Short-Term Outcomes of Gonioscopy-Assisted Transluminal Trabeculotomy in Patients with Advanced Glaucoma

Abstract

Purpose:

To review the results of gonioscopy-assisted transluminal trabeculotomy (GATT) in patients with advanced glaucoma. GATT has shown efficacy in mild to moderate cases of glaucoma. Its efficacy in advanced glaucoma is uncertain, though recent studies show promising short-term outcomes.

Methods:

A retrospective chart review of advanced glaucoma patients undergoing GATT with or without cataract surgery from January 2018 to April 2023 was conducted. Complete success was defined as intraocular pressure (IOP) <14 mmHg with ≥30% reduction from baseline (Criteria A) or IOP <18 mmHg and/or ≥20% reduction (Criteria B), without antiglaucoma medications. Qualified success was defined as attainment of the same with a maximum of 2 medications.

Results:

A total of 69 eyes from 60 patients (mean age, 61.2 ± 15.3 years) were included in the study. The mean IOP decreased from 24.9 ± 10.7 mmHg preoperatively to 12.1 ± 3.0 mmHg at 6 months postoperatively. At 6 months, the complete success rate (based on Criteria A) was 66.7%, with an overall success rate of 81.9%. By 1 year (n = 52), these rates were 51.9% and 65.4%, respectively. Using Criteria B, the complete success rate was 74.2%, and the overall success rate was 95.4% at 6 months, decreasing to 57.7% and 82.7% at 1 year. Patients who had used antiglaucoma medications for <2 weeks before surgery showed significantly higher success rates at both 6 months (P = 0.01) and 1 year (P = 0.019). Combining GATT with cataract surgery also improved success rates at 6 months (P = 0.005). In addition, six eyes with previously failed trabeculectomy achieved success at 6 months, with five remaining successful at 1 year. Three eyes experienced delayed bleeds with an IOP spike, all of which met failure criteria during the follow-up.

Conclusions:

GATT demonstrates encouraging short-term outcomes in advanced glaucoma, particularly in treatment-naïve patients and those undergoing combined cataract surgery. However, delayed bleeds can lead to IOP spikes and treatment failure. GATT may be considered as a viable surgical option in treatment naïve patients with advanced glaucoma. Its efficacy is bolstered when combined with cataract surgery.

INTRODUCTION

Gonioscopy-assisted transluminal trabeculotomy (GATT) introduced by Grover et al., is an ab-interno conjunctival sparing surgery that works by bypassing the trabecular meshwork and the inner wall of the Schlemm’s canal, which contribute around 71% of resistance to aqueous outflow.1 It has been suggested as a cost-effective logical step before performing filtering surgery for glaucoma with high success rates of up to 97%.2 Four-year results of GATT have been recently published and supports its role as a viable treatment option for glaucoma.3 However, much of the literature pertains to mild or moderate glaucoma.4 Grover et al. reported high failure rates for patients with advanced glaucoma and a mean deviation of −15 dB or worse.5 Recent studies by Dar et al., Aktas et al., Ruparelia et al. and Gunay et al. show more promising results in patients with advanced glaucoma.6,7,8,9 Atrophy of collector channels has been suggested as a possible cause for failure of this procedure in advanced glaucoma.10 A comparable reduction in intraocular pressure (IOP) in patients with pseudoexfoliation glaucoma (PXFG) has been reported albeit with a reduced success rate when the target IOP was set at <12 mmHg.11 GATT has also shown promising results in patients with a failed trabeculectomy and in patients with advanced age.12,13 The available literature on the efficacy of GATT in Asian patients, especially those with advanced glaucoma, remains limited [Table 1].14,15

Table 1.

