Intraocular pressure (IOP) is the fluid pressure inside the eye and is measured by the use of a tonometer during an eye examination. In the general population, normal intraocular pressure is between 10 mmHg and 20 mmHg, with an average value of IOP at 15.5 mmHg, with fluctuations of about 2.75 mmHg.
Glaucoma is a disorder that causes damage to the optic nerve of the eye, which carries visual information to the brain. Most cases of glaucoma are caused by increased IOP due to poor filtration of aqueous humor, a clear, watery fluid that flows between and nourishes the lens and the cornea. Due to this increased IOP and/or loss of blood flow to the optic nerve, the nerve fibers of the optic disc begin to die. This causes the center portion of the optic disc, known as the "cup," to become larger in comparison to the optic disc. Individuals with glaucoma tend to have a greater cup-to-disc ratio. Undetected glaucoma leads to permanent optic nerve damage, with resulting visual field loss, which will progress to blindness. Once lost, this damaged visual field cannot be recovered. The World Health Organization (WHO) reports that glaucoma is the second leading cause of blindness after cataracts. Additionally, the National Eye Institute indicates that glaucoma is the leading cause of blindness among African-Americans.
The two main types of glaucoma are open-angle and closed-angle. Open-angle glaucoma (OAG), which accounts for most of the US glaucoma cases, is chronic and tends to progress slowly; the individual may not even notice the loss of vision until there has been significant disease progression. It is caused by the slow clogging of the drainage canals, resulting in increased IOP. Closed-angle glaucoma typically appears suddenly, is very painful, and visual loss can progress quickly; however, the discomfort associated with closed-angle glaucoma usually will prompt an individual to seek medical attention before permanent damage can occur. It is caused by blocked drainage canals, which result in a sudden rise in IOP.
GLAUCOMA IN ADULTS
In the treatment of OAG, the goal is to reduce the risk of damage to the optic nerve by keeping IOP from rising above a certain target pressure level. The target pressure is based on the degree of optic nerve damage, the amount of visual field loss and, to a lesser degree, the initial pressure in the eye and how widely it varies each time it is measured. This pressure will vary from person to person. The ability to maintain the target pressure may help slow the progression of glaucoma by reducing the risk of optic nerve damage. Laser procedures, such as laser trabeculoplasty, can be used in most types of open-angle glaucoma, and laser peripheral iridectomy can often lower pressure in chronic and acute-angle glaucoma. When medication or laser treatment, if indicated, are not effective in reducing the IOP, guarded filtration surgery (e.g., trabeculectomy) may be indicated. Trabeculectomy, the most established glaucoma surgery procedure, allows aqueous humor drainage from within the eye to underneath the conjunctiva, where it is absorbed. However, trabeculectomy can result in the development of filtering blebs on the eye (a vesicular outpocketing of the scleral membrane), and is associated with numerous complications (e.g., leaks or bleb-related endophthalmitis) and long-term failure of IOP reduction. Other surgical interventions for glaucoma may include aqueous shunting, minimally invasive glaucoma surgeries (MIGS), canaloplasty or viscocanalostomy.
GLAUCOMA IN CHILDREN
Glaucoma in infants and children, while unusual, is a significant cause of blindness and includes congenital and pediatric glaucoma. Primary congenital glaucoma results from abnormal development of the ocular drainage system and occurs in about 1 out of 10,000 births in the United States. It is the most common form of glaucoma in infants. About 10% of congenital glaucoma is present at birth, with most of the remaining congenital glaucoma diagnosed before one year of age. Secondary glaucoma results from other disorders of the body or the eye and may or may not be genetic. Both primary and secondary glaucoma can be associated with other medical conditions or syndromes.
Pediatric glaucoma typically presents when the child is in the primary grades and is frequently discovered at the time of school vision screening. The presentation is not acute, and the child may appear quite comfortable in spite of a very high IOP at the time of diagnosis. The term "pediatric glaucoma" is not limited to a singular cause of an increased IOP, as there are several angle closure variants in pediatric glaucoma, as well as secondary causes, including inflammation and trauma. Moreover, one variant, called juvenile OAG, believed to be inherited from one parent, presents from age 7 up to age 30, and can create diagnostic confusion with adult glaucoma.
