Gonioscopy-Assisted Transluminal Trabeculotomy: A Review

Abstract Aim To review the recent evidence in the literature regarding the efficacy and safety of gonioscopy-assisted transluminal trabeculotomy (GATT) in the management of pediatric and adult glaucoma. Methods A literature search was performed in the electronic databases of PubMed, Google Scholar, Embase the Register of Controlled Trials, and Ovid Medline for studies evaluating the safety and outcomes of GATT in glaucoma. Results GATT is a novel minimally invasive glaucoma surgery that allows the incision of the inner wall of Schlemm’s canal increasing aqueous drainage through the physiologic outflow pathway with subsequent intraocular pressure reduction in different types if of glaucoma. Conclusion GATT demonstrated favorable results in a wide range of both primary and secondary open-angle and angle-closure glaucoma.


Introduction
Glaucoma is the most common cause of irreversible blindness worldwide. 1,2 Elevated intraocular pressure (IOP) is the cardinal risk factor for the development and progression of glaucoma. 3,4 The only treatment has proven to slow the progression of glaucoma and subsequent visual field loss is the reduction of IOP, either medically or surgically. 4 The major site of resistance to aqueous humor outflow lies in the trabecular meshwork (TM), specifically the juxtacanalicular tissue. [5][6][7] Conventional surgical options to treat glaucoma rely on bypassing this resistance using filtering procedures, namely trabeculectomy or aqueous shunts. [8][9][10][11] Over the past decade, there has been a rising interest in addressing the main site of resistance to aqueous outflow through minimally invasive procedures that target the physiologic outflow pathway through the TM and the adjacent inner wall of the Schlemm's canal (SC). 10,12,13 Angle-based goniotomy and trabeculotomy are standard procedures in the management of childhood glaucoma. [13][14][15] The concept to "incise the angle of the iris in glaucoma" was first described in 1893 by Carlo de Vincentiis, but it was performed without direct visualization. 16 In 1938, Otto Barkan introduced goniotomy (Greek: gonio ¼ angle; tomein ¼ cut) utilizing gonioscopy and a knife to incise the trabecular tissue. 17 M€ oller (1977) and Shaffer (1982) have reported the effectiveness of goniotomy in infantile glaucoma. 18 However, Luntz and Livingstone (1977) described unfavorable results of trabeculotomy in adult-onset openangle glaucoma (OAG). 19 Then Chihara and Tanihara in1993 redirected the spotlight on trabeculotomy which was found to have comparable results to trabeculectomy in the adult age group and good results in pseudoexfoliation glaucoma. 20,21 Recently, there has been a growing body of evidence to support their effectiveness in adult primary and secondary glaucoma as well, especially with circumferential incisional procedures that address the whole angle. 10,13,[22][23][24][25] Gonioscopy-assisted transluminal trabeculotomy (GATT) is a minimally invasive glaucoma surgery that allows the cannulation of the SC through a single corneal incision, thus avoiding the postoperative complications of filtering surgery. 22 GATT was first introduced in 2014 by Grover and colleagues 22 and has since shown promising results in most types of glaucoma, both primary 26 and secondary 27 open angle 26 and angle closure glaucoma (ACG) 28 as well as in paediatric 29 and juvenile glaucoma. 26,28 The current study aims to systematically review the current literature evaluating the efficacy and safety of GATT in the management of the different types of glaucoma.