Summary of published literature on gonioscopy-assisted transluminal trabeculotomy in advanced glaucoma

Author	Number of advanced glaucoma patients (%)	Mean IOP reduction (%)	Mean AGM reduction	Success (%)	Follow-up	Degree of GATT (°)	Factors studied that influence outcome
Grover et al.5	Not mentioned	POAG: 37.3 SOAG: 49.8	POAG: 1.43 SOAG: 2.0	Cumulative proportion of failure: 0.18–0.48	24 months	360 (mostly)	Mean deviation –15 db or worse-high proportion of failure
Dar et al.6	47 (100)	32	2.1±0.0	A: 91 B: 75	6.8 months	180–360	IOP spikes-did not corelate with failure and mean deviation-did not find association with increased risk of failure
Aktas et al.7	Not mentioned	GATT: 41.34 GATT+CE: 38.5 GATT with prior CE: 36.5	2.2±0.1	Overall: 83.7	19.4 months	360	Preoperative duration of usage of AGM-did not find significant correlation
Ruparelia et al.8	35 (100)	32	2.03±0.34	Overall: 71.4	12 months	180	Mean deviation (worse than - 12 dB) was associated with higher risk of failure
Gunay et al.9	31 (100)	53.1±18.6	1.3±1.4	Overall: 82.5	12 months	360	None
Fasna Asafali et al.36	5 (14.7)	40.9	1.95±0.7	Overall: 92.6	45±14.6 weeks	360: 35.3% 180-270:–58.8% 90: 5.9%	Degree of trabecular shelf-success rates not significantly different
Maheshwari et al.14	46 (54.1)	GATT: 23.1 GATT+CE: 30	GATT: 2.03±0.06 GATT+CE: 1.33±0.38	Complete success: GATT: 91.2 GATT+CE: 96.7 Success rates not separately mentioned for advanced glaucoma	6 months	360	None
Siddhartha et al.15	25 (83.3)	38	2.9±0.6	Complete: 20 Qualified: 57	15 months	360	Age-older age significant risk factor for failure
Our study	69 (100)	43.7	72.6%	CS-A: 51.9 CS-B: 57.7 QS-A: 13.5 QS-B: 25 Overall success: 82.7	12 months	360	Preoperative duration of AGM-, delayed bleed in the angle-increases risk of failure

IOP: Intraocular pressure, AGM: Antiglaucoma medications, POAG: Primary open angle glaucoma, SOAG: Secondary open angle glaucoma, GATT: Gonioscopy-assisted transluminal trabeculotomy, CE: Cataract extraction, CS-A: Complete success criterion A, CS-B: Complete success criterion B, QS-A: Qualified success criterion A, QS-B: Qualified success criterion B

The factors that predict the success of GATT are poorly understood but better success may be expected in patients with less severe glaucoma, patients with secondary open angle glaucoma, younger patients (where the collector channels may be robust), those with a good episcleral fluid venous wave (which testifies to the patency of collector channels)16 and those without a postoperative IOP spike (postoperative IOP spikes are thought to have a greater chance of trabecular meshwork fibrosis or peripheral anterior synechiae).17 Our study aims to assess the role of GATT in patients with advanced glaucoma and to elucidate the factors that may influence the outcome.

METHODS

A retrospective chart review of patients with advanced glaucoma defined as per the Hodapp Anderson Parish criteria18 who underwent GATT with or without phacoemulsification and completed a minimum of 6 months of follow-up between January 2018 and April 2023 was done. The study was approved by the institutional review board of our hospital. Information on antiglaucoma medication use, visual acuity, IOP, gonioscopy, visual fields, and anterior and posterior segment examinations was retrieved from the medical records.

All included patients had undergone a comprehensive ocular evaluation including vision, refraction, measurement of IOP, anterior and posterior segment evaluation, gonioscopy, visual fields by Humphrey (30-2,10-2) and optical coherence tomography (Spectral Domain, Optovue Inc. California, USA). All patients were either poorly controlled by more than two antiglaucoma medications or poorly compliant/intolerant to antiglaucoma medications. An informed consent was obtained from all patients. Cataract surgery was combined with GATT in the presence of a visually significant cataract. Patients with primary open angle glaucoma (POAG), angle recession glaucoma, steroid induced glaucoma and PXFG were included in the study. Patients with primary angle closure glaucoma (PACG) were included if the nasal anterior chamber angle was visible for at least two clock hours. Patients with PACG with visually insignificant cataracts were not scheduled for GATT. Only those patients who underwent combined cataract surgery for a visually significant cataract or those who were already pseudophakic from a prior cataract surgery were included in the study. In normal tension glaucoma defined as open-angle glaucoma characterized by glaucomatous optic neuropathy and corresponding visual field defects despite an untreated IOP consistently ≤21 mmHg, patients were included if the untreated baseline IOP was more than 14 mmHg. Patients on anticoagulants/antiplatelet medications stopped these prior to surgery in consultation with their cardiologist/neurologist. Patients with diabetes mellitus were included only if their systemic control was satisfactory and they were cleared for surgery by their treating physician. Patients thought to be at high risk for future neovascular glaucoma like those with active or previously documented proliferative diabetic retinopathy were not included in the study.

