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Gonioscopy-assisted Transluminal Trabeculotomy: An Ab Interno Circumferential Trabeculotomy 24 Months Follow-up

Grover, Davinder, S., MD, MPH*; Smith, Oluwatosin, MD*; Fellman, Ronald, L., MD*; Godfrey, David, G., MD*; Gupta, Aditi, MD*; Montes de Oca, Ildamaris, MD; Feuer, William, J., MS

doi: 10.1097/IJG.0000000000000956
Original Studies

Purpose: The purpose of this study was to provide 24-month follow-up on surgical success and safety of an ab interno circumferential 360-degree trabeculotomy.

Methods: Chart review of patients who underwent a gonioscopy-assisted transluminal trabeculotomy (GATT) procedure was performed by 4 of the authors (D.S.G., O.S., R.L.F., and D.G.G.). The surgery was performed in adults with various types of open-angle glaucoma with preoperative intraocular pressures (IOPs) of ≥18 mm Hg.

Results: In total, 198 patients aged 24 to 89 years underwent the GATT procedure with at least 18 months follow-up. Patients with primary open-angle glaucoma had an average IOP decrease of 9.2 mm Hg at 24 months with an average decrease of 1.43 glaucoma medications. The mean percentage of IOP decrease in these primary open-angle glaucoma groups at 24 months was 37.3%. In secondary open-angle glaucoma, at 24 months there was an average decrease in IOP of 14.1 mm Hg on an average of 2.0 fewer medications. The mean percentage of IOP decrease in the secondary open-angle glaucoma groups at 24 months was 49.8%. The cumulative proportion of failure at 24 months ranged from 0.18 to 0.48, depending on the group. In all 6 study groups, at all 5 postoperative time points (3, 6, 12, 18, and 24 mo) the mean IOP and reduction in glaucoma medications was significantly reduced from baseline (P<0.001) with the exception of one time point.

Conclusions: The 24-month results demonstrate that GATT is relatively safe and effective in treating various forms of open-angle glaucoma. The long-term results for GATT are relatively equivalent to those previously reported for GATT and ab externo trabeculotomy studies.

*Glaucoma Associates of Texas, Dallas, TX

Montego Ophthalmology Center Ltd. Barquisimeto, Venezuela

University of Miami, Bascom Palmer Eye Institute, Miami, FL

Disclosure: The authors declare no conflict of interest.

Reprints: Davinder S. Grover, MD, MPH, Glaucoma Associates of Texas, 10740 N. Central Expressway, Suite 300, Dallas, TX 75231 (e-mail:

Received March 16, 2017

Accepted March 21, 2018

For over the past 50 years, numerous authors have demonstrated the efficacy of trabeculotomy in lowering the intraocular pressure (IOP) by improving the flow of aqueous through Schlemm’s canal and adjacent collector channels without bleb formation.1–5 Recently, Grover et al6 described a novel, minimally invasive approach to a circumferential ab interno trabeculotomy, which they termed the GATT (gonioscopy-assisted transluminal trabeculotomy) procedure and reported short-term outcomes in adults and children.7 The purpose of this manuscript is to provide midterm follow-up for this surgery and report on the surgical safety and efficacy.

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A retrospective chart review was performed for all patients who underwent a GATT procedure by 4 of the authors (D.S.G., O.S., R.L.F., and D.G.G.) at Glaucoma Associates of Texas between 2011 and 2013 with the goal of evaluating at least a 2-year follow-up during our initial experience with this technique. This study cohort includes the patients that were initially included in our initial report however is more inclusive in terms of patients and follow-up.6 The study followed the tenants of the Declaration of Helsinki and was approved by the affiliated hospital’s institutional review board. Written consent was obtained from all patients before surgery.

All patients had a preoperative gonioscopic examination that revealed an open angle and identifiable landmarks, notably the scleral spur and trabecular meshwork. The surgery was performed in adults with various types of open-angle glaucoma. Patients either had this surgery performed in isolation or in combination with a cataract surgery. For the purpose of this study, hyphema was defined as either a layered hyphema or a 4+ microscopic hyphema. In addition, preoperative IOP was defined to be the average of the 2 most recent measurements obtained via Goldmann applanation.

