Canaloplasty (Schlemm's Canal Surgery)

Key Points at a Glance

Highlights of This Disease

• Canaloplasty is a non-penetrating glaucoma surgery that dilates Schlemm’s canal with a microcatheter to restore physiological aqueous outflow.
• The conventional ab externo approach reduces intraocular pressure by 30–42%, and up to 47% when combined with cataract surgery.
• The ab interno approach (ABiC) does not require conjunctival or scleral incisions, is highly minimally invasive, and is classified as MIGS (minimally invasive glaucoma surgery).
• Compared to trabeculectomy, the intraocular pressure reduction is inferior, but complications related to hypotony and filtering blebs are significantly fewer.
• In the VENICE trial (the first RCT), the 6-month outcomes of STREAMLINE canaloplasty and iStent inject W were equivalent.
• The main complication is transient anterior chamber hemorrhage (microhyphema), which usually resolves spontaneously.

1. What is Canaloplasty?

Canaloplasty is a non-penetrating glaucoma surgery that uses a flexible microcatheter to dilate Schlemm’s canal (SC) 360 degrees, promoting drainage through the physiological aqueous outflow pathway ¹⁾.

While trabeculectomy creates a new outflow pathway (fistula) from the anterior chamber to the subconjunctival space, canaloplasty restores the physiological outflow pathway: trabecular meshwork → Schlemm’s canal → collector channels → aqueous veins ¹⁾⁸⁾. Because aqueous humor does not drain below episcleral venous pressure, hypotony and its related complications are less likely to occur.

Canaloplasty is a modification of viscocanalostomy. In viscocanalostomy, a metal cannula is used to dilate only a limited portion of Schlemm’s canal, whereas canaloplasty uses a flexible microcatheter to dilate the entire length of the canal ⁸⁾.

According to the EGS guidelines, canaloplasty is classified as a non-penetrating glaucoma surgery along with deep sclerectomy and viscocanalostomy ¹⁰⁾. The AAO PPP describes it as a procedure characterized by circumferential viscoelastic dilation of Schlemm’s canal and placement of an intraluminal tension suture ⁹⁾.

Q What is the fundamental difference between canaloplasty and trabeculectomy?

A

Trabeculectomy creates a fistula to the subconjunctival space to bypass aqueous humor. Canaloplasty dilates Schlemm’s canal to restore the physiological outflow pathway and does not form a filtering bleb ¹⁾. Therefore, complications related to hypotony and filtering blebs are significantly fewer, but the achievable intraocular pressure is generally higher than with trabeculectomy.

2. Main Symptoms and Clinical Findings

Subjective Symptoms

Canaloplasty is indicated for patients with open-angle glaucoma (OAG) who present with the following symptoms and conditions.

• Visual field defect: Progressive glaucomatous visual field loss
• Eye pain/headache: May be associated with high intraocular pressure
• Intolerance to eye drops: Due to side effects or poor adherence

Clinical Findings

The following findings should be evaluated to determine surgical indication.

• Intraocular pressure: Surgery is indicated when target pressure is not achieved with medication or when eye drops are not tolerated
• Angle findings: Gonioscopy must show open angle of Shaffer grade 3–4. Closed angle is an absolute contraindication
• Optic disc: The degree of cupping and RNFL thinning determines the timing of surgery
• Visual field test: Surgery is considered when progressive visual field loss is confirmed

3. Causes and Risk Factors

Indications and contraindications for canaloplasty are as follows.

Indications

• Open-angle glaucoma (including primary open-angle glaucoma [POAG], exfoliation glaucoma, pigmentary glaucoma, and steroid glaucoma) ¹⁾⁸⁾
• When target intraocular pressure is not achieved with medication or laser therapy
• Young patients with clear lens (low risk of cataract progression)
• High myopia (when trabeculectomy carries a high risk of hypotony)
• Aphakic eyes (when vitreous may block the fistula)
• Failed trabeculectomy cases ²⁾

Contraindications

• Absolute contraindication: Angle-closure glaucoma ¹⁾
• Relative contraindications: Congenital glaucoma, angle recession, neovascular glaucoma, elevated episcleral venous pressure ¹⁾

Expanding indications

In recent years, the indications for canaloplasty have expanded beyond mild to moderate glaucoma to include severe glaucoma ¹⁾²⁾. Particularly good outcomes have been reported for exfoliation glaucoma with canaloplasty ²⁾.

