Numerous space-maintaining implants have been developed to increase long-term success of NPGS by avoiding secondary collapse of the intrascleral space. They are placed in the scleral bed before suturing the superficial scleral flap. The first to be used was a highly purified lyophilized porcine collagen cylinder (Aquaflow, Staar Surgical AG, Mideau, Switzerland) [8, 10, 11]. It is placed radially at the center of the deep sclerectomy and secured with a single 10.0 Nylon suture to the scleral bed before closure of the superficial flap. It swells rapidly when exposed to aqueous humor and is progressively resorbed within 6–9 months after surgery [12, 13]. Another resorbable implant has been proposed in the form of a reticulated hyaluronic acid triangular implant (SK-GEL, Corneal, Paris, France) [14]. The advantage of this implant is that it does not need to be sutured to the scleral bed and it can occupy a large volume in the intrascleral space given its rheological and physical characteristics [15–17]. Another hyaluronic acid device has been proposed in the form of a slowly resorbable viscoelastic implant that can be injected under the scleral flap and the conjunctiva to maintain both subconjunctival and intrascleral filtration routes (Healaflow, Anteis, Geneva, Switzerland) [18]. A nonresorbable hydrophilic implant has also been developed to maintain the intrascleral space (T-Flux, IOLTech laboratories, La Rochelle, France). Each arm of this T-shaped designed implant is inserted into one of the surgically created ostia of Schlemm’s canal to increase aqueous outflow through this route [16, 19–21].

Once the conjunctiva is opened, a 4 × 4 mm superficial scleral flap is created with a depth corresponding to one-third of the scleral thickness. A radial cut is performed at the edge of the flap to localize Schlemm’s canal. The change of orientation of the scleral fibers can be a good landmark for this step. Once Schlemm’s canal is identified, it is unroofed. The inner wall of the canal is removed using fine forceps until percolation of aqueous humor occurs. This technique is similar to a sinusotomy except for the presence of a superficial scleral flap and the removal of the inner wall of Schlemm’s canal [5, 22]. It differs from a deep sclerectomy since a deep scleral flap is not created and dissection of a TDM window is not performed.

In the 1990s Stegmann introduced a variant of NPGS consisting of injecting high-viscosity sodium hyaluronate (Healon GV) into Schlemm’s canal to improve aqueous drainage through this pathway [9]. Injection of viscoelastic into the canal not only dilates the canal and associated collectors but also disrupts the internal and external walls of Schlemm’s canal and adjacent trabecular layers, thus increasing trabecular outflow facility and making the procedure act as a trabeculotomy [23]. Increased outflow occurs also at the TDM window into the scleral lake where it can diffuse into the subchoroidal and subconjunctival space. External filtration and filtering blebs are uncommon in viscocanalostomy because of tight suturing of the flap and can be detected only in up to one-third of the eyes [24], whereas a supraciliary hypoechoic area suggesting subchoroidal drainage has been shown in UBM studies [25]. The challenge in viscocanalostomy is to maximize circumferential outflow and this has been possible with the recent advances in cardiac stent technology. Canaloplasty consists of inserting a flexible catheter into Schlemm’s canal (iTrack, iScience Interventional, Menlo Park, CA) [26]. This facilitates Healon GV injection followed by a 10.0 Prolene tensioning suture in the canal to enhance outflow [27, 28].

Numerous authors have reported the use of different kinds of lasers to facilitate the dissection of the deep corneoscleral flap. The use of erbium:Yag and excimer lasers for performing a deep sclerectomy have been described in the literature as relatively effective alternatives to the conventional surgical procedure [29–34]. More recently CO₂ Laser-assisted deep sclerectomy has been developed using an OT-135 beam manipulator system, conjugated with a CO₂ laser (“IOPtiMate”; IOPtima Ltd., Ramat Gan, Israel). This system, which was first described by Assia et al., enables deep tissue ablation with minimal risk of perforation [35, 36]. The far infrared radiation of this laser (wavelength of 10,600) is absorbed in water and thus is ineffective when applied over wet tissues [37]. Applying CO₂ laser energy on the dried scleral and trabecular tissue results in a localized ablation until the point at which fluid begins to percolate through the inner wall of Schlemm’s canal. Once percolation occurs, further laser application is ineffective because of the wavelength characteristics. This technique offers a potential alternative to the manual NPGS, making the procedure simpler and less surgeon dependent [38].

