 to 4 × 3 mm for a thickness of approximately 200–250 μmThe dissection can be performed with standard bevel-up crescent knives or with a specially created Grieshaber bevel-up knives
Fig. 4.4
Deep scleral flap. The preparation of the deep scleral flap is a crucial phase in the procedure: only the correct depth of the dissection plane will allow the surgeon to identify the Schlemm Canal. The margins of the deep flap are cut approximately 0.5 mm inside the edges of the superficial flap, to guarantee better closure when the superficial flap is sutured at the end of surgery. Considering the variability of the scleral thickness between individual patients, it is difficult to establish the right depth of the dissection plane: the use of precalibrated blades is a relative contraindication in this phase of the surgery. The more appropriate approach is to reach the edge of the choroid: the blue-gray color must be visible through the residual scleral lamellas. To this end it is essential to work under maximum magnification with extremely low traumatizing instruments that will allow the surgeon to penetrate deeper or remain more superficially when creating the dissection plane with great precision (depending on the surgeon’s requirements). If the dissection plane is correct, the surgeon proceeds forward and will automatically reach the point for identifying the Schlemm canal, that becomes ‘unroofed’ because it is part of the deep flap
Fig. 4.5
Identification of the Schlemm Canal . Once opened, the Schlemm Canal appears as a dark line, positioned immediately in front of the scleral spur, represented visually by the concave line that anteriorly delimits the scleral bed. In front of the Schlemm Canal, proceeding with the dissection in a centripetal direction, the surgeon identifies the sclera-corneal trabeculate, with its typical granular appearance
Fig. 4.6
Dilation of the Schlemm Canal. Once the Schlemm Canal has been identified and opened, the surgeon injects some high viscosity sodium hyaluronate inside (Healon GV). This maneuver is performed using a special Grieshaber cannula of external diameter 165 μm and internal lumen of diameter 90 μm. The surgeon must proceed with as little trauma to the structures as possible, allowing the cannula to penetrate just 0.5–1 mm. Simultaneously, the surgeon applies moderate traction on the deep flap. The viscoelastic must be injected slowly through the opening that has been created, avoiding that a sudden increase in the pressure inside the Schlemm Canal ruptures the internal wall, inducing an undesired trabeculectomy. The viscoelastic mechanically dilates the lumen of the canal (that is pathologically reduced in primary open-angle glaucoma), increasing it from 25–30 to over 200 μm. Immediately after the injection, the surgeon will observe that the episcleral collectors will whiten as the viscoelastic passes through the lumen. To achieve maximum dilation on a greater portion of the circumference of the Schlemm, Stegmann suggests repeating this injection two or three times into both ends of the Schlemm. Now the surgeon can proceed with the successive phase, the creation of the Descemet’s window
Fig. 4.7
Anteriorization of the sides of the deep scleral flap. A Descemet’s window of suitable depth can only be created if, prior to applying pressure on the trabeculate with the sponge, the surgeon has anteriorized the sides of the deep scleral flap for about 1 mm in clear cornea. For this maneuver, Vannas scissors appear to be the ideal instrument to provide the best control during the cut: however, as these scissors are extremely sharp, there is a risk that they may perforate the underlying Descemet’s membrane. Micro-scissors with blunt tips would appear to be more suitable for this maneuver
Fig. 4.8
Creation of the Descemet’s window. First, a paracentesis is created at 2 o’clock: this will reduce the pressure in the anterio r chamber (AC) and in the posterior chamber (PC); it will decrease the risk of perforation and consequently the prolapse of the iris through the window itself. The tonometric drop that follows the creation of the paracentesis will often eliminate the pressure gradient between the AC and the venous collectors, determining an inversion of the venous flow and blood reflux, that from the cut ends of the Schlemm Canal slowly drains onto the exposed surface of the trabeculate. This sign gives us proof that the canal has been identified correctly. In the event of combined surgery, the paracentesis is used to allow the introduction of a second instrument in the AC. The Descemet’s