Key words intraocular pressure - ab-interno canaloplasty - MIGS - canaloplasty - minimally invasive glaucoma surgery - open-angle glaucoma
Schlüsselwörter Augeninnendruck - Kanaloplastik ab interno - MIGS - Kanaloplastik - Offenwinkelglaukom
Introduction
Glaucoma is the leading cause of blindness worldwide, with an estimated prevalence of 76 million people in 2020 and is predicted to affect around 112 million people by 2040 [1]. The most
common type of glaucoma is open-angle glaucoma (OAG) [1 ] and its primary treatment involves the reduction of intraocular pressure (IOP), which has been shown to be
the most relevant and variable risk factor for progression of glaucomatous optic neuropathy as well as field loss [2 ]. The traditional treatment algorithm in OAG
consists of topical antiglaucoma medications, followed by laser treatment (selective laser trabeculoplasty or SLT) and penetrating filtration surgeries such as trabeculectomy when IOP is no
longer controlled with topical therapy [3 ].
Although a wide variety of effective topical medications are available, poor patient compliance and intolerance are common and can lead to irreversible disease progression [4 ], [5 ]. Potential barriers to adherence include difficulty administering eye drops, forgetfulness, intolerance, and cost of the medications
[6 ]. Furthermore, topical medications are associated with a variety of side effects such as ocular surface irritation, blurred vision, pain, bronchoconstriction, and
cardiovascular effect [7 ]. For example, benzalkonium chloride, a preservative common to most topical medications, can cause apoptosis in the endothelial cells and
along the trabecular columns [8 ] as much as the disease itself. This can lead to the fusion of the trabecular meshwork, with the collapse of Schlemmʼs canal, thereby
compromising the function of the natural outflow pathway and limiting the viability of future treatments [9 ]. For these reasons, patients with mild to moderate
glaucoma may benefit from a reduction in the number of medications required to control IOP.
While filtering surgeries are clinically effective for managing glaucoma [10 ], these procedures are associated with a high rate of complications, including
hypotony, failure, and bleb-related infections [11 ].
Over the last decade, minimally invasive glaucoma surgery (MIGS) has been developed to fill the treatment gap between topical therapy and invasive glaucoma surgery [3 ], [12 ]. In comparison to filtration surgery, MIGS provides several advantages such as minimal disruption to the eyeʼs anatomy and physiology and is
therefore associated with an improved safety profile and rapid postsurgical recovery [13 ]. These advantages have supported the adoption of MIGS in todayʼs glaucoma
practice. Although MIGS was originally designed to be performed in combination with cataract surgery, it has since evolved and some MIGS procedures now offer the possibility to be performed as
a standalone surgery.
Canaloplasty offers a comprehensive approach by addressing all aspects of potential outflow resistance in the trabecular meshwork, Schlemmʼs canal, and the collector channel systems [14 ]. During canaloplasty, a microcatheter is inserted into Schlemmʼs canal over the entire 360 degrees. The microcatheter is withdrawn whilst injecting an ophthalmic
viscoelastic device (OVD) to dilate the full circumference of the canal and a suture is left tensioned in Schlemmʼs canal.
Ab-interno canaloplasty (ABiC) is a development of the traditional ab-externo canaloplasty that does not require any disruption of the conjunctiva. The procedure has the same proposed
mechanism of action as traditional ab-externo canaloplasty by breaking obstructions and adhesions in the canal and restoring the physiological aqueous outflow pathway but does not employ a
tensioning suture. During the ab-interno procedure, the iTrack microcatheter is inserted through either a clear corneal or a limbal micro-incision. This has the added advantage of preserving
the conjunctiva for future glaucoma treatments, despite a reduced efficacy when compared to the ab-externo technique, which is the reason that the ab-interno is often performed on mild to
moderate glaucoma patients. It can be performed as a standalone procedure or in combination with cataract surgery. Previous studies have reported good efficacy in terms of IOP reduction and a
reduced need for glaucoma medications whilst reporting low risks of complications up to 12 – 24 months after surgery [15 ], [16 ], [17 ].
The aim of this study was to investigate the efficacy and safety over a 4-year period of ABiC using the iTrack microcatheter (Nova Eye Medical, Fremont, CA, USA) performed as a standalone
procedure or in conjunction with cataract surgery. The primary clinical outcomes were a reduction in IOP and glaucoma medications. Additional outcomes were visual field and visual acuity.