Visual acuities were converted from Snellen to approximate logMAR [−1×log (Snellen fraction)] for analysis. Average logMAR acuities were compared with preoperative measurements with the paired t test. Unlike IOP and medications, visual acuities were not censored following reoperation (following the convention of trabeculotomy versus tube study, Ahmed baerveldt comparison study).

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Surgical Procedure

The full details of the surgical procedure were previously reported therefore a brief summary of the key portions of the procedure will be described.6,7 A 23-G needle paracentesis track is placed in either the superonasal or inferonasal quadrant. A viscoelastic (sodium hyaluronate, specifically Healon GV) is injected into the anterior chamber through this site. A temporal paracentesis is created with a 15-degree blade. A 5-0 prolene suture (Ethicon) or microcatheter (Ellex) is inserted into the anterior chamber through the entry site. The microscope and the patient’s head are then oriented to allow proper visualization of the nasal angle with a Swan-Jacob goniolens (specifically, the left gonioprism by Ocular Trabectome). A goniotomy is created in the nasal angle using a 25 g microvitreoretinal blade. Microsurgical forceps are then used to direct the distal tip of the microcatheter into Schlemm’s canal at the goniotomy site. Within the anterior chamber, the microsurgical forceps (Microsurgical technology handle and microsurgical technology MH 23 gauge) are used to advance the catheter through the canal circumferentially 360 degrees. The distal tip of the catheter, once it has circumnavigated the entire canal, is retrieved and externalized, thereby, creating a 360-degree trabeculotomy. The viscoelastic is then removed from the anterior chamber using bimanual irrigation aspiration. A 15% to 40% anterior chamber fill with viscoelastic is instilled to help tamponade bleeding from the canal and protect against immediate postoperative hypotony. The amount of viscoelastic left in the eye is based on the degree of blood reflux as well as the presence and degree of an episcleral venous fluid wave (EVFW).8 Briefly, the EVFW is a temporary blanching of the episcleral vasculature while balance salt solution is irrigated into the anterior chamber. The presence and quality of the EVFW has been correlated with IOP lowering and surgical success.8 Postoperative steroid (subconjunctival or intracameral) and antibiotic drops are given per the surgeon’s discretion.

In certain cases, the microcatheter cannot be passed 360 degrees in 1 direction and stops at around 180 to 70 degrees. In these cases, a limited trabeculotomy is created by cutting down (ab interno) over the catheter at the stopping point. The distal end is retrieved and externalized, thus creating a 180- to 270-degree trabeculotomy. The microcatheter is then passed in the opposite direction through an additional 23-G needle incision and the remaining trabecular tissue is cleaved (Fig. 1). In nearly all of these cases in this study, 360 degrees of the angle was successfully treated. The number of cases with 360 degrees treated was 177 (89.4%), an additional 11 (5.5%) had between 300 and 360 degrees treated, 9 cases (4.5%) had between 180 and 280 treated. The goal of each surgeon was to treat as much of the angle as possible.



For the cases in which cataract extraction (CE) and intraocular lens (IOL) implantation was performed, the GATT procedure was performed first followed by a clear corneal CE. The initial manuscript describing this technique has an illustration demonstrating the key portions of this procedure.6

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Postoperative Care and Follow-up

After surgery, all patients were given topical broad-spectrum antibiotics and topical steroids. The topical antibiotics were stopped at postoperative week 1. The topical steroids were tapered per the surgeons’ discretion, with the main goal of controlling inflammation and preventing a steroid IOP response. The patient’s IOP was treated during the postoperative period per the surgeons’ discretion. Clinical information, obtained through chart review, was collected for the following postoperative visits; 1 day, 1 week, 2 to 3 weeks, 1, 3, 6 months and thereafter every 3 to 6 months. At each follow-up visit, the following data were collected: visual acuity, IOP, number of glaucoma medications, surgery-related complications, and gonioscopic findings. Humphrey Visual Field data were obtained within 9 months from the date of surgery.