Surgical techniques
The GATT technique described by Grover and colleagues aims at creating a 360-degree incision in the inner layer of SC and the adjacent TM, usually through a temporal corneal incision. This is performed under a direct gonioscopic view. First, a nasal goniotomy is performed, then microsurgical forceps are used to introduce a prolene (or nylon) suture or an illuminated microcatheter into SC through one end of the goniotomy incision. The microcatheter or suture is circumferentially advanced into SC until the distal end reaches the opposite end of the goniotomy incision. Both ends of the sutures are then held and pulled simultaneously creating a circumferential trabeculotomy ( Figure 1) (Supplementary Video). 22 A modification of the conventional GATT technique uses the ablation of the Trabectome (NeoMedix Inc., Tustin, CA, USA) instead of the microsurgical blade to create the initial incision through which the microcatheter or suture is introduced. The rationale is that the Trabectome's ablation of tissue reduces the risk of bleeding which may impede proper visualization in addition to allowing easier insertion of the microcatheter into the SC. 30 Grover and Fellman described another modified technique in which they used a marked and thermally blunted suture. The tip of a clear 4-0 nylon suture was first marked with a surgical marking pen allowing proper visualization and then low-temperature ophthalmic cautery was used to blunt the tip, incorporating the ink into the suture. This allowed for visualization of the end of the suture internally and through the sclera if it stopped or became misdirected. 31 New methods were recently adopted to identify SC intraoperatively. Gupta and colleagues used a method of enhancement of SC by external jugular vein (EJV) compression during endoluminal angle surgeries. In their technique, the anaesthetist was asked to compress the EJV on the same side as the eye being operated upon, for 15-20 seconds. This resulted in engorgement of the SC, seen as a circumferential tangential red streak along with its anatomical site. In their study of 10 eyes with primary congenital glaucoma (PCG) undergoing GATT, cannulation of SC was successful in 10 eyes representing 80% of eyes in their study. 32 Panigrahi and colleagues used indocyanine green dye to delineate and identify SC intraoperatively. They performed their study on 5 eyes of 5 PCG patients in which the dye was injected intracamerally at the beginning of surgery. Successful identification of the site of goniotomy incision could be done in all the patients, although patchy TM pigmentation was noted in two cases with subsequent successful circumferential cannulation of the SC. 33

Gonioscopy-assisted transluminal trabeculotomy in different types of glaucoma
Primary open angle glaucoma Supplementary Table 1 summarizes the studies evaluating the outcomes of GATT in primary open angle glaucoma (POAG). In the first study conducted by Grover and colleagues to evaluate the GATT procedure, 57 out of the 85 study eyes had POAG. In those eyes, the authors reported a mean IOP reduction of 7.7 ± 6.2 mmHg (30% IOP reduction from preoperative values (p < 0.001)) with a mean reduction of glaucoma medications by 0.9 (p < 0.001) at the 6-months postoperative follow-up visit. At 12 months, the mean IOP was reduced by 11.1 ± 6.1 mmHg (39% IOP reduction (p < 0.001)) with a mean reduction of glaucoma medications by 1.1 (p ¼ 0.013). Hyphema was the most common postoperative complication in the first week and was resolved in all cases by the end of the first month. 22 The authors attributed surgical failure in some of their patients to a dysfunctional distal collector channels system which is difficult to be identified preoperatively. 6,22 Four years later, Grover and colleagues presented their 24-month follow-up results of patients with OAG who underwent GATT or GATT combined with cataract extraction (CE). They included 198 eyes, 119 of which had POAG. Forty-six eyes were phakic and underwent GATT only with a mean IOP reduction of 10.4 mmHg and an average decrease in glaucoma medications of 1.4 at the 24 month follow-up visit (p < 0.001). There were 37 eyes in the  pseudophakic POAG group who underwent GATT alone and had a decrease in the mean IOP by 8.9 mmHg on 1.0 fewer glaucoma medications (p < 0.001). On the other hand, 36 eyes underwent GATT combined with CE with a mean IOP reduction of 8.4 mmHg with an average decrease in glaucoma medications by 1.9 at 24 months (p < 0.001). 34 They reported that eyes with visual field mean deviation (MD) worse than À15 had a higher risk of surgical failure (the cumulative proportion of failure reached 0.8) following GATT and postulated that this could be attributed to the atrophy of the collector channels in eyes with advanced glaucoma. 