Safety data was obtained at all time points.
Methods
Study design
This was a single center, retrospective, consecutive case series analysis of 27 eyes that were treated with an ABiC between the 28th of October 2014 and the 24th of October 2016. All
surgeries were undertaken at the Augencentrum Köln-Porz, Germany, and were performed by a single surgeon (N. K.). The study was conducted according to the principles stated in the
Declaration of Helsinki and all patients provided written informed consent.
Patient selection
All patients aged 18 years or older with a diagnosis of either uncontrolled or controlled OAG were eligible for inclusion. Controlled IOP was defined as an IOP equal to or less than
18 mmHg. Cases of pseudoexfoliative glaucoma (PEX) due to progression despite treatment were also eligible for inclusion. Eyes with secondary forms of glaucoma such as those with neovascular
disease, uveitis, peripheral anterior synechiae, angle closure, narrow-angle glaucoma, or traumatic glaucoma were excluded.
Patients were allocated to undergo ABiC in combination with phacoemulsification if they presented with a cataract.
Clinical examinations
Preoperative and postoperative assessments included medication use, IOP, gonioscopy, slit lamp, and fundus examinations. IOP was measured using Goldman applanation tonometry and was the
average of three measurements at each time point. Data was collected at baseline, and 12, 24, 36, and 48 months after surgery. Best-corrected distance visual acuity (CDVA) and visual field
were recorded at baseline and at the last available follow-up. Visual field was measured with a Humphrey machine (Carl Zeiss Meditec, Jena, Germany) using the 24/2 testing strategy. We
excluded patients with more than 30% either false positives, false negatives, or fixation losses. Visual field deterioration was defined by loss of the mean deviation (MD).
Surgical procedure
All eyes underwent ABiC under local anesthesia consisting of a retrobulbar injection of carbocaine and lidocaine. The iTrack 250-micron canaloplasty microcatheter with a fiber optic light
and lumen was passed through a clear corneal paracentesis and then through a small longitudinal opening in the trabecular meshwork to circumferentially viscodilate and intubate Schlemmʼs
canal ([Fig. 1 ]). The meshwork opening was made with a 24 G needle with a 20° bent tip ([Fig. 2 ]). Surgeon-controlled delivery of a
high-molecular weight OVD (Healon GV, Johnson & Johnson, New Brunswick, NJ, USA) during withdrawal of the microcatheter allowed for the compressed tissue planes of the trabecular
meshwork to separate, and any herniated inner wall tissue to withdraw from the collector channels (see [Video 1 ] and [2 ]). In combined
cases, phacoemulsification and IOL implantation were performed prior to the ABiC procedure. At the end of the procedure, the OVD was removed from the anterior chamber, and the pupil was
constricted using Miochol. A subconjunctival dose of gentamicin and dexamethasone was then applied.
Fig. 1 The iTrack microcatheter intubates and viscodilates the entire 360 degrees of Schlemmʼs canal. The red dot is the illuminated tip of the microcatheter in the trabecular
meshwork.
Fig. 2 The 24 G needle enters the trabecular meshwork with a 20° bent tip.
Video 1
The iTrack microcatheter intubates and viscodilates the entire 360 degrees of Schlemmʼs canal.
Video 2
The red dot is the illuminated tip of the microcatheter in the trabecular meshwork.
Postoperatively, patients received combined drops of gentamicin and dexamethasone 4 times daily for 1 week, followed by diclofenac drops 4 times daily for 1 month.
Outcomes
The primary efficacy outcomes were IOP and the number of glaucoma medications. Secondary efficacy outcomes were corrected distance visual acuity and visual field mean deviation. Safety data
was obtained by recording intraoperative and postoperative complications.
Statistics
Descriptive statistics (mean, standard deviation, and range) were used for IOP and number of medications at each visit. Comparative analysis between visits was performed using the
repeated-measures ANOVA test followed by a post hoc Tukey test for multiple comparisons and nonparametric Friedman test followed by Wilcoxon signed-rank test for multiple comparisons. A p
value of < 0.05 was considered statistically significant and p values are indicated where applicable.
