physiology of the drainage pathways of the eye. In other words, the surgery aims at increasing the outßow facility of the aqueous humor between the source of the humor and the extraocular drainage routes while minimizing alterations of the natural structure of the eye [1Ð6]. Achieving this goal is challenging, for the surgery should be efÞcient enough to ensure a signiÞcant lowering the intraocular pressure (IOP), which and should remain stable over time. The surgery should be equally safe, not leading to excessive egress of aqueous humor, or resulting in prolonged overÞltration, but providing durable ocular IOP control. In order to achieve these combined goals, nonpenetrating deep sclerectomy requires a sound knowledge of the eye structures being dissected, a good set of instruments, and a great deal of surgical precision.

8.1.2 Anesthesia

The anesthesia may be either local or general [7Ð9]. The choice of the anesthesia technique depends on the clinical situation. For pediatric patients, anxious patients, or subjects unable to stay calm and cooperate with the surgeon, general anesthesia is recommended. In most of the other cases, a local procedure provides enough relaxation and pain relief to ensure safe and precise surgery. A retrobulbar or peribulbar injection can be given to achieve efÞcient anesthesia. In this case, it is important to limit the volume of anesthetic solution to about 3Ð4 mL. Greater volume in the orbital cone would likely tend to expand the orbital structures, thus leading to a signiÞcant rise in the IOP. This should be avoided in glaucoma patients, who already have compromised optic nerve head blood supply [7, 9]. Expansion of intraorbital structures could also impede free movements of the eye ball, potentially exposing the region of the sclera that will be operated (see below, anchoring of the eye using a silk thread).

egress of aqueous humor, and by leaving an anatomical structure that will act as a barrier to prevent excessive outßow, the IOP may be gently lowered without abrupt perioperative changes.

8.1.3.1 Preparation

The eye is secured by a silk 6.0 thread that may either be inserted through the upper rectus muscle, or through the cornea, taking care not to penetrate into the anterior chamber. The advantage of using such a thread is that the eye can be moved easily and tilted down to expose the sclera for dissection. The conjunctiva is dissected at the level of the limbus for a fornixbased ßap, a technique which gives a good result in terms of subconjunctival bleb function and prevention of postoperative scarring. Conversely the conjunctiva may as well be opened 8Ð10 mm from the limbus, next to the upper rectus muscle, for a limbus-based ßap. In this latter case, it is very important to safely suture the conjunctiva at the end of the surgery to prevent bleb leakage, wound dehiscence, and the risk of blebitis and possible endophthalmitis. The TenonÕs layer is cut to a 10 mm length and the subTenon space is undermined using blunt scissors. Care is taken to not perforate either the TenonÕs layer or the conjunctiva. The sclera surface is then carefully cleaned, removing any remaining portion of tissue, Þbrosis, or scarring, using a hockey knife (Fig. 8.1). This is an important step, preventing excessive postoperative scarring, eventually leading to bleb Þbrosis. Hemostasis is generally achieved after using the hockey knife (Fig. 8.2). In cases of persistent bleeding, gentle wet Þeld elec- tro-coagulation might be performed, or cellulose

sponges soaked with vasoconstrictive solution might be used.

8.1.3.2 Superficial Flap Preparation

A superÞcial scleral ßap measuring 5 × 5 mm is delineated with a steel knife (Fig. 8.3). The depth of the incision represents about one-third of the entire sclera thickness. Using a ruby crescent knife, the superÞcial ßap is then dissected further toward the cornea. It is important to extend this dissection into clear cornea for about 1Ð1.5 mm (Fig. 8.4). Depending on the clinical situation, e.g., in young patients, patients prone to develop hypertrophic scars, and patients of African origin, it might be useful to prevent excessive scarring of the subconjunctival space and the ßap by applying antiÞbrotic agents. Such agents are generally antimetabolit chemicals, e.g., Mitomycin C or 5-ßuorouracil.

A square cellulose sponge soaked with a 0.2 mg/mL solution of Mitomycin C is applied at the interface between the superÞcial ßap and the remaining deeper sclera, as well as between the ßap and the conjunctiva, taking care not to touch the edges of the conjunctiva opening, for 30Ð60 s. The cellulose sponge is then removed and the tissues are thoroughly rinsed using a balanced salt solution.