34 They also reported that the pseudophakic POAG group tended to have a higher cumulative proportion of failure (0.5 compared to 0.3 in the phakic group) and a higher cumulative proportion for reoperation (0.4 compared to 0.25 in the phakic group) after the 24-month followup visit. 34 Rahmatnejad and colleagues reported results of GATT in 66 eyes with OAG of which 48 eyes had POAG. The success rate for GATT in POAG patients was approximately 71.7%, defined as IOP reduction >20% from baseline or a postoperative IOP between 5 and 21 mmHg, with no need for further glaucoma surgery. 26 The mean IOP reduction was 42.4% (p < 0.001) at 6 months and about 38.0% (p < 0.001) at 12 months postoperatively. They found equal success rates in eyes that underwent GATT combined with CE compared to eyes that underwent GATT alone (p ¼ 0.999). 18 The overall success rates of the surgery among white and black patients were 69 and 42%, respectively (p < 0.05). The authors reported a postoperative IOP spike in 24.7% of patients which was associated with a less favorable final outcome. This was observed more often in the surgical failure group (44%) compared to the surgical success group (13%) (p ¼ 0.003). Transient hyphema was the most common postoperative complication. At one week postoperatively, 38% of the participants had hyphema compared to 6% at the 1month follow-up visit. 26 Another retrospective study conducted by Aktas and colleagues evaluated the outcomes of GATT in eyes with moderate to advanced OAG. 35 The study included 104 eyes, of which 65 eyes had POAG. They compared the IOP reduction in patients who had GATT alone to those who underwent GATT combined with CE as well as those with prior CE who underwent GATT. Success, defined as IOP <18 mmHg and !20% IOP reduction from the preoperative value, was 65% in the whole cohort with a mean postoperative IOP of 15.9 ± 4.3 mmHg. 35 The percentage of IOP reduction in the GATT alone, combined GATT and CE, and GATT after prior CE groups was 41.34±17.54%, 38.51±20.72%, and 36.52±17.22%, respectively (p > 0.1). 35 In contrast to Grover and colleagues, 34 they did not find any significant correlation between visual field MD values and IOP reduction (p ¼ 0.456). They also reported that patients with POAG had better IOP reduction (40.1%) compared to patients with secondary OAG (27.6%) at the 18 months follow-up visit (p < 0.001). 35 Loayza-Gamboa and colleagues retrospectively evaluated the efficacy of GATT combined with CE in patients with OAG. They included 32 eyes with visually significant cataract, best corrected visual acuity (BCVA) <20/40, and OAG. Twenty-seven of the 32 eyes were diagnosed with POAG. Success rate, defined as IOP between 5 and 18 mmHg and !20% reduction of IOP, was about 65% and the mean IOP reduction was 21% at 12 months. They attributed this to relatively low baseline mean IOP (15.9 ± 3.6 mmHg) and that nearly a quarter (22%) of patients underwent trabeculotomy of 270 or less. There was a significant improvement in the BCVA from 1.1 ± 0.7 log MAR preoperatively to 0.4 ± 0.7 log MAR at 6 months postoperatively (p < 0.0001). 36 In another retrospective study of 32 eyes with cataract and POAG, Baykara and colleagues found that GATT combined with CE reduced IOP by 65% at 6 months follow-up with mean IOP decreasing from 34.2 ± 10.6 mmHg to 11.2 ± 2.4 mmHg at 6 months postoperatively (p < 0.0001). 37 Additionally, the need for postoperative medications dropped from a mean of 2.8 ± 0.4 to 0.2 ± 0.4 at 6 months postoperatively (p < 0.000). No patients developed a postoperative IOP spike. One patient developed hypotony (3.1%) which was managed with topical and oral steroids, subtenon steroid injection, and administration of autologous serum into the anterior chamber, yet deep hypotony could not be avoided with any of these treatments. 37 Postoperative hyphema occurred in 31% of eyes.
Another retrospective study by Buzkart et al. included 108 eyes with OAG, of which 48 eyes had POAG. 38 They compared GATT alone to GATT combined with CE and found that the IOP reduction was significantly higher in the GATT group (44.25 ± 21.32%) compared to the GATT combined with the CE group (32.29 ± 23.41%) (p ¼ 0.009). 38 They attributed this difference to higher preoperative IOP values in the GATT group (27.75 ± 8.29 mmHg vs. 23.65 ± 7.56 mmHg in the GATT-CE group, p ¼ 0.004). Another hypothesis was the possible inflammatory reaction associated with CE that may have adversely influenced the IOP reduction. 38 Also the mean age of the patients in the GATT-CE group was higher which could correlate with more liability of atrophy of collector channels. Despite this, fewer postoperative glaucoma medications were used in the GATT-CE group (0.25 ± 0.85 compared to the GATT group (0.88 ± 1.45) (p ¼ 0.008) and subsequent surgical interventions were also performed less often in the GATT-CE group (four trabeculectomies in GATT group, one trabeculectomy in GATT-CE group). 38 They found that the efficacy of GATT was less in the older age group.
