glaucoma

Novel Devices and Procedures in Glaucoma Management

Evolving surgical interventions and advanced IOP monitoring devices promise to improve the health and vision of patients who have glaucoma.

RANDALL R. MCPHERRAN, O.D., F.A.A.O.
Paradise, Calif.

Glaucomatous optic neuropathy is the second leading cause of blindness worldwide and the most common optic neuropathy. Although intraocular pressure (IOP) is the most readily modifiable risk factor, the individual threshold at which the damage cascade is evident is widely variable. Because of this variability, we need a better understanding of the pathophysiology, biomechanical and biometric properties to enable us to provide treatment tailored to the individual glaucoma patient. In this article, I review novel devices and procedures likely to impact glaucoma care.

IOP measurement challenges

The current gold-standard adjustable-force tonometer is a 50-year-old, analog, mechanical relic, living in a digital age. In addition to the known inherent errors in interpretation imposed by the limitations of the Imbert-Fick Law,¹ numerous factors influence the accuracy of applanation tonometry, including:

► Tear film volume and composition (adhesion and cohesion properties draw the tonometer tip more firmly onto the cornea)
► Corneal hydration
► Central corneal thickness
► Corneal curvature
► Hysteresis
► Connective tissue composition (degree of age-related or diabetic glycation cross-linkage)
► Bioelasticity
► Resistance.

What's more, the manufacturer's recommendation for monthly calibration is rarely followed in clinical practice, which also impacts accuracy.²

Clearly, the human cornea is a complex biomechanical structure. Glaucoma specialist George Baerveldt, M.D, F.C.S. (SA) estimates at least a 10% error rate with our current gold-standard tonometer.³ Additionally, although the average glaucoma patient will have three to four IOP evaluations per year, the actual applanation event lasts only moments per eye at each visit. These IOP “snapshots” are then extrapolated to all time periods, with assumptions that are not likely to be valid throughout a 24-hour cycle.

Although the newest generation of tonometers attempts to overcome the corneal biomechanical challenges, they do not address the diurnal and nocturnal IOP considerations. Importantly, most glaucoma clinicians practicing outside of research centers are unable to evaluate the increasingly important effects of nocturnal IOP variation on glaucoma progression or conversion with the currently available technology. We need a new approach to continuously and accurately monitor a patient's IOP 24 hours per day.

Figure 1. Graphic of Sensor Concept.

Figure 2. The SendSor System Ready for Surgical Implantation.

Seeking the perfect tonometer

For more than two decades, researchers have been refining microfabrication design techniques with the goal of producing an implantable telemetry system that overcomes the biomechanical challenges while providing extended IOP monitoring. Experimental research tonometers have been incorporated into intraocular lenses (IOLs), inserted into the suprachoroidal space, iris-attached or built into contact lenses. Contact lens-mounted tonometers will need to overcome similar corneal biomechanical concerns as tonometers currently in use, and IOL-attached systems may interfere with new generations of astigmatism correction and multifocal lens designs.

An implantable or intraocular-designed tonometer addresses many of the biomechanical issues by inherently being “where the action is.” A critical design objective for these devices is longterm biocompatibility to obviate inflammatory concerns and decrease the risk for device explantation. Parylene C (polyparaxylene C) is the material most often used because of its proven properties of biocompatibility, fabrication flexibility and chemical inertness.⁴

With funding from SOLX (Waltham, Mass.), researchers in association with Purdue and Boston Universities have developed a self-powered, advanced, implantable tonometer system. The system (SendSor) provides continuous 24-hour-a-day IOP monitoring with sampling every five minutes. Minimal patient interaction is required, and stored IOP data is uplinked during a single short period each day.⁴ According to the manufacturer (D. Adams, oral communication, February 2010), the first human implant with the SOLX Sensor IOP monitor is scheduled for April 2010. (See figures 1 and 2.)

In parallel research at the California Institute of Technology, Chen and colleagues, in a remarkable engineering achievement, developed a microfabricated, sutureless, 19-gauge needle-implantable parylene-based IOP monitor.⁵ The micro-device is delivered into the anterior chamber and fixed to the anterior iris face with anchoring feet. Although this monitor lacks telemetry, the microfabrication techniques have allowed the development of an implantable, wireless passive IOP monitor from this same research group. Ongoing research and development is focused on power supply considerations, decreasing package size and increasing telemetry range.

