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.
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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Keywords:Copyright © 2018 Wolters Kluwer Health, Inc. All rights reserved.
circumferential trabeculotomy; GATT; ab interno; gonioscopy-assisted transluminal trabeculotomy