For glaucoma occurring within the first few years of life, initial surgical therapy is generally more effective than medical treatment, which is utilized as a temporizing measure. Angle surgery (either goniotomy, if trabecular meshwork is visible, or trabeculotomy ab externa, if the cornea is not clear and angle structures cannot be clearly seen) is the initial procedure indicated. As noted in Current Opinions in Ophthalmology, trabeculectomy and goniotomy have success rates of 75% to 90%; however, within a few years, 20% will fail to maintain IOP at an acceptable level. Thus, if angle incision operations fail to provide adequate control of intraocular pressure, filtering surgery (involving trabeculectomy with an anti-fibrotic agent or implantation of a drainage device) is justified. Cyclodestructive procedures (i.e., contact and non-contact transcleral cyclophotocoagulation, endoscopic ciliary ablation) are indicated when all other methods have failed or when vision is extremely poor.
AQUEOUS SHUNTS AND STENTS
In addition to laser trabeculoplasty and trabeculectomy, aqueous shunts and stents can be implanted to increase the outflow of fluid and reduce the IOP via procedures performed outside the eye (ab externo) or inside the eye (ab interno).
AB EXTERNO AQUEOUS SHUNTS
Insertion of shunts from outside the eye (ab externo) is another surgical option to lower IOP. Aqueous shunts are placed in the anterior or posterior chamber to facilitate drainage of aqueous humor between the anterior chamber and the suprachoroidal space. Examples of ab externo devices cleared by the U.S. Food and Drug Administration (FDA) include the Ahmed, Baerveldt, Molteno, and EX-PRESS Mini-Shunt. These devices differ by explant surface areas, shape, plate thickness, presence or absence of a valve, and details of surgical installation. The primary indication for aqueous shunts is for failed medical or surgical therapy, although some ophthalmologists have advocated their use as a primary surgical intervention, particularly for selected conditions such as congenital glaucoma, trauma, chemical burn, or pemphigoid.
For individuals who have refractory open-angle glaucoma who receive ab externo aqueous shunts, the evidence includes randomized controlled trials (RCTs), retrospective studies, and systemic reviews. Relevant outcomes are change in disease status, functional outcomes, medication use, and treatment-related morbidity. RCTs assessing U.S. Food and Drug Administration (FDA)‒approved shunts have shown that the use of large externally placed shunts reduces intraocular pressure (IOP) to slightly less than standard filtering surgery (trabeculectomy). Reported shunt success rates show that these devices are noninferior to trabeculectomy in the long term. FDA approved shunts have different adverse event profiles and avoid some of the most problematic complications of trabeculectomy. Two trials have compared the Ahmed and Baerveldt shunts. Both found that eyes treated with the Baerveldt shunt had slightly lower average IOP at 5 years than eyes treated with the Ahmed, but the Baerveldt also had a higher rate of serious hypotony-related complications. The evidence is sufficient to determine that the technology results in a meaningful improvement in the net health outcome.
AB INTERNO AQUEOUS STENTS
For individuals who have refractory open-angle glaucoma who receive ab interno aqueous stents, the evidence includes a nonrandomized retrospective comparative study and several single-arm studies. Relevant outcomes are change in disease status, functional outcomes, medication use, and treatment-related morbidity. The comparative study reported that individuals receiving the stent experienced similar reductions in IOP and medication use as individuals undergoing trabeculectomy. The single-arm studies, with 12-month follow-up results, consistently showed that individuals receiving the stents experienced reductions in IOP and medication use. Reductions in IOP ranged from 4 mm Hg to over 15 mm Hg. In addition, the FDA has given clearance to the XEN gel stent based on equivalent IOP and medication use reductions as seen with ab externo shunts. Clearance for the stent was based on a review in which the FDA concluded that while there were technical differences between the stent and predicate devices (shunts), the differences did not affect safety and effectiveness in lowering IOP and medication use. The evidence is sufficient to determine that the technology results in a meaningful improvement in the net health outcome.
MINIMALLY INVASIVE GLAUCOMA SURGERIES
Minimally invasive glaucoma surgeries (MIGS) are alternative, less invasive techniques that are being developed and evaluated. The objective of MIGS is to lower IOP by improving outflow of eye fluid; however, MIGS, which use microscopic-sized equipment and smaller incisions, involves less surgical manipulation of the sclera and the conjunctiva compared with other surgical techniques. There are several categories of MIGS, such as miniaturized trabeculectomy, trabecular bypass, milder laser photocoagulation, and totally internal or suprachoroidal stents (ab interno).