4. Surgical technique

Ab externo approach (conventional method)

In the ab externo approach, after conjunctival incision and creation of a scleral flap, Schlemm’s canal is exposed and a microcatheter (iTrack, Nova Eye Medical) is inserted 360° ¹⁾⁸⁾.

The main steps are as follows:

1. Perform a conjunctival incision at the limbal base.
2. Create a 5×5 mm superficial scleral flap.
3. Create a 4×4 mm deep scleral flap to expose the trabecular meshwork and Descemet’s window.
4. Identify Schlemm’s canal and insert a microcatheter with an optical fiber 360°.
5. Inject viscoelastic material (OVD) during catheter withdrawal to dilate Schlemm’s canal.
6. Place a 10-0 polypropylene (Prolene) suture inside the canal to apply tension to the trabecular meshwork.
7. Close the scleral flap in a watertight manner (to avoid bleb formation).

The intraluminal tension suture has a dual effect of maintaining Schlemm’s canal patency and applying tension to the trabecular meshwork ⁹⁾.

Ab interno method (ABiC)

Ab interno canaloplasty (ABiC) is a minimally invasive technique that does not require conjunctival or scleral incisions ¹⁾¹⁰⁾.

• Approach via a clear corneal incision under gonioscopy.
• Insert a microcatheter through a limited goniotomy and inject OVD while withdrawing.
• No tension suture is used.
• Classified as MIGS (minimally invasive glaucoma surgery) ⁵⁾.

OMNI system

The OMNI Surgical System is a technique that allows canaloplasty (Schlemm’s canal viscodilation) and trabeculotomy to be performed with a single device ¹⁾⁴⁾. In addition to 360° catheter insertion and viscodilation, it simultaneously performs a 180° trabeculotomy.

STREAMLINE System

The STREAMLINE Surgical System (FDA approved in 2021) is an ab interno method that punctures the trabecular meshwork and injects OVD directly into Schlemm’s canal ³⁾⁶⁾. It dilates Schlemm’s canal segmentally with 3 to 8 injections (approximately 7 µL each).

Q Which is better, the ab externo method or the ab interno method?

A

The ab externo method allows placement of an intraluminal tension suture, which is expected to maintain long-term patency of Schlemm’s canal, but requires conjunctival and scleral incisions. The ab interno method (ABiC) is a minimally invasive technique that preserves the conjunctiva and is classified as MIGS, but does not use a tension suture ¹⁾⁵⁾. A study comparing three modified techniques reported no significant difference in intraocular pressure reduction ⁵⁾.

5. Standard Treatment Outcomes

Schlemm's canal cannulation in canaloplasty

ScientificWorldJournal. 2014;2014:469609. Figure 4. PMCID: PMC3915493. License: CC BY.

Intraoperative image showing cannulation of Schlemm’s canal while confirming the tip of the microcatheter through the sclera. It succinctly illustrates the key maneuver of canaloplasty.

Intraocular Pressure Lowering Effect

The intraocular pressure reduction rate of canaloplasty varies depending on the surgical technique and baseline IOP, but is generally 30–42% ¹⁾⁸⁾.

The main study results are summarized below.

Study/Surgery	IOP Reduction Rate	Final IOP
Ab externo alone (3 years)	34%	15.1 mmHg
Ab externo + cataract (3 years)	43%	13.8 mmHg

Bull et al. (2011) reported in a multicenter trial of 109 eyes that IOP decreased from 23.0 to 15.1 mmHg (-34.3%) with ab externo canaloplasty alone, and from 24.3 to 13.8 mmHg (-43.2%) with combined cataract surgery⁵⁾⁷⁾.

In the GEMINI study (Gallardo et al. 2022), 120 eyes treated with the OMNI system plus cataract surgery showed a reduction in medication-free IOP from 23.8 to 15.6 mmHg (-34%) at 12 months, and 80% were medication-free⁴⁾.

