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Background
Viscocanalostomy is an ophthalmic surgical procedure that has been developed as an alternative to trabeculectomy. In some cases, viscocanalostomy has been used in conjunction with cataract removal via phaco-emulsification.
Although there are a variety of viscocanalostomy techniques, the procedure basically involves production of superficial and deep scleral flaps, excision of the deep scleral flap to create a scleral reservoir, and unroofing of Schlemm's canal. A high-viscosity viscoelastic, such as sodium hyaluronate, is used to open the canal and create a passage from a scleral reservoir to the canal. The superficial scleral flap is then sutured water tight, trapping the viscoelastic until healing takes place. However, the peer-reviewed published medical literature reveals that viscocanalostomy has only been studied in a relatively small number of patients at a few centers. There are no published peer-reviewed data on long-term outcomes of viscocanalostomized patients, and what few clinical studies have been published directly comparing viscocanalostomy with trabeculectomy show that trabeculectomy is more effective at lowering intra-ocular pressure (IOP).
Randomized controlled multi-center clinical trials directly comparing viscocanalostomy with trabeculectomy are needed before viscocanalostomy can be accepted as an established alternative to trabeculectomy. As one of the leading investigators of viscocanalostomy in North America commented: "As with all glaucoma studies, long term follow-up evaluation will be required to prove its [viscocanalostomy's] real efficacy. In addition, randomized studies which would compare viscocanalostomy and phacoemulsification with other combined glaucoma procedures are also in order" (Henahan, 1998).
In a technology assessment on non-penetrating glaucoma surgery, the American Academy of Ophthalmology (AAO, 2001) stated that non-penetrating glaucoma surgery (viscocanalostomy is one of the 2 major variations of this procedure) has the potential to reduce IOP) while minimizing the risk of post-operative relative hypotony and the complications associated with hypotony. The authors found, however, that the majority of the published literature on non-penetrating glaucoma surgery contains information from case series, which are not randomized and lack a control group. The authors concluded that randomized clinical trials (RCTs) are needed to assess these procedures and to determine their role in the clinical management of glaucoma patients.
The AAO's practice guideline on the management of primary open-angle glaucoma (2005) recognized viscocanalostomy as being a non-penetrating surgery used by some physicians as an alternative to trabeculectomy, but it stated that the precise role of non-penetrating surgeries (i.e., viscocanalostomy and non-penetrating deep sclerectomy) has yet to be determined.
Shaarawy and associates (2003) studied prospectively the success rate and complications of viscocanalostomy in patients with medically uncontrolled primary and secondary open angle glaucoma (n = 57). These investigators concluded that viscocanalostomy appears to be a promising modification of filtering surgery.
Randomized controlled clinical studies comparing viscocanalostomy with trabeculectomy in glaucomatous patients have shown that trabeculectomy is more effective in lowering IOP than viscocanalostomy. In a RCT (n = 20) comparing viscocanalostomy and trabeculectomy for the treatment of patients with open angle glaucoma, Jonescu-Cuypers et al (2001) found that trabeculectomy was more effective than viscocanalostomy in lowering IOP in glaucomatous eyes of such patients. This is in agreement with the finding of Luke et al (2002) who examined the IOP-lowering effectiveness and the post-operative complication profile of viscocanalostomy versus trabeculectomy in a prospective randomized trial (n = 60). The authors concluded that in eyes with open-angle glaucoma, viscocanalostomy is less effective in reducing IOP than standard filtering surgery. However, post-operative complications are less frequent after viscocanalostomy. In a prospective clinical study, Kobayashi and colleagues (2003) compared the IOP-lowering effect and safety of viscocanalostomy and trabeculectomy with mitomycin C in patients with bilateral primary open-angle glaucoma (n = 25). The eyes of each patient were randomly assigned to receive viscocanalostomy in 1 eye and trabeculectomy with mitomycin C in the other eye. The patients were followed-up for 12 months. These researchers reported that trabeculectomy with mitomycin C may be more effective than viscocanalostomy in lowering IOP in patients with primary open-angle glaucoma, while eyes undergoing viscocanalostomy experience a lower incidence of complications, and they stated that further investigation of more cases is needed.