Examples of ab interno devices either approved or given marketing clearance by the FDA include the iStent, which is a 1-mm long stent inserted into the end of the Schlemm canal through the cornea and anterior chamber; the CyPass suprachoroidal stent; and XEN gelatin stent. It has been proposed that stents such as the iStent, CyPass, and Hydrus Microstent may be useful in individuals with early-stage glaucoma to reduce the burden of medications and problems with compliance. One area of investigation is individuals with glaucoma who require cataract surgery. An advantage of ab interno stents is that they may be inserted into the same incision and at the same time as cataract surgery. Also, most devices do not preclude subsequent trabeculectomy if needed. It may also be possible to insert more than one stent to achieve desired IOP. Therefore, health outcomes of interest are the IOP achieved, reduction in medication use, ability to convert to trabeculectomy, complications, and device durability.
AQUEOUS MICROSTENTS WITH CATARACT SURGERY
Several stents have the FDA approval for use in conjunction with cataract surgery. The iStent inject device is preloaded with two stents. An additional stent, the CyPass, had FDA approval but was voluntarily recalled by the manufacturer in 2018, as follow-up data had shown significant endothelial cell loss among individuals receiving the CyPass in conjunction with cataract surgery compared with individuals receiving cataract surgery alone.
Aqueous microstents, minimally invasive glaucoma surgical devices, drain aqueous humor from the anterior chamber into the Schlemm canal, the suprachoroidal or subconjunctival space. For individuals who have mild-to-moderate open-angle glaucoma who receive aqueous microstents during cataract surgery, the evidence includes RCTs. Relevant outcomes are change in disease status, functional outcomes, medication use, and treatment-related morbidity. Implantation of one or two microstents has received FDA approval for use in conjunction with cataract surgery for reduction of IOP in adults with mild-to-moderate open-angle glaucoma currently treated with ocular hypotensive medication. RCTs have been conducted in individuals with cataracts and less advanced glaucoma, where IOP is at least partially controlled with medication. Study results have shown that IOP may be lowered below baseline with decreased need for medication through the first 2 years. The evidence is sufficient to determine that the technology results in a meaningful improvement in the net health outcome.
AQUEOUS SHUNTS OR MICROSTENTS FOR GLAUCOMA TREATMENT OTHER THAN CATARACT SURGERY OR REFRACTORY OPEN-ANGLE GLAUCOMA
For individuals with indications for glaucoma treatment other than cataract surgery or refractory open-angle glaucoma who are treated with aqueous shunts or microstents, the evidence includes an RCT and an observational study. Relevant outcomes are change in disease status, functional outcomes, medication use, and treatment related morbidity. Several RCTs have evaluated the use of multiple microstents, but comparators differed. One RCT compared a single microstent with multiple microstents. This study reported no difference on the primary outcome (percentage of individuals with ≥20% reduction in IOP); secondary outcomes favored the multiple microstent groups. An observational study described implantation of two or three stents, at the discretion of the operating surgeon. The evidence is insufficient to determine the effects of the technology on health outcomes.
Canaloplasty is a surgical procedure that involves dilation and tension of Schlemm’s canal with a suture loop between the inner wall of the canal and the trabecular meshwork. This ab externo procedure accesses and dilates the entire length of Schlemm’s canal to pass the suture loop through the canal.
In 2017, the National Institute for Health and Care Excellence (NICE) updated its 2008 guidance on canaloplasty for primary open-angle glaucoma. The current recommendation is that the “evidence on the safety and efficacy of ab externo canaloplasty for primary open-angle glaucoma is adequate is support the use of this procedure.…”
In a prospective study, Grieshaber et al. (2010) examined the safety and efficacy of canaloplasty (360º viscocanalostomy) in 60 patients with POAG. Patients were followed for a mean of 30.6 ± 8.4 months. The mean preoperative IOP was 45.0 ± 12.1 mmHg. The mean IOP at 12 months was 15.4 ± 5.2 mmHg (n=54), 16.3 ± 4.2 mmHg (n=51) at 12 months, and 13.3 ± 1.7 mmHg (n=49) at 36 months. 77.5% of patients had an IOP ≤ 21 mmHg. Cox regression analysis showed that preoperative IOP (HR=1.003, 95% CI=0.927 to 1.085; p=0.94), age (HR=1.000, CI=0.938 to 1.067; p=0.98), and sex (HR=3.005, CI=0.329 to 27.448; p=0.33) were all not significant predictors of IOP reduction to ≤ 21 mmHg. Complications included Descemet’s detachment (n=2), elevated IOP (n=1), and false passage of the catheter (n=2), though the overall complication rate was low. The authors concluded that canaloplasty resulted in a sustained reduction of IOP in patients with primary OAG, independent of preoperative IOP. Limitations of the study include its small sample size and short-term follow-up period.