Results
Demographics and baseline characteristics
A total of 27 eyes from 21 patients met eligibility criteria and were enrolled in the study. Eleven eyes presented pseudoexfoliation syndrome. Mean age was 77.3 ± 5.8 years, ranging from 66
to 90 years old. There were 11 females and 10 males.
Classification of glaucoma was performed with the Hodapp-Parrish grading system [18 ] as recommended by the European Glaucoma Society [19 ] and based on the MD measured using the Humphrey visual field scale. Baseline analysis showed that ten eyes (37.0%) were mild cases (< − 6 dB), nine eyes (33.3%) moderate cases
(from 6 dB to 12 dB), and four eyes (14.8%) severe cases (> − 12 dB). Baseline visual field data was not available for four eyes (14.8%).
The majority of eyes, 23 out of 27 eyes (85%), were treated with ABiC combined with phacoemulsification and 4 eyes (15%) were treated with ABiC as a standalone procedure. Nine patients were
lost to follow-up for the 48-month visit as they had returned to their referring ophthalmologist.
Primary outcomes
Intraocular pressure
Mean IOP at baseline and at each postoperative visit are presented in [Table 1 ] and [Fig. 3 ]. There was a statistically significant
reduction in IOP between baseline (19.85 ± 5.2 mmHg; n = 27) and all postoperative visits (p < 0.001). At the 12-month follow-up, mean IOP was 14.98 ± 2.6 mmHg, with no statistically
significant difference between all postoperative visits (p = 0.35) – 15.58 ± 3.3 mmHg at 24 months, 14.71 ± 3.8 mmHg at 36 months, and 14.56 ± 3.0 at 48 months (n = 18).
Table 1 Intraocular pressure (IOP) in mmHg at baseline and all postoperative visits.
All eyes
Baseline
12 M
24 M
36 M
48 M
Mean ± SD
19.85 ± 5.2
14.98 ± 2.6
15.58 ± 3.3
14.71 ± 3.8
14.56 ± 3.0
Min
13.00
10.00
8.00
7.00
10.00
Max
37.00
20.00
26.00
21.00
21.00
N (eyes)
27
26
25
21
18
Standalone
Baseline
12 M
24 M
36 M
48 M
23.5 ± 9.26
17.67 ± 2.3
17.25 ± 7.37
16.33 ± 4.73
17.67 ± 4.16
Combined with phaco
Baseline
12 M
24 M
36 M
48 M
19.22 ± 4.20
14.63 ± 2.51
15.27 ± 2.01
14.44 ± 3.73
13.93 ± 2.43
Fig. 3 Mean IOP (mmHg) at baseline and postoperative visits. IOP was statistically significantly reduced between baseline and all postoperative visits (p < 0.001). There was
no statistically significant difference in IOP between all postoperative visits (p = 0.35).
[Table 2 ] shows the percentage of eyes with at least a 25% reduction in IOP and the percentage of eyes with a postoperative IOP of 15 mmHg or 17 mmHg.
Table 2 Distribution of IOP success factors.
At 12 M
At 24 M
At 36 M
At 48 M
% of eyes with:
≥ 25% reduction in IOP
46.2%
40.0%
57.1%
50.0%
IOP ≤ 17 mmHg
80.8%
80.0%
71.4%
77.8%
IOP ≤ 15 mmHg
53.8%
48.0%
57.1%
66.7%
Glaucoma medications
The mean number of glaucoma medications is presented in [Table 3 ] and [Fig. 4 ] and decreased from a preoperative average number of
1.93 ± 1.00 (n = 27) to 0.89 ± 0.83 at 48 months (n = 18). This reduction was determined to be statistically significant at all postoperative visits (p < 0.05). The number of
medications was stable between 12 and 24 months, with no statistically significant difference in number of medications (p ≥ 0.135) but then increased between 12 and 36 months (p = 0.030)
and 48 months (p = 0.048).
Table 3 Number of medications at baseline and all postoperative visits.