Cataract surgery with intraocular lens (IOL) implantation (when combined) preceded GATT. All patients were operated under sub-Tenon’s anaesthesia by the second author (MR) with a left handed open access Hill direct goniolens. Sitting on the temporal side, two paracentesis incisions were made at 7 o’clock and 11 o’clock for the right eye (1 o’clock and 5 o’clock for the left eye). The anterior chamber was filled with 2% Hydroxy propyl methyl cellulose. The microscope was tilted towards the surgeon by 30° and the patient’s head was tilted away from the surgeon by approximately 30°. The thermally blunted end of a 5-0 polypropylene suture was introduced into the anterior chamber through a separate paracentesis. Under direct gonioscopic view, the nasal angle was incised for 1 clock hour with a microvitreoretinal (MVR) blade and the 5-0 polypropylene suture was threaded into the Schlemm’s canal with a micro forceps. The suture was advanced by small pushes. When the distal end of the suture was seen it was retrieved with the micro forceps and GATT was completed. If the distal end of the suture was not seen after 25 pushes, the surgeon moved to the nasal side to perform a goniotomy and retrieve the distal end of the suture. One percent sodium hyaluronate was used to tamponade bleeding during surgery. If only a partial goniotomy was achieved, reverse cannulation was done to cleave the remaining angle. In rare instances, when both forward and reverse cannulation of the nasal angle failed, the surgeon moved to the nasal side of the patient – the above process was repeated to cannulate and cleave the temporal angle [Video 1]. This enabled us to achieve 270°–360° goniotomy in all patients. An anterior chamber wash was done and the side ports hydrated. The anterior chamber was filled up to 20% with 1% sodium hyaluronate. 0.1 ml of intracameral moxifloxacin (Kitmox, Sentiss Pharma Private Limited, Solan, India) was injected at conclusion of the surgery. Postoperatively patients were placed on a tapering regimen of topical moxifloxacin (Moxflu, Senses Pharmaceuticals Private Ltd., Bengaluru, India) for 1 week, loteprednol (Lotenate, Senses Pharmaceuticals Private Ltd., Bengaluru, India) for 2 months starting at 8 times/day on the 1^st week, and pilocarpine (Pilocar 2%, FDC Limited, Aurangabad, India) four times a day for a month. Patients were reviewed on the 1^st postoperative day, 1^st and 6^th postoperative week, 3^rd and 6^th postoperative month. Subsequent reviews were timed by the operating surgeon based on the extent of glaucomatous damage. At each review, the vision was recorded and the applanation IOP measured. An anterior and posterior segment examination was done. Gonioscopy was done to record the extent of the trabecular shelf at the 6^th postoperative week.

Complete success according to Criteria A (CS-A) was defined as an attainment of IOP <14 mmHg without the use of antiglaucoma medications and ≥30% reduction from baseline, while qualified success according to Criteria A (QS-A) was defined as an attainment of the same with the use of a maximum of two antiglaucoma medications. Complete success according to Criteria B (CS-B) was defined as an attainment of IOP <18 mmHg and/or 20% reduction from baseline without the use of antiglaucoma medications, while qualified success (QS-B) was defined as an attainment of the same with the use of a maximum of two antiglaucoma medications. Failure according to either of these criteria was defined as requirement of more than two antiglaucoma medications or surgery to attain the target IOP and/or a reduction in IOP of <20% from baseline or an IOP <6 mmHg with hypotonous maculopathy. A postoperative IOP spike was defined as an IOP of >30 mmHg within the first 6 weeks of surgery.

