Fundamental principles

Deep sclerectomy (DS) aims to reduce intraocular pressure by external filtration of aqueous humor. Unlike trabeculectomy, aqueous exits the eye not through a patent hole, but by slow passage through the ‘sclerodescemetic membrane’ formed by the internal portion of the posterior and anterior trabecular meshwork and by the adjacent Descemet’s membrane. The membrane is created by removing the inner wall of Schlemm’s canal and juxtacanalicular trabeculum (sites of the increased outflow resistance in glaucoma) to expos the anterior trabecular meshwork and Descemet’s membrane. Percolating from the anterior chamber, aqueous fills an intrascleral ‘lake’ or ‘decompression chamber’, from where it drains into the subconjunctival space and/or is partially reabsorbed into the suprachoroidal space (Fig. 38.2).

Fig. 38.2 Scheme of deep sclerectomy: aqueous is exiting the eye by filtering through the sclerodescemetic membrane into the intrascleral chamber and then into the subconjunctival space.

Goals of surgery

Deep sclerectomy aims to reduce IOP by creating a new aqueous outflow pathway, by removing a deep scleral flap, the external wall of Schlemm’s canal, and the corneal stroma behind the anterior trabecular meshwork and Descemet’s membrane, without perforating the eye.

Indications for surgery

Deep sclerectomy is indicated in eyes with IOP uncontrolled despite maximal tolerable medical therapy and/or laser trabeculoplasty, or with poor adherence to prescriptions.

Based on its high safety profile and on its mechanism of action, deep sclerectomy is indicated in primary open angle, pseudoexfoliative, and pigmentary glaucomas. Being non-penetrating, it can be useful particularly in aphakic eyes with vitreous in the anterior chamber, or in-patients where a sudden drop in IOP or long-lasting hypotony should be avoided, such as eyes with uncontrolled high pressure, eyes with high myopia, or even eyes with very advanced glaucoma. The procedure was also found to be effective in uveitic glaucoma3.

Specific contraindications are conditions which can impair aqueous filtration through the sclerodescemetic membrane: angle closure glaucoma, extensive peripheral anterior synechiae in the surgical quadrant, neovascular glaucoma, and iridocorneal endothelial (ICE) syndrome. Eyes with diffuse scarring in the surgical quadrant secondary to trauma or previous surgeries might have damaged sclera and non-functional trabecular meshwork and Schlemm’s canal. This increases both difficulty and the risk of complications. Similarly, eyes treated by argon laser trabeculoplasty are more at risk for ruptures of the sclerodescemetic membrane, which often requires conversion into trabeculectomy. There is no information as to whether eyes that have undergone selective laser trabeculoplasty share this risk.

Preoperative assessment

Preoperative assessment is similar to penetrating surgery. A week before surgery, anticoagulants (e.g. warfarin) or anti-platelet medications (such as aspirin) should be discontinued in order to minimize intraoperative bleeding. Long-lasting ocular hypotensive medications (e.g. beta-blockers) should be discontinued, if possible, and chronic blepharo-conjunctivitis and dry eye syndrome must be treated.

Anesthesia

As in penetrating surgery, anesthesia may vary based on surgeon preference and on the patient. As deep sclerectomy is a long and difficult procedure, effective and long-lasting anesthesia and akinesia is preferred, such as provided by a peribulbar block. In young patients or those with poor cooperation, general anesthesia is recommended.