Lazcano-Gomez et al. (2024) reported a decrease from 23.0 to 15.3 mmHg (-33.5%) at 12 months in 40 eyes undergoing STREAMLINE plus cataract surgery, with 70% being medication-free⁶⁾.

Comparison with trabeculectomy

In the TVC study (Matlach et al. 2015, RCT, 62 eyes), the 2-year IOP was 14.4 mmHg with canaloplasty and 10.8 mmHg with trabeculectomy²⁾⁵⁾. However, the trabeculectomy group had high complication rates: transient hypotony 37.5%, hypotony lasting ≥90 days 18.8%, and choroidal detachment 12.5%. The long-term hypotony rate with canaloplasty was less than 1%²⁾.

Comparison with iStent inject W (VENICE study)

Goldberg et al. (2024, VENICE study) conducted the first RCT comparing STREAMLINE (35 eyes) and iStent inject W (37 eyes) combined with cataract surgery³⁾. At 6 months, IOP was 16.5 mmHg with STREAMLINE vs. 16.1 mmHg with iStent inject W (no significant difference, P=0.596). Medication-free rates were 81.8% and 78.4%, respectively³⁾.

Modified technique: Suprachoroidal drainage (ScD)

Szurman (2023) reported a comparative study of 417 eyes in which canaloplasty plus suprachoroidal drainage (ScD) achieved a 35.9% IOP reduction (20.9 to 13.1 mmHg), surpassing the 31.2% reduction (20.8 to 14.0 mmHg) with the conventional method²⁾. Combined cataract surgery plus ScD reached a 47.4% IOP reduction (23.2 to 12.2 mmHg), for the first time yielding results comparable to successful trabeculectomy²⁾.

Complications

The main complications are as follows¹⁾⁵⁾⁷⁾.

• Microhyphema (microscopic anterior chamber hemorrhage): Most frequent (1.6–12.8%). Most resolve spontaneously within 1 month.
• Descemet membrane detachment: 1.6–6.1%. Caused by excessive injection of viscoelastic material.
• Transient intraocular pressure elevation (>30 mmHg): 1.6–8.7%.
• Hypotony: Less than 1%. Extremely rare compared to trabeculectomy (18.8%)²⁾.

Postoperative management and attention to complications

Postoperatively, antibiotics are given for 1 week and topical steroids are tapered over 4 weeks. If the target intraocular pressure is not reached by 1 month postoperatively, conversion to full-thickness filtration surgery is possible with YAG goniopuncture or needling (in the ab externo method). Corneal endothelial cell density has been reported to decrease to a similar extent as with cataract surgery alone.

Q Is canaloplasty effective for severe glaucoma?

A

Traditionally, mild to moderate glaucoma was the main indication, but recent reports indicate that even in severe glaucoma, a similar intraocular pressure reduction rate (approximately 33%) can be achieved as in mild to moderate cases¹⁾. Furthermore, improved techniques such as suprachoroidal drainage (ScD) are expected to provide further intraocular pressure reduction²⁾.

6. Pathophysiology and detailed mechanism

Anatomy and resistance of aqueous humor outflow pathways

The main pathway of aqueous humor (conventional outflow pathway) is trabecular meshwork → Schlemm’s canal → collector channels → aqueous veins → episcleral veins → systemic circulation. The main pathway accounts for 83–96% of total aqueous outflow⁵⁾.

The main site of outflow resistance is the juxtacanalicular tissue (JCT) and endothelial cell layer of the inner wall of Schlemm’s canal⁵⁾. In glaucoma, elevated intraocular pressure causes collapse of Schlemm’s canal, leading to chronic structural changes in the canal wall and trabecular meshwork, further increasing resistance, forming a vicious cycle¹⁾.

Mechanism of action of canaloplasty

Canaloplasty reduces outflow resistance through the following mechanisms¹⁾⁸⁾:

• Viscoelastic dilation of Schlemm’s canal: Physically expands the collapsed lumen and reopens more collector channel openings.
• Tension from the tension suture (ab externo method): Applies continuous inward tension on the inner wall and trabecular meshwork, maintaining patency of the lumen. It also improves the aqueous humor permeability of the trabecular meshwork.
• Microruptures: The pressure during OVD injection creates small ruptures in the inner wall of Schlemm’s canal and adjacent trabecular meshwork, establishing direct communication between the anterior chamber and the canal lumen.