window is not created by dissection: the use of instruments that are even moderately sharp will considerably increase the risk of penetrating the AC. The window must be created using blunt instruments, using a sponge tip to exert mild pressure on the anterior edge of the trabeculate and simultaneously applying modest upward traction on the deep flap (black arrow). At this point, a reduced IOP will facilitate the success of the maneuver. The surgeon separates the Descemet membrane, that will remain attached to the sclera-corneal trabeculate from the corneal stroma that continues in the scleral tissue of the deep flap. The Descemet’s membrane must be at least 500 μm long. Once the Descemet window has been created, in some patients it will already be possible to observe the percolation of aqueous across the Descemet membrane and the exposed portion of the trabeculate
Fig. 4.9
Stripping the internal (deep) wall of the Schlemm Canal. In the event percolation is insufficient or is absent, it will be necessary to remove the obstruction to this filtration: the obstruction will usually be the juxtacanicular trabeculate, or rather the thin layer of connective tissue that separates the endothelium of the Schlemm Canal from the remaining layers of the trabeculate. The juxtacanicular trabeculate is removed—along with the anterior wall of the Schlemm—with a delicate ‘stripping’ maneuver; at this point, the Descemet’s window has been created in front of the Descemet membrane and behind the more internal layers of the trabeculate. The ‘stripping’ can be repeated several times using fine forceps until adequate percolation has been achieved. If necessary, light stripping can be associated using the bevel-up knife used for the inspection.Stripping of the canal’s internal wall can be facilitated if the area is kept dry.Once the stripping of the internal wall of the Schlemm Canal has been completed, the surgeon will observe an increase in filtration across the Descemet’s window
Fig. 4.10
Excision of the deep flap and suture. Once percolation of the aqueous has been achieved, the deep flap can be removed. Vannas scissors are used, again with the risk of inadvertently perforating the Descemet’s window below with the sharp tips of the instrument. In this maneuver, it is essential to correctly position the sharp blades of the scissors in parallel with the Descemet window to cut the flap, keeping it distant from the window itself
Fig. 4.11
Suture of the superficial flap. The superficial flap is sutured with 5 or 7 tight nylon 10–0 sutures to prevent aqueous filtering to the sub-Tenonian layer below; alternately, the surgeon can use 11–0 polyester sutures (Mersilene). It is essential that the suture provides good closure to prevent the bleb formation. High viscosity sodium hyaluronic can then be injected into the intrascleral lake with the objective of preventing the collapse and reduce scarring in the early postoperative period.Mersilene or nylon are also used for the closure of the conjunctiva, with two sutures applied on the sides of the conjunctival-capsular flap. Alternately, vicryl 8-0 can be used.Finally, a subconjunctival injection of a steroid-antibiotic combination can be performed in the inferior fornix
Fig. 4.12
Single access phaco-viscocanalostomy . Combined phaco-viscocanalostomy procedure: as with the phaco-trabeculectomy, in the combined phaco-viscocanalostomy procedure, the surgeon may opt for either a single or a double access. In the event the surgeon prefers the single access in the superior position at 12 o’clock, following creation of the superficial and deep flaps and identification the Schlemm Canal, the viscocanalostomy is suspended temporarily: with the deep flap positioned on the scleral bed, the surgeon creates the tunnel as described below and performs the phacoemulsification. When this step has been completed, the surgeon resumes the viscocanalostomy. In practice, the surgeon begins the viscocanalostomy (having identified the Schlemm Canal); the phaco procedure is performed (through a tunnel created below the superficial flap in the most anterior portion); finally, the surgeon creates the Descemet’s window and the viscocanalostomy is concluded. Opting for a single access (at 12 o’clock), the tunnel created must also satisfy the requisites of the phaco procedure (intraoperative maintenance of the chamber depth, good closure of the incision at the end of the procedure) and those associated with the viscocanalostomy (that requires absolute integrity of the scleral flaps and the Descemet’s window). The tunnel for