Glaucoma surgery developments
New technology and techniques help make procedures safer and more individualized.
BY THOMAS W. SAMUELSON, MD
After years of stagnation, we’ve seen more innovation within the glaucoma surgical space in the last five years than the previous 30 years combined.
For most of my career, the glaucoma management paradigm has been relatively straightforward. Initial treatment for open-angle glaucoma included medicines or laser. If the patient’s glaucoma progressed, trabeculectomy was the recommended next step. Although trabeculectomies and tube shunts effectively lower intraocular pressure (IOP), they subject patients to considerable risk, especially those with mild to moderate glaucoma.
Several recent developments have improved glaucoma surgery and, in particular, have helped reduce the level of risk of incisional glaucoma surgery.
Modern clear corneal phacoemulsification cataract surgery often effectively lowers IOP, which has become fairly accepted within mainstream glaucoma circles. Removal of the cataract (native lens) is now considered by most to be a favorable step in glaucoma management for several reasons, including a deeper anterior chamber, improved vision, a more favorable refraction, and, perhaps most importantly, a typically lower IOP after cataract surgery.
This concept has become the foundation for microincisional glaucoma surgery or minimally invasive glaucoma surgery (MIGS), a glaucoma surgical strategy that has become a very important option for patients with mild to moderate glaucoma. While rather loosely defined, to qualify as a MIGS procedure, an operation must be minimally tissue disruptive and safe.
The MIGS procedures have helped fill the “safety gap” because they are extremely safe, but they are also generally less efficacious than traditional procedures. Moreover, unlike procedures that completely bypass the eye’s physiological outflow system, canal-based surgeries are generally synergistic with the favorable effect of cataract surgery and are typically performed through the same incision.
Only one MIGS device, the Glaukos iStent, is FDA approved (see Figure 1). The iStent is an extremely small (0.3 mm x 1.0 mm) stent that is placed within the eye’s natural outflow system, enhancing the eye’s natural, physiological drainage of aqueous humor.
Figure 1: The iStent (Glaukos) is the first FDA-approved device intended to increase outflow through Schlemm’s canal. It is the smallest device implantable in the human body, measuring 0.3 mm x 1.0 mm.
COURTESY DR. SAMUELSON
I had the privilege of being an investigator in the landmark U.S. Premarket Approval (PMA) trial that led to FDA approval. In this trial, patients with mild to moderate glaucoma were prospectively randomized to cataract surgery alone or combined with an iStent. The visual acuity outcomes, complication rate, and adverse event rate were not measurably significant between the two arms of the study. However, patients receiving the iStent were statistically more likely to be medication-free or take less medication compared to the control arm receiving cataract surgery alone.
My experience in clinical practice has generally been even more favorable than the PMA trial, because patients receiving iStent often require less medication and have lower IOP than those who have cataract surgery alone. Moreover, in the hands of an experienced iStent surgeon, patients do not experience a measurable increase in surgical risk.
The opportunity to perform combined cataract and glaucoma surgery without a measurable increase in surgical risk is a breakthrough development in glaucoma management. Until now, we did not have a low-risk surgical option for glaucoma management.
Another strategy, the ab interno trabeculectomy or Trabectome (NeoMedix, Inc.) procedure, utilizes the MIGS philosophy similar to the iStent. This approach employs microincisional technology to enhance the physiological outflow system of the eye. An electrocautery-like device is used to ablate the inner wall of the canal of Schlemm or trabecular meshwork over a portion of the anterior chamber angle, generally 120 degrees. This provides a direct conduit for aqueous humor to leave the eye, again synergistic with the favorable effect of cataract surgery on IOP.
The Trabectome procedure, unlike the iStent, is also approved for use as a stand-alone procedure and can be performed without co-incident cataract surgery.
Using suprachoroidal space
While procedures involving Schlemm’s canal are extremely safe, the IOP reduction is limited by the inherent resistance within the system. The canal of Schlemm and trabecular meshwork (physiological outflow system) drain into the episcleral vasculature – the venous system surrounding the eye. Therefore, the IOP reduction cannot become lower than the eye’s inherent episcleral venous pressure (8 mmHg to 12 mmHg).
Other MIGS strategies utilize alternative outflow pathways, such as the suprachoroidal space, and the more traditional subconjunctival space, which are not limited by episcleral venous pressure. The Glaukos iStent Supra and the Transcend CyPass each utilize the suprachoroidal space for aqueous drainage. Such procedures can achieve lower IOP than canal-based procedures. Both are in clinical trials and will be under FDA review in the near future. Early results are very encouraging.
Less invasive options
The AqueSys XEN 45 Gel Stent and the InnFocus MicroShunt procedure utilize transscleral outflow of aqueous humor, which makes them most analogous to trabeculectomy. However, they are more elegant and far less tissue disruptive.
These procedures each generate a filtration bleb, which has been the gold standard option for aggressive pressure reduction. These procedures will likely lower IOP better than the previously mentioned MIGS options but likely subject the patient to greater surgical risk.
In March 2015, AqueSys announced the completion of enrollment in the U.S. investigational study of the XEN 45 Gel Stent. In November 2015, InnFocus Inc. received FDA approval to expand to the final phase of MicroShunt’s randomized clinical study. Both procedures’ roles, at least initially, will be for those with moderate or advanced glaucoma.
Managing aqueous humor
While outflow procedures are the mainstay of surgical glaucoma management, procedures that suppress aqueous production are also important and have improved in recent years. Endoscopic cyclophotocoagulation (ECP) employs laser energy delivered to the ciliary processes to reduce the amount of aqueous humor the eye produces.
The MicroPulse laser (Iridex) is another recently developed technology. Reportedly, the micropulse delivery of this laser creates less collateral tissue injury and more gently reduces ciliary body aqueous production by applying laser energy through the sclera or directly to the ciliary processes.
The recent, long-overdue glaucoma surgical renaissance gives glaucoma surgeons a portfolio of procedures to offer patients with varying levels of safety and efficacy.
Patients can receive surgery with a risk/benefit profile to match their condition, making glaucoma surgery safer and more individualized than ever before. OP
Thomas W. Samuelson, MD, specializes in glaucoma and anterior segment surgery at Minnesota Eye Consultants in Minneapolis where he is one of the founding partners. He serves on the executive boards of the American Glaucoma Society and the American Society of Cataract and Refractive Surgery and received the AAO’s 2015 Secretariat Award. Dr. Samuelson has been involved in numerous MIGS trials and is an investigator and/or scientific advisor to Glaukos, AqueSys, Ivantis, Transcend, and InnFocus.