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Statistical Methods

In this report we provide GATT outcomes for 6 groups of patients all of which had no prior glaucoma surgery and had preoperative IOP≥18 mm Hg.

Group 1: primary open-angle glaucoma (POAG) with no prior CE, receiving only GATT.

Group 2: POAG with no prior CE, receiving combined GATT and CE.

Group 3: POAG with prior CE, receiving only GATT.

Group 4: Other glaucoma with no prior CE, receiving only GATT.

Group 5: Other glaucoma with no prior CE, receiving combined GATT and CE.

Group 6: Other glaucoma with prior CE, receiving only GATT.

Eyes with juvenile open-angle glaucoma were excluded from this report.

For efficacy evaluation only 1 randomly selected eye with ≥6 months of follow-up per patient was included.

Postoperative measurements were compared with preoperative ones with the paired t test. Dichotomous and polychotomous variables, for example, sex and diagnosis, were summarized with percentages. The statistical significance of differences between study groups with respect to pressure lowering and decreases in medication use from preoperative levels was assessed with analysis of variance.

Kaplan-Meier methods were used to calculate cumulative rates of reoperation and failure. Failure was defined as reoperation to control pressure, a postoperative IOP not lowered by at least 20% from preoperative levels at or after 6 months follow-up, or an IOP >21 mm Hg at or after 6 months follow-up. Eyes were considered failures by high IOP with a single high pressure refractory to glaucoma medications.

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Table 1 describes the study groups, including number of eyes per group, mean age, sex, and distributions of diagnoses per group.



Efficacy data are reported in Table 2 for 198 eyes of 198 patients.



There were 46 patients with POAG who underwent an isolated GATT. In this group, the mean IOP decreased by 10.4 mm Hg with an average decrease in glaucoma medications of 1.4 at 24 months. In the POAG group that underwent concurrent cataract surgery, there were 36 eyes. In this group, the mean IOP decreased by 8.4 mm Hg and these eyes required 1.9 fewer glaucoma medications at 24 months. There were 37 eyes in the pseudophakic POAG group. The eyes in this group had a decrease in mean IOP of 8.9 mm Hg on 1.0 fewer glaucoma medications. There were 30 eyes in the secondary open-angle glaucoma (SOAG) group that underwent isolated GATT procedures. In this study group, the mean decrease in IOP was 17.1 mm Hg on 2.0 fewer glaucoma medications. In the SOAG group that underwent concurrent cataract surgery, there were 25 eyes. The eyes in this group had a mean IOP decrease by 11.2 mm Hg on 1.9 fewer glaucoma medications. There were 24 eyes in the SOAG group that were pseudophakic. In this group, the mean IOP decreased by 13.4 mm Hg on 2.1 fewer glaucoma medications. The cumulative proportion of failure at 24 months ranged from 0.18 to 0.48, depending on the group. If 1 excluded the outlier group (POAG Prior CE), the cumulative proportion of failure ranged from 0.18 to 0.35. Similarly, the cumulative proportion of reoperation at 24 months ranged from 0.09 to 0.43; however, excluding the outlier group (POAG Prior CE) the range was 0.09 to 0.28. In all 6 study groups, at all 5 postoperative time points (3, 6, 12, 18, and 24 mo) the mean IOP was highly significantly reduced from baseline (P<0.001). Similarly, in all 6 study groups at all 5 postoperative time points the average number of IOP-lowering medications used was also highly significantly reduced (P<0.001). The only exception is the POAG Prior CE group 24 months medication value, which had a P value of 0.059.

Table 3 represents the intraoperative and postoperative complications. Hyphema was a common occurrence within the first week but nearly all resolved at the first postoperative month. Steroid induced IOP spike did occur rarely at the first postoperative month visit. There were 3 eyes that had an inadvertent iridodialysis. The 3 cases of iridodialysis resolved without long-term sequelae.