the phacoemulsification is created following identification of the Schlemm Canal, and prior to creating the Descemet’s window. The fragile structure of the window could be damaged by the traumatic maneuvers of the phacoemulsifier. The most suitable location is the space that lies between the superficial flap and the deep flap (indicated in the figure). As the dissection for the deep flap extends approximately 1 mm into clear cornea, the tunnel will also lie in clear cornea. It is approximately 2 mm long with the width depending on the gauge of the tip fitted to the phaco handpiece. The principle advantage of the single access is logistics, as the surgeon will not have to change his position during the procedure. It is important to prevent the IOP rising excessively during phacoemulsification to avoid rupturing the filtration structures in the sclerectomy site with the consequent undesired sectorial perforation of the eye bulb. When the cataract procedure has been completed, the AC is filled with medium or low resting viscosity viscoelastic, and the glaucoma surgery can be resumed with the exposure and opening of the Schlemm Canal. At this point, percolation of aqueous humor is usually observed in phakic eyes; however, following cataract surgery, only a marginal quantity of liquid will be observed. At the end of the phaco procedure, and once the residual viscoelastic has been removed, it is advisable to leave the eye in a condition of hypotonia to reduce the thrust of the iris on the Descemet’s window. An injection of miotic into the AC will reduce the risk of iris prolapse in the event of a perforation of the Descemet’s window (an event that would warrant a conversion to a trabeculectomy). Finally, the walls of the tunnel and the paracentesis are edemized
Phaco-viscocanalostomy with a double access : in the event of phaco-viscocanalostomy with double or separate accesses, the surgeon can begin the procedure with a phaco in a temporal position, complete it and then proceed with the viscocanalostomy in a superior position. Alternately, in a similar way to a single access, the surgeon can begin with the viscocanalostomy in a superior position and continue with the preparation of the deep flap and the identification of the Schlemm Canal, with the phaco performed in a temporal position and the viscocanalostomy completed superiorly.
Results
Available literature on viscocanalostomy and on the deep sclerectomy was initially scarce, though the quantity has increased in recent years. Regarding the viscocanalostomy (not combined with phaco), Stegmann reported a 3-year success rate of 82.7%. These figures refer to a relative young population of non-white patients (mean age 54 years), with a high preoperative IOP (47 mmHg). These results appear to be exceptional.
Encouraging results have also been reported in Europe and America with percentage success rates lower than the trabeculectomy in the majority of cases.
Compared to the trabeculectomy with NPGS (deep sclerectomy, viscocanalostomy, canaloplasty), the literature has reported contradictory findings. On this subject, as mentioned previously, some authors have recently underlined how the non-penetrating techniques (with or without phacoemulsification) have similar outcomes in terms of a reduction in the IOP, while others have highlighted the greater efficacy of the trabeculectomy (with or without MM-C) compared to the non-penetrating techniques (see also the results for the deep sclerectomy).
Complications
Intraoperative
The procedure (like the deep sclerectomy and canaloplasty) thins the wall of the eyebulb without opening the AC and will avoid a sudden drop in pressure that is traditionally observed with the classical filtering procedures. Consequently, complications associated with this drop in pressure are less frequent (intraoperative bleeding and post-operative detachment of the choroid).
The perforation of the Descemet’s window is the most commonly observed intraoperative complication, particularly when the surgeon is learning the technique. If on the one hand, any microperforations can be ignored or even created intentionally to increase the percolation, on the other important solutions of continuity of the window necessitate the conversion to trabeculectomy (or phacotrabeculectomy).
The two factors that condition the management of the perforation of the Descemet’s window are the depth of the AC and an iris prolapse. In the event of tiny holes with no iris prolapse or loss of AC depth, the surgeon can continue the procedure without having to convert the technique.