Baseline
12 M
24 M
36 M
48 M
IQR: interquartile range
Mean ± SD
1.93 ± 1.00
0.30 ± 0.54
0.40 ± 0.64
0.80 ± 0.83
0.89 ± 0.83
Median
1.00
0.00
0.00
1.00
1.00
IQR
2.00
0.50
1.00
1.25
1.75
Min
0
0
0
0
0
Max
4
2
2
2
2
N (eyes)
27
27
25
20
18
Fig. 4 Mean number of medications at baseline and postoperative visits. Number of medications was statistically significantly reduced at all postoperative visits compared to
baseline (p < 0.001). The number of medications was stable between 12 and 48 months (p ≥ 0.135).
At 12 months, 26 out of 27 eyes (96.3%) were on 1 medication (n = 6) or 0 medications (n = 20). The percentage of eyes on 1 medication or less decreased over time: 92% (23 out of 25 eyes)
at 24 months, 75% at 36 months (15 out of 20 eyes), and 72.2% at 48 months (13 out of 18 eyes). All eyes were on two medications or less at all postoperative visits ([Fig. 5 ]).
Fig. 5 Distribution of medications at all visits.
Standalone vs. combined with cataract surgery
Twenty-three eyes received ABiC combined with cataract surgery and four eyes as a standalone procedure. In both groups, there was a reduction in IOP and medication burden at each time point
compared to baseline.
The combined group had a baseline IOP (mmHg) of 19.22 ± 4.2, which decreased to 14.63 ± 2.5, 15.27 ± 2, 14.44 ± 3.7, and 13.93 ± 2.4 at 12, 24, 36, and 48 months, respectively, with a net
decrease of 5.28 mmHg. Medication use was reduced from 1.9 ± 1 at baseline to 0.3 ± 0.6, 0.4 ± 0.7, 0.9 ± 0.9 and 0.9 ± 0.8 at 12, 24, 36, and 48 months, respectively.
For eyes in the standalone group, IOP was reduced from a baseline value of 23.5 ± 9 to 17.67 ± 2.3, 17.25 ± 7.4, 16.33 ± 4.7 and 17.67 ± 4.1 (n = 3) at 12, 24, 36, and 48 months,
respectively, with a net decrease of 5.83 mmHg ([Table 1 ]). Medication use was reduced from 2 ± 0.8 at baseline to 0.3 ± 0.5, 0.3 ± 0.7, 0.3 ± 0.7 and 1 ± 0.9 at
12, 24, 36, and 48 months, respectively. One of the 4 eyes in the standalone group presented with pseudoexfoliation syndrome and had a preoperative IOP of 37 mmHg, which reduced to 21 mmHg
at 48 months. The other 2 eyes in this groups that reached the 48-month follow-up showed a reduction in IOP of a minimum of 3 mmHg at 48 months.
Reducing the medication burden in cases of controlled glaucoma
Patients who had a baseline IOP equal to or lower than 18 mmHg recorded a mean number of medications of 1.77 ± 0.93 at baseline. In this group, the number of medications administered in the
postoperative phase was reduced to 0.83 ± 0.98 after 48 months ([Table 4 ]). In addition, at 48 months, 3 of the 6 eyes in this group were on 0 medications
(preoperatively they were on 2, 2, and 1 medication, respectively). Eyes that were uncontrolled with medications at baseline (IOP higher than 18 mmHg) also experienced a significant
reduction in the number of medications and 4 eyes out of 12 eyes in this group were on 0 medications (33%) at 48 months. In total, 7 eyes out of 18 (39%) were on 0 medications at 48
months.
Table 4 Number of medications for group ≤ 18 mmHg and > 18 mmHg.
Number of medications ≤ 18 mmHg and > 18 mmHg
Baseline
12 M
24 M
36 M
48 M
IQR: interquartile range
IOP ≤ 18 mmHg
Mean ± SD
1.77 ± 0.93
0.23 ± 0.44
0.27 ± 0.47
0.88 ± 0.99
0.83 ± 0.98
p = 0.85 between the 2 groups at 48 months
Median
1.00
0.00
0.00
0.50
0.50
IQR
2.00
0.00
0.50
2.00
1.75
N (eyes)
13
13
11
8
6
IOP > 18 mmHg
Mean ± SD
2.07 ± 1.07
0.36 ± 0.63
0.50 ± 0.76
0.75 ± 0.75
0.92 ± 0.79
Median
1.00
0.00
0.00
1.00
1.00
IQR
1.75
0.75
1.00
1.00
1.25
N (eyes)
14
14
14
12
12
All 9 eyes but 1 that were lost between the 12- and 48-month visits were on 0 medications at 12 or 24 months.
