In most cases, CP can be performed safely under topical anesthesia, although peribulbar or retrobulbar anesthesia may also be used. A superior approach is ideal to allow for coverage of a bleb by the upper lid in case one is formed, and also for less discomfort postoperatively. If a traction suture is used to improve exposure, it should be placed a few clock hours away from the surgical site to avoid obstruction of view into the AC and/or distortion of perilimbal anatomy.

A fornix-based peritomy is created using sharp-tipped Westcott scissors while leaving a small anterior skirt of conjunctiva at the limbus (Fig. 14.2). Blunt dissection is directed posteriorly to allow the posterior edge of the peritomy to be sufficiently relaxed for creation of scleral flap (Fig. 14.3). Location of ciliary veins should be considered when deciding where to perform the dissection. One percent plain Xylocaine is injected under Tenon’s capsule to apply further local anesthesia (Fig. 14.4). Cautery should be performed lightly only as needed, and avoiding these veins and collector channels (Fig. 14.5).

Given the high accuracy required to create the scleral flaps, diamond blades can be helpful in making incisions and dissections. A 5 mm × 5 mm superficial scleral flap of parabolic shape (which in our experience provides good watertight closure) is outlined using a diamond trifacet blade (Fig. 14.6). The flap is then fashioned with a diamond crescent knife at about one-third scleral thickness (200–300 μm) (Fig. 14.7), dissecting 1–2 mm into clear cornea (Fig. 14.8).

Then under high magnification, again using the diamond trifacet blade, a deep flap is outlined from 1 mm inside the posterior edge of the superficial flap (Fig. 14.9). This deep flap is also fashioned using the diamond crescent blade, but now at 90 % scleral thickness, leaving only about 100 μm of sclera covering the choroid (one may cut down into suprachoroidal space just at the back end of the flap to allow better determination of the required depth) (Fig. 14.10). At the desired depth, you should see irregular scleral fibers with an underlying purple hue coming from the choroid (Fig. 14.11). Sometimes a full-thickness dissection may inadvertently occur, in which case care must be taken to restart a new tissue plane leaving behind a thin layer of scleral bed. If dissection is too deep, then one risks penetration into the choroid or even the vitreous; yet if it is too superficial (more often the case), when dissection reaches the limbus, one may pass right over SC into clear cornea without exposing the canal (where it becomes even more difficult to reestablish a new plane or flap). Creation of a good deep scleral flap exposing SC is thus likely the most technically challenging step of CP. Identification of proper anatomical landmarks is crucial, and this step must be performed under high magnification. This is particularly true when dissection reaches the base of the flap at the limbus, where unintentional penetration into the AC can occur. If that is the case, which would be evident with a gush of aqueous coming from the AC, one can convert to a penetrating procedure (trabeculectomy).

The deep flap is carried anterior enough when one sees scleral fibers changing from random to organized circumferential arrangement parallel to the limbus. At this point the scleral spur is reached. One may then either slowly and carefully dissecting forward to unroof SC (Fig. 14.12) or simply use a pair of toothed forceps to lift the deep flap up to expose fibers of the outer wall of SC at the base of the flap. Good exposure is confirmed when one sees aqueous percolating through SC, or blood reflux from cut ends of SC (Fig. 14.13).

A paracentesis is then made through the clear cornea away from the surgical site (Fig. 14.14). This would allow IOP to drop gradually to almost single digits, and thus preventing bulging of Descemet’s membrane and the inner wall of SC. One should ensure gradual entry and retraction of the blade while making the paracentesis, so as to avoid sudden decompression of the eye which would risk sudden shallowing of the AC and/or intraoperative hemorrhage. This is particularly true if the reason of choosing CP (a non-penetrating procedure) instead of a penetrating procedure was because the eye is prone to such issues.

The deep flap is advanced forward for another 1 mm into the clear cornea, where further advancement only requires little force as it gets into the plane between Descemet’s membrane and the corneal stroma. The trabeculo-Descemet’s window (TDW) is thereby fashioned. To allow better advancement and exposure of TDW, one may use the diamond trifacet blade to gently create vertical relaxing incisions into the clear cornea at the lateral edges of the flap (with the cutting edge of the blade facing you, thus reducing the chance of penetration into the AC) (Fig. 14.15).