The perforations with the reduction or the abolition of the AC depth, but without iris prolapse must be treated to prevent the iris prolapsing or anterior synechias forming. The surgeon then proceeds with the introduction of viscoelastic into the AC through a paracentesis, taking care to inject it below the perforated window to distance the iris. The surgeon should inject the smallest amount of viscoelastic to avoid postoperative hypertonia. Moreover, some authors suggest affixing an implant (scleral or corneal patch) on the perforation site to close the hole.
The perforation of the scleral bed, caused by an excessive depth of the dissection plane, in theory can have serious consequences, particularly if it causes bleeding of the ciliary bodies. Nevertheless, this is an extremely rare occurrence. The superficial flap is closed with several sutures (6–8) in 10.0 nylon, when the AC has reformed and the iris repositioned (if it had prolapsed).
In the event an iris prolapse accompanies a large Descemetic perforation, the surgeon should perform a peripheral iridectomy. The superficial flap must be sealed with sutures once the viscoelastic material has been introduced into the surgically-created scleral space to increase the resistance to drainage. Considering that the scleral space created reduces the resistance to drainage of the aqueous humor, it is extremely important that the superficial flap is completely watertight.
Post-operative
In the literature and from the experience of a number of surgeons, it emerges that the postoperative pathway prior to the viscocanalostomy shows an extremely low incidence of complications (see also the complications of the deep sclerectomy). Some complications appear early, others in the late postoperative period.
Redness (hypoema) was reported by a number of authors: it was normally mild and extended for less than 4 mm) but even in the more serious cases, it will reabsorb within three days.
The surgeon may detect pressure spikes in the initial post-operative period (with the pathogenesis probably correlated to Healon GV remaining in the Schlemm Canal, though this is not completely clear). Leakage may be observed, associated with the incorrect placement of the sutures on the conjunctival flap.
Hypotonia and associated consequences are extremely infrequent, even considering that the application of antimetabolites has no place in this surgery.
Postoperative hypertonia is not a frequent complication and should be managed differently based on the cause. There may be several causes:
An incomplete surgical dissection of the deep flap: in this case, the surgical incision can be reviewed. Revision of the incision site may prove to be difficult and this is one of the reasons many surgeons prefer to intervene on a completely different site.
Intrascleral hemorrhage: this will be resolved within a few days.
Excessive viscoelastic in the AC (particularly after combined surgery or the re-formation of the AC following a microperforation): this will be resolved spontaneously within a few days.
Rupture of the Descemet’s window with iris prolapse secondary to hypertonia caused by rubbing the eye, Valsalva’s maneuver etc. It is managed with miotics and the gonio-YAG laser on the prolapsed iris. If this is ineffective, the surgeon must proceed with an iridectomy.
Formation of anterior synechias at the site of the Descemet’s window, often secondary to intraoperative microperforations.
Steroid-induced hypertonia (during the first post-operative week). Even the formation of the filtering bleb (5% of cases according to Stegmann) is considered to be a sort of failure, even though this is normally associated with good tonometric control.
Deep Sclerectomy
The deep sclerectomy is a type of non-penetrating glaucoma surgery . It differs from viscocanalostomy and canalostomy mainly because its goal is to obtain the filtration of the aqueous humor into the intrascleral lake and from here into the sub-Tenonian space (and not to facilitate drainage through the dilation of the Schlemm Canal with viscoelastic, as happens in the viscocanalostomy) (Fig 4.13). Regarding the tonometric results from the deep sclerectomy, numerous studies have demonstrated that the deep sclerectomy (as with the viscocanalostomy and canaloplasty), either when performed alone or in combination with phacoemulsification, can produce a satisfactory reduction in the IOP, with a greatly reduced percentage of complications compared to trabeculectomy. Many papers published in the literature—that have increased quite considerably over the last 8 years—support the efficacy of this technique, that has the same indications as the viscocanalostomy technique. Even though the efficacy of the deep sclerectomy is generally considered to be lower than the effectiveness of the trabeculectomy, additional techniques such as the intraoperative use of antimetabolites, implants and laser gonio-perforation would appear to increase the efficacy. In the past, the addition of collagen to the deep sclerectomy provided contrasting results and it is not possible to state definitely if it will really guarantee an improvement in the filtration.