Figure 2 presents the cumulative proportion of failure over time, separated by the 6 different study groups. Figure 3 presents the cumulative proportion of eyes reoperated for pressure control and failing either by virtue of reoperation or failure to control pressure, defined as pressure not reduced by at least 20% from baseline or >21 mm Hg at or after 6 months.





Figure 4 presents the cumulative proportion of survival for eyes in the POAG group, separated by mean deviation (MD) obtained from preoperative Humphrey Visual Field data. This curve demonstrates that eyes with a preoperative MD of −15.0 or worse had a very large proportion of failure early on, approaching over 0.8. Alternatively, in eyes with a preoperative MD of better than −15.0, the cumulative proportion of failure ranged between the 0.2 and 0.4 depending on the group. The eyes with the least severe glaucoma (better than −3.0) had the lowest proportion of failure.



Table 2 reveals that the follow-up in each study group at 24 months was relatively good, after taking into consideration, the number of eyes that failed due to requiring further IOP-lowering surgery. For example in the POAG group, at 24 months, there were 26 eyes in the analysis and 15 eyes failed due to further glaucoma surgery; therefore, 41 eyes of the 46 eyes (89.1%) are accounted for at 24 months. Similarly, in the POAG Prior CE group, 19 eyes are in the analysis at 24 months and 16 required further glaucoma surgery; therefore 35 of 37 eyes (94.6%) are accounted for at 24 months. Figure 5 represents a flow chart to clearly document follow-up throughout the 24 and demonstrate the patients lost to follow-up as well as the patients who failed due to further surgery. This figure demonstrates that of the 198 eyes, 44 were censored due to reoperation for uncontrolled glaucoma and 22 were lost to follow-up after 24 months.



Median and mean acuities decreased for a week following surgery but returned to preoperative levels and remained essentially through follow-up (Table 4). Average logMAR acuity at month 24 was ~1 Snellen line worse than preoperative, which was statistically significant (P=0.029). At 2 years follow-up, 18 eyes (9%) had lost 3 lines of acuity, 14 (78%) of these had undergone reoperation to lower IOP.



Figure 6 represents the scatterplot of postoperative IOP at 24 months versus preoperative IOP demonstrating marked reductions in postoperative IOP in the vast majority of cases. In addition, this plot demonstrates that most eyes had postoperative IOP<20 mm Hg. There were a small number of cases, especially those with preoperative mean IOP<25 mm Hg, with worse postoperative IOP’s at 24 months.



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Safety and efficacy of angle-based glaucoma surgery continue to improve. Over the past several decades, the field of glaucoma has also witnessed an evolution in trabeculotomy, from Harm’s trabeculotomes and segmental suture trabeculotomies to circumferential ab externo suture trabeculotomies. Recently, Grover et al published on a novel surgical technique, the GATT, an ab interno, conjunctival sparing circumferential trabeculotomy.6 The authors now report on 24 months safety and efficacy of this procedure.

Our results with circumferential ab interno trabeculotomy utilizing a microcatheter in adults are similar to the previously published studies on trabeculotomies in adults. In our study, patients with POAG (study groups 1 to 3) had an average IOP decrease of 9.2 mm Hg at 24 months with a decrease, on average, of 1.43 glaucoma medications. The mean percentage of IOP decrease in these POAG groups at 24 months was 37.3%. In patients with SOAG (study groups 4 to 6), at 24 months there was an average decrease in IOP of 14.1 mm Hg on an average of 2.0 fewer medications. The mean percentage of IOP decrease in these SOAG groups at 24 months was 49.8%.

There is an apparent limitation in follow-up however, when one considers the number of eyes accountable at 24 months, including those eyes that failed due to requiring further glaucoma surgery, the follow-up is relatively decent. Figure 5 demonstrates the follow-up and eyes that were lost to follow-up and censored after reoperation. At 24 months, 88.9% of eyes are accounted for (176 of 198 eyes).