In a single-masked, parallel-group, prospective, randomized 24-month trial, Carassa et al (2003) compared the effectiveness and safety of viscocanalostomy and trabeculectomy in adults with uncontrolled open-angle glaucoma (n = 50). Eyes were assigned randomly to either viscocanalostomy (group 1) or trabeculectomy (group 2) with no intraoperative anti-fibrotics in the study eye. In group 1, no further intervention was allowed, whereas trabeculectomy eyes could receive subconjunctival 5-fluorouracil (5-FU) injections or laser suture lysis after surgery. It was found that viscocanalostomy is an effective IOP-lowering procedure in adults affected by open-angle glaucoma. Trabeculectomy with post-operative 5-FU can probably provides lower IOPs but, with more numerous complications, greater discomfort, and more intensive post-operative management. The authors concluded that large, multi-center controlled studies are needed to define the role of viscocanalostomy in the surgical management of glaucoma.
An assessment prepared for the Cochrane Collaboration comparing surgical and medical management of glaucoma found no studies comparing viscocanalostomy with medical management (Burr et al, 2004). The assessment noted that although viscocanalostomy may have fewer complications than trabeculectomy, viscocanalostomy may have limited effectiveness at lowering IOP.
Guidelines from the Royal College of Ophthalmologists (2004) conclude that "[a]t the present time there is insufficient evidence from prospective studies that these operations [viscocanalostomy and deep sclerectomy] have a lower incidence of long-term complications while maintaining good IOP control to advocate their use in routine glaucoma practice."
In a prospective randomized 1-year study, Kobayashi and Kobayashi (2007) compared the IOP-lowering effect of combined viscocanalostomy and phaco-emulsification and combined trabeculectomy and phaco-emulsification with mitomycin C in eyes with primary open-angle glaucoma. A total of 40 consecutive patients (40 eyes) with primary open-angle glaucoma and cataract were enrolled in this study. Eyes were assigned randomly either to trabeculectomy with mitomycin C or to viscocanalostomy in combination with phaco-emulsification and intra-ocular lens implantation. Mean baseline IOP was 24.0 +/- 2.0 mm Hg in the viscocanalostomy group and 23.7 +/- 2.6 mm Hg in the trabeculectomy group (p = 0.7). Mean post-operative IOP was 13.7 +/- 2.2 mm Hg at 3 months, 14.8 +/- 3.3 mm Hg at 6 months, and 14.9 +/- 3.0 mm Hg at 12 months in the viscocanalostomy group and 12.1 +/- 4.0 mm Hg at 3 months, 13.8 +/- 4.7 mm Hg at 6 months, and 14.1 +/- 4.4 mm Hg at 12 months in the trabeculectomy group. There was no significant difference in the mean IOP between the groups at any time. At 12 months, 17 patients (85 %) in the viscocanalostomy group and 16 patients (80 %) in the trabeculectomy group achieved an IOP of 20 mm Hg or less without medication (p = 0.7). Complications included 2 cases (10 %) of flat/shallow anterior chamber and 4 cases (20 %) of hypotony in the trabeculectomy group, whereas intra-operative microperforation of Descemet's membrane occurred in 3 cases (15 %) in the viscocanalostomy group. The authors concluded that there was no significant difference in IOP reduction between viscocanalostomy and trabeculectomy with mitomycin C in combination with phaco-emulsification and intra-ocular lens implantation in patients with primary open-angle glaucoma. They also stated that future study of a large population is needed to verify these observations.
In a long-term, prospective, randomized study, Gilmour et al (2007) compared the lowering effects of viscocanalostomy and trabeculectomy without anti-metabolite on IOP. The results of this study showed that at 40-month follow-up, trabeculectomy was more likely to achieve IOP levels below 18 mm Hg than viscocanalostomy. The study enrolled 21 primary open angle glaucoma patients who underwent trabeculectomy and 22 who underwent viscocanalostomy by a single surgeon familiar with both procedures. The mean pre-operative IOP was 25 mm Hg, and the trabeculectomies were performed without anti-metabolite intra-operatively, although some patients received 5-FU in the post-operative period. The follow-up period was 4 years, and the post-operative treatment was similar in both groups.