Al Habash and colleagues conducted a prospective study of 19 patients (20 eyes) with POAG to study the efficacy of GATT and ab interno canaloplasty combined with CE. 39 An illuminated microcatheter was passed circumferentially through the goniotomy site using microsurgical forceps with the assistance of Healon/Healon GV injection (2 clicks for each clock hour while sliding the illuminated tip of the microcatheter). 39 The success rate was 100% with success defined as postoperative IOP reduction >20% from baseline and/or IOP between 5 and 21 mmHg without the need for further glaucoma surgery. 39 There was a significant reduction in IOP from 19.75 ± 4.68 mmHg preoperatively to 13.30 ± 1.30 mmHg postoperatively (32.7%) at 12 months (p < 0.001). The number of glaucoma medications was also significantly reduced from 3.4 ± 0.6 preoperatively to 1.1 ± 1.0 at 12-month follow-up visit (p < 0.001). 39 Another prospective study by Sato and colleagues included 64 OAG eyes, of which 28 eyes had POAG. They combined CE with GATT in patients with visually significant cataract (45 eyes). 40 At 24-months, the mean IOP was significantly reduced from 18.4 ± 2.9 mmHg to 13.4 ± 3.0 mmHg (27.2%) (p < 0.001). 40 In the 19 eyes that underwent GATT alone, the mean IOP was significantly reduced from 19.1 ± 2.2 mmHg preoperatively to 14.6 ± 2.9 mmHg at 24 months (23.6% IOP reduction from baseline) (p < 0.001). In the 45 eyes that underwent GATT combined with CE mean IOP was significantly reduced from 18.1 ± 3.2 mmHg preoperatively to 12.7 ± 2.9 mmHg at 24 months (28.5% IOP reduction from the baseline (p < 0.001). The difference in the mean IOP reduction was not statistically significant between groups (p ¼ 0.200). 40 Surgical success in eyes with POAG, defined as IOP 15 mmHg with or without glaucoma medications and IOP reduction !20%, was 39%. 40 They reported higher failure rates in patients who were on !2 glaucoma medications before undergoing GATT.
Juvenile open angle glaucoma Several paediatric glaucoma surgeons consider 360 circumferential ab-externo trabeculotomy as their initial surgical treatment for PCG as well as juvenile open angle glaucoma (JOAG). [41][42][43] Grover and colleagues reported the efficacy of GATT for the treatment of JOAG and PCG. 29 Their study included 14 eyes (4 eyes with PCG and 10 eyes with JOAG) with a mean follow-up period of 20.4 months after GATT. In the JOAG patients, there was a statistically significant reduction in the mean IOP from 25.8 ± 10.9 mmHg to 12.9 ± 3.6 mmHg (p ¼ 0.003) and the mean number of glaucoma medications decreased from 3.1 ± 0.8 to 1.2 ± 1.1 (p ¼ 0.0008) at the last follow-up visit. 29 Wang and colleagues 44 retrospectively reviewed 59 eyes with JOAG who underwent GATT. They reported an IOP reduction from 26.5 ± 9.0 mmHg on 3.7 ± 0.9 medications preoperatively to 14.7 ± 3.0 mmHg on 0.7 ± 1.2 medications at 12 months and to 14.1 ± 2.3 mmHg on 0.4 ± 0.8 medications at 18 months postoperatively (p < 0.001). The percentage of IOP reduction was 37.2 and 47.9% at 12 (p < 0.05) and 18 months (p < 0.05), respectively. Complete and qualified success were defined as an IOP 18 mmHg and a reduction of IOP by ! 20% from baseline with (qualified success) or without (complete success) glaucoma medications and were reported to be 70.8 and 81.2% at 12 months and 58.6 and 81.2% at 18 months, respectively. Success in eyes with and without prior glaucoma surgery did not differ significantly (p ¼ 0.759). In addition, GATT was effective for both mild-to-moderate and severe cases; the latter achieved a surgical success of 79.1% with no statistically significant difference when compared to mild-to-moderate cases (p > 0.05). The authors also found that a postoperative IOP spike (defined as a postoperative IOP of !30 mmHg within 1 month of surgery) was a risk factor for surgical failure (hazard ratio (95% confidence interval (CI)): 3.189 (1.204, 8.448)). 44 Shi and colleagues 45 prospectively studied the surgical outcome of GATT on 70 eyes with JOAG. The mean IOP decreased from 31.3 ± 9.5 mmHg preoperatively to 15.8 ± 2.7 mmHg at 12 months postoperatively (p < 0.001). They defined success as a postoperative IOP 21 mmHg with !20% IOP reduction from baseline at 12 months follow-up, with (qualified success) or without (complete success) medications. Success was achieved in 91.4 and 74.3% of the qualified and complete success groups respectively with 78.6% of eyes demonstrating IOP reduction >30%. 45 They did not find any correlation between disease severity and success. However, older age and longer duration of postoperative IOP spikes were the most important risk factors for failure (p ¼ 0.009). A significant correlation was found between lower postoperative IOP and the degree of cyclodialysis cleft on anterior segment optical coherence tomography. 45 Table 1 summarizes studies evaluating the outcomes of GATT in JOAG and PCG.