Interestingly, the pseudophakic POAG group tended to have a higher cumulative proportion of failure and a higher cumulative proportion for reoperation after the 24-month time point. Although it is not immediately clear why this group is such an outlier, there are a few aspects of this group that tend to be different from the other groups. The pseudophakic POAG group is older than the other groups with a worse average MD on preoperative visual fields (VFs) than most of the other groups (Table 1). Given the more advanced nature of this group, the surgeons likely had a lower target pressure for these eyes and thus, were more likely to reoperate on this group, despite the fact that this group, overall, had a fairly similar postoperative mean IOP compared with the other groups. However, these patients required greater postoperative glaucoma medications relative to the other groups. This finding is demonstrated in Figure 3 and Table 2, which show that the pseudophakic POAG group had a high proportion of reoperation for IOP control compared with other groups. The vast majority of the eyes in this group failed due to requiring further surgery. Other authors have reported that pseudophakic POAG eyes tended to have worse surgical outcomes with other glaucoma procedures like trabeculectomy.9 These authors cite various reasons for failure related to the inflammatory response, fluid dynamics, however, no clear evidence has been provided as to the reason this subset of patients tend to have worse surgical outcomes.

These results compare favorably with Tanito et al, who reported success rates near 58.7% at 1 year in patients who underwent trabeculotomy combined with phacoemulsification and IOL implantation.10 More recently, Chin et al11 reported on a modified circumferential ab externo trabeculotomy with a mean postoperative IOP of 13.1 mm Hg on a mean number of 0.5 glaucoma medications at 1 year. Compared with these prior studies, our patient groups had roughly similar (if not better) IOP lowering and proportions of success.

Figure 4 demonstrates a very interesting finding that is consistent with clinical intuition when considering which patients tend to do well with angle-based glaucoma surgery. This Kaplan-Meier curve shows that patients with more advanced glaucoma tend to do substantially worse with GATT when compared with patients with mild to moderate glaucoma. Perhaps MD obtained from preoperative VF data can be used as a proxy for the health of the inherent canal system of an eye and can help predict, preoperatively, the likelihood of a successful surgical outcome. Although this study was not designed to answer this exact question, based on Figure 4, one may consider not performing a GATT procedure in POAG patients with a MD worse than 15 given the high proportion of failure in this group. Nesterov12 and Theobald13 demonstrated that in advanced glaucoma, the collector channels and the intrascleral plexus are sclerotic and destroyed. Further studies are required to further elucidate this connection and whether it has clinical significance.

The eyes in this study reflect the initial experience with this novel technique of all the surgeons with this surgery. The authors feel this data set represents the most conservative estimate of the safety of efficacy of this procedure. During these initial cases, the surgeons were perfecting their technique, learning to handle the nuances of postoperative care and further understanding how to best select patients for this procedure. For example, given the high rate of hyphema, the authors routinely leave a modest amount of viscoelastic in the anterior chamber to protect against postoperative hypotony. In addition, the authors will not routinely perform a GATT surgery in patients who cannot be taken off anticoagulation or who have a bleeding diathesis. The authors also learned that having the patient take normal hyphema precautions (elevate their head of bed by at least 30 degrees) during the initial 1 to 2 postoperative weeks, the chance of persistent hyphema decreased.

In this study, we considered a single IOP>21 mm Hg at last follow-up a failure, whereas typically many retrospective and prospective studies require 2 consecutive high IOP’s before classifying a case as a failure. The authors felt it was important to use more conservative definitions of success and failure in this study in order to present the data in a standardized way, similar to other surgical studies.

Gonioscopy demonstrates a number of anterior chamber angle findings, both in the operating room and postoperatively in the clinic. Intraoperatively, the tension on both ends of the filament causes it to move anteriorly in the canal, juxtaposed against Schwalbe’s line. Once cleaved, the trabeculotomy opening is usually directly behind Schwalbe’s line, not in the middle of the canal. This causes a shelf of cleaved trabecular tissue, hinged at the scleral spur, to open up “like a draw-bridge” and extend over the peripheral iris, previously termed a trabecular shelf.6 This configuration becomes more permanent as the peripheral iris attaches to the shelf.14 As the canal heals, the superior and nasal angle, typically demonstrate this persistent configuration, while some of the angle, especially inferiorly, appears to reapproximate. The authors postulate that postoperative anterior chamber angle appearance may be able to prognosticate outcomes however, this needs to be further studied in a more controlled manner and is beyond the scope of this study.