Survival analysis suggested that patients who underwent trabeculectomy had a 42 % chance of experiencing lower IOP compared to a 21 % chance in the viscocanalostomy group at 40 months. Success was defined as IOP below 18 mm Hg, and qualified success was IOP below 18 with medications. The trabeculectomy group, however, had more complications, and interventions such as needling and 5-FU injections were needed in the early post-operative period. Although the trabeculectomy group was less likely to need glaucoma medications to control IOP in the post-operative period, IOP with medication was well-controlled in the viscocanalostomy group. Of interest, only 1 patient required goniopuncture in the viscocanalostomy group compared to other studies in the literature where goniopuncture was required for around 30 % of patients. It is unclear how this might have influenced the results.
The authors stated that the findings of this study confirmed previous reports that the likelihood of obtaining lower IOP over the long-term with viscocanalostomy is lower than that with a standard trabeculectomy. The re-introduction of canal based surgery in adults has generated intense interest in the glaucoma community to develop new approaches to improve filtration through the trabecular and other pathways. They noted that prospective, randomized, long-term, clinical trials will hopefully provide some insight as to which of these new procedures might be most beneficial to glaucoma patients.
More recently, a glaucoma canaloplasty (enhanced viscocanalostomy) has been introduced, which involves modification of the viscocanalostomy procedure. Canaloplasty uses viscoelastic and a specialized microcatheter (e.g., iScience Surgical Ophthalmic Microcannula, Menlo Park, CA) to forceably open the Schlemm's canal. The procedure is intended to restore the natural drainage of fluid from the eye, thus reducing IOP in persons with glaucoma.
Similar to the viscocanalostomy, canaloplasty is completed under a scleral flap. The canal is identified then intubated with a flexible microcatheter which has a lighted tip to identify its location as it passes through the Schlemm's canal. The microcatheter also has a lumen to allow for the passage of high viscosity sodium hyaluronate for dilation of the canal. Once the cannula has passed the full length (360º through) of the Schlemm's canal, a suture is tied to the cannula and as the cannula is withdrawn the suture is tied off and left in place. The intracanalicular suture cinches and stretches the trabecular meshwork inwards and permanently opening the Schlemm's canal. The scleral flap is tightly closed as well as the conjunctiva. Before, during and after the surgery, a special ultrasound imaging system is used to help identify the canal and the instrumentation in the canal.
An important difference between viscocanalostomy and canaloplasty is that canaloplasty aims at opening the entire length of the Schlemm's canal, not just one section of it. Canaloplasty is currently under investigation. Several trials are currently underway to further support the benefits and safety of this technique.
Although a relatively new procedure, canaloplasty seems effective in lowering IOP when used in glaucoma patients as an alternative to trabeculectomy. The choice between these procedures relates to the degree of IOP lowering required by a patient as well as the patient's risk factors for complications. Canaloplasty will likely be used most often in earlier stages of glaucoma and in patients in whom bleb infection and leakage would put them at higher risk for infection-associated blindness (endophthalmitis). This procedure may also be indicated in patients who may not need IOP-lowering to the degree that is achievable with trabeculectomy. Canaloplasty and other procedures that lower IOP without creating an aqueous filtering hole in the eye with a conjunctival bleb may have an increasing role in the surgical management of patients with glaucoma because of their potentially improved safety profile.
