Traditionally, ophthalmologists have treated open-angle glaucoma with IOP-lowering medications, laser therapy of the angle, and bleb-based filtering surgery. A new technique that uses products developed by iScience Interventional (Menlo Park, CA), however, offers an alternative therapy designed to enhance or restore the eye's physiologic outflow mechanism. According to the company's founders, the technology for the interventional canaloplasty procedure was inspired by advances in another medical specialty: cardiology.
Adapt and Improve
When iScience Interventional (originally iScience Surgical) was founded in 1999, the company's goal was to combine expertise in biomaterials, ultrasound, and ophthalmology to develop interventional technologies that could deliver anatomically intuitive, site-specific treatments. As the founders talked to ophthalmologists about the challenges they encountered during surgery, they learned that anterior and posterior segment specialists had similar difficulties locating and accessing compromised ocular structures.
“We saw several parallels between ophthalmology and interventional cardiology of decades past,” said Stan Conston, Co-founder and Vice President of Research and Development for iScience Interventional, in an interview with Glaucoma Today. “The addition of new imaging technologies and catheter-based treatments changed the focus of cardiology from open surgery to less invasive procedures such as balloon angioplasty and fluoroscopically guided stent placement. We adapted that model for ophthalmology to produce anterior segment access technology, which can be used in a wide range of procedures, including canaloplasty.”
Tools and Techniques
The canaloplasty procedure uses three product platforms from iScience Interventional: the iTrack-250A microcatheter; the iLumin microillumination system; and the iUltrasound imaging system. Other materials include Healon GV (Advanced Medical Optics, Inc., Santa Ana, CA) and a10–0 Prolene suture (Ethicon Inc., Somerville, NJ).
“Although the instruments were not designed with a specific procedure in mind, they have found a home in anterior and posterior segment surgery,” said Michael Nash, the company's co-founder, president, and CEO,
in an interview with GToday.
The glaucoma specialists whom iScience Interventional surveyed were interested in a minimally invasive surgical technique that could improve aqueous outflow through the trabecular meshwork and Schlemm's canal.
“A surgical procedure that uses the distal outflow system to lower IOP is very appealing,” wrote Thomas W. Samuelson, MD, a private practitioner from Minneapolis who also serves as a consultant and investigator for iScience Interventional in an email to GToday.
Canaloplasty is not the first such approach. An early attempt to restore this physiological pathway was the viscocanalostomy, in which the surgeon injects a viscoadaptive device into the surgical ostia of Schlemm's canal. The canaloplasty procedure evolved from concepts elucidated in viscocanalostomy and deep sclerectomy.
Dilating Schlemm's Canal
To access all 360º of Schlemm's canal, surgeons needed a cannula that was small and flexible enough to travel through the entire structure but large enough to facilitate the infusion of a viscoelastic material. “The iTrack has a shaft that is approximately 250 µm in diameter, the equivalent of several strands of hair from your head, but it is also complex,” said Mr. Nash. “It is not just a tube. It also has an infusion pathway, a wire that controls how the catheter tracks, and a fiber optic for illuminating the distal tip” (Figure 1).
Figure 1. This diagram shows the structures contained in the iTrack microcatheter.
During the canaloplasty procedure, the surgeon advances the microcatheter around the canal and injects Healon GV in a controlled manner. After dilating the canal with the viscoelastic, the surgeon attaches a 10–0 Prolene suture to the microcatheter. He then withdraws the microcatheter, leaving the suture in the canal. Tying the suture to itself in a loop puts tension on the trabecular meshwork, opens the canal, and reduces resistance to aqueous outflow.1
Co-founder and Chief Scientific Officer for iScience Ron Yamamoto points out, however, that it is not advisable to insert a catheter into an anatomic space without knowing exactly where it is going. Surgeons can follow the iTrack's progress through Schlemm's canal by watching the illuminated tip through the sclera (Figure 2) or by capturing intraoperative images of the canal with the iUltrasound system.
Figure 2. The black arrow indicates the location of the illuminated tip of the iTrack microcatheter during the canaloplasty procedure. (Courtesy of Norbert Koerber, MD.)