Pseudoexfoliation glaucoma
Pseudoexfoliation glaucoma is the most common form of secondary glaucoma worldwide with a worse prognosis than POAG. 48 Medical or laser treatment has been recommended as first-line therapy, however, surgical treatment is often required. 49 Sharkawi and colleagues 27 prospectively reported the surgical outcome of GATT in 103 eyes with pseudoexfoliation glaucoma; 50 eyes underwent combined GATT and CE while 53 eyes were pseudophakic. The mean preoperative IOP was 27.1 (95% CI; 25.5-28.7) mmHg using 2.9 ± 1.1 glaucoma medications which decreased postoperatively to 13.0 (95% CI 11.6-14.4) mmHg and 1.0 ± 1.1 glaucoma medications at 24 months (p < 0.001). 19 Over 24 months, there was approximately a 52% reduction of postoperative IOP after GATT either alone or when combined with CE in pseudoexfoliation glaucoma patients with no statistically significant difference found between groups. 27 Bozkurt and colleagues 38 conducted a retrospective study on 108 eyes, 66 of which had pseudoexfoliation glaucoma and underwent either GATT alone or with CE. Success was defined as IOP reduction !20% from baseline and IOP between 5-21 mmHg. The authors found a statistically significant reduction of IOP from 25.35 ± 8.52 mmHg preoperatively to 15.06 ± 4.39 mmHg postoperatively (p < 0.001) at 12 months. Like Sharkawi and colleagues, they also did not find a statistically significant difference in the success rates between GATT alone (87.5%) compared to GATT combined with CE (83.8%) (p ¼ 0.811) 38 In a retrospective case series by Vez and colleagues 50 the efficacy of GATT in 31 eyes with OAG, and 20 with pseudoexfoliation glaucoma, was studied. 50 At 6 months followup, there was an average IOP reduction of 58% in the cohort of 31 eyes. The mean preoperative IOP was significantly reduced from 33 ± 8.0 mmHg to 13.9 ± 2.2 mmHg at the 6-month follow-up visit and 13.9 ± 3.1 mmHg at the 12-month follow-up (p < 0.001). 50 They also found that GATT lowers IOP more effectively in patients with secondary OAG (especially pseudoexfoliation glaucoma) compared to POAG.

Steroid induced glaucoma
Steroid induced glaucoma is a common cause of secondary OAG. Although the exact mechanism is unknown, increased IOP may be due to reduced outflow facility, specifically at the level of the TM. 51,52 Boese and Shah 53 conducted a retrospective study of 13 eyes with steroid induced glaucoma who underwent GATT. They found a significant reduction in IOP from 28.0 ± 8.2 mmHg preoperatively to 10.5 ± 1.0 mmHg by postoperative month 24, which represented an IOP decrease of 63%. They reported a reduction of postoperative medications to <1 throughout all postoperative visits, ranging from 0.3 to 0.8 with most of the patients on topical steroids chronically. 53 Uveitic glaucoma Sachdev and colleagues performed GATT in 3 eyes of 3 patients who had juvenile idiopathic arthritis with secondary uveitis and glaucoma. 54 They found that GATT was effective in reducing IOP by 40-66% beyond 10 months postoperatively. The first patient had preoperative IOP between 25 and 30 mmHg which decreased to 6-10 mmHg on no glaucoma medications at the 14 month follow-up visit. The second patient had IOP fluctuating between 28 and 34 mmHg preoperatively using 4 medications which decreased to 10 mmHg on 3 medications at 10 months follow-up visit. The third patient showed fluctuating IOP between 21 and 32 mmHg on 4 glaucoma medications; postoperatively IOP remained stable at 13 mmHg at 21 months follow-up on no medications. 54 Glaucoma following pars plana vitrectomy The incidence of glaucoma after uncomplicated pars plana vitrectomy (PPV) ranges from 15-to 20%. 44 The etiology may be due to increased molecular oxygen in the anterior chamber angle of post-vitrectomy patients which causes alterations in the TM by inducing oxidative stress, ultimately leading to reduced aqueous outflow and a resultant rise in IOP. 45 Emulsified silicone in the anterior chamber also causes destruction of the TM and subsequent elevation of IOP. 44 Quan and colleagues 55 in their retrospective study of 8 eyes with glaucoma following PPV who underwent GATT, found that the mean preoperative IOP was 32.7 ± 5.1 mmHg and improved to 13.6 ± 1.8 mmHg postoperatively (p < 0.001) with an average reduction of 50%, at the last follow-up visit (1-25 months; median 4). The average number of IOP-lowering medications preoperatively was 4.8 ± 0.9 which decreased significantly to 1.6 ± 1.4 postoperatively (p < 0.001). 55 GATT in these cases has the advantage of being a conjunctival sparing procedure in such patients with extensive conjunctival scarring, however, this study was limited by the short-term follow-up.