In a multi-center prospective study, Bull et al. (2011) evaluated the safety and efficacy of canaloplasty to treat patients with OAG. The study included 109 adult patients with historical IOPs of at least 21 mmHg with or without medical therapy and preoperative IOPs of at least 16 mmHg who underwent canaloplasty or combined cataract and canaloplasty surgery. Patients with successful suturing were followed for 3 years. Primary outcome measures included post-operative IOP, glaucoma medication usage, and the number of adverse events. Patients treated with canaloplasty had a mean baseline IOP of 23.0 ± 4.3 mmHg and mean glaucoma medication usage of 1.9 ± 0.7 medications, which decreased to a mean IOP of 15.1 ± 3.1 mmHg on 0.9 ± 0.9 medications at 3 years postoperatively. Patients treated with combined cataract and canaloplasty surgery had a mean baseline IOP of 24.3 ± 6.0 mmHg on 1.5 ± 1.2 medications, which decreased to a mean IOP of 13.8 ± 3.2 mmHg on 0.5 ± 0.7 medications at 3 years. Postoperative IOP and medication use for all patients significantly decreased from baseline (p < 0.00001). Late postoperative complications included cataracts (19.1%) and transient IOP elevation (1.8%). The authors concluded that canaloplasty was a safe and effective procedure, demonstrating significant and sustained IOP reductions in adult patients with OAG. Limitations of the study include its small sample size and heterogeneous patient group, short-term follow-up period, and patient loss to follow-up.
In a non-randomized, retrospective study, Ayyala et al. (2011) compared the operative outcomes of patients after canaloplasty (n=33) and trabeculectomy (n=46). Patients were followed for 12 months. Outcome measurements included changes in IOP and the number of post-operative medications used. Failure was defined as IOP > 18 mmHg or < 4 mmHg at 1-year follow-up or the need for a subsequent operative procedure. The mean percent reduction in IOP was not significantly different between the groups, reducing 32% for the canaloplasty group compared with 43% for the trabeculectomy group at 1-year follow-up (p=0.072). The median reduction in the number of medications needed was 2 in the canaloplasty group and 3 in the trabeculectomy group at 1-year follow-up. A higher percentage of patients treated with canaloplasty than trabeculectomy (36% vs. 20%) required post-operative medications, though this was not statistically significant. Failure based on IOP was 12.1% (n=4) for the canaloplasty group and 4.3% (n=2) for the trabeculectomy group. There was no statistically significant difference in failure rates between the two groups. The authors concluded that both canaloplasty and trabeculectomy achieved significant reduction in IOP at 12 months. The study is limited in its small sample size and short-term follow-up period.
In a multi-center prospective study, Lewis et al. (2011) evaluated the safety and efficacy of canaloplasty to treat patients with OAG. The study included 157 adult eyes with historical IOPs of at least 21 mmHg with or without medical therapy and preoperative IOPs of at least 16 mmHg who underwent canaloplasty or combined cataract and canaloplasty surgery. Patients were followed for 3 years. Primary outcome measures included post-operative IOP, glaucoma medication usage, and the number of adverse events. At 3-year follow-up, all study eyes had a mean IOP of 15.2 ± 3.5 mmHg and mean glaucoma medication use of 0.8 ± 0.9 compared with a baseline pre-operative mean IOP of 23.8 ± 5.0 mmHg on 1.8 ± 0.9 medications. Eyes with combined cataract and canaloplasty surgery had a mean IOP of 13.6 ± 3.6 mmHg on 0.3 ± 0.5 medications compared with a baseline mean IOP of 23.5 ± 5.2 mmHg on 1.5 ± 1.0 medications. IOP and medication use were significantly decreased from baseline at every follow-up period (p < 0.001). Late postoperative complications included cataract (12.7%), transient IOP elevation (6.4%), and partial suture extrusion through the trabecular meshwork (0.6%). The authors concluded that canaloplasty was a safe and effective procedure, leading to significant and sustained IOP reduction in adult patients with OAG. Limitations of the study include its small sample size and heterogeneous patient group, short-term follow-up period, and loss to follow-up.
In a review, Grieschaber (2012) indicated that canaloplasty is a valuable alternative to glaucoma filtration surgery as it targets the abnormally high resistance to outflow in the trabecular meshwork and re-establishes the physiologic outflow system. IOP reduction to the lower teens can be expected and the majority of complications seen in filtering surgery are largely eliminated by the non-penetrating and bleb-independent approach. Moreover, postoperative care is minimal and contributes to a high safety profile as bleb management is required and hypotony-related complications are largely avoided.