“We looked at the current state-of-the-art technologies for imaging the anterior segment, but they did not provide the resolution we needed to see a structure as small as Schlemm's canal,” Mr. Yamamoto told GToday. “We tapped our previous experience with ultrasound to create the iUltrasound imaging system.” Cross-sectional images of Schlemm's canal captured before and after the interventional procedure show differences in the lumen's patency (Figure 3). In one study, the canal's postoperative diameter ranged between 83 and 373 µm.2
Figure 3. Images captured with the iUltrasound system show the same eye before (left) and after (right) canaloplasty. Note the increased diameter of Schlemm's canal (SC) postoperatively.
Dr. Samuelson has performed approximately 12 canaloplasty procedures with favorable results. “One procedure was a complete failure, but that patient converted from POAG [primary open-angle glaucoma] to neovascular glaucoma during the study period and required the implantation of an aqueous drainage device,” he wrote in an email to GToday. “All the other patients are maintaining IOPs within their target range, almost all without medications. None of my patients has developed significant complications or lost best-corrected acuity. My longest follow-up is 19 months.”
Mapping New Territory
According to Mr. Yamamoto, the goal of iScience Interventional is to provide surgeons with the tools they need to perform less invasive interventional procedures. “This strategy makes sense for ophthalmology, just like it did for interventional radiology and cardiology,” he said.
Richard A. Lewis, MD, a glaucoma specialist in Sacramento, California, and consultant to iScience Interventional, is encouraged by the results of the canaloplasty's early clinical trials, but he remains cautious about the procedure's long-term utility. “Prior to the introduction of the microcatheter, I had never been able to access the entire canal,” he said. “I think it is too early to say whether canaloplasty will become a reliable first-line therapy for glaucoma. We need to know more about its long-term effects before we can reach that conclusion.”
Dr. Lewis cited ophthalmologists' lack of knowledge about the structure of Schlemm's canal as another reason to investigate the canaloplasty procedure thoroughly. “We basically understand the function of Schlemm's canal and the trabecular meshwork, but we still know relatively little about the canal as an anatomic structure,” he said. “In a sense, we have to depend on our imagination to visualize Schlemm's canal because it is such a small space. We can see it in histologic sections and electron micrographs, but we cannot dissect it.”
Dr. Samuelson expressed concern that episcleral venous pressure and the inherent resistance of the distal outflow system might limit canaloplasty's efficacy for lowering IOP in some patients. Nevertheless, he is comfortable offering this procedure to patients who have glaucoma in its early-to-moderate-stage, who have special circumstances such as high myopia, or who wear contact lenses.
Echoing the designers' inspiration for the microcatheter, Dr. Samuelson uses the analogy of cardiac surgery to educate prospective canaloplasty candidates. “In some patients, cardiac surgeons can stent or dilate the obstructed vessel with a less invasive angioplasty,” he tells them. “In other patients with more serious disease, surgeons may have to open the chest and perform a more complex procedure.” For glaucoma patients, this means transitioning from canaloplasty to trabeculectomy.
Just having access to new interventional treatments for glaucoma is beneficial, said Dr. Samuelson. “I believe we may eventually get to the point where we replace our ‘one-size-fits-all' approach to glaucoma surgery with one that emphasizes per-patient treatment,” he noted. “This is certainly an exciting time in glaucoma surgery, but much remains to be resolved, and I would not abandon time-tested trabeculectomy yet. However, we may be much more selective in choosing this procedure over less-invasive approaches in the future.”
Richard A. Lewis, MD, may be reached at (916) 649-1515; rlewiseyemd@yahoo.com.
Thomas W. Samuelson, MD, may be reached at (612) 813-3628; twsamuelson@mneye.com.
Stan Conston, Michael Nash, and Ron Yamamoto may be reached at info@iscienceinterventional.com.
1. Tightening Schlemm's canal reduces IOP. Ophthalmology Times. Available at: http://www.ophthalmologytimes.com/ophthalmologytimes/article/articleDetail.jsp?id=370911&searchString=tetz. Accessed January 11, 2007.
2. Cameron B, Field M, Ball S, Kearney J. Circumferential viscodilation of Schlemm's canal with a flexible microcannula during non-penetrating glaucoma surgery. Digital J Ophthalmol. Available at: http://www.djo.harvard.edu/site.php?url=/physicians/oa/929. Accessed January 5, 2007.