Statistical analysis

Descriptive analysis was carried out by frequency and proportion for categorical variables. Continuous variables were presented as mean ± standard deviation (SD) for normally distributed random variable and median (IQR) for non-normal variable. The Chi-square test was used to test the statistical significance of cross-tabulation between categorical variables. Wilcoxon sign test was used to compare the median (IQR) of continuous paired data between two time points. Repeated measures analysis of variance (ANOVA) was used to compare the mean ± SD of continuous paired data between more than two time points. ANOVA was used to compare the mean ± SD of continuous variables between more than two groups. P < 0.05 was considered statistically significant. Courtesy: Coguide Research Enablement and Productivity Version 1.2 (BDSS, Bengaluru, India; https://reapv2.coguide.in/)

RESULTS

A total of 69 eyes of 60 patients (51 males and 18 females) with a mean age of 61.2 ± 15.3 years were included in the study. The mean baseline IOP was 24.9 ± 10.6 mmHg. Information about the duration of antiglaucoma medication usage before presentation was available for 59 patients. The average mean deviation in POAG, PXFG, PACG groups was −19 dB, −20.6 dB, and −18.4 dB, respectively and overall average mean deviation was −19.82 dB ± 5.72. Twenty-three patients had GATT only (33.3%) and the rest had combined phacoemulsification with IOL implantation and GATT. In POAG group, 20 eyes had combined surgery while 12 had GATT alone. In PXFG group 11 had combined surgery while 4 had GATT alone surgery. All patients of PACG group (n = 9) underwent combined surgery. Among them two patients underwent sequential procedure, phacoemulsification of cataract with IOL followed by GATT at 2 weeks in view of severe positive pressure during surgery. A summary of baseline characteristics is detailed in Table 2. Fourteen eyes had no trabecular shelf at 6 weeks.

Table 2.

Summary of clinical characteristics of study population (n=69)

Parameters	Summary statistics, n (%)
Age (years), mean±SD	61.23 ± 15.30
Gender
Male	51 (73.9)
Female	18 (26.1)
Surgery
GATT	23 (33.3)
Phaco GATT	46 (66.7)
Usage of AGM
No usage ≤2 weeks	37 (53.6)
>2 weeks	22 (31.9)
NA	10 (14.5)
Type of glaucoma
POAG	32 (46.4)
PXFG	15 (21.7)
PACG	9 (13)
NTG	5 (7.2)
Steroid induced glaucoma	4 (5.8)
JOAG	1 (1.4)
Eye with microspherophakia post SFIOL implantation	1 (1.4)
Posttraumatic angle recession (no PAS noted in preoperative gonioscopy)	1 (1.4)
Uveitic glaucoma (no PAS noted in preoperative gonioscopy)	1 (1.4)
Shelf (°)
<180	40 (58)
≥180	22 (31.9)
NA	7 (10.1)
IOP spike
Yes	8 (11.6)
No	61 (88.4)
MD on fields, mean±SD	−19.82±5.72
PACG – phaco GATT	9 (100)
PACG-GATT	0

SD: Standard deviation, GATT: Gonioscopy-assisted transluminal trabeculotomy, AGM: Antiglaucoma medications, NA: Not applicable, POAG: Primary open angle glaucoma, PXFG: Pseudoexfoliation glaucoma, PACG: Primary angle closure glaucoma, NTG: Normal tension glaucoma, JOAG: Juvenile open angle glaucoma, SFIOL: Scleral fixated intraocular lens, IOP: Intraocular pressure, MD: Mean deviation, PAS: Peripheral anterior synechiae

The decrease in IOP was significant at all follow-ups and for subtypes POAG, PXFG, and PACG eyes at 6 months [Table 3]. There was a significant decrease in requirement for antiglaucoma medications from 2.6 ± 1.5 at baseline to 0.4 ± 0.9 at 6 months (P < 0.001). Overall, an IOP drop of 51% and a reduction in need for antiglaucoma medications of 84.4% was noted at the 6-month review. Complete success was noted in 66.7% and qualified success in 15.2% patients according to CS-A, totaling to an overall success of 81.9% at 6 months. Applying CS-B complete success was 74.2% and qualified success was 21.2% totaling to an overall success of 95.4%. Failure at 6 months was 5.4%. Six patients with previous failed trabeculectomy were successful at 6 months review (CS-A: 2, QS-A: 2, QS-B: 2) and five at 1 year (CS-A: 1, QS-A: 2, CS-B: 1, QS-B: 1). The sixth patient had IOP in high teens on two antiglaucoma medications.

