Intraluminal dilation of Schlemm’s canal has been a longstanding surgical approach to increase aqueous outflow through the trabecular meshwork/canalicular pathway. Early approaches, such as viscocanalostomy, involved the injection of a viscoelastic agent to dilate the canal, with ab externo techniques showing modest efficacy and limited clinical adoption.

Dilation combined with secondary implantable reinforcement of the canal (eg, canaloplasty with a device or stent) represents a further advance aimed at achieving more sustained canal expansion through durable structural support. Originally designed as an ab externo technique, canaloplasty as a surgical treatment for glaucoma was first developed around 2008 and has continued to evolve.

Traditional canaloplasty involves transluminal dilation of Schlemm’s canal with a microcatheter, followed by placement of permanent scaffolding or reinforcement material (eg, suture) to physically expand the canal and facilitate aqueous outflow. The original technique described by Robert Stegmann involved microcatheter insertion circumferentially up to 360°.¹ The microcatheter, with an outer diameter of 200 µm, can significantly dilate an atrophied canal, which in some cases may collapse to less than 120 µm in diameter in patients with open-angle glaucoma.^2,3 In some cases, the canal may collapse completely as the trabecular meshwork herniates into the collector channels.⁴ After initial dilation, a reinforcement suture or other scaffolding device is placed to maintain durable canal expansion.

As many ophthalmic procedures are evolving to less invasive approaches, ab externo canaloplasty is also transitioning to an ab interno, bleb-free, conjunctival- sparing approach. Ab interno canaloplasty has demonstrated similar efficacy with a more favorable microinterventional safety profile.⁵ Canaloplasty, using either suture or other scaffolding materials, appears to provide greater and more sustained intraocular pressure (IOP) reduction compared with viscoelastic-only dilation (eg, viscocanalostomy), in which no permanent reinforcement is present; this has been supported by a recent meta-analysis.⁶

Billing for Canaloplasty

The CPT codes for canaloplasty with and without a device or stent are as follows:

• 66174 Transluminal dilation of aqueous outflow canal; without retention of device or stent
• 66175 Transluminal dilation of aqueous outflow canal (eg, canaloplasty); with retention of device or stent.

According to the RUC database procedure description for CPT code 66175, the essential elements of the procedure include the following steps:

• Mechanical circumferential dilation of the canal up to 360°, with or without adjunctive viscoelastic; and
• Placement of a permanent stenting or scaffolding material within the canal to provide expansile tension.

CPT code 66175 was created in 2011 and has had relatively low utilization, because canaloplasty techniques have continued to evolve and ab interno interventional approaches have come into greater focus. Although still a low-volume procedure, there has been renewed interest in recent years, particularly with the availability of different implantable scaffolding materials for intraluminal dilation.⁵

Ab Interno Canaloplasty Procedures

Newer ab interno approaches are bringing canaloplasty into the interventional era. An emerging variation is advancing clinically and may become available in the United States, using nitinol-based intraluminal mechanical dilation of Schlemm’s canal up to 360°, followed by biologic tissue–based intracanalicular scaffolding to maintain durable aqueous outflow. These ab interno approaches include all essential elements of CPT code 66175, with key procedural steps as follows:

1. Ab interno insertion and advancement of a flexible nitinol microfilament through the trabecular meshwork into Schlemm’s canal, achieving dilation up to 360°, but no less than 180°;
2. Following dilation, implantation of a bioexpandable allograft scaffold to permanently increase canal lumen diameter;
3. Retention of the implant within the canal to provide expansile tension, facilitating aqueous outflow through the trabecular meshwork into the canal and distal outflow channels, resulting in reduced IOP.

Conclusion

Advances in ab interno canaloplasty techniques, including the implantation of novel canal scaffolding materials, show promise for optimizing trabecular outflow in patients with glaucoma. These approaches may represent a meaningful addition to the glaucoma surgical armamen-tarium. GP

Nathan M. Radcliffe, MD, is a clinical associate professor of ophthalmology at the New York Eye and Ear Infirmary of Mount Sinai and a surgeon at the New York Eye Surgery Center. He is the editor in chief of Glaucoma Physician. He did not disclose any relevant conflicts of interest for this article.

Cynthia Mattox, MD, is an emeritus associate professor and glaucoma surgeon at Tufts University School of Medicine in Boston. She did not disclose any relevant conflicts of interest for this article.

References

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3. Chen Z, Sun J, Li M, et al. Effect of age on the morphologies of the human Schlemm’s canal and trabecular meshwork measured with swept source optical coherence tomography. Eye (Lond). 2018;32(10):1621-1628. doi:10.1038/s41433-018-0148-6

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