Three-year follow-up from an independent series of 214 patients treated with canaloplasty in Europe was reported by Brusini (2014). Mean IOP was reduced from 29.4 mm Hg at baseline to 17.0 mm Hg, after excluding 17 (7.9%) patients who later underwent trabeculectomy. At 3 years, IOP was 21 mm Hg or lower in 86.2% of patients, 18 mm Hg or lower in 58.6%, and 16 mm Hg or lower in 37.9%. There was a decrease in mean medication use, from 3.3 at baseline to 1.3 at follow-up. Complications, which included hyphema, Descemet membrane detachment, IOP spikes, and hypotony, were fewer than typically seen with trabeculectomy. Several disadvantages of the procedure were noted, including the inability to complete the procedure in 16.4% of eyes.
Voykov et al (2015) reported on 5-year follow-up on patients (20 eyes) with open-angle glaucoma who underwent canaloplasty at a single center in Germany. Mean IOP decreased from 25.7 mm Hg at baseline (n=33) to 15.5 mm Hg (n=19) at 1 year, 15.1 mm Hg (n=18) at 3 years, and 14.2 mm Hg (n=18) at 5 years. At each time point, reductions in mean IOP were statistically significant vs baseline (p<0.001). Mean number of medications used was 3.4 at baseline, 1.5 at 1 year, 1.6 at 3 years, and 1.7 at 5 years. At each time point, medication use was significantly lower than baseline (p<0.001). Thirteen (65%) of 20 eyes underwent another surgical procedure due to inadequate IOP control. Median length of time before additional surgery was 24 months (95% confidence interval, 1 to 51 months). The complication rate was low, with the most common being hyphema (7/20 [35%] eyes). No sight-threatening complications were reported.
In a prospective, randomized clinical trial, Matlach et al. (2015) evaluated the comparability of canaloplasty to the gold standard, trabeculectomy, in the treatment of OAG. Sixty-two patients were randomized to receive either trabeculectomy (n=32) or canaloplasty (n=30) and were followed-up for two years. Primary outcome measures included complete (without medication) and qualified (with or without medication) success defined as an IOP of < 18 mmHg for a complete success and an IOP < 21 mmHg and > 20% IOP reduction for a qualified success. Any success also required no vision loss and no further glaucoma surgery. Intraocular pressure was reduced in both groups after 2 years of follow-up (p < 0.001). Complete success was achieved in 74.2% and 39.1% (p = 0.01) and qualified success was achieved in 67.7% and 39.1% (p=0.04) of the trabeculectomy and canaloplasty groups, respectively. Mean IOP reduction was 10.8 + 6.9 mmHg and 9.3 + 5.7 mmHg, and mean IOP was 11.5 + 3.4 mmHg and 14.4 + 4.2 mmHg in the trabeculectomy and canaloplasty groups, respectively. The trabeculectomy group experienced a more frequent rate of complications including late hypotony (18.8%), choroidal detachment (12.5%), and elevated IOP (25.0%), however, these rates were not found to be significantly more frequent than the rates that occurred in the canaloplasty group. The authors concluded that while trabeculectomy is associated with larger rates of complete and qualified success, there may be a greater risk of complications. The authors also noted that canaloplasty may be an appropriate treatment for OAG in those patients for whom only a moderate IOP reduction is required.
In summary, the available peer-reviewed literature and recommendations from medical guidelines indicate that in individuals diagnosed with POAG, canaloplasty provides successful outcomes, particularly in patients who have failed previous medical therapy. Recent studies provide mid-term results (e.g., up to 3 years) that suggest canaloplasty provides modest IOP reductions with minimal intraoperative or post-operative complications.
Viscocanalostomy is a variant of deep sclerectomy and is an ab externo procedure that unroofs and dilates Schlemm's canal without penetrating the trabecular meshwork or anterior chamber. A high-viscosity viscoelastic solution, such as sodium hyaluronate, is used to open the canal and create a passage from the canal to a scleral reservoir. Viscocanalostomy has been performed in conjunction with cataract removal. An important difference between viscocanalostomy and canaloplasty is that viscocanalostomy attempts to open just one section of Schlemm’s canal, whereas canaloplasty attempts to open the entire length of Schlemm’s canal. While viscocanalostomy is able to open just 120º of Schlemm’s canal, limiting its efficacy and enhancing the procedure’s potential failure, canaloplasty uses a 360º tensioning suture, which attempts to maintain increased permeability of the inner wall region, increase outflow facility, and reduce the risk of re-collapse.