Fig. 4.13
Deep sclerectomy: possible filtration pathway for the aqueous humor . The procedure involves the removal of a portion of the sclera, including the part that covers the opened portion of the Schlemm Canal, and a more anterior portion that lies immediately above the Descemet membrane. The endothelial wall of the canal itself is removed towards the corneo-scleral trabeculate. Therefore, there will be no fistula between the AC and the subconjunctival space. The Schlemm Canal is expected to be in a slightly more scleral position with respect to the surgical limbus (Fig. 4.14). As with the viscocanalostomy, drainage is facilitated by the aqueous humor seeping through the thinned eye bulb wall in correspondence with the corneo-scleral trabeculate. The aqueous humor is collected in an intrascleral drainage chamber where it is transported by the uveoscleral discharge (blue arrows). The bleb almost appears to be almost an unintentional additional tool for reducing the IOP, transforming the procedure almost into a classical filtering surgery. As with the other glaucoma procedures, the deep sclerectomy can also be combined with phacoemulsification
Fig 4.14
Deep sclerectomy: schematic representation
Surgical Technique
As for the viscocanalostomy, the first phase of the procedure involves the appropriate exposure of the bulb and the preparation of the scleral bed: the conjunctiva and the Tenon capsule are generally cut at the limbus, creating a flap that is approximately 8 mm wide. Hemostasis of the episcleral vessels is performed by bipolar diathermy . It is important not to exaggerate with the settings used to avoid inducing an excessive scarring reaction, with possible retraction of the scleral tissue; however, it is not necessary to protect the venous collectors as in the viscocanalostomy, as they do not play an essential role in the mechanism of action of the deep sclerectomy.
The preparation of the superficial scleral flap more or less reiterates what has already been described for viscocanalostomy. The shape of the flap is usually quadrangular and measures approximately 4 × 4 mm; the surgeon can also prepare flaps that are slightly larger or slightly smaller.
Fig. 4.15
Creation of the deep flap . The method used to create the deep flap is similar to that used to create the flap for the viscocanalostomy. The figure illustrates the pentagon-shaped deep flap before (a), during (b) and after (c) the dissection and the opening of the Schlemm Canal. If the objective is to achieve greater filtration, a deep flap can be created with the side margins corresponding to those of the superficial flap. When the Schlemm Canal has been identified and the posterior wall, that remains adhered to the inferior surface of the deep flap, has been removed, the Descemet’s window can also be created in the deep sclerectomy. The process to create the Descemet’s window is the same as the procedure described for the viscocanalostomy
Fig. 4.16
Positioning of the inserts in the scleral lake. To ensure better filtration in the medium- and long-term postoperative period, the surgeon can position two inserts in the scleral lake following removal of the deep flap; this will facilitate maintenance of the intrascleral space and prevent occlusion by scar processes. An insert in collagen (Aqua-Flow, Staar Surgical AG, Ch-2560 Nidau, Switzerland) consists of a cylinder of lyophyllized pig collagen (see the figure). This substance has a considerable degree of hydration and excellent biocompatibility. The cylinder is positioned radially at the bottom of the scleral lake, and is sutured with 10-0 nylon; the anterior tip of the cylinder should lie above the Descemet’s window. The insert will be slowly re-absorbed over a period of about 6 months. The insert in reticulate hyaluronic acid (SK-Gel, Corneal, Paris, France) is a solid gel. The mechanism of action is correlated with maintenance of the space of the intrascleral lake. The insert is positioned on the bottom of the lake prior to suturing the superficial flap. According to the manufacturer, the insert will be reabsorbed within 3–6 months with decomposition delayed by the reticular agent. However, the insert in reticulate hyaluronic acid is not widely used
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