Aktas and colleagues 56 conducted a retrospective study of 15 eyes of 15 patients with silicone-induced glaucoma who underwent GATT after silicone oil removal. They found that the mean IOP decreased from 31.0 ± 4.1 mmHg at baseline to 15.6 ± 4.6 mmHg at 37.5 months follow-up visit (p < 0.001). They reported that 93.3% had qualified success (IOP 21 mmHg on medications) but only 26% achieved complete success (an IOP !6 and 21 mmHg without glaucoma medications). 56 The long-term effectiveness of the procedure is unknown because emulsified silicone oil droplets cannot be completely removed from the eye and these particles may obstruct other parts of the aqueous outflow pathway such as the distal collector channels after GATT surgery. 56 After failed glaucoma surgery Grover and colleagues performed GATT after failed trabeculectomy, glaucoma drainage device (GDD), Trabectome, and cyclophotocoagulation. 57 In their retrospective study of 35 eyes in 35 patients, they found a significant decrease in IOP from 24.6 ± 6.4 mmHg on 3.2 ± 1.0 medications to 16.7 ± 5.6 mmHg (p ¼ 0.027) on 2.1 ± 1.4 glaucoma medications (p ¼ 0.063) at 24 months in the prior trabeculectomy group. In the GDD group, IOP showed a significant reduction from 27.0 ± 7.1 mmHg on 3.4 ± 1.1 glaucoma medications preoperatively to 12.9 ± 2.6 mm Hg (p < 0.001) on 2.1 ± 1.2 glaucoma medications postoperatively (p ¼ 0.080). 57 Cubak and colleagues 58 studied the surgical outcomes of GATT after previously failed trabeculectomy for POAG and pseudoexfoliation glaucoma cases in 26 eyes. 50 Twelve eyes had POAG. There was a reduction of IOP from 26.2 ± 6.3 mmHg preoperatively to 17.8 ± 6.1 mmHg at 12 months of follow-up (p ¼ 0.013). Eyes with pseudoexfoliation glaucoma had a better reduction of IOP (24.6 ± 4.5 mmHg preoperatively to 13.1 ± 4.5 mmHg at 12 months postoperatively, p ¼ 0.006) compared to POAG patients. They attributed that to the abnormal collector channels that may be responsible for aqueous outflow resistance in POAG. At a mean follow-up of 17.8 months, surgical success, defined as final IOP 15 mmHg and !20% IOP reduction from baseline without any further glaucoma surgery, was achieved in 16 of 26 eyes (61.5%). 58 Chronic angle closure glaucoma Based on the EAGLE study, lens extraction was recommended as a first line treatment in cases of chronic angle closure glaucoma (CACG). 59 However, in a subset of patients lens extraction alone was not sufficient to control IOP. Fontana and colleagues 28 retrospectively studied the outcomes of GATT in 15 pseudophakic eyes with CACG. They performed sectoral goniosynechiolysis using a smooth spatula to gain visualization and access to the iridocorneal angle allowing them to perform ab-interno trabeculotomy in the same way performed in OAG. They reported a significant reduction in the mean IOP from 30.27 ± 4.20 mmHg preoperatively to 15.20 ± 2.08 mmHg at 1-year following surgery (p < 0.001). The mean percentage of IOP reduction from baseline was 49±9.41% (p ¼ 0.001). The success rate, defined as IOP reduction >30% from baseline at 6 and 12 months with (qualified) or without (complete) glaucoma medications, was 27 and 73%, respectively. 28 Chira-adisai and colleagues 60 retrospectively studied 39 eyes including 37 with CACG, one with a post-acute attack of ACG, and one with plateau iris syndrome who underwent GATT. 52 The mean preoperative IOP was 21.8 ± 5.4 mmHg which decreased postoperatively to 15.1 ± 3.8 mmHg at 1 month (p < 0.001), 14.4 ± 1.2 mmHg at 3 months (p ¼ 0.0012), 14.8 ± 2.1 mmHg at 6 months (p ¼ 0.001), and 14.5 ± 0.8 mmHg at 1 year (p ¼ 0.028). Glaucoma medications were significantly reduced from a mean of 3.5 ± 1.4 preoperatively to 1.5 ± 1.4 at 1 month (p < 0.001), 0.9 ± 0.9 at 3 months (p ¼ 0.01), 1.4 ± 1.4 at 6 months (p ¼ 0.002), and 1.5 ± 0.5 at 1 year (p ¼ 0.028) follow-up. 60 The success rate, defined as IOP 15 mmHg with or without glaucoma medications, was 71.8% at the last follow-up. The results of this study were comparable to other studies assessing the outcomes of CE with goniosynechiolysis only. [61][62][63] Supplementary Table 2 summarizes the studies evaluating the outcomes of GATT in glaucoma other than POAG, JOAG, or PCG.