In an international multi-center prospective study (14 sites in Geramny and in the United States of America), Lewis et al (2007) assessed the safety and effectiveness of circumferential viscodilation and tensioning of the inner wall of Schlemm's canal (canaloplasty) for the treatment of open-angle glaucoma (OAG). Adult patients having glaucoma surgery, patients with qualifying pre-operative IOP of at least 16 mm Hg or higher and open angles were eligible. Evaluation was performed at baseline and 1 day, 1 week, and 1, 3, 6, and 12 months post-operatively. After a non-penetrating dissection technique to expose Schlemm's canal was performed, a flexible microcatheter was used to dilate the full circumference of the canal by injecting sodium hyaluronate 1.4 % during catheterization. A suture loop was placed in the canal to apply tension to the trabecular meshwork. High-resolution ultrasound imaging was used to evaluate Schlemm's canal and anterior segment angle morphology, including distension of the trabecular meshwork caused by the tensioning suture. Data analysis was performed in 2 groups: Group 1, in which patients met all inclusion criteria, and Group 2, made up of Group 1 patients who had successful suture placement. Group 1 comprised 94 patients and Group 2, 74 patients. The mean baseline IOP in Group 1 was 24.7 mm Hg +/- 4.8 (SD) on a mean of 1.9 +/- 1.0 medications per patient. In Group 2 (patients with sutures), the mean IOP was 16.1 +/- 4.7 mm Hg 3 months post-operatively, 15.6 +/- 4.0 mm Hg at 6 months, and 15.3 +/- 3.8 mm Hg at 1 year. Medication use dropped to a mean of 0.6 +/- 0.9 per patient at 12 months. Suture tensioning was an apparent contributing factor in achieving surgical success. Patients with measurable trabecular meshwork distension from suture tension had a mean IOP of 15.9 +/- 5.2 mm Hg at 6 months and 14.5 +/- 3.0 mm Hg at 12 months. Surgical and post-surgical adverse events were reported in 15 of 94 patients (16 %) and included hyphema (n = 3), elevated IOP greater than 30 mm Hg (n = 3), Descemet's tear (n = 1), hypotony (n = 1), choroidal effusion (n = 1), and exposed closure suture with eyelid edema and erythema epiphora (n = 1); 4 patients were subsequently converted to trabeculectomy. The authors concluded that canaloplasty was a safe and effective procedure to reduce IOP in adult patients with OAG. The major drawbacks of this study included the lack of randomization and a control group, as well as the learning curve associated with performance of the procedure. Other limitations include the small, heterogeneous patient group, short-term follow-up, and the number of patients lost to follow-up.
Shingleton et al (2008) evaluated the safety and effectiveness of canaloplasty combined with clear corneal phaco-emulsification and posterior chamber intraocular lens (IOL) implantation in in treating OAG. This international multi-center prospective study comprised adult patients with OAG having combined glaucoma and cataract surgery. Patients with qualifying treated pre-operative IOP of at least 21 mm Hg or higher and open angles were eligible. Evaluation was performed at baseline and 1 day, 1 week, and 1, 3, 6, and 12 months post-operatively. Intra-operative and post-operative high-resolution ultrasound imaging was used to assess Schlemm canal and anterior segment angle morphology, including distension of the trabecular meshwork due to the tensioning suture. Data from 54 eyes that had combined glaucoma and cataract surgery performed by 11 surgeons at 9 study sites were analyzed for this interim analysis. The mean baseline IOP was 24.4 mm Hg +/- 6.1 (SD) with a mean of 1.5 +/- 1.0 medications per eye. In all eyes, the mean post-operative IOP was 13.6 +/- 3.8 mm Hg at 1 month, 14.2 +/- 3.6 mm Hg at 3 months, 13.0 +/- 2.9 mm Hg at 6 months, and 13.7 +/- 4.4 mm Hg at 12 months. Medication use dropped to a mean of 0.2 +/- 0.4 per patient at 12 months. Surgical complications were reported in 5 eyes (9.3 %) and included hyphema (n = 3, 5.6 %), Descemet tear (n = 1, 1.9 %), and iris prolapse (n = 1, 1.9 %). Transient IOP elevation of more than 30 mm Hg was observed in 4 eyes (7.3 %) 1 day post-operatively. The authors concluded that canaloplasty combined with clear corneal phaco-emulsification and posterior chamber IOL implantation was a safe and effective procedure to reduce IOP in adult patients with OAG.
The National Institute for Health and Clinical Excellence (NICE, 2008) stated that canaloplasty for the treatment of primary OAG should be used only in the context of research or formal prospective data collection. It noted that current evidence on the safety and effectiveness of canaloplasty is inadequate in quality and quantity. Specialist advisors to NICE considered theoretical adverse events to include anterior chamber perforation, tearing of Descemet’s membrane resulting in corneal opacification or retinal damage, intra-ocular inflammation caused by the suture, cataract formation, sustained increases in IOP, hypotony, and bleb formation or suture exposure with endophthalmitis.