Controlled intraocular pressure pre- and postoperatively
About half of the eyes (n = 13) were defined as successfully controlled with medications at baseline, with an IOP equal to or less than 18 mmHg. In these eyes, ABiC was performed to reduce
patient reliance on medications due to intolerance to medications.
Out of the 27 eyes of the total cohort, 4 eyes had an IOP higher than 18 mmHg for 2 consecutive follow-up visits: 1 eye was below 18 mmHg at baseline, then IOP increased at 24 – 36 months
but decreased to 14 mmHg at 48 months; the remaining 3 eyes were above 18 mmHg and had an IOP above target for 2 consecutive follow-up visits. However, the IOP of 1 of those 3 eyes started
at 37 mmHg at baseline and fell to 21 mmHg at 48 months (PEX).
[Fig. 6 ] describes the cumulative probability of failure defined as an IOP higher than the target of 18 mmHg on consecutive visits as defined in Lewis et al.
[20 ].
Fig. 6 Kaplan-Meier plot of the cumulative probability of failure for (a ) all eyes and for (b ) eyes that had a preoperative IOP above target (18 mmHg). Failure was
defined as an IOP higher than 18 mmHg on 2 consecutive visits (Lewis, 2011).
Visual Acuity
There was a statistically significant improvement in CDVA between baseline and the last postoperative visit (p < 0.001). The mean logMAR CDVA was 0.36 ± 0.37 (20/45 Snellen) at baseline
and 0.15 ± 0.31 (between 20/25 and 20/32 Snellen) postoperatively, representing an average gain of 2 lines of visual acuity.
Visual Field
The mean deviation recorded from the Humphrey visual field was not statistically significantly different between baseline and the last postoperative visit (p = 0.634), indicating that there
was no deterioration of the visual field. Mean baseline was − 7.86 ± 6.90 dB and mean postoperative was − 6.95 ± 5.72 dB.
All eyes except one showed no change or improvement. One eye demonstrated a deterioration of the visual field. In this patient, IOP was lowered from 20 mmHg to 13 mmHg at 12 months, but
then increased to 15 and 17 mmHg at subsequent follow-ups, whilst gaining 3 lines of visual acuity at the last follow-up.
Safety Outcomes
No secondary surgical interventions were required in order to control IOP in any of the eyes. No significant complications were noted, except for a single case of limited descemetolysis
near the limbus. This was likely caused by a slower withdrawal of the catheter with a subsequent local over-delivery of the OVD in the canal that resulted in a detachment of the Descemet
membrane. This resolved spontaneously after 6 weeks, with no sequalae and a stable IOP. No other adverse events were reported.
Discussion
In this retrospective case series study, ABiC performed as a standalone procedure or in conjunction with cataract surgery was found to be a safe and effective option for maintaining a
sustained IOP reduction in patients with OAG up to 4 years, from 19.8 ± 5.2 (n = 27) mmHg at baseline to 14.6 ± 3.0 mmHg (n = 18) at the 48-month follow-up.
It was also effective in reducing glaucoma medication dependence, with approximately 39% of eyes medication free and 72% of the eyes on 1 medication or less at 4 years. Overall, no eye was on
more than two medications 48 months after the procedure. About half of the eyes (n = 13) were defined as successfully controlled with medications at baseline, with an IOP equal to or less than
18 mmHg. In these eyes, ABiC was performed to reduce patient reliance on medications due to intolerance of the medications. At 48 months, 50% of the eyes in this group were medication free and
the 2 eyes that were missed between the 36 – 48-month follow-up were medication free at 36 months.
All patients who were lost to follow-up returned to their referring ophthalmologist because they showed no complications, and the overall outcome was satisfying; all but one patient was
medication free at their last (12, 24, or 36 month) follow-up. Our conclusion is that ABiC appears effective in patients with controlled glaucoma who wish to reduce the reliance on medications
due to intolerance or noncompliance.
The cohort also achieved an average gain of two lines of CDVA. Whilst we assume that this improvement is largely due to cataract removal, none of eyes lost any line of visual acuity, proving
that ABiC does not jeopardize visual acuity.