Primary congenital glaucoma
Grover and colleagues 29 demonstrated that GATT is safe and effective in the surgical management of PCG and JOAG. In a retrospective study that included 14 eyes of 10 patients, 4 eyes had PCG, with a mean follow-up of 20 months. The mean IOP decreased from 31 ± 10.4 preoperatively to 19.5 ± 4.3 mmHg and the mean number of medications decreased from 1.5 ± 1.6 preoperatively to 0. However, this study was limited by its small sample size. 29 Song and colleagues performed GATT in both eyes of a 3-year-old PCG patient and found that the IOP was reduced from 27 mmHg right eyeand 34 mmHg left eye to less than 10 mmHg at one week postoperatively and remained controlled at 6 months postoperatively. 46 Lehmann-Clarke and colleagues 47 reported the outcome of microcatheter-assisted GATT in four eyes with PCG, two of which had previous goniotomy. The preoperative IOP was above 25 mmHg in all cases, on medications, which dropped to less than 17 mmHg without medications after 30 months of follow-up (Table 1). 47 Chen and colleagues in their retrospective study reviewed the risk factors for failure after GATT in a young cohort of patients. They included 122 eyes of 88 patients, 17 eyes had PCG with a median follow-up duration of 7 months. They reported that patients who used corticosteroids postoperatively were more likely to have IOP spikes than those using non-steroidal anti-inflammatory drugs (NSAIDs) alone (HR ¼3.34, p ¼ 0.042). Patients with non-circumferential trabeculotomy were 2.56 times more likely to fail (p ¼ 0.002) compared to those with circumferential surgery. Also, patients with IOP spikes have a higher incidence of failure, however, no specific analysis for PCG patients was conducted in the study. 64

Gonioscopy-assisted transluminal trabeculotomy versus other glaucoma surgeries
When comparing the outcome of 360 vs. 180 GATT, Sato and Kawaji 65 conducted a prospective study of 99 eyes that were randomly assigned to one of three groups: the 360 incision group (34 eyes), the upper-180 incision group (34 eyes) and the lower-180 incision group (31 eyes). Authors found a reduction in IOP from 18.6 ± 5.9 mmHg with 3.1 ± 1.1 medications to 13.7 ± 3.4 mmHg (20.8% IOP reduction; p < 0.001) with 1.4 ± 1.3 medications (p < 0.001) at the postoperative 12-month and no significant differences were found between the 3 groups throughout 12 months of follow-up in either the IOP measurements or the number of postoperative glaucoma medications. Success,defined as IOP 21 mmHg and !20% IOP reduction, was 36.7% in the 360 group, 26.5% in the upper-180 group, and 25.5% in the lower-180 group (p ¼ 0.320). 65 When compared to ab-externo trabeculotomy, Yalinbas and colleagues 66 retrospectively compared 33 eyes who underwent 360 ab-externo trabeculotomy to 22 eyes who underwent 360 ab-interno trabeculotomy for OAG. The mean IOP in the ab-externo group was 26.2 ± 10.4 mmHg, and the mean number of antiglaucoma medications was 3.2 ± 1.0 preoperatively. This decreased to 11.2 ± 3.0 mmHg and 0.1 ± 0.4 at 12 months, respectively (p < 0.001). 66 This was comparable to the ab-interno group in which the mean IOP was 28.3 ± 10.4 mmHg, and the mean number of glaucoma medications was 3.5 ± 0.9 preoperatively, decreasing to 13.3 ± 6.5 mmHg and 0.8 ± 1.0 at 12 months, respectively (p < 0.001). There was no statistically significant difference in IOP reduction between groups at 12 months (51% in the ab-externo group compared to 49% in the ab-interno group, p ¼ 0.7). 66 Qiao and colleagues 67 reported superior results for GATT when compared to Kahook dual-blade (KDB) assisted abinterno trabeculectomy (or excisional goniotomy) in patients with uncontrolled JOAG in a retrospective study. 67 Thirtysix eyes were treated with GATT versus 13 eyes were treated with KDB. The mean preoperative IOP was 30.48 ± 12.9 mmHg and 26.08 ± 13.1 mmHg (p ¼ 0.164) on 3.71 ± 0.46 and 3.08 ± 0.86 (p ¼ 0.023) glaucoma medications in the GATT and KDB groups, respectively. 