According to the Preferred Practice Guidelines published by the American AAO in 2010, viscocanalostomy is a non-penetrating type of glaucoma surgery. The rationale for non-penetrating glaucoma surgery is that by avoiding a continuous passageway from the anterior chamber to the subconjunctival space, the incidence of complications such as bleb-related problems and hypotony can be reduced. However, according to the AAO, “randomized clinical trials comparing viscocanalostomy with trabeculectomy generally suggest greater IOP reduction with trabeculectomy, but more complications with viscocanalostomy.”
In a prospective, randomized controlled trial, Yalvac et al. (2004) compared the safety and efficacy of viscocanalostomy and trabeculectomy in patients with POAG. Patients were randomized to have viscocanalostomy (n=25) or trabeculectomy (n=25). Patients were followed for 3 years. Outcome measurements included post-operative IOP and the number of post-operative complications. Successful outcomes were defined as qualified (IOP between 6 and 21 mmHg with medication) or complete (IOP between 6 and 21 mmHg without medication). At 3-year follow-up, the mean IOP was 16.0 ± 7.07 mmHg in the trabeculectomy group and 17.8 ± 4.6 mmHg in the viscocanalostomy group. Complete success was achieved in 66.2% of eyes at 6 months and 55.1% at 3 years in the trabeculectomy group and in 52.9% and 35.3%, respectively, in the viscocanalostomy group. Qualified success was achieved in 95.8% of eyes at 6 months and 79.2% at 3 years in the trabeculectomy group and in 90.7% and 73.9%, respectively, in the viscocanalostomy group. There were no statistically significant differences between the two groups with either outcome measurement. Postoperative hypotony and cataract formation occurred more frequently in the trabeculectomy group than in the viscocanalostomy group (p=0.002). The authors concluded that in patients with POAG, trabeculectomy resulted in a greater IOP reduction than with viscocanalostomy. The study is limited in its small sample size and short-term follow-up period.
In a non-comparative, prospective study, Stangos et al. (2007) evaluated the safety and efficacy of combined viscocanalostomy and phacoemulsification (cataract) surgery to treat 50 eyes of 50 patients with medically uncontrolled OAG and a concomitant age-related cataract. Patients were followed for a mean of 29.02 ± 7.09 months. Outcome measurements included IOP reduction ≥ 30% compared to pre-operative IOP and post-operative IOP < 21 mmHg with medications (qualified success), post-operative IOP < 21 mmHg without medications (complete success), and the number of adverse events. Mean pre-operative IOP significantly decreased from 23.51 ± 4.48 mmHg to 14.06 ± 1.64 mmHg at the last follow-up (p < 0.001). The overall success was 94% at 12 months, 92% at 24 months, and 82% at 36 months. Complete success was 74% at 12 months and 67% at 24 and 36 months. No serious complications were documented. The authors concluded that phacoviscocanalostomy can be considered a safe and effective alternative surgical treatment for patients with medically uncontrolled OAG and a concomitant age-related cataract. The study is limited in its small sample size, short-term follow-up, and lack of a comparison group. The authors called for larger, randomized, comparative studies to provide stronger evidence for the safety and efficacy of phacoviscocanalostomy.
In a non-randomized, prospective study, David et al. (2008) examined the success rates and complications associated with viscocanalostomy. Forty-six eyes of 46 patients with medically uncontrolled primary and secondary OAG underwent viscocanalostomy. Patients were followed for a mean of 60 months. Outcome measurements included post-operative IOP < 21 mmHg with medications (qualified success), post-operative IOP < 21 mmHg without medications (complete success), and the number of adverse events. At 60 months, qualified success was achieved in 82% (n=37) of patients and complete success was achieved in 54% (n=25). No sight-threatening complications were observed in these patients. The authors concluded that viscocanalostomy appeared to be a safe and effective treatment in lowering IOP in eyes with POAG and certain types of secondary OAG. The study is limited in its small sample size, short-term follow-up period, and heterogeneity of the patient population.
In a prospective, randomized controlled trial, Gilmour et al. (2009) compared the safety and efficacy of viscocanalostomy with trabeculectomy in the management of POAG. Patients were randomized to have viscocanalostomy (n=25) or trabeculectomy (n=25). Patients were followed for a mean of 40 months (6 to 60 months). Outcome measurements included post-operative IOP and the number of post-operative complications. Post-operative IOP < 18 mmHg without treatment was deemed a successful outcome. 42% (n=10) of the patients in the trabeculectomy group had a successful outcome at their last follow-up visit, compared to 21% (n=5) in the viscocanalostomy group. Mean IOP was lower in the trabeculectomy group when compared to the viscocanalostomy group, with differences being statistically significant at 12, 24, and 36 months (p < 0.001), and at 48 months (p=0.018). The trabeculectomy group required less post-operative IOP medications (p=0.011) when compared to the viscocanalostomy group. The authors concluded that trabeculectomy was more effective at lowering IOP than viscocanalostomy in patients with POAG. The study is limited in its small sample size and short-term follow-up period.