Table 3.

Comparison of mean intraocular pressure and antiglaucoma medication

	Mean IOP±SD (mmHg)	Mean number of AGM±SD	Post hoc-test P (compared to baseline)
Baseline (SD)	24±10.6	2.8±1.6
1 week	14.6±3.7		<0.001
6 weeks	12.6±5.6		<0.001
3 months	13.6±5.6		<0.001
6 months	12.1±2.9	0.5±0.9	<0.001
1 year	14.0±5.1	0.7±1.0	<0.001
POAG
Baseline	−21.6±4.7	−2.9±1.5	<0.001
6 months	−12.9±2.7	−0.4±0.6
PXFG
Baseline	−30.8±15.0	−2.7±1.4	<0.001
6 months	−11.5±2.6	−0.2±0.8
PACG
Baseline	−27.6±13.0	−2.1±0.8	<0.001
6 months	−11.4±2.5	−0.4±0.7

IOP: Intraocular pressure, SD: Standard deviation, AGM: Antiglaucoma medication, POAG: Primary open angle glaucoma, PXFG: Pseudo exfoliation glaucoma, PACG: Primary angle closure glaucoma

At 1 year, an IOP drop of 43.7% was noted (n = 52) and with a reduction of 72.6% in need for antiglaucoma medications (P < 0.001). Complete success was noted in 51.9% and qualified success in 13.5% according to CS-A. According to CS-B complete success of 57.7% and qualified success of 25% was achieved. Failure was seen in nine patients (17.3%). Notably, four patients initially successful at 6 months were later deemed failures upon review at 1 year (CS-A: 1, QS-A: 2, QS-B: 1). Three patients needed trabeculectomy for IOP control at 6 months review while two more patients underwent trabeculectomy by 1 year review.

The outcome at 6 months or 1 year according to Criteria A/B did not differ significantly according to gender (P = 0.687, P = 0.429), mean deviation on fields at presentation (P = 0.283, P = 0.844), or the degrees of trabecular shelf at 6 weeks (P = 0.828, P = 0.844). A higher success rate was noted among patients with POAG (POAG subgroup attained a complete success of 40.8% while it was 28.6% in PXFG and 12.2% in PACG). The qualified success in POAG was 71.4% compared to in PACG which was 21.4% (P < 0.001). The mean IOP reduction from baseline to 6 months follow-up was 12.9 mmHg, 19.3 mmHg, and 16.1 mmHg in POAG, PXFG, and PACG groups, respectively. There was a significant decrease in requirement for antiglaucoma medications in all the three groups (P < 0.001) (POAG: 84.6%, PXFG: 92.1%, PACG: 79.4%). Patients who were not on antiglaucoma medications or had used them for <2 weeks (n = 37) before surgery had a higher success rate at 6 months (CS-A 72.7% vs. 27.3%) (P = 0.01) and 1 year (CS-A 69.2% vs. 30.8%) (P = 0.012) compared to those who were on antiglaucoma medications for >2 weeks (n = 22). Patients who had combined cataract surgery (n = 45) had higher success (P = 0.005) at 6 months (CS-A 77.5%) compared to those who had a GATT alone (n = 21) (22.4%). The success rates (CS-A) at 1 year were 70% for combined surgery (n = 31) and 30% for GATT alone surgery (n = 21) but this did not reach statistical significance (P = 0.204).

A total of 30 complications were documented, with 13 attributed to hyphema. Eleven patients needed an anterior chamber wash in the first 2 weeks after surgery. No patient had ocular hypotony requiring treatment. One patient needed an anterior chamber washout twice. On the second instance, accidental puncture of the anterior lens capsule resulted in total cataract. This happened at another center, where the surgeon had made a separate entry with MVR blade and attributed this to a shallow anterior chamber and obscured visibility due to total hyphema. Cataract surgery was done with good visual recovery. Four months after cataract surgery, the patient developed severe cystoid macular edema resulting in loss of best-corrected visual acuity.