In a meta-analysis, Hondur and colleagues (2008) evaluated the effectiveness of non-penetrating glaucoma surgery for OAG with respect to target IOP and severity of glaucoma. Studies encompassing only combined glaucoma and cataract surgery were excluded. Measurement of effectiveness was determined on the basis of achievement of target IOP. Data related to post-operative goniopuncture and needling with anti-metabolite application were noted. The percentage of cases achieving less than or equal to 21 mm Hg was 48.6 % after primary deep sclerectomy (DS), 68.7 % after DS with implant, 67.1 % after DS with anti-metabolite, 51.1 % after primary viscocanalostomy, and 36.8 % after viscocanalostomy with anti-metabolite or implant. Visual field parameters were almost exclusively not available; whereas cup/disk ratio and target IOP lower than 21 mm Hg were available in very few reports. With lower set IOP targets, the rates of success varied between 35 % and 86 % for DS, and between 10 % and 67 % for viscocanalostomy. The mean follow-up of the studies were mostly in the range of 3 years. The authors concluded that non-penetrating glaucoma surgery seems to provide IOP reduction into the high teens. Its potential to achieve lower target IOPs seems to be low. They stated that longer-term studies, with data related to glaucoma severity and proper target IOPs are needed.
Mendrinos et al (2008) noted that non-penetrating glaucoma surgeries have been developed in recent years in order to improve the safety of conventional filtering procedures. The goal of non-penetrating filtering procedures is to reduce IOP by enhancing the natural aqueous outflow channels, while reducing outflow resistance located in the inner wall of the Schlemm's canal and the juxta-canalicular trabecular meshwork. In the last few years, viscocanalostomy and DS with external trabeculectomy have become the most popular non-penetrating filtering procedures. Both involve removal of a deep scleral flap, the external wall of Schlemm's canal and corneal stroma behind the anterior trabeculum and Descemet's membrane, thus creating an intrascleral space. The aqueous humour leaves the anterior chamber through the intact trabeculo-Descemet's membrane into the scleral space, from where it will egress into different pathways. The technique is associated with a long learning curve. Published clinical trials comparing non-penetrating glaucoma surgery to full-thickness trabeculectomy have a consensus on the superior safety profile of non-penetrating glaucoma surgery but are not in agreement when it comes to efficacy, where conflicting results have been found.
Lewis and colleagues (2009) assessed 2-year post-surgical safety and efficacy of canaloplasty (circumferential viscodilation and tensioning of the inner wall of Schlemm canal) to treat OAG. This international prospective study comprised adult OAG patients having glaucoma surgery or combined glaucoma-cataract surgery. Qualifying pre-operative IOP was at least 16 mm Hg and historical IOP, at least 21 mm Hg. The full circumference of the canal was viscodilated and a trabecular tensioning suture placed with a micro-catheter. Primary outcome measures included IOP and glaucoma medication use. At 24 months, all 127 eyes (127 patients) had a mean IOP of 16.0 mm Hg +/- 4.2 and mean glaucoma medication use of 0.5 +/- 0.8 (baseline values 23.6 +/- 4.8 mm Hg and 1.9 +/- 0.8 medications). Eyes with canaloplasty alone had a mean IOP of 16.3 +/- 3.7 mm Hg and 0.6 +/- 0.8 medications (baseline values 23.2 +/- 4.0 mm Hg and 2.0 +/- 0.8 medications). Eyes with combined glaucoma-cataract surgery had a mean IOP of 13.4 +/- 4.0 mm Hg and 0.2 +/- 0.4 medications (baseline values 23.1 +/- 5.5 mm Hg and 1.7 +/- 1.0 medications). The IOP and medication use results at all time points were statistically significant versus baseline (p < 0.001). The late post-operative follow-up identified 3 patients with elevated IOP. No other serious ocular or non-ocular complications were reported. The authors concluded that canaloplasty was safe and effective in reducing IOP in adult patients with OAG. These are "interim" results of an ongoing clinical study; the clinical value of canoplasty awaits the completion and final analyses of results.
Griesbacher et al (2010) reported on a prospective study evaluating 32 consecutive eyes that had canaloplasty and at least 1 year of follow-up. The mean pre-operative IOP (with medications) of 27.3 +/- 5.6 mm Hg improved to a mean post-operative IOP (without medications) of 12.8 +/- 1.5 mm Hg at 12 months. The success rate where the IOP reached less than 21, less than 18, and less than 16 mm Hg was 93.8 % (95 % confidence interval [CI]: 0.86 to 1.0), 84.4 % (95 % CI: 0.73 to 0.98), and 74.9 % (95 % CI: 0.61 to 0.92) at 12 months, respectively.