Operation technique

For good exposure of the surgical area, place a traction suture in the cornea or under the superior rectus. Raise a fornix-based conjunctival flap in the upper quadrant and lightly cauterize superficial blood vessels carefully, so as to preserve collector channels and to avoid scleral shrinkage and damage to Schlemm’s canal. Dissect a 5 × 5 mm superficial flap, approximately ⅓ scleral thickness, and advance anteriorly into clear cornea for about 1–1.5 mm (Fig. 38.3). To make the deep sclerokeratectomy, dissect a second 4 × 4 mm deep scleral flap. The dissection should be just deep enough to leave a thin layer of sclera (50–100 µm) over the choroid and the ciliary body with a dark reflex just visible below the scleral fibers. Start the deep sclerectomy posteriorly and carry it anteriorly until Schlemm’s canal is deroofed (Fig. 38.4). Advance the dissection into clear cornea to create the sclerodescemetic membrane, the site of aqueous filtration. Deepen the two lateral radial cuts and advance into clear cornea without touching the anterior trabeculum or Descemet’s membrane. By gently pulling the deep scleral flap with forceps and with counter traction on the bed of the canal using a triangular cellulose sponge, detach the anterior portion of the deep flap from the anterior trabecular meshwork and from Descemet’s membrane. Advance this 1–1.5 mm anteriorly. At this stage, there should be aqueous percolation through the membrane. To optimize outflow, peel the internal wall of Schlemm’s canal (the juxta-canalicular trabeculum and the canal endothelium) by grabbing it with thin forceps (‘external trabeculectomy’) (Fig. 38.5). This removes a homogeneous external trabecular membrane in one coherent plane and allows aqueous humor to egress through the remaining inner trabecular layers. Excise the deep flap by cutting it anteriorly.

Fig. 38.3 The superficial flap should be dissected anteriorly until clear cornea is reached.

Fig. 38.4 The deep scleral flap should be dissected anteriorly close to the choroid until the Schlemm’s canal is reached and deroofed.

Fig. 38.5 In order to increase aqueous outflow, the anterior wall of Schlemm’s canal (the iuxtacanalicular trabecular meshwork) is often stripped.

To maintain the created space (‘intrascleral lake’ or ‘decompression chamber’) and to avoid postoperative scarring, different implants are used4. Absorbable porcine collagen implant (Aquaflow™, Staar surgical AG, Nidau, Switzerland), reticulate hyaluronic acid implant (HealaFlow™, Anteis, Geneve, CH), non absorbable implant (T Flux™, Ioltech Laboratoires, La Rochelle, France) or PMMA implant (Homdec SA, Belmont, Switzerland) can be sutured or positioned in the intrascleral space (Fig. 38.6).

Fig. 38.6 In order to achieve lower intraocular pressures after deep sclerectomy, an implant should be placed into the intrascleral chamber. In this picture a T-flux was sutured.

Reposition and suture the superficial scleral flap with two loose 10-0 nylon sutures, and finally close the conjunctiva tightly.

Before dissecting the deep scleral flap or just after opening Schlemm’s canal, in all or in selected cases, some surgeons apply a sponge soaked with mitomycin C (0.1–0.3 mg/ml) over the sclera for 1 to 3 minutes, to minimize excessive scarring, so as to increase the success rate.

Intraoperative complications

The two most common intraoperative complications are the inability to find Schlemm’s canal and perforation of the sclerodescemetic membrane. These complications are most common during the initial learning phase on the first 15–20 cases, where they can affect up to 30% of operated eyes. This falls to 3% with experience5. Inability to find the canal relates to an improper dissection of the deep scleral flap. Usually, fear of being too close to the choroid causes the surgeon to be too superficial, thus dissecting over the top of the canal. Generally, a careful deepening of the deep scleral flap suffices to reach and open the canal. Sclerodescemetic membrane ruptures can be as small as little holes or linear and transverse. Small ruptures without iris prolapse have no consequence. With shallow or flat anterior chambers, the external flap should be sutured tightly (consider releasable sutures). Long ruptures of Descemet’s membrane mostly occur at the junction with the anterior trabeculum, and are followed by iris prolapse. An iridectomy is always required with a conversion to trabeculectomy. This can be done by re-suturing into position the internal flap (in order to cover the rupture), and by doing a sclerectomy in its anterior portion. The superficial flap can then be sutured as in standard trabeculectomy. This complication is more common in eyes previously treated with argon laser trabeculoplasty.

Postoperative care

For at least 6 weeks postoperatively, eyes are medicated with topical antibiotics and corticosteroids. Considering the very weak inflammatory reaction, cycloplegics are unnecessary and relatively contraindicated as they increase the risk of iris incarceration.

