While the United States and Europe see a higher prevalence of POAG, in Asia the incidence of PACG is much greater and needs a prompt response [3, 15]. Currently, the traditional methods to treat PACG include laser iridotomy and trabeculectomy. Laser iridotomy may be effective in reducing the risk of acute attacks but it does not address the underlying cause of the disease. Trabeculectomy is effective but is also an invasive procedure that creates a new drainage pathway with a higher risk of complications such as hypotony, bleb-related infections, and cataract formation [16].

On the other hand, the on-label recommendations for MIGS do not include PACG and this has prevented many glaucoma surgeons from performing these procedures in PACG patients. A major reason why canaloplasty was thought to be ineffective in treating primary angle-closure glaucoma was the lack of physical access to the trabecular meshwork, caused by a narrowing of the angle between the iris and the cornea. Furthermore, angle closure can prevent the aqueous in the anterior chamber from reaching the canal, theoretically making canaloplasty an ineffective treatment option.

Recently, this thinking seems to be changing. It is true that canaloplasty is a MIGS that was specifically designed for POAG. The entrance of the microcatheter, its insertion, and circumnavigation seem a perfect fit for an open angle, but the diagnosis of open angle versus angle closure is not a clear line, rather it is a nuanced line in which a closed angle may be open just enough to allow the procedure to be performed. This allows enough maneuverability to incorporate canaloplasty via an ab-interno surgical technique in angle closure glaucoma patients, especially those with at least 180 degrees of continuous/non-continuous angle open. Alternatively, another method to incorporate canaloplasty for angle closure patients can be achieved by performing it in conjunction with phacoemulsification or in pseudophakic eyes. Indeed, extracting the lens may cause a significant deepening of the anterior chamber thus resolving the angle [17], ensuring both visibility of and access to the angle structures in PACG patients.

One reason why canaloplasty can work in PACG is that it addresses the underlying mechanism of the disease, that is, the impaired outflow of aqueous humor, which is now considered as the primary cause of increasing IOP, not only in POAG but in PACG cases. Histopathological studies have also confirmed that PACG and POAG share similar pathophysiological mechanisms [18], which provides a theoretical basis for the use of canaloplasty via an ab-interno surgical technique in PACG. Research has shown that continuous contact between the iris and trabecular meshwork and the gradual development of peripheral anterior synechia (PAS) can lead to damage of the TM-SC in PACG over time. These mainly consist of (1) interference with the blood-aqueous barrier, which affects the functioning of TM cells; (2) mitochondrial dysfunction in trabecular cells, which leads to TM fibrosis; (3) damage to the endothelium of the Schlemm's canal (SC), which causes a decrease in its diameter; and (4) mechanical blockage of the canal by pigment granules and trabecular endothelial cells [19]. By focusing on the TM-SC dynamic, canaloplasty can help alleviate the resistance of aqueous outflow.

The volume of OVD delivered into Schlemm’s canal was found to be positively correlated with the reduction of the number of medications: a higher volume of OVD delivered in the canal was associated with a larger reduction of the number of medications. In addition, in the eyes that were IOP uncontrolled at baseline (above 18 mmHg), the reduction of medications was associated with an IOP reduction, meaning that canaloplasty was able to lower both IOP and medications at the same time and that medication reduction was correlated with the OVD volume delivered in Schlemm’s canal.

Despite being off-label, MIGS has been attempted in PACG, often in combination with cataract surgery. Wang et al. evaluated 22 eyes of 20 PACG subjects for efficacy and safety of cataract surgery combined with goniosynechialysis and Trabectome at 12 months: IOP decreased from 22.07 ± 6.62 mmHg at baseline to 15.06 ± 3.39 mmHg (p = 0.001) and the number of antiglaucoma medications decreased from 2.68 ± 1.17 to 0.78 ± 0.73 (p < 0.01) [20]. Bussel et al. reported a 24% reduction in the mean IOP and 0.8 fewer medications after cataract surgery combined with Trabectome in patients with narrow angles [17].

Studies focusing on the use of iStent in PACG showed that cataract surgery combined with iStent implantation showed significantly greater reductions in IOP and the number medications than cataract surgery alone. In a recent matched cohort study comparing IOP reduction at one year following cataract surgery versus cataract surgery and iStent implantation in 158 eyes with PACG, mean IOP decreased by 13% in the cataract surgery group and by 27% in the cataract surgery and iStent implantation group [21]. In another study, Chansangpetch et al. showed that cataract surgery and iStent implantation, with respect to cataract surgery alone, significantly improved the complete surgical success (IOP < 18 mmHg without glaucoma medications) in patients with PACG after iridotomy. It also reduced the number of antiglaucoma medications [4]. In a randomized controlled trial, Chen et al. reported that cataract surgery with iStent implantation had a higher success rate (87.5%) than cataract surgery alone (43.8%) in 32 patients with PAC/PACG and cataract [22]. Another few studies have reported similar results using a stent in PACG [23, 24]. Despite their use being “off-label”, the scientific literature reports the potential of MIGS in reducing IOP and number medications.

To the author’s knowledge, in the scientific literature the use of canaloplasty via an ab-interno surgical technique [7] has not been reported in PACG eyes. The scientific literature does report on the use of canaloplasty performed via an ab-externo surgical technique in PACG eyes [25,26,27], with good results—not surprisingly, these studies have been conducted in Asia where PACG is more frequent. However, canaloplasty via an ab-interno surgical technique is less invasive and preserves conjunctiva [28].

We decided to perform canaloplasty via an ab-interno surgical technique in PACG patients considering that it targets the root cause of IOP elevation (which is the same for both POAG and PACG) and works to enhance the natural physiological outflow of aqueous humor, instead of creating an artificial window for fluid drainage. Our data confirms canaloplasty is effective: the mean IOP reduced by 33.3% at latest postoperative follow-up (average 26 months), dropping from 21.9 ± 7.3 mmHg to 14.6 ± 3.73 mmHg.

However, since cataract surgery has also been proposed to be an effective treatment for PACG [29,30,31], we decided to observe the outcomes in those eyes that received canaloplasty alone versus those eyes in which canaloplasty was combined with phacoemulsification. It appears that the iTrack-alone group, despite containing only 9 eyes, achieved a reduction in mean IOP and mean number of medications comparable to the iTrack + phaco group (n = 51).

The IOP reduction was also observed in those eyes that were IOP uncontrolled at baseline as well as across all glaucoma severities. The medications reduction was significant either in uncontrolled and controlled eyes but only in mild glaucoma eyes and not in moderate and severe glaucoma eyes.

This effectiveness recommends that canaloplasty, both alone as well as combined with phacoemulsification, can be a good surgical modality not only for POAG, but for PACG patients as well, offering a safe and effective alternative to traditional methods of treatment, with fewer complications and a quicker recovery time [5, 8, 9]. It can be performed on pseudophakic eyes, otherwise, whenever possible, it is advisable to remove the lens to widen the angle. Furthermore, as it is conjunctiva sparing, it allows for a further surgical intervention, such as trabeculectomy or valve implantation, to be performed at a later date, as required.

Limitations of our study include the small sample size, the loss at follow-up, and the retrospective nature of the study. A prospective study with a larger sample size and the inclusion of wider range of ethnic groups (especially Asian PACG patients) would further underline the usefulness of this canaloplasty via an ab-interno surgical technique in PACG patients.In conclusion, canaloplasty via an ab-interno surgical technique represents a new treatment paradigm for primary angle-closure glaucoma. Its ability to address the underlying cause of the disease, restore the natural flow of aqueous humor through the conventional outflow pathway, and provide long-term benefit makes it a promising option for those patients.