The procedure can provide prolonged IOP lowering in primary open-angle glaucoma with an excellent safety profile.
Canaloplasty is a minimally invasive glaucoma surgery that essentially involves 360° catheterisation as well as viscodilation of the Schlemm canal. Not only is canaloplasty designed to treat the entire conventional outflow pathway, facilitating flow through the trabecular meshwork (TM), the Schlemm canal, and collector channels, but it does so without a stent and without removing tissue.
Birth of canaloplasty and the concept behind it
As our understanding of glaucoma has evolved, we have come to appreciate that it involves a pathophysiology of the entire outflow pathway. Canaloplasty
began as part of the movement to microinvasive glaucoma surgery and was introduced by Robert Stegmann, MD, whose procedure involved ab externo insertion of a rigid metal cannula, followed by injection of high-molecular-weight sodium hyaluronate into the Schlemm canal.1
This concept was revolutionary because it built on the concept that the resistance to outflow in glaucomatous eyes occurs not only at the trabecular meshwork (TM), but also at Schlemm’s canal and the collector channels. Cha et al showed that up to 90% of collector channels are blocked with herniated TM tissue in glaucomatous eyes.2
During the past several years, a plethora of procedures havebeen proposed and occasionally grouped under the designation of “viscoelastic delivery procedures,” but canaloplasty is more than that. Canaloplasty involves the combination of microcatheterisation and viscodilation. The process of 360° catheterisation of Schlemm’s canal results in mechanical relief of microadhesions within the canal. It further works by pushing out the trabecular meshwork herniations, removing or reducing the outflow resistance in these areas. The second step is the pressurised injection of a high-molecular-weight ophthalmic viscosurgical device (OVD), which further stretches the trabecular plates of the TM while simultaneously dilating Schlemm’s canal and the collector channels.3-5 This viscoelastic material remains for a period after the procedure, further facilitating the newly enhanced outflow.
An important point to remember is that canaloplasty is not simply focal injection of viscoelastic. Catheterisation and viscodilation optimally address the entire canal. An early study by Smit et al suggests that dilation of the Schlemm canal can only extend circumferentially 6 mm to 14 mm beyond the point of OVD delivery.5 Therefore, to effectively treat the entire outflow pathway, delivery of the OVD would be required along the full 360° of the Schlemm canal.
OVDs: What, how, and why?
Depletion of hyaluronic acid in the TM has been linked to primary open-angle glaucoma (POAG).2 Reduced hyaluronic acid levels can lead to downregulation of matrix metalloproteinases and disruption of the extracellular matrix, leading to pathologic changes that increase outflow resistance.6,7 During canaloplasty, the delivery of synthetic hyaluronic acid via injection of the OVD helps restore the hyaluronic acid levels within the TM, reversing the pathologic changes and removing outflow resistance.
The pressurised delivery of the high-molecular-weight OVD not only breaks the adhesions in the canal, but also results in an amount of OVD entering the collector channels, also stretching and relieving blockage throughout the distal outflow system. Extending the same concept, it is safe to presume that the more volume of OVD injected, the more stretching of the outflow system that will occur, resulting in a greater improvement in outflow facility.
Currently 2 devices are on the market to perform canaloplasty: the iTrack microcatheter (Nova Eye Medical), which delivers +100 μl of OVD over 360 degrees of the canal; and the Omni Surgical System (Sight Sciences), which delivers 5.5 μl of OVD across 180 degrees of the canal (twice for a total of 360 degrees and 11 μl) and is coupled with trabeculotomy as per the
US FDA 510(k) clearance. Another device, the Streamline Surgical System (New World Medical), delivers 7 μl of OVD per application8 for several applications nasally but cannot be included under the canaloplasty definition because it does not deliver OVD over the entire 360° of the canal and it does not perform microcatheterisation.
Canaloplasty and its effect on the TM
The impact of canaloplasty in dilating the canal and the collector channels is well understood, but its impact on the TM is often a point of conjecture. Pressurised delivery of high-molecular-weight OVD into Schlemm’s canal during viscodilation stretches the TM, possibly causing microperforations extending into the anterior chamber, improving outflow.3
The accumulation of extracellular matrix proteins and banded fibrillar elements is known to compromise the function of the TM cells.6,7 TM endothelial cells counteract these imbalances by regulating hyaluronic acid levels within the outflow pathway.9 The replenishment of depleted hyaluronic acid during viscodilation may help to restore this regulatory function of TM cells. With a deeper understanding of the functioning of TM cells, it is apparent that canaloplasty has a clinically meaningful impact on the TM.
Building on the understanding of TM endothelial cell function, we can now also understand that in procedures involving excision of TM tissue, the removal of these important TM cells in patients with POAG can further hinder the regulation of hyaluronic acid levels in eyes, not only adding to the trauma of postoperative inflammation with angle scarring, but also working against improving the natural outflow mechanism.4
Canaloplasty: safe and effective
Several recent studies have added to the growing body of clinical evidence in support of the effectiveness of canaloplasty. Gallardo published a study highlighting 36-month effectiveness of canaloplasty as a stand-alone procedure and combined with cataract surgery performed with the iTrack canaloplasty microcatheter. A significant reduction was noted in intraocular pressure (IOP) and in the number of medications postoperatively in both groups, highlighting the clinical usefulness of the procedure.10
Khaimi published similar results with canaloplasty (iTrack) in patients with controlled POAG11 and Koerber confirmed the findings of Gallardo and Khaimi, highlighting a comparable reduction in IOP and medications 48 months postoperatively with canaloplasty performed as a stand-alone procedure or with cataract surgery.12 In the GEMINI study, which examined 12-month outcomes of 360° canaloplasty and 180° trabeculotomy using the Omni Surgical System with phacoemulsification, 84.2% of eyes achieved IOP reductions greater than 20% from baseline and 80% of eyes were medication free, further highlighting the usefulness of canaloplasty as a treatment modality for POAG.13
There is renewed interest in canaloplasty because it treats the entire outflow pathway and allows surgeons to target underlying patient physiology—not work around it. It does so without a stent and without removing or tearing precious ocular tissue.
However, to be “real” canaloplasty, the procedure must combine the mechanical breakdown of adhesions via microcatherisation with the restorative and hydrostatic effect of viscodilation. When done correctly, canaloplasty offers prolonged IOP lowering in POAG with an excellent safety profile.