In the case of a small membrane tear, miotics are recommended for 3–4 weeks, and the patient must be instructed to avoid any pressure or digital massage on the eye.

Postoperative complications

Deep sclerectomy is a safe procedure, affected by few complications. Expect early hypotony with IOPs around 5 mmHg on the first postoperative day; it is a positive prognostic factor. If there is no perforation, hypotony is short lasting, the anterior chamber remains deep, and small peripheral choroidal effusions might be observed in up to 5% of eyes. Small hyphemas, mostly secondary to blood reflux from Schlemm’s canal, are sometimes seen and are mostly associated with hypotony. IOPs spikes can occur for several reasons: an insufficient surgical dissection of the sclerodescemetic membrane (detectable during surgery by lack of aqueous filtration), which can be resolved with laser goniopuncture; an iris prolapsed through an undetected membrane tear, which can be treated with miotics and iridotomy or iridoplasty, or with a surgical iridectomy; a transient hemorrhage under the scleral flap not requiring any intervention; a steroid induced response (generally after several weeks of treatment), which is solved by halting the medication; and finally very rarely malignant glaucoma, which must be treated medically and/or surgically. As in penetrating surgery, wound leaks with a positive Seidel test can occur with inadequate conjunctival closure, especially if anti-metabolites were used. This complication is less frequent with NPGS than with trabeculectomy. Early or late failure of the procedure, from excessive scarring, is more likely in high risk eyes. As in trabeculectomy, the success rate can be increased by using intraoperative anti-metabolites or by treating the operated eye with needling and/or subconjunctival anti-metabolite injections.

A progressive increase in IOP can be observed in up to 60% of the cases during the first year postoperatively, related to a decrease of permeability of the sclerodescemetic membrane. This complication, most frequent 6–8 months after surgery, must be treated with Nd:YAG goniopuncture. The aiming beam is focused on the semi-transparent trabecular–Descemet’s membrane, which often has a concave appearance, through a gonioscopic contact lens. In the free running Q-switched mode with a power of 4–8 mJ, 4–5 YAG laser shots are applied. This will create small holes in the membrane increasing its permeability and thus lowering IOP.

Even if rare, blebitis can occur, but endophthalmitis has not yet been reported. Few cases of implant migration into the anterior chamber have occurred. This complication might occur in eyes with a perforated membrane and without, or with inadequate, fixation of the implant to the scleral bed. Cataract may occur late, but with a significantly lower incidence than that following trabeculectomy6,7. Scleral ectasia has been seen occasionally.

Results of surgery

Reported results vary from different follow-ups and techniques. In 2004, a meta-analysis of 29 articles found deep sclerectomy yielded an IOP <21 mmHg without medications in 69.7% of eyes without implant, 59.4% with collagen implant, and 71.1% with reticulated hyaluronic acid implant. No significant differences were found between the three techniques8. In 2008, a meta-analysis reported an IOP <21 mmHg in 68.7% of eyes after deep sclerectomy with implant, 48.6% after deep sclerectomy without implant, and 67.1% after deep sclerectomy with anti-metabolites (with or without implant)9. While this confirmed a higher success rate with a collagen implant, the role of anti-metabolites was confusing, in part because it included surgeries performed with different anti-metabolites (intraoperative mitomycin C, and intraoperative or postoperative 5-fluorouracil) and operations with and without implants. Randomized controlled studies comparing deep sclerectomy with and without mitomycin C (MMC) showed a better outcome when the anti-metabolite was used. Kozobolis et al. showed a success rate (IOP <22 mmHg without medications) of 72.5% without MMC and 95% with MMC, without significant different complication rates between the groups10. Neudorfer et al. reported a mean IOP decrease at 24 months of 48% and of 32% in the MMC and no-MMC groups respectively11.