In another patient, bleeding in the anterior chamber (after completion of cataract surgery and incising the trabecular meshwork) hampered further visualization. Uneventful GATT was done 2 weeks later. One had severe positive pressure in both eyes during cataract surgery right from the time of initial paracentesis. This was likely due to very shallow anterior chamber and poorly controlled angle closure glaucoma. GATT was done 2 weeks later after premedicating the patient with oral glycerol and intravenous mannitol.

One of the patients had two episodes of delayed bleed in the anterior chamber with a decline in vision and rise in IOP. This resolved with conservative measures. At his last review (3 years after surgery), the IOP was 14 mmHg on maximal medical therapy with a best-corrected visual acuity of 20/60. It may be noted that the baseline IOP was 32 mmHg on maximal medical therapy.

Another patient experienced a decline in vision in the left eye to 20/40 at 1 year and was found to have dispersed hyphema in the anterior chamber with an IOP of 38 mmHg. A week later the right eye was noted to have an asymptomatic IOP spike of 24 mmHg. Fine new vessels were noted in the superior and temporal angles on gonioscopy [Figure 1]. IOP at last review (1 year and 5 months after surgery) was 14 mmHg in both eyes and she needed three antiglaucoma medications for IOP control. No rubeosis iridis or neovascularization of the retina was noted in either patient. The coagulation profile was normal and a magnetic resonance imaging/magnetic resonance angiography of the brain was normal. Both patients denied history of head down yoga/intense exercise or Valsalva maneuver.

Figure 1.

(a) Gonioscopic photograph of the temporal angle of the left eye showing fine new vessels (arrow) that could have contributed to the delayed bleeding. (b) Gonioscopic photograph of superior angle of the left eye showing fine new vessels (arrow)

DISCUSSION

Advanced glaucoma at presentation is known to have a higher risk of progression and irreversible vision loss. There is certainly evidence that initial surgical management in patients with advanced glaucoma can slow or stop subsequent field loss when compared to medical therapy.19 The NICE guidelines and the TAGS study recommend initial surgical treatment of patients with advanced glaucoma.20,21 Trabeculectomy was naturally the procedure of choice until the advent of minimally invasive glaucoma surgery (MIGS). Trabeculectomy, however has a high incidence of complications, some of which can be sight threatening and this continues to spur the search for alternate treatment options.22 MIGS, with its favorable safety profile and high efficacy, has emerged as the procedure of choice particularly for mild and moderate glaucoma. Among the plethora of available MIGS procedures, GATT emerges as one of the most cost-effective treatment options available.23 While the extent of goniotomy remains a matter of contention, it would be reasonable to assume that at least in advanced glaucoma, a complete circumferential trabeculotomy increases the odds of success as this leads to maximal reduction in outflow resistance.24,25 Video 1 demonstrates the technique to achieve the same when both forward and reverse cannulation fail in the nasal angle. To the best of our knowledge, cannulating the temporal angle to achieve complete goniotomy has not been reported before. Four-year results of GATT have recently been published and these reinforce the choice of GATT as a viable treatment option for this category.3 It may be noted that GATT appears to have a favorable learning curve as well.23