In terms of visual acuity, 18 eyes lost 3 lines of acuity. The vast majority of these patients (78%) underwent reoperation for further IOP control. Given the advanced nature of the disease stage in a large portion of the patients in this cohort, the authors acknowledge that this is a rare risk of this procedure and any glaucoma procedure in patients with advanced glaucoma and uncontrolled IOP. The authors feel that the loss of visual acuity is not specific to GATT as it has not been reported as a specific complication following ab externo cirumferential trabeculotomy over the past 50+ years that this surgery has been used.1–5 In addition, a large number of glaucoma studies evaluating visual acuity in patients with relatively advanced glaucoma report a small group of patient that lost vision again, likely due to the disease process and the stage of disease.15,16

In addition, the GATT surgery can be performed safely with either the microcatheter (US$700 to US$1000) or with a 5-0 prolene suture (US$5) with a thermally blunted tip. Although it is easier to learn the procedure with a microcatheter, once one is comfortable with the technique, one can easily use a 5-0 prolene suture. The fact that this surgery can be performed safely and effectively with a suture that costs US$5 has tremendous implications for the cost-effective delivery of surgical glaucoma care in the US and in other developed and developing countries.

One weakness of our study is the limited follow-up. However, compared with our initial GATT paper, the results are relatively similar. Moreover, as described above, the authors are able to account for 88.9% of eyes (Fig. 5). Moreover, several authors have reported on the long-term follow-up in many adults and children treated with external circumferential trabeculotomy with relatively good success.11,17–23 Our group’s clinical experience over the past 30 years has mirrored the high success rates of the published studies. The authors found that ab interno circumferential opening of the canal and trabecular meshwork is useful for many types of glaucoma within a variety of clinical settings. This approach is promising given the absence of conjunctival and scleral incisions and scarring, which may contribute to failure of future filtration surgery.

This study also has the weaknesses inherent to all retrospective clinical studies. The decision for a surgical intervention was purely at the individual surgeon’s discretion. A wide variety of types of glaucomas were treated, however, the groups are relatively large enough that we feel reliable data can be obtained from each of the groups. In addition, because there was not a standard definition for how hyphema was documented in the chart, there is some variability. The vast majority of the authors defined hyphema as a layered hyphema, however, some documented hyphema when a microhyphema was observed. This issue can be better addressed when we a prospective study is performed with standardized grading criteria. In general, a macroscopic hyphema is not usually present after 2 weeks and any comments about a hyphema at postoperative month 1 is usually a microhyphema. On the basis our current experience and knowledge, the authors recommend washing out a layered hyphema if a significant hyphema is still present on postoperative day 7 to 10.

On the basis of our experience over the past few years, we feel there are absolute and relative contraindications for this procedure. Absolute contraindications include an unstable IOL, difficulty finding Schlemm’s canal, a bleeding diathesis, extensive peripheral anterior synechiae, severe endothelial compromise and the inability to stop anticoagulant drugs. The relative contraindications for this surgery include a difficult angle view due to corneal pathology or an inability to properly identify key angle anatomy. If a patient cannot elevate their head for at least the first 14 postoperative days, the authors will reconsider performing a GATT as this may prolong the presence of a hyphema. In addition, we now add preoperative VF defect of >−15 dB as a deterrent to canal-based glaucoma surgery as this may be a proxy for irreversibly compromised collector channels.

In conclusion, we report on the 24-month results in eyes that initially underwent the GATT procedure. Proportions of success are at least equivalent to previously published studies of ab externo circumferential trabeculotomy. The procedure was safe and found to be effective in a majority of eyes in a variety of clinical settings.

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circumferential trabeculotomy; GATT; ab interno; gonioscopy-assisted transluminal trabeculotomy

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