Our clinical outcomes are in accordance with those reported in other studies. A consecutive case series by Gallardo et al. demonstrated comparable clinical efficacy and safety of the same
ABiC device (iTrack) over a 12-month period [16 ]. Another case series by Gallardo, which included a 24-month follow-up, also showed similar results: at 24 months
(n = 60), the mean IOP and baseline medications were reduced from 20 ± 4.9 mmHg and 2.77 ± 0.91 to 13.5 ± 2.6 mmHg and 1.7 ± 1.29, respectively. Twenty-five percent of eyes were medication
free and 47% of patients were on one or fewer medications at 24 months compared to only 10% at baseline [21 ].
In another study, Kazerounian et al. observed a similar substantial and sustained lowering of IOP, which was comparable with our cohort [17 ] ([Table 5 ]).
Table 5 Results at 12 months for different MIGS studies in the literature.
Publication
IOP baseline (in mmHg)
IOP 12 months (in mmHg)
Number of medications baseline
Number of medications 12 months
*At 24 months. ** MIGS with cataract surgery results only.
Our study (n = 27) (ABiC)
19.8 ± 5.2
15.0 ± 2.6
1.92 ± 1.00
0.30 ± 0.54
Gallardo et al. [16 ] (n = 75) (ABiC)
20.4 ± 4.7
13.3 ± 1.9
2.8 ± 0.9
1.1 ± 1.1
Kazerounian et al. [17 ] (n = 25) (ABiC)
20.24 ± 5.92
12.55 ± 3.33
1.92 ± 1.04
0.05 ± 0.23*
Pahlitzsch et al. [25 ] (n = 130) (Trabectome)
19.2 ± 4.0
11.8 ± 3.1
2.3 ± 0.8
2.3 ± 1.4
Fea [26 ] (n = 12) (iStent)
17.9 ± 2.6
14.8 ± 1.2
2.0 ± 0.9
0.4 ± 0.7
Hengerer et al. [27 ] (n = 81) (iStent)
22.6 ± 6.2
14.3 ± 1.7
2.5 ± 1.1
0.8 ± 0.9
Pfeiffer et al. [24 ] (n = 50)** (Hydrus)
18.9 ± 3.3
16.1 ± 3
2.0 ± 1.0
0.5 ± 1.1
We therefore conclude that the significant reduction in IOP at 12 months that we observed in our study is comparable with other 12-month studies. This IOP reduction was maintained in our
study at 48 months with no significant change from the 12-month follow-up (p = 0.35), indicating that the IOP outcomes were stable over time. We also observed a significant reduction of
medications at 12 – 24 – 36 – 48 months, which was in line with two of the three studies mentioned, although after 24 months, the analysis showed an increase in the number of medications
between 12 and 36 months and a borderline (p = 0.048) increase between 12 and 48 months.
In our study, we were able to demonstrate that the efficacy of ABiC and the reduction in IOP was maintained up to 4 years after the procedure. This is an encouraging result and, to the
authorsʼ knowledge, the longest follow-up available to date for the ABiC procedure performed with the iTrack microcatheter.
Other licensed MIGS that specifically target the conventional outflow pathway include the Trabectome (NeoMedix Corporation, Tustin, CA, USA), the iStent (Glaukos, Burlington, MA, USA) and the
Hydrus Microstent (Ivantis, Inc., Irvine, CA, USA) [22 ], [23 ], [24 ]. Pahlitzsch et al. showed a mean IOP
reduction for Trabectome [25 ] that was comparable with our current series. Fea [26 ] and Hengerer et al. [27 ]
found similar results using the iStent. The study by Pfeiffer et al. [3 ], [24 ] also yielded comparable results implanting the Hydrus in
conjunction with cataract surgery (see [Table 5 ]). In contrast, these procedures are focal in their approach, whereas ABiC addresses all aspects of the conventional
outflow system without leaving an implant in place.