56 At 3-months postoperatively, the mean IOP was 15.48 ± 5.93 mmHg in patients who underwent GATT and 20.0 ± 10.8 mmHg in those who underwent KDB (p ¼ 0.072). The cumulative proportion of partial and complete success were 65.6 and 44.7% in the GATT group and 30.8 and 15.4% in the KDB group at 6 months. This could be explained by the extent of angle treatment being more in GATT compared to KDB. They also found that IOP spike was a risk factor for failure of GATT. Patients with a shorter axial length before surgery had a longer survival time. 67 Hamze and colleagues 68 compared the outcomes of 37 eyes of 37 patients who underwent combined GATT and CE to 30 eyes of 30 patients who underwent combined iStent Inject insertion and CE in a retrospective study. At 12 months follow-up, they found that the GATT group achieved a significantly greater mean IOP reduction of 40.2% (11.4 ± 9.1 mmHg) compared to 14.9% (3.3 ± 4.5 mmHg) in the iStent group (p < 0.01). 68 In patients with OAG, Olgun and colleagues 69 retrospectively compared 114 eyes who underwent XEN implantation to 107 eyes who underwent GATT. They found that the complete surgical success rates defined as IOP 21 mmHg and 20% ! IOP reduction from baseline without medications were 34.2 and 50.5% in XEN and GATT groups, respectively (p ¼ 0.039). The mean postoperative IOP reduction was more in the XEN group (57.9%) when compared to the GATT group (37.1%) (p < 0.001). However, the postoperative medication dependence was more in the XEN group 1.8 ± 1.8 when compared to the GATT group 1.2 ± 0.4 (p ¼ 0.009). 69 Fontana and colleagues 70 found that IOP lowering was greater after mitomycin C-augmented trabeculectomy than after GATT at 18 months, however, both groups showed a significant reduction in the number of medications in a retrospective study of 110 OAG eyes. The mean baseline IOP was 30.04 ± 7.5 in the trabeculectomy group and 27.59 ± 4.70 in the GATT group (p ¼ 0.072). At 18 months, the mean IOP was 12.48 ± 4.58 mmHg and 15.26 ± 3.47 mmHg after mitomycin Caugmented trabeculectomy and GATT, respectively. Complete and qualified success were defined as a percentage decrease !30% and absolute IOP 18 mm Hg at 18 months with (qualified) or without (complete) medications. Complete and qualified successes were 59% and 27% after mitomycin C-augmented trabeculectomy and 46 and 31% after GATT (p ¼ 0.353) with a higher reintervention rate in the GATT group. The most frequent complication after trabeculectomy was hypotony and after GATT was hyphema. 70 In conclusion, studies on GATT demonstrated favorable results in a wide range of both primary and secondary OAG and ACG. The IOP reduction was found to be in many studies more than 30% with final IOP reaching mid-teens, however, further studies are recommended to study the effectiveness of the procedure to halt glaucoma progression in conditions requiring very low teen IOP such as advanced cases and normal-tension glaucoma. The unfavorable outcome was noted in patients with postoperative IOP spikes and non-circumferential trabeculotomy. Also, patients older than 60 years had been reported to have a significantly greater chance of failure. 71 More studies are needed to assess the long-term results of GATT and compare it to other minimally invasive glaucoma surgeries. In the meantime, GATT can be consideres a safe, effective surgical option in most types of glaucoma.

Methods of literature search
We performed a literature search in the electronic databases of PubMed CENTRAL, Google Scholar, EMBASE the Register of Controlled Trials, and Ovid MEDLINE between January 2014 to May 2022 using the following terms: "gonioscopy assisted transluminal trabeculotomy," "GATT," "trabeculotomy," "primary open angle glaucoma," "secondary open angle glaucoma," "angle closure glaucoma," "juvenile open angle glaucoma," "primary congenital glaucoma," "uveitic glaucoma," and "pseudoexfoliation glaucoma." There was no limitation on language or year of publication.

Disclosure statement
No potential conflict of interest was reported by the author(s).