Intraoperative perforation of the TDM is likely the most common complication of NPGS. It occurs during the anterior deep dissection, which is a delicate and complex technique resulting in the relatively flat learning curve of NPGS. It is not uncommon to encounter a 30 % perforation rate during the first 10–20 cases and this rate drops to 2–3 % after the learning phase [78]. Management of this complication depends on the magnitude of perforation, the depth of the anterior chamber, and the presence or absence of iris prolapse [79]. A small hole with a deep anterior chamber can be disregarded and the surgery is completed normally. These small perforations can be usually tamponaded by positioning the collagen implant next to the hole. If a larger crack in the membrane occurs upon deep dissection with a flat or deep anterior chamber and no iris prolapse, viscoelastic can be injected through a paracentesis to restore a normal configuration. This prevents further iris prolapse or PAS. In the worst situation, transverse tears occur at the weakest point of the TDM which is the junction between the anterior trabeculum and Descemet’s membrane. Such tears are immediately followed by iris prolapse. Protruding iris tissue must be excised by a peripheral iridectomy. NPGS is then converted to a trabeculectomy and the superficial scleral flap is tightly sutured with 6–8 10.0 Nylon sutures and outflow resistance is created by injecting viscoelastic into the intrascleral reservoir.

Late postoperative complications can also occur at the level of the TDM. PAS can develop late after intraoperative microperforations. They can also be caused by iris prolapse following goniopuncture [80] or any other traumatic disruption of the TDM (e.g., blunt trauma, valsalva maneuver followed by corectopia and iris prolapse). PAS can disrupt aqueous outflow resulting in increased IOP. Failure of Nd:YAG laser synechiolysis to reposition the iris is an indication for medical or surgical treatment if the IOP is still high.

Early postoperative inflammation is relatively less common in NPGS compared to trabeculectomy. Chiou et al. compared flare levels after deep sclerectomy with collagen implant and conventional trabeculectomy [12]. Postoperative flare was significantly lower in the deep sclerectomy group and there was a quick return to the preoperative level within 1 week whereas the intense inflammation following trabeculectomy persisted for 1 month. Eyes with a compromised blood–aqueous barrier such as those with uveitic glaucoma or PEXG may benefit from this advantage that NPGS offers.

Early postoperative intraocular bleeding is a rare complication because the IOP is reduced gradually with nonpenetrating techniques. Hyphema has a low incidence after NPGS [79, 81] and does not require any particular treatment. It is usually due to a rupture of small iris vessels or leakage of red blood cells from the scleral bed or Schlemm’s canal ostia through the TDM.

Cataract progression seems to be significantly reduced with NPGS. Cataract formation rate after trabeculectomy is 78 % at 5 years and the risk of cataract increases considerably in the setting of a flat anterior chamber or significant postoperative inflammation [51]. Shaarawy et al. followed 105 patients after deep sclerectomy and found no surgery-induced cataracts. The mean follow-up was 64 months and 25 % of eyes showed progression of a preexisting age-related cataract [13].

Early postoperative IOP spikes have been described following trabeculectomy when the scleral flap has been too tightly sutured. This complication seldom occurs following NPGS if the dissection of the membrane has been done properly. Insufficient surgical dissection in the hands of an inexperienced surgeon can be a potential cause of hypertony. Hemorrhage in the scleral bed or even excessive viscoelastic in the anterior chamber can cause IOP spikes that are usually self-resolving in a few days. Hypertony can also be caused by malignant glaucoma following NPGS as reported by Shaarawy et al. in a case that was successfully treated with cycloplegics [82]. Steroid response during the first postoperative weeks is also a potential cause of hypertony when other conditions have been ruled out. Iris prolapse and PAS should always be ruled out by gonioscopy in the setting of a postoperative IOP spikes and should be managed with laser synechiolysis or surgical management if the former doesn’t work [80].

Hypotony occurs in 50 % of patients for some days following NPGS and the average IOP on the first postoperative day has been reported to be 5 mmHg [9, 82]. This indicates that the dissection was adequately performed and is generally considered as a favorable predictive factor for surgical success [13]. Prolonged hypotony should be carefully monitored to identify a cause such as wound leakage. The risk of prolonged hypotony remains minimal following NPGS [9, 82] but still can lead to hypotony maculopathy if left untreated [10, 83]. Choroidal effusion is a rare complication [9, 79, 84] and the incidence rate has been reported to be 5 % after NPGS which is four times lower than following trabeculectomy [85]. Suprachoroidal hemorrhage has been rarely reported after deep sclerectomy [86] or viscocanalostomy [87] and the main risk factor seems to be the occurrence of a prolonged hypotony [88]. Hypotony is more common in the first weeks following NPGS when compared to trabeculectomy but complications associated with prolonged hypotony are much less frequent in the former. The safer profile of NPGS is also due to the absence of an abrupt IOP decrease during the procedure [77].

Early postoperative Descemet’s membrane detachment occurs in about one of 250–300 operated eyes after NPGS [77]. In viscocanalostomy, this complication is due to a misdirection of the viscoelastic canula during injection into Schlemm’s ostia [89]. Descemet’s membrane detachments are less frequent after deep sclerectomy and occur when the membrane is undermined by aqueous humor, viscoelastic, and/or blood [77]. This can be seen several weeks after surgery as a bulla under the cornea at the level of the dissection site and the adjacent stroma becomes opalescent. Reflux of fluid in the sub-Descemet space can be caused by an elevation of intrableb pressure in the setting of a trauma or bleb fibrosis. Treatment consists of curing the cause if the problem is an encysted bleb or dealing with the detachment itself by descemetopexy.