8.1.3.3 Deep Flap Preparation

The superÞcial scleral ßap is lifted to allow good access to the deep sclera. The sclera is cut radially with a safety margin of about 0.5 mm on each side, to obtain a 4 mm wide deep ßap (Fig. 8.5). The posterior cut is performed last, ideally using a diamond knife, the safety margin being then slightly wider. The depth of the dissection extends close to the choroid.

Fig. 8.6 Opening of the choro•dal space for determination of the total scleral thickness

Fig. 8.7 Dissection of the deep ßap with a ruby blade

Fig. 8.8 Opening of SchlemmÕs canal

Fig. 8.9 Scleral spur seen posterior to SchlemmÕs canal with horizontal Þbers

A Þne layer of remaining sclera is what forms the ßoor of the Þltering scleral bed. A good means to precisely determine the depth of the dissection consists of performing small cuts of the sclera down to the underlying choroid at the edges of the ßap (Fig. 8.6). These minute perforations do not induce any complication or supplementary difÞculties. The ßap is then carefully and progressively dissected forward using the crescent ruby knife (Fig. 8.7). It is important at this stage in the surgery to proceed with great care to remain at the same level during the entire dissection. Unlevel dissection could lead to large perforation of the sclera with protrusion of the choroid, or could result in too-thin a deep scleral ßap with subsequent reduction in the Þltering function of the scleral bed.

8.1.3.4Dissection of the Trabeculo-Descemet’s Membrane

Further dissection of the deep scleral ßap allows access to the SchlemmÕs canal (Fig. 8.8). The scleral spur is an excellent landmark for reckoning the anatomical features of this region and to help in identifying the precise location of SchlemmÕs canal. Indeed the collagen Þbers forming the posterior sclera are randomly organized, while the Þbers in the more anterior sclera become better aligned and organized, to form a ligament (the scleral spur) running parallel to the limbus just posterior to SchlemmÕs canal (Fig. 8.9). Further dissecting unveils this canal. Before opening SchlemmÕs canal along its posterior border, a paracentesis is

Fig. 8.10 Anterior dissection to expose the DescemetÕs membrane

Fig. 8.11 Lateral cuts with number 11 blade upside down

Fig. 8.12 Excision of the deep ßap with Galan scissors

performed through the cornea to decrease the IOP. This paracentesis would allow introduction of viscoelastic material to maintain the anterior chamber

should a large perforation of the trabeculo-Descemet window occur. After derooÞng SchlemmÕs canal, the dissection is terminated 1Ð1.5 mm forward to remove the sclerocorneal tissue in front of the anterior trabeculum and trabeculo-DescemetÕs membrane (TDM) (Fig. 8.10). This step is very delicate and difÞcult, for there is a high risk of breaking this very thin membrane. To ease dissection of the sclerocorneal tissue, two thin radial incisions are made in the corneal tissue. The metal blade is held upside down, the sharp edge in the upper position. The TDM is gently dissected in the anterior plane (Fig. 8.11). A wet triangular cellulose sponge, or a Þne metal blade, a spatula, or a ruby knife can be used to undermine the membrane to complete the exposure by pressing down while simultaneously lifting up the deep scleral ßap with the other hand. Upon completion of the anterior dissection, the scleral ßap is initially cut using a diamond knife. The cutting is extended further using GalanÕs scissors (Fig. 8.12). Percolation of aqueous humor from the anterior chamber through the TDM is a good indicator for an effective and correct dissection of the latter.

8.1.3.5Peeling of Schlemm’s Canal and Juxtacanalicular Meshwork

To enhance Þltration of aqueous humor through the TDM, the major obstacles to egress of aqueous humor, i.e., the inner wall of SchlemmÕs canal and the juxtacanalicular trabeculum, are removed [13]. These structures are thought to offer the highest resistance to aqueous humor outßow in primary and probably other types of secondary open-angle glaucoma. This is a crucial step in the surgical procedure that will warrant success or failure of nonpenetrating surgery. In the course of ab externo trabeculectomy, both the inner wall of SchlemmÕs canal and the juxtacanalicular trabeculum are peeled away using blunt forceps. After drying out the surface of SchlemmÕs canal, the blunt tips of the forceps are used to grasp the edge on the ßoor of the canal. A gentle lift of the forceps will peel off about 4 mm of continuous membrane, resulting in a signiÞcant decrease in the resistance to outßow and consequently a notable increase in the previously mentioned percolation of aqueous humor (Fig. 8.13).