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Though much focus for stents has been in the trabecular meshwork, many companies are currently developing suprachorodial devices, which are in various stages of FDA review.
By John Berdahl, MD, Special to Ophthalmology Times
Dr. BerdahlMicroinvasive glaucoma surgery (MIGS) has revolutionized the treatment of glaucoma. With compliance concerns regarding topical medications, the benefits of MIGS may even outweigh those of medical therapies in patients with pressures adequately controlled on medications.
Several companies are currently striving to add more tools to the MIGS armamentarium (including Glaukos with the iStent Trabecular Micro-Bypass device).
Past focus for stents has been in the trabecular meshwork. However, companies are currently developing suprachoroidal devices, which are in various stages of FDA review. These include the iStent SUPRA (Glaukos), the CyPass Micro-Stent (Alcon Laboratories), and the Solx Gold Shunt (Solx). These additions will provide more surgical options and allow surgeons to individualize treatments to patients more effectively.
The eye has two outflow pathways: the trabecular outflow and the uveoscleral outflow pathway. The trabecular outflow pathway is more pressure dependent and is where 75% of the aqueous humor outflow occurs,1 making this area a logical place to insert a stent to restore natural physiologic aqueous outflow.
The episcleral venous pressure present is in the range of 6 to 12 mm Hg, and while distal outflow resistance adds a few mm Hg to the IOP,2 trabecular bypass stents are still capable of producing effective IOP lowering. The essential benefit of the episcleral venous pressure is that it virtually ensures no hypotony will occur.
The iStent has a strong safety and efficacy profile, as proven in the pivotal FDA trial.3 Results have only improved with time as surgeons continually become more experienced with the procedure.
Courtesy of GlaukosIn a recently published study,4 a single stent in combination with cataract surgery resulted in a post-treatment mean IOP of 14.9 mm Hg ± 2.3 mm Hg at 3 years, a 36% reduction in IOP from medicated baseline and an 86% reduction in the mean number of medications. This was accomplished with no cases of hypotony, IOP spikes, or hyphema within 1 month. Similar studies have shown corroborating results.5
Further studies demonstrate the efficacy of multiple stents in reducing IOP and medications, illustrating the possibility of titrating treatment with the stent.5-7
Optimizing patient benefit-to-risk is the cornerstone of every glaucoma clinician’s management of glaucoma disease and iStent has arguably demonstrated the highest benefit-to-risk profile of any MIGS device to date.
Suprachoroidal devices bypass the episcleral venous pressure system and shunt fluid directly into the suprachoroidal space to be reabsorbed into the body. The intrinsic pressure differential between the anterior chamber and the suprachoroidal space may aid in transferring aqueous from the anterior chamber.6 However, without the episcleral venous pressure back-stop there may be a greater risk of hypotony. A 2-year study of the CyPass demonstrated hypotony prior to 1 month in 15.4% of patients and hypotony after 1 month in 1.9% of patients.7 With suprachoroidal stents there is also potential for corneal edema8 as well as hyphema.9-10
Courtesy of Glaukos
Published clinical evidence on suprachoroidal shunts has at best shown comparable efficacy to a single trabecular bypass stent. Initial data for a CyPass inserted at the time of cataract surgery in patients with an IOP of > 21 mm Hg show a reduction from a baseline mean IOP of 25.5 mm Hg to a mean of 15.8 mm Hg at 24 months which is meaningful10 and it is possible that safety and efficacy rates will improve over time as surgeons gain experience but further long-term study will be needed.
Like trabecular stents, implanting suprachoroidal shunts requires a good understanding of the anatomy. Surgeons should be deliberate about placing a shunt in such a highly vascularized space as it does present the possibility of serious intraoperative complications. Risks can include bleeding from the highly vascular iris and ciliary body and from the anterior ciliary arteries while unintended cyclodialysis clefts may result from implantation of suprachoroidal shunts.
Other possible complications include peripheral anterior sequelae (PAS), small areas of scarring around the device, which could impair long-term efficacy. The creation of passageways in the suprachoroidal space could cause abrupt closures of the cleft, resulting in high-pressure spikes. The CyCLE trial showed PAS formation at 10%,10 while the COMPASS trial showed PAS formation reducing to 3.1% over a 2-year period.12
However, suprachoroidal shunts may be a valuable tool, especially as an adjunct to a trabecular outflow procedure in cases where disease progression required additional pressure lowering. I would like to see a diagnostic that could easily determine episcleral venous pressure.
For low episcleral venous pressures, a trabecular meshwork stent would be more logical, while higher episcleral venous pressures might yield better results with suprachoroidal stents. Perhaps in the future that technology will be available.
Our collective experience and the accumulation of data will determine the best use for suprachoroidal and trabecular stents. Exploiting both the conventional and uveoscleral pathways will enable enhanced treatment of patients with progressive glaucoma disease who require lower target pressures than can be obtained by trabecular bypass treatment alone.
John Berdahl, MD
Dr. Berdahl is an ophthalmologist at Vance Thompson Vision, Sioux Falls, SD. He specializes in advanced corneal and glaucoma surgery.
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