New surgical management approaches

The current gold standard for glaucoma surgery is trabeculectomy, and data support the contention that surgical intervention more adequately controls IOP over 24 hours than medical management.⁶ Although trabeculectomy has undergone considerable refinement in the past four decades, there still exist infrequent but significant surgery-related concerns and complications, such as purulent bleb/blebitis, hypopyon, spontaneous leakage/erosion, acceleration of cataract formation, hypotony and late bleb failure.

The rationale for current and emerging glaucoma surgeries is the need to bypass the area of highest aqueous flow resistance, the juxtacanalicular region and the inner wall of Schlemm's canal.⁷ Approximately 50% of all aqueous flow resistance is concentrated within these structures in the normal eye, and resistance dramatically increases in glaucomatous pathology. Several new surgical approaches are being evaluated that modify or bypass these flow resistive areas, without the need for external blebs or the use of antifibrotic agents.

Canaloplasty has evolved from the visco-canalostomy procedure developed by pioneering glaucoma surgeon Robert Stegmann, M.D. Canaloplasty is a nonpenetrating limbal flap procedure that introduces a flexible microcatheter into Schlemm's canal. The canal is dilated by injecting sodium hyaluronate through the catheter tip, which is circumferentially advanced through the canal, with recovery at the surgical ostia. The surgeon attaches 10-0 Prolene suture to the catheter, which is fully withdrawn and the Prolene is surgically tied, resulting in a continuous 360° tensioning of the canal.⁸ The success of the procedure appears related to the dilation and tensioning of the canal, which is believed to stretch and then fissure the juxtacanalicular region and the inner wall of Schlemm's canal, allowing increased aqueous outflow.

In a different approach, the Trabectome (NeoMedix, Tustin, Calif.) directly modifies the trabecular resistance.⁹ It is an advancement of the goniotomy procedure, which is used in infantile glaucoma but produces poor results in adults. In the Trabectome procedure, after gaining access to the anterior chamber, the surgeon introduces a pointed, ceramic-coated footplate into and along Schlemm's canal, thereby protecting the collector channel ostia from damage. An arc gap is created between the footplate and a 19-gauge microelectrical pulse surgical ablation device, which is used to remove a 30° to 90° area of the “captured” trabecular meshwork and the inner wall of Schlemm's canal. Continuous irrigations and aspirations are maintained, and electro-ablation is controlled by foot pedal. The procedure has the potential to be repeated in those quadrants without previous application.

Advances in aqueous shunts

A new generation of aqueous shunt devices is giving glaucoma specialists more latitude in performing trabecular bypass surgery. The Ex-Press Mini Glaucoma Shunt (Optonol, Inc., Kansas City, Kans.) was originally designed for placement under a conjunctival flap. Complications arising from hypotony and conjunctival erosion necessitated a modification of the procedure. The device is now placed under a superficial scleral flap. Access to the anterior chamber is through a 26-gauge needle pre-perforation under the scleral flap in the center of the blue-gray transition zone between the white sclera and the clear cornea and parallel to the iris plane. The Ex-Press shunt is inserted into the pre-perforation site, the scleral flap is sutured, and the conjunctiva is manipulated into place.

The Gold Shunt GMS Plus implant (SOLX, Waltham, Mass.) follows an alternate suprachoroidal approach to bypass the restrictive trabecular meshwork and inner wall of Schlemm's canal. The 24k gold device (5.6 mm x 3.2 mm x 80 microns, or about the thickness of a human hair) is inserted into the anterior chamber through a 3-mm limbal incision and bridges into the supraciliary space. The shunt is designed with tubular baffled channels, not all of which are open at the time of implantation. Additional channels can be opened by laser application to the visible portion of the shunt in the anterior chamber. (See figure 3, below.)

Figure 3. Anterior Chamber View of Implanted GMS Plus Shunt.

Three key benefits

The development of implantable telemetry tonometers will allow enhanced IOP-associated progression analysis, especially during the critical and difficult to assess nocturnal period. Advances in surgical shunts and nonpenetrating procedures may decrease or, in some cases, eliminate chronic medication use. The convergence of these technologies will result in improved patient care, decreased compliance issues and healthcare cost containment. OM

Dr. McPherran is optometric coordinator at Castle Family Health Center and assistant clinical professor at UC Berkeley School of Optometry. He discloses no relationships with any products or companies mentioned in his article. E-mail him at opto1@sunset.net.

Acknowledgment: The author thanks Ann McPherran, O.D., for advice and insightful comments on this manuscript.