Advantages of Using the Physiological Outflow Pathway

Since aqueous humor drains through Schlemm’s canal, intraocular pressure cannot fall below the episcleral venous pressure (normally 8–10 mmHg). Therefore, hypotony (<5 mmHg) and associated complications such as choroidal detachment, shallow anterior chamber, and maculopathy, which are problematic in filtration surgery, are less likely to occur¹⁾²⁾.

7. Latest Research and Future Perspectives

For Patients: Please Read Carefully

The following content is currently under research or in clinical trials and is not standard treatment available at general hospitals. It is reference information for professionals regarding future medical developments.

Suprachoroidal Drainage + Collagen Implant

Szurman (2023) reported outcomes of canaloplasty + ScD + Ologen collagen implant²⁾. In a retrospective study of 1034 eyes, the IOP reduction rate at 12 months was 45.8% (12.7 mmHg), which remained stable at 45.1% at 4 years.

Eyemate-SC Intraocular Pressure Sensor

A technique has been developed to implant a telemetric IOP sensor (Eyemate-SC) into the suprachoroidal space simultaneously with canaloplasty + ScD²⁾. A 12-month study showed high agreement with GAT, with a mean difference of 0.23 mmHg.

Long-Term Follow-up of the VENICE Trial

The VENICE trial (STREAMLINE vs iStent inject W) is at the 6-month interim analysis stage³⁾, and future long-term follow-up data will provide important information for comparing the two procedures.

Mitomycin C-Augmented Canaloplasty

In the “filtration-type” canaloplasty with mitomycin C, a 42.7% IOP reduction and zero medications were reported at 12 months, but the hypotony rate was 15%, significantly higher than the conventional method (1.1%)²⁾. Meta-analysis showed improved IOP reduction in the mitomycin C group, but no significant difference in complication rates.

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8. References

1. Wagner IV, Gessesse BA, Garg SJ, et al. A Review of Canaloplasty in the Treatment and Management of Glaucoma. J Curr Glaucoma Pract. 2024;18(2):59-73.
2. Szurman P. Advances in Canaloplasty — Modified Techniques Yield Strong Pressure Reduction with Low Risk Profile. J Clin Med. 2023;12(9):3287.
3. Goldberg JL, Gallardo MJ, Heersink M, et al. A Randomized Controlled Trial Comparing STREAMLINE Canaloplasty to Trabecular Micro-Bypass Stent Implantation (VENICE Trial). Clin Ophthalmol. 2024;18:3015-3027.
4. Gallardo MJ, Supnet RA, Ahmed IIK, et al. Canaloplasty and Trabeculotomy Combined with Phacoemulsification for Glaucoma: 12-Month Results of the GEMINI Study. Clin Ophthalmol. 2022;16:1225-1234.
5. Cwiklinska-Haszcz A, Bak E, Wierzbowska J. Revolution in Glaucoma Treatment: A Review Elucidating Canaloplasty and Gonioscopy-Assisted Transluminal Trabeculotomy. Front Med. 2025;12:1547345.
6. Lazcano-Gomez G, Muñoz-Villegas P, Ruiz-Lozano RE, et al. Safety and Efficacy of STREAMLINE Canaloplasty with Phacoemulsification in Hispanic Adults with Open-Angle Glaucoma. Clin Ophthalmol. 2024;18:3619-3630.
7. Golaszewska K, Skrzypczak-Jankun E, Jankun J. Evaluation of the Efficacy and Safety of Canaloplasty and iStent Bypass Implantation in Patients with Open-Angle Glaucoma: A Review. J Clin Med. 2021;10(22):5309.
8. Beres A, Scharioth GB. Canaloplasty in the Spotlight: Surgical Alternatives and Future Perspectives. Rom J Ophthalmol. 2022;66(3):185-195.
9. American Academy of Ophthalmology. Primary Open-Angle Glaucoma Preferred Practice Pattern. Ophthalmology. 2021;128(1):P51-P110.
10. European Glaucoma Society. Terminology and Guidelines for Glaucoma, 5th Edition. Br J Ophthalmol. 2025.