It is of particular importance to understand the effect of cataract surgery on IOP. A recent study by Samuelson et al. [28 ] estimated cataract surgery alone was
able to lower IOP 5.4 mmHg at 24 months but other longer-term (3 – 5 years) studies conducted in relevant patient populations, such as mild and moderate OAG patients, suggest a more modest
long-term IOP reduction of 1 – 2 mmHg [29 ], [30 ], including a dedicated study by Shingleton et al. who reported an IOP decrease of
1.8 ± 3.5 mmHg at 5 years [31 ] and Kozera et al. who found an IOP decrease of 0.90 mmHg and 0.43 mmHg at 24 months; groups were divided according to the initial IOP
(after the washout period) into IOP < 26 mmHg and IOP ≥ 26 mmHg [32 ].
The patient cohort in this study demonstrated a net IOP decrease of 5.3 mmHg at 48 months for all eyes, a decrease of 5.28 mmHg for eyes with cataract surgery, and a decrease of 5.83 mmHg for
standalone eyes. In addition, all eyes in the standalone group were able to reduce IOP of at least 3 mmHg at 48 months. This suggests an additional IOP benefit of ABiC above that expected for
phacoemulsification alone, especially when we consider that approximately half of the eyes (n = 13) were defined as successfully controlled with medications at baseline. For these patients,
the primary treatment goal was to reduce medication dependence whilst maintaining IOP within the target range. Gallardo [21 ] as well as previous studies of patients
undergoing ABiC have demonstrated similar IOP reduction in eyes treated with either a standalone ABiC procedure or combined procedure, which recommends that ABiC offers utility beyond the
reduction of IOP [16 ], [33 ], [34 ]. Nevertheless, in order to assess the IOP lowering effect of ABiC outside
of cataract surgery, a study with a sufficient number of patients would be required.
In this current series, no severe complications occurred either intraoperatively or postoperatively, although the study was underpowered to identify low-risk complications. In one patient, a
limited descemetolysis near the limbus was observed, likely due to an over-delivery of viscoelastic during a slower withdrawal of the catheter through Schlemmʼs canal. A potential complication
that was not observed in our case series, but which occurs in approximately 5% of the cases [35 ], is where the microcatheter can divert into a collector channel.
This complication can be managed by making a paracentesis 180° away and catheterizing in the opposite direction in order to achieve a successful 360° viscodilation. In our experience, we found
that, because the bulbous tip of the microcatheter is atraumatic, there were no instances of choroidal damage. The illuminated LED tip allows the surgeon to track the microcatheter as it
passes through the full length of Schlemmʼs canal and prevents misdirection into the collector channels or suprachoroidal space. Furthermore, temporary misdirection of the microcatheter may be
resolved by applying pressure to the affected area or again, by making a paracentesis 180° away from the site of the misdirection and catheterizing in the opposite direction.
The limitations and potential biases of this case series include the lack of randomization, a small sample size, its retrospective nature, and the inclusion in the same group of eyes treated
with combined cataract surgery as well as eyes treated with a standalone procedure.
Further analysis in a larger number of eyes will be needed to confirm the findings of this study. However, our findings indicate that the efficacy of ABiC in lowering IOP and reducing
medication dependency was comparable to previous 12-month ABiC studies, and IOP reduction was sustained up to 4 years following surgery.
Despite its efficacy and safety profile, these results do not suggest that ABiC should supplant ab-externo canaloplasty or traditional glaucoma surgery. Traditional ab-externo canaloplasty
has a rich history, particularly in Germany, with more advanced glaucoma cases, and some studies have suggested that its efficacy is comparable with trabeculectomy [36 ].
Rather, we recommend the deployment of ABiC earlier in the treatment paradigm of mild to moderate glaucoma patients in order to delay more invasive treatments.
Further studies that employ a larger case series are required to assess its efficacy and to investigate patient-center outcomes. A prospective, multicenter, randomized, single-masked clinical
trial to evaluate the effective outcomes of ABiC, with iTrack performed as a standalone procedure, is currently under way in the United States [37 ].
Conclusion
This 48-month data showed that ABiC performed in combination with phacoemulsification or as a standalone procedure results in a significant reduction in IOP compared to the preoperative
baseline along with a significant decrease in mean glaucoma medications. Our results showed that performing ABiC with the iTrack is safe and effective in patients with OAG up to 4 years.
Further studies with prospective multicenter trials with a larger patient cohort are needed to evaluate the outcomes of ABiC as a standalone procedure and combined with cataract surgery.
