Advances in minimally invasive glaucoma surgery

Take-home message: One surgeon’s experiences with the iStent trabecular micro bypass; an implant that forms a patent opening through the trabecular meshwork into Schlemm’s canal, restoring physiological aqueous outflow and providing a significant and sustained decrease in IOP.

By Dr Fritz Hengerer

The glaucoma surgical therapy field has grown enormously in the last few years, providing ophthalmologists and their patients myriad options that are less invasive and safer than the traditional trabeculectomy. While many pharmacotherapies address high intraocular pressure (IOP) by reducing aqueous production, most surgical therapies are aimed at improving aqueous outflow. The conventional pathway for aqueous humor includes the trabecular meshwork, Schlemm’s canal and collector channels, with the trabecular meshwork contributing the majority of resistance to aqueous outflow in both normal and glaucomatous eyes.1-3 

iStent trabecular micro bypass

The iStent trabecular micro bypass (Glaukos Corp., California, USA) forms a patent opening through the trabecular meshwork into Schlemm’s canal, restoring physiological aqueous outflow and providing a significant and sustained decrease in IOP. iStent inject is the second-generation trabecular bypass system by Glaukos that has been available only in Germany for the past year. While the iStent was already the smallest device known to be implanted in humans at 1 mm long by 0.33 mm in height, the new iStent inject is one-third of the size at 0.4 mm tall with a diameter of 0.3 mm (Figure 1). 

Multiple clinical trials have established the efficacy of a single stent placement in combination with phacoemulsification to provide a significant, long-term decrease in IOP and the number of medications required.4-8 These have been followed by a growing body of literature demonstrating efficacy of iStent as a stand-alone procedure, as well as the placement of multiple stents to titrate therapy.9,10 A recent pan-European, multicentre, prospective, controlled study of 192 open angle glaucoma (OAG) patients uncontrolled on one medication compared placement of 2 iStent injects to pharmacotherapy with 2 medications.11 Patients receiving the stent (n = 94) had a mean IOP drop of 8.1 mmHg, resulting in a reduction > 20% from baseline in 95% of patients. In comparison, patients in the medical therapy cohort saw a mean IOP drop of 7.3 mmHg, with 92% achieving an IOP reduction > 20%.

iStent inject delivers compelling clinical benefits with no medication burden or doubts about compliance, along with a highly favorable safety profile.

Next: Surgical methods explored

“Over the last twelve months, I have implanted 70 stents,” said Dr Hengerer. “All have primary OAG and were uncontrolled on two or more medications, with many having prior glaucoma surgery. It is significant that I am performing a micro invasive glaucoma surgery (MIGS) on patients that have failed trabeculectomy, and seeing favorable results.”

Surgical methods

As a stand-alone procedure in phakic and pseudophakic eyes, I begin with a peribulbar injection of topical anesthesia to eliminate any pain patients may feel when I enter the anterior chamber. Precision when placing the stent is very important to the success of the procedure, thus I prefer to create a very relaxed situation. I then place an intracameral miotic agent to constrict the pupil and flatten the iris plane. I create a single paracenthesis temporally and fill the anterior chamber with viscoelastic. A gonioscope contact lens is used to visualise the anterior chamber angle on the opposite side from the paracenthesis. This position allows me full access with a lid speculum to rotate the eyeball in any direction I need.

The injector is placed across the anterior chamber and I locate Schlemm’s canal opposite of my paracenthesis. I puncture Schlemm’s canal with the injector then implant the first stent. When I see a little outflow of blood through the stent opening, I know the stent will function properly. The iStent inject inserter allows the surgeon to clearly visualise the stent passing through the needle and entering Schlemm’s canal, providing excellent control.  I then move one clock hour below the first stent and implant the second stent in the same manner.

I have found that the best position is to sit temporally and place the stent on the nasal side, as there is a larger concentration of collector channels in the nasal region and entering from the nasal side with a straight injector would be quite difficult due to the orb projecting into the path. In my patients, I see a lowering of IOP the following day.

Episcleral back-pressure

Restoring the natural outflow channels provides the highest clinical benefit–to–risk ratio, making it the safest site for implantation. A stent through the trabecular meshwork overcomes the site of resistance while assuring an active Schlemm’s canal to minimize atrophy and extracellular matrix associated with disuse. Episcleral venous back-pressure eliminates the potential for iatrogenic hypotony, adding an additional safety factor to the procedure. However, it also makes it difficult to achieve IOP lower than 14 mmHg. If a patient needs IOP lower than 14 or 15, I would consider performing a trabeculectomy or implanting the XEN Gel Stent (Aquesys).

Next: Patient results

My patients’ results have been better than I anticipated, and the elegant procedure creates minimal disruption of the angle anatomy. Importantly, the iStent inject is a very flexible tool.

“I have used it in mild to moderate patients to reduce the drug burden as well as in patient that need additional treatment following a trabeculectomy,” explained Dr Hengerer, “The only patients that are not candidates are those that previously received canaloplasty, as Schlemm’s canal now contains the cord from the previous procedure.” Dr Hengerer concluded with the following statement: “My patients are content to have a procedure with a minimal complication rate that can reduce their medications while controlling the disease.”

References

• M. Johnson. Exp. Eye Res. 2006;82(4):545-57

• J.S. Schuman et al., Invest. Ophthalmol. Vis. Sci. 1999;40(8):1676-80.

• R. Rosenquist et al., Curr. Eye Res. 1989;8:1233–1240.

• T.W. Samuelson et al., for the US iStent Study Group. Ophthalmology. 2011;118:459–467.

• A.M. Fea. J. Cataract. Refract. Surg. 2010;36:407–412.

• E.R. Craven et al., for the iStent Study Group. J. Cataract. Refract. Surg. 2012;38:1339–1345.

• A.M. Fea et al., Prospective, randomized, double-masked trial of trabecular bypass stent and cataract surgery vs. cataract surgery alone in primary OAG: long-term data. Presented at the 2012 European Society of Cataract and Refractive Surgeons, Milan, Italy, September, 2012.

• P. Arriola-Villalobos et al., Br. J. Ophthalmol. 2012;96(5):645-9.

• I.K. Ahmed et al. J. Cataract. Refract. Surg. 2014;40:1295–1300.

• L. Voskanyan et al., Adv. Ther. 2014;31(2):189–201.

• A.M. Fea et al., Clinical Ophthalmology. 2014;8:875-882.

Dr Fritz Hengerer

e: fritz.hengerer@kgu.de

Dr Fritz Hengerer is Vice Chairman and Senior Head Physician at the University Eye Hospital, Frankfurt, Germany.

The author has declared no financial conflicts relating to the content of this