When compared with trabeculectomy, randomized controlled studies, even if consistently showing a better safety profile for deep sclerectomy, do not agree on which procedure provides the better outcome. This relates to factors such as differences in technique, surgical experience, the use of anti-metabolites, and the use of goniopuncture. Overall, trabeculectomy provides lower IOP than deep sclerectomy , but when goniopuncture and/or anti-metabolites are added to DS, most studies show similar final IOP and success rates12–15.

Fundamental principles

Viscocanalostomy (VC) and canaloplasty (CP) are non-penetrating surgeries that reduce IOP by increasing aqueous outflow through both the sclerodescemetic membrane and Schlemm’s canal and collector channels.

Similar to deep sclerectomy, the difference is to dilate Schlemm’s canal with injection of high molecular weight sodium hyaluronate through its opened ostia or through a probe inserted into the canal with a tensioning suture left inside canaloplasty. Schlemm’s canal dilation disrupts its inner and outer walls and disorganizes the juxta-canalicular zone, with resultant increase in conventional aqueous outflow facility, and uveoscleral outflow16. Unlike in deep sclerectomy, the superficial scleral flap is sutured watertight. This avoids external subconjunctival filtration: aqueous exits the decompression or intrascleral chamber mainly through the two ostia of Schlemm’s canal, and partly via the suprachoroidal space. Occasionally there is some external subconjunctival filtration.

Goals of surgery

Viscocanalostomy and canaloplasty aim to reduce IOP by increasing conventional trabecular facility and by creating a new aqueous outflow pathway. They create focal micro-ruptures in the walls of Schlemm’s canal by dilating it with sodium hyaluronate (VC) or with a tensioning suture (CP), and, as in deep sclerectomy, by removing a deep scleral flap, the external wall of Schlemm’s canal, and the corneal stroma behind the anterior trabecular meshwork and Descemet’s membrane, without perforating the eye.

Indications for surgery

Indications and contraindications are the same as in deep sclerectomy. In particular, for VC and CP to succeed, Schlemm’s canal and collector channels should not have been damaged by previous surgeries. Since the procedure provides final IOPs in the mid–high teens, it is mainly indicated in open angle glaucomas when target IOP is not very low. The absence (or very reduced) external filtration theoretically makes the technique safer, especially for eyes with chronic blepharitis, in contact lens wearers, or when the surgery has to be performed in the lateral or inferior quadrants. VC has been shown to be effective in uveitic glaucomas with well controlled inflammation17, in juvenile glaucomas18, and in congenital glaucomas19.

Preoperative assessment

As for DS.

Anesthesia

As for DS.

Operation techniques

The surgical steps of viscocanalostomy and canaloplasty are mostly the same as deep sclerectomy. The conjunctival flap, the superficial and deep scleral flaps, and the sclerodescemetic membrane are as described for deep sclerectomy. During these steps, cautery should be minimized to prevent damage to Schlemm’s canal and collector channels; in dissecting the internal scleral flap, Schlemm’s canal needs to be fully opened and deroofed leaving two patent and clean ostia on the lateral edges of the cut.

Unlike in deep sclerectomy, dilation of Schlemm’s canal is the critical part of the procedure.

During viscocanalostomy, using the specific 165 µm cannula, high molecular weight sodium hyaluronate is slowly injected into Schlemm’s canal through the two ostia at the lateral edges of the inner flap (Fig. 38.7). To avoid damage to the canal endothelium, do not insert the cannula more than 1–1.5 mm from the ostia. The slow injection should be repeated 6–7 times on each side to avoid tears and ruptures of the canal.

Fig. 38.7 In viscocanalostomy, Schlemm’s canal is dilated by injecting sodium hyaluronate through its opened ostia.

During canaloplasty, a 200 µm microcatheter with a 250 µm distal tip incorporating an illuminating optical fiber is inserted into Schlemm’s canal through its open ostium. The catheter is gently pushed through the canal until its tip exits from the contralateral ostia (Fig. 38.8). A 10-0 prolene suture is tied to the distal tip and the catheter is withdrawn, pulling the suture into and along the canal. While withdrawing, 4–6 mg of sodium hyaluronate 1.4% should be injected every 2 hours. Once the suture has been pulled right around, it is cut from the catheter and tied to form a tight loop encircling Schlemm’s canal. This technique provides both viscodilation and a tensioning of the inner wall of the canal20.

