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New technology changes the course of glaucoma therapy
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Abnormality of trabecular meshwork function is the primary cause of elevated IOP in primary open-angle glaucoma. The trabecular meshwork is lined with endothelial cells that facilitate the outflow of aqueous humor from the eye. Injury or death of endothelial cells increases resistance to outflow of aqueous through the trabecular meshwork and results in elevation of IOP. Recent advances in glaucoma treatment are aimed at enhancing the function of the trabecular meshwork and Schlemm’s canal.
Drug therapy has been the standard of care for first-line glaucoma treatment for more than 30 years, followed by laser trabeculoplasty when medications are not sufficient. Although selective laser trabeculoplasty is effective in lowering IOP and has a relatively benign safety profile, most patients continue to require medical treatment after the procedure. In addition, results from long-term studies indicate that 30% to 50% of eyes require additional surgical treatment within 5 years after laser trabeculoplasty. In cases in which laser trabeculoplasty fails after an initial adequate response, a second laser trabeculoplasty procedure proves unsuccessful in nearly 90% of eyes at the 2-year follow-up period. Compared with initial laser trabeculoplasty, there is an increased risk of IOP spikes after repeat laser trabeculoplasty.
IOP reduction in cataract surgery
Of the 3.3 million annual cataract procedures performed in the U.S., 20.5% of patients have comorbid glaucoma and/or ocular hypertension. Studies in which I participated showed successful long-term lowering of IOP in 588 normotensive and ocular hypertensive eyes after phacoemulsification and IOL implantation. We demonstrated that that IOP reduction with cataract surgery was positively correlated with presurgical IOP. This finding has been duplicated in subsequent studies, as well.
Indeed, considerable evidence is mounting that cataract surgery lowers IOP and should be considered a step in the management of glaucoma. Moreover, knowing that cataract surgery is likely to lower IOP in most patients, surgeons may be reluctant to perform a combined procedure that significantly increases the risk of cataract extraction alone unless such risk is justified by the severity of the glaucoma. Yet, cataract extraction presents a major opportunity to manage both conditions simultaneously if the glaucoma portion of the procedure can be accomplished without compromising the safety of cataract surgery when performed as a standalone procedure. The clinical literature shows evidence that cataract surgery lowers IOP and strong evidence that combined glaucoma and cataract surgical procedures control IOP better than cataract surgery alone.
Thomas W. Samuelson
Combined surgery
Studies involving the iStent (Glaukos) provide evidence that combining a glaucoma procedure with cataract surgery can indeed add to the effectiveness of cataract surgery in lowering IOP and reducing the need for medication without compromising safety. The 1-mm stent is inserted into Schlemm’s canal using the same temporal incision used during cataract surgery. The temporal cataract incision is optimal for placement of the iStent in the inferonasal portion of the canal in which collector channels are most abundant. The iStent helps restore physiologic outflow of aqueous humor through Schlemm’s canal. By utilizing the physiological outflow pathway, the risk of hypotony is reduced due to the natural episcleral back pressure of 8 mm Hg to 11 mm Hg. By the same token, the physiological system will not yield pressures as low as transscleral filtration procedures such as trabeculectomy.
In the first paper published utilizing a randomized, controlled study design, Fea compared phacoemulsification alone (control) to phacoemulsification and iStent implantation (combined) in eyes with primary open-angle glaucoma. According to this data, it appears that implantation of an iStent lowers IOP an additional 3 mm Hg above and beyond cataract surgery alone.
I participated in the multicenter U.S. Food and Drug Administration investigational device exemption clinical trial for the iStent, which found additional lowering of IOP beyond what was seen in the control patients who received phacoemulsification alone. Sixty-six percent of the patients who received the iStent had an IOP reduction of 20% or greater vs. 48% of patients who had cataract surgery alone. Further, 72% of patients in the iStent group were able to remain medication free vs. 50% of patients who had cataract surgery alone.
No increased risk
Importantly, the safety profile and recovery time of cataract surgery were not affected by the iStent implantation. To be able to perform a glaucoma operation without compromising the safety and visual outcomes of cataract surgery, arguably one of the best operations in all of medicine, is important. It is promising that we now have operations that work synergistically with cataract surgery to achieve additional glaucoma benefit without adding measurably to the risk.
The American Academy of Ophthalmology’s Preferred Practice Guidelines state that the goals of glaucoma treatment are to achieve stable optic nerve or retinal fiber layer status, control IOP, and stabilize visual fields while maintaining the patient’s quality of life. In order to achieve these goals, safe surgical options such as iStent implantation and other minimally invasive glaucoma procedures are much-needed additions to the well-established and efficacious but far riskier procedures such as trabeculectomy and implantation of aqueous drainage devices. While the efficacy is more modest, the reduced risk profile fills an important void while still providing the potential to eliminate or reduce the need for drug therapy, which is attractive to both clinicians and patients.