In a meta-analysis, Chai and Loon (2010) compared the efficacy and safety profile of viscocanalostomy when compared with trabeculectomy in medically uncontrolled glaucoma. Ten randomized controlled trials were selected and included in the analysis, with a total of 458 eyes in 397 patients with medically uncontrolled glaucoma. Outcome measurements included mean IOP difference at 6 months, 12 months, and 24 months, mean difference in the number of postoperative medications, and relative risk of adverse events. Mean IOP difference was 2.25 mmHg at 6 months (95% CI: 1.38 to 3.12), 3.64 mmHg at 12 months (95% CI: 2.74 to 4.54), and 3.42 mmHg at 24 months (95% CI: 1.80 to 5.03). Trabeculectomy was found to have a significantly better pressure-lowering outcome (p < 0.00001). The relative risk of having an adverse event such as an intraoperative perforation of Descemet membrane when having a viscocanalostomy was 7.72 times the risk when having a trabeculectomy (95% CI: 2.37 to 25.12). The trabeculectomy group did have a statistically significantly larger number of postoperative adverse events (p < 0.008). The authors concluded that trabeculectomy had a greater pressure-lowering effect when compared with viscocanalostomy. However, viscocanalostomy had a significantly better risk profile. The study was limited in its short-term follow-up period and the heterogeneity of the patient populations as patients across the included studies had different types of medically uncontrolled glaucoma.
In a retrospective study, Kay et al. (2011) examined the outcomes in 39 eyes of 24 consecutive pediatric patients with childhood glaucoma. Surgical success was defined as a postoperative IOP of < 23 mmHg with or without glaucoma medication and without further surgical intervention. The mean age at the time of surgery was 66 ± 66 months, with a mean preoperative IOP of 40.4 ± 10.2 mmHg. Surgical success was achieved in 69% (n=27) of eyes with an average follow-up of 22 months. In patients without history of previous surgery but with a diagnosis of congenital or juvenile glaucoma, surgical success was achieved in 89% (n=17) of eyes with an average follow-up of 20 months. There were no serious complications associated with the procedure. The authors concluded that viscocanalostomy appeared to be a safe and effective procedure to lower IOP in patients with congenital or juvenile glaucoma. The study is limited in its small sample size, short-term follow-up period, and the heterogeneity of the patient population.
In a comparative case study, Eid and Tantawy (2011) compared combined viscocanalostomy-trabeculectomy to trabeculectomy alone for the management of advanced glaucoma. Eighteen patients with bilateral advanced glaucoma underwent the combined procedure in the right eye and trabeculectomy alone in the left eye. Successful outcome measurements included postoperative IOP < 14 mmHg or >30% lowering of IOP without devastating complications. At 1-week and 3 month follow-up, mean IOP was statistically significantly lower after the combined procedure with viscocanalostomy when compared to trabeculectomy alone. However, there was no significant difference at the final follow-up (9 months) and no significant difference in the number of glaucoma medicines used. There were more hypotony-related complications after trabeculectomy alone. Target IOP was achieved in 83.3% of the patients who had the combined procedure compared to 55.6% of the patients who had trabeculectomy alone. The authors concluded that combined viscocanalostomy and trabeculectomy was at least as effective as trabeculectomy alone in reducing IOP for patients with advanced glaucoma. Patients undergoing the combined procedure also had a better safety profile. The study is limited in its small sample size and short-term follow-up period. These results are also only generalizable to patients with advanced glaucoma in both eyes.
In a retrospective study, Yu et al. (2012) evaluated the clinical effect of viscocanalostomy in 51 eyes of 42 patients with primary congenital glaucoma. Outcome measurements included postoperative IOP, corneal diameter, cup/disc ratio, and adverse events. Patients were followed 1 week, 1 month, 3 months, 6 months, and 12 months after surgery. The results revealed that postoperative IOP decreased from 38.57 ± 13.61 mmHg preoperatively to 10.53 ± 3.91 mmHg, 14.89 ± 5.26 mmHg, 15.42 ± 5.11 mmHg, 13.82 ± 3.46 mmHg, and 13.16 ± 5.29 mmHg, at each respective time point postoperatively (p < 0.001). The postoperative corneal diameter also had a statistically significant reduction (p=0.002). There was no statistically significant difference in cup/disc ratio, but there was a low number of adverse events. The authors concluded that viscocanalostomy improved outcomes in patients with primary congenital glaucoma, resulting in higher success rates, lower postoperative mean IOP, and fewer complications. The study is limited in its lack of a comparative group, small sample size, and short-term follow-up period. These results are also only generalizable to patients with primary congenital glaucoma.