Fig. 38.8 During canaloplasty, a probe is passed through Schlemm’s canal.

Courtesy of P. Brusini M.D.

To prevent trabecular ruptures, a paracentesis to lower IOP, is made before the cannulation of the canal both during viscocanalostomy and canaloplasty.

In contrast with DS, the superficial flap is sutured watertight to avoid external filtration. To seal the ‘intrascleral chamber’, the outer scleral flap is sutured with 6 or 7 10-0 nylon stitches. The different sizes of the two flaps allows a tight apposition of the external flap (Fig. 38.9). Finally, to minimize bleeding and to prevent collapse and scarring, high molecular weight sodium hyaluronate is injected underneath the flap (Fig. 38.10).

Fig. 38.9 UBM showing an eye after viscocanalostomy. The sclerodescemetic membrane and the intrascleral chamber are clearly visible.

Fig. 38.10 At the end of viscocanalostomy, the superficial scleral flap is watertight sutured, and the intrascleral chamber is filled with sodium hyaluronate.

An implant has not been shown to benefit the outcome of viscocanaloplasty and canaloplasty. This probably relates to the differences in the mechanisms of action between the procedures.

Intraoperative complications

As the viscocanalostomy surgical technique is very similar to deep sclerectomy, intraoperative complications are mostly the same. An excessive or too-rapid injection of sodium hyaluronate into Schlemm’s canal can rupture the trabecular meshwork with viscoelastic entering the anterior chamber. At the end of the procedure, an excessive amount of viscoelastic into the intrascleral chamber can break the sclerodescemetic membrane or detach Descemet’s membrane. During canaloplasty, complications include the inability to catheterize Schlemm’s canal in full, partial breaks in Schlemm’s canal with the creation of a false path, and micro or gross hyphemas21.

Postoperative care

As for DS.

Postoperative complications

Viscocanaloplasty is safe with the few postoperative side effects mainly related to intraoperative technical complications. Postoperative management is less demanding with significantly less refractive change21 and eye discomfort than trabeculectomy22, as could be expected given the absence of a filtering bleb in most cases. Postoperative complications are similar to deep sclerectomy. Since viscocanalostomy is mostly independent of external filtration, excessive healing does not affect the outcome and does not require any specific intervention. Viscocanalostomy, due to its different mechanisms of action, requires fewer goniopunctures compared with deep sclerectomy.

Canaloplasty can be affected by suture extrusion through the trabecular meshwork into the anterior chamber20.

Results of surgery

As with deep sclerectomy, a direct comparison between different studies is difficult because criteria for success, length of follow-up, and techniques are different. Viscocanalostomy has been found effective to lower IOP with a good safety profile. In 2004, a meta-analysis of eight articles reported a success rate for viscocanalostomy (IOP <21 mmHg without medications) of 72.0%8. More recently, meta-analysis of 14 articles reported an IOP below 21 mmHg in 51.1% of the eyes at 25.6 months9. The mean final IOP at 20.4 months was 16.4 mmHg (range 12–20 mmHg). This meta-analysis was biased by inclusion of articles from a group operating within their learning curve, with a success at the beginning as low as 0%23, which increased to 30% by enlarging the sample size24, thus affecting the final result.

The only reported study on canaloplasty showed, at 24 months, a mean final pressure of 16.3 ± 3.7 mmHg (15.7 ± 3.1 mmHg in eyes with greater distension of the canal)25. An IOP <18 mmHg with or without medications in all postoperative visits from 6 months onward was found in up to 50% of the eyes.

When compared with trabeculectomy in randomized controlled trials, viscocanalostomy provided higher final mean IOPs and a lower success rate for IOPs in the low teens. In most of these studies, these differences did not reach statistical significance22,26–29.

No studies to date, have compared canaloplasty with trabeculectomy.