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Suprachoroidal devices an exciting step in treatment of POAG
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Approximately 3.5 million people in the U.S. have primary open-angle glaucoma, and the number in the world is at least 60 million. In addition, it is estimated that only 50% of patients with this disease have been diagnosed. Glaucoma is the second leading cause of blindness, cataract being No. 1. In African-Americans, glaucoma is the leading cause of blindness.
POAG is defined by most as an optic neuropathy associated with elevated IOP above 21 mm Hg with progressive loss of ganglion cells. The many National Eye Institute-sponsored clinical trials, including OHTS, EMGTS, CIGTS, GLT and AGIS, have taught us much. Several risk factors have been confirmed, including elevated IOP, increased age, a thin cornea, female sex, a positive family history, especially on the maternal side, migraine and myopia. It is noteworthy that the only risk factor in the prior list that can be modified with medical or surgical treatment is elevated IOP. Thus, all current approved therapies lower IOP, and according to the AGIS study, every 1 mm Hg reduction in IOP reduces the risk of progressive ganglion cell loss 10%.
In most studies of POAG, a washout pressure off all meds approximates 25 mm Hg to 26 mm Hg. Thus, a reduction in IOP of 10 mm Hg results in an IOP near 15 mm Hg, which using the 10% reduction in risk per mm Hg should generate a high likelihood progressive damage will cease. There is some confirmation for this in the AGIS study, in which an IOP less than 15 mm Hg was associated with a stable visual field.
These same clinical trials teach us which tests are most helpful. Of course, gonioscopy is required to make the diagnosis of POAG. It is interesting that in chart surveys, confirmation of this examination is often absent. The other especially useful diagnostic findings include an increased vertical cup, disc hemorrhage, an elevated pattern standard deviation or mean standard deviation on a visual field examination, and reduced optic nerve fiber layer thickness on optic nerve OCT, especially in the superior and inferior location. These are the most useful tests in diagnosing POAG and in monitoring patients for efficacy of treatment.
The AAO Preferred Practice Pattern recommends that a target pressure be placed in the patient’s chart, and while many use a percentage reduction in IOP as a target, the AGIS study suggests to me that most patients need a target pressure less than 18 mm Hg to prevent progressive damage and that less than 15 mm Hg would be ideal. Starting at the typical washout pressure of 25 mm Hg to 26 mm Hg, an 8 mm Hg to 9 mm Hg reduction in IOP is needed to get to less than 18 mm Hg and 11 mm Hg to 12 mm Hg for the ideal less than 15 mm Hg. Most studies suggest that a prostaglandin analogue will reduce IOP only 6 mm Hg to 7 mm Hg, which will result in a less than ideal outcome with a single medication for the typical POAG patient.
The addition of a second or third topical medication may get us there, but a recent study suggested that only 15% of patients are compliant with topical glaucoma medications at 4 years after initiation of treatment. Development of extended-release glaucoma devices that ensure compliance will be a welcome addition to our treatment alternatives, but none are currently available. Argon or selective laser trabeculoplasty can be helpful, but IOP has a tendency to drift back up a few years after these laser treatments. Surgery, including trabeculectomy and tube shunts, generate very good pressure reduction and can be combined with medical therapy.
In the much referenced Tube Versus Trabeculectomy Study, IOPs after both procedures with the addition of one topical medication reached 13 mm Hg to 14 mm Hg, which according to AGIS is the magic number. Unfortunately, the complication rate with these two relatively invasive and technically demanding surgical procedures was quite high. Thus, the quest for more microinvasive glaucoma surgery, or so-called MIGS.
I currently perform the Glaukos iStent in my practice in combination with cataract surgery in mild to moderate glaucoma, and I have been quite happy with the outcomes. Some have suggested that the additional IOP lowering achieved with a single stent is small, but studies outside the U.S. suggest even a single stent can reduce IOP an additional 2 mm Hg to 3 mm Hg over cataract surgery alone. In addition, global studies suggest that up to three stents may be placed with additional IOP lowering. So-called “intelligent placement” of the stent near collector channels as pioneered by Ike Ahmed also appears capable of enhancing outcomes.
I had the honor of traveling to Armenia and participating in a clinical trial with two iStents in patients with phakic and pseudophakic POAG, and with a single medication, usually a prostaglandin analogue, mean pressures were less than 15 mm Hg on average. On my trip to Armenia, I was also allowed to implant the iStent Supra, which is similar to the Transcend CyPass. I found the surgical technique straightforward, and again, IOPs less than 15 mm Hg were common with this device. Complication rates with the iStent in the U.S. clinical trial were near zero, and with the suprachoroidal devices, while somewhat higher, far lower than with a trabeculectomy or tube shunt. In addition, it appears that the IOP-lowering effect of trabecular bypass with an iStent can be combined with a suprachoroidal device with additional IOP lowering. Also, the addition of a topical medication can enhance IOP lowering further.
While we have much to learn, I can imagine starting with two to three of the somewhat safer iStent trabecular bypass implants to enhance physiologic outflow. If IOP control is inadequate, a suprachoroidal device such as the iStent Supra or CyPass could be placed. Of course, at any stage we can add in drops, and in the future, a device such as prostaglandin-releasing punctal plugs will help eliminate compliance issues. In extremely severe cases, or those responding less than desired to these less invasive approaches, we can still do a trabeculectomy or tube shunt. Finally, not mentioned in the accompanying cover story, AqueSys is developing a minimally invasive ab interno approach to filtration surgery called the Xen implant with a much lower complication rate than standard trabeculectomy.
All in all, it is a very exciting time with amazing advances already here and more coming for the medical and surgical treatment of POAG.
Disclosure: Lindstrom reports he is a consultant for and investor in AqueSys, Glaukos and Transcend Medical.