Minckler and Hill (2009) described the rationales and initial clinical outcomes in studies to date on Glaukos iStent, iScience (canaloplasty), Solx (supra-choroidal shunt), and Trabectome, which are newly developed surgical technologies for the treatment of OAG. These new approaches to angle surgery have been demonstrated in preliminary case series to safely lower IOP in the mid-teens with far fewer complications than expected with trabeculectomy and without anti-fibrotics. Trabectome and iStent are relatively non-invasive, aim to improve access of aqueous to collector channels and do not preclude subsequent standard surgery. Canaloplasty, modified from viscocanalostomy, is thought to improve trans-trabecular flow. Solx potentially offers an adjustable aqueous outflow into the supra-choroidal space.
In a prospective and non-randomized study, Chakib and colleagues (2010) evaluated the short-term clinical results and complications of viscocanalostomy. A total of 107 consecutive eyes of 67 patients who underwent viscocanalostomy were analyzed. The surgeon conducted post-operative care. The minimal follow-up was 1 year, with a mean follow-up of 13.1 months (range of 12 to 18 months). The criteria for success were defined as IOP less than 21 mm Hg without treatment. The mean pre-operative IOP was 28.3 mm Hg while the mean post-operative IOP was 5.4 mm Hg on the first day and 10.2 mm Hg at 13 months. The rate of patients who had IOP below 21 mm Hg with or without treatment was 98 % at 13 months. The complete success rate without treatment was 80 % at 13 months. Seven cases of ocular hypotony lasting more than 1 month were noted. The authors concluded that viscocanalostomy is a promising procedure because in the short-term it provides good tonometric results in glaucomatous patients without the complications of trabeculectomy. However, it remains a technique with a learning curve.
Tian and Kaufman (2013) stated that since the inner wall of Schlemm's canal (SC) is directly in contact with the trabecular meshwork (TM) for 360 degrees and the catheter device used in canaloplasty allows viscoelastic to be injected into the entire length of SC, canaloplasty might also be used to perform SC/TM-targeted delivery of transgene vectors for glaucoma gene therapy. This hypothesized new method for transgene delivery may give the transgene access to the entire inner wall of SC and the whole juxta-canalicular region of the TM and allow the transgene to be expressed in both the TM and SC without affecting the cornea, iris and ciliary body. The authors concluded that this strategy might have a greater trabecular outflow resistance-decreasing effect than either the genetic or surgical approach alone.
Aktas et al (2014) noted that SC inner wall is adjacent to the juxta-canalicular TM over their entire circumference. These researchers attempted to transfer reporter and therapeutic genes to these outflow-modulating tissues via canaloplasty surgery in live monkeys. A standard canaloplasty surgical approach was performed in cynomolgus monkeys using flexible canaloplasty catheters, modified for monkey eyes with a 175-μm outer diameter and an LED-lighted tip. A 6-0 prolene suture was used for the exact localization of SC. Trypan blue was injected during catheter withdrawal to document catheter placement within SC and to determine ease of injecting fluid into SC. Before, during, and after the injection, the position of the catheter and the anatomic details were video-captured with an externally positioned non-contact endoscopic imaging system and 50 mHz ultrasound biomicroscopy (UBM). A 360-degree catheterization and injection of dye into SC was achieved. Suture, catheter, and trypan blue were imaged with the endoscope camera system and the catheter was also visualized with UBM. Trypan blue was seen in the SC over 5 clock hours after a 1 clock-hour insertion of the catheter. The authors concluded that a modified canaloplasty catheter device might be used for gene delivery to the SC/TM area without circumferential catheterization. Moreover, they stated that further studies comparing different delivery methods of the vector/transgene into the SC using canaloplasty are needed.
An AAO‘s clinical practice guideline on “Primary open-angle glaucoma” (AAO, 2010) as well as an AAO report on “Novel glaucoma procedures” (Francis et al, 2011) did not mention phacoviscocanalostomy as an therapeutic option. Furthermore, in an UpToDate review on “Open-angle glaucoma: Treatment” (Jacobs, 2014), viscocanalostomy and phacoviscocanalostomy are not mentioned as therapeutic options.
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