Our study shows promising results in patients with advanced glaucoma in line with other studies in this regard. Dar et al. reported attainment of IOP in low teens in 75% of patients and an IOP of <18 mmHg in 91% at 6 months.6 Aktas et al. reported a success of 83.7%, while Ruparelia et al. reported in 71.4% for patients with advanced glaucoma [Table 1].7,8 Unlike most other studies, our research specifically focused on patients with advanced glaucoma. This bears relevance as GATT is a safer option to trabeculectomy, traditionally the gold standard in managing these patients. Higher success rates were noted in treatment naïve patients and in those undergoing combined phacoemulsification with GATT. This would help us stratify patients with advanced glaucoma who may be better candidates for choosing this surgical modality. There is some evidence that the use of antiglaucoma medications could increase the population of inflammatory cells in the subconjunctival space thereby increasing chances of conjunctival scarring and failure after trabeculectomy.26,27,28,29,30 We are not aware of any published literature on the impact of preoperative antiglaucoma medications on the success of GATT, though there is evidence that the long-term use of antiglaucoma medications reduces the success of ab-interno microhook trabeculotomy.31 Proposed mechanisms include asymptomatic inflammation caused by benzalkonium chloride in antiglaucoma medications that may affect the Schlemm’s canal and distal collector channels. Another suggested mechanism is disuse atrophy of the conventional outflow pathway due to ocular hypotensives.31 Our success rates did not correlate with the mean deviation on visual fields. Perhaps, it is not the stage of disease as reflected by the mean deviation, but rather the length of time an eye has used topical medications which explain the difference in outcomes between different studies. It is logical to presume that patients with advanced glaucoma are likely to have used more antiglaucoma medications for a longer period. Thus the stage of the disease is a possible confounding factor. Likewise deepening of the anterior chamber with GATT could aid the IOP reduction thereby explaining the synergy which has also been previously reported. It is important to note that combined cataract surgery with GATT reduces IOP and the need for antiglaucoma medications more than GATT alone in PACG, with better success rates and survival, suggesting an additive effect of GATT.32 We noted a higher success rate in patients with POAG but this could potentially be due to a higher number of subjects in this subgroup.

It may be admitted that none of these factors proved to be significant in a univariate regression analysis. A possible explanation could be the small number of patients in our study. Table 1 clearly shows that several factors influence the outcome of GATT in advanced glaucoma, but there is no consensus, as factors deemed significant in one study did not reach statistical significance in others. All studies, including ours, had small sample sizes and limited follow-up. Larger, multi-center studies with longer follow-up are needed to establish clear guidelines for managing advanced glaucoma in the MIGS era.

Of concern is the late bleeding and IOP spikes noted in 3 eyes of two patients. Delayed bleeding with a spike in IOP after GATT following pupillary dilation has been reported previously.33 Grover et al. also reported delayed bleeding after GATT, though further details were not furnished.5 Delayed onset hyphema has been reported after other MIGS procedures. Proposed mechanisms include ocular compression and decompression during sleeping on the surgical side and increase of episcleral venous pressure attributed to postural effect/physical activity.34,35 Figure 1 suggests that the bleed may be because of new vessels that appear as part of the healing process after GATT. Overall, this appears to be a rare complication – nevertheless it bears great significance in advanced glaucoma. Another potential problem is confusion with neovascular glaucoma. Loss of best-corrected visual acuity in one patient was probably related to the IOP spikes. Patients undergoing GATT should be counseled to report any vision loss to the treating physician immediately. This finding offers an insight into healing process in the angle and also offers an alternate explanation for delayed hyphema. If proven by other studies and histopathology, delayed bleed could be a predictor for failure. Gonioscopy should be done at regular intervals to monitor the healing process and the appearance of new vessels in the angle should alert the physician to monitor these patients closely.

The success rates after GATT did not correlate with the trabecular shelf and this has been reported previously.36 Sixteen eyes had complete disappearance of the trabecular shelf. This may obscure signs of a previous glaucoma surgery and could confuse future surgical decisions if the patient loses access to his/her medical records.

The limitations of our study include its retrospective nature, and small number of patients with limited follow-up. The study was also done at a single center and is noncomparative in nature. A comparison with other surgical techniques like trabeculectomy or other MIGS procedures would provide context to the reported outcomes. Subgroup comparisons (e.g. POAG vs. PXFG vs. PACG) have limited statistical power due to small sample sizes, especially in the PACG group.

In summary, GATT appears to be a viable treatment option for advanced glaucoma especially so when patients are treatment naïve or when combining cataract surgery with glaucoma. We also show that delayed bleeds and failure could be the result of healing process in the anterior chamber angle. Our study thus adds to the existing literature on managing patients of Asian ethnicity with advanced glaucoma. Further studies on a larger number of patients with extended follow-up are required before recommending GATT for advanced glaucoma.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.