One may use a pair of Mermoud forceps to delicately strip the inner wall of SC away to further increase aqueous percolation (Fig. 14.16). Surgical sponges such as Weck-cel (Medtronic, Minneapolis, Minnesota) can be used to carefully push down on Schwalbe’s line and Descemet’s membrane to separate corneal stroma from TDW. However, any excessive downward pressure or sudden movement may easily perforate TDW and cause inadvertent penetration into the AC. Thus, we recommend moistening the tip of the Weck-cel prior to use, as it will reduce the rigidity of the tip when it is placed on the TDW, allowing a “soft touch.” Once a satisfactory TDW is created, the trifacet diamond blade can be used to score the underside of the base of the deep scleral flap (Fig. 14.17). A pair of Vannas scissors is then used to excise the deep flap (Fig. 14.18), allowing formation of scleral lake in its place later on.

The two cut ends of SC can now be intubated with a special 150-μm diameter viscocanalostomy cannula (Grieshaber, Switzerland) (Fig. 14.19), through which a cohesive high viscosity viscoelastic such as Healon GV (Advanced Medical Optics Inc., Santa Ana, California) is injected to widen the ends of SC for easier entry of the microcatheter. The iScience microcatheter can be secured to the drape with Steri-strips (3M, St. Paul, Minnesota). Using two non-toothed forceps, the catheter is passed into one of the cut ends of SC and is advanced for 360° until the tip emerges from the other cut end of the canal (Fig. 14.20). During passage, one must pay attention to any resistance or false passage into the suprachoroidal space by observing the blinking red light at the leading tip of the microcatheter. If strong resistance or false passage is encountered, one should stop pushing further, retract, and reattempt with scleral depression at the site of resistance, or simply pull out and try passage in the opposite direction through the other cut end of SC.

Once the catheter has passed 360° and the tip emerges, a 10-0 Prolene suture (polypropylene) with needles cut off is tied around the shaft of the device near its tip (by tying the two loose ends to the loop) (Fig. 14.21). The microcatheter is then withdrawn slowly in the reverse direction to which it was passed (Fig. 14.22); meanwhile viscoelastic is injected into SC using the screw mechanism by a surgical assistant. One must avoid injecting excessive viscoelastic which can cause Descemet’s detachment (approximately 1/8th turn of the screw for every 2 clock hours of passage is recommended). Reflux of blood into the AC can be expected during passage or withdrawal of catheter. Once the entire catheter is removed (Fig. 14.23), the 10-0 Prolene suture is released from the catheter tip using Westcott scissors. By now there should be 2 single 10-0 Prolene sutures in SC with 4 loose ends exposed. After confirming the corresponding ends, each suture is tied to itself with a slip knot (Fig. 14.24) with a back-and-forth movement in SC known as “flossing.” Suture tension is adjusted such that one sees a small indentation of TDW by the suture, which should sit anteriorly in SC – this is confirmed by pulling the knot posteriorly until it is barely able to reach the scleral spur (Fig. 14.25). More tension of the suture would result in more distention of SC, thus theoretically more IOP reduction (Fig. 14.26) [13]. Once satisfactory suture tension is achieved, the knots are locked and trimmed with Vannas scissors (Fig. 14.27).

Watertight closure of the superficial scleral flap is achieved with about five 10-0 nylon sutures placed in simple interrupted fashion. A cohesive high viscosity viscoelastic, such as Healon GV, may be placed under the flap to maintain the scleral lake (Fig. 14.28). It has been suggested that viscoelastics may limit fibrinogen migration [18], thereby minimizing scar formation which may obliterate scleral lake over time. Aqueous will percolate through the TDW and accumulate in this “reservoir” before being absorbed by the episcleral, scleral, and choroidal veins.

Finally, the conjunctiva is closed in a watertight fashion using 10-0 or 9-0 Vicryl suture in a running horizontal mattress fashion (Fig. 14.29) as in the case of trabeculectomy. The paracentesis and conjunctiva are checked for leaks with a Weck-cel.