In a comparative case study, Koerber (2012) evaluated the safety and efficacy of canaloplasty when compared with viscocanalostomy when performed in both eyes of patients with bilateral OAG. Thirty eyes of 15 adult patients with bilateral OAG had canaloplasty performed in one eye and viscocanalostomy performed in the contralateral eye. The requirement for preoperative IOP was at least 18 mmHg with a historical IOP of at least 21 mmHg. Primary outcome measures included IOP, glaucoma medication usage, and incidence of adverse events. Patients were followed for 18 months. Both the canaloplasty and viscocanalostomy groups showed statistically significant reductions in mean IOP (p < 0.01) and the number of IOP medications (p < 0.01) when compared to preoperative values. In the canaloplasty group, eyes had a mean IOP of 14.5 ± 2.6 mmHg on 0.3 ± 0.5 medications at 18 months postoperatively when compared to preoperative levels of 26.5 ± 2.7 mmHg on 2.1 ± 1.0 medications. In the viscocanalostomy cohort, eyes had a mean IOP of 16.1 ± 3.9 mmHg on 0.4 ± 0.5 medications at 18 months when compared to preoperative levels of 24.3 ± 2.8 mmHg on 1.9 ± 0.8 medications (p=0.02). Significant complications were not reported in either group. The authors concluded that canaloplasty and viscocanalostomy were safe and effective in the surgical management of OAG with canaloplasty demonstrating superior efficacy to viscocanalostomy in the reduction of IOP (p=0.02). This study design is unique in that each patient underwent both procedures for each respective eye. However, the study is limited in its small sample size and short-term follow-up period.
Stangos et al (2012) reported the effect of the learning curve on the surgical outcome of viscocanalostomy from a retrospective series of 180 consecutive cases performed by 2 surgeons at a single center in Europe. Overall success (no visual field deterioration with an IOP ≤20 mm Hg) and IOP reduction of 30% or more compared with baseline values improved from 64% to 91% when comparing the first and the last 45 cases of the series. Complete success (no medications required) improved from 38% to 73%. Surgical complications did not differ significantly between the first and last 45 cases (16 vs 10, respectively).
A Cochrane review on the effectiveness of non-penetrating trabecular surgery, specifically viscocanalostomy or deep sclerectomy, compared with conventional trabeculectomy in participants with open-angle glaucoma was published in 2014. Five randomized and quasi-randomized controlled studies with a total of 311 eyes (247 participants) were included in the systematic review. There were 160 eyes which had trabeculectomy compared to 151 eyes that had non-penetrating glaucoma surgery, of which 101 eyes had deep sclerectomy and 50 eyes had viscocanalostomy. The findings from this review suggest that trabeculectomy is better in terms of achieving total success (pressure controlled without eyedrops) than non-penetrating filtering procedures. The analysis for the outcome of partial success (pressure controlled with additional eyedrops) was more imprecise, and the results could not exclude one surgical approach being better than the other. The authors concluded that the trials demonstrate the lack of use of quality of life outcomes and the need for higher methodological quality RCTs to address these issues.
Grieshaber et al (2015) reported long-term results of viscocanalostomy in a series of 726 patients. Mean IOP before surgery was 42.6 mm Hg. Mean IOP was 15.4 mm Hg at 5 years, 15.5 mm Hg at 10 years, and 16.8 mm Hg at 15 years. Qualified success (with or without medications) at 10 years (≤of 18 mm Hg) was 40% in the European population and 59% in the African population. Laser goniopuncture was performed postoperatively on 127 (17.7%) eyes. Fifty-three (7.3%) eyes were considered failures and required reoperation. There were no significant complications.
In summary, the available peer-reviewed literature and recommendations from medical guidelines indicate that the current evidence is insufficient for viscocanalostomy. There exist few high-level studies that compare viscocanalostomy with trabeculectomy. Meta-analysis of these trials indicates that trabeculectomy has a greater pressure-lowering effect than viscocanalostomy. Studies are limited in their short-term follow-up and heterogeneity in design and patient populations.