Modified glaucoma value reduces outflow resistance

Nancy Groves

Fort Lauderdale, FL-A modified glaucoma valve made of a porous polymer material reduced outflow resistance in tests conducted in an animal model. The reduced resistance should in turn result in improved control of IOP over conventional implants, explained R. Rand Allingham, MD, at the annual meeting of the Association for Research in Vision and Ophthalmology.

Dr. Allingham and colleagues from the ophthalmology and plastic surgery departments at Duke University Medical Center, Durham, NC, developed the experimental implant, which should be ready for a phase I clinical study this year.

For the recent study, a control implant was placed into one eye and the experimental implant, a similar device covered with porous material, was placed in the contralateral eye of 10 New Zealand rabbits. At 10 weeks, the anterior chamber tubes were cannulated in situ and perfused with saline at a flow rate (Q) of 2 μL/min for 30 minutes, followed by perfusion at a flow rate of 5 μL/min for 30 minutes. The average outflow resistance of the experimental implant was significantly lower than the control device at both perfusion pressures in these experiments.

The theory behind the new valve was that an implant constructed of a porous polymer material would result in an altered healing response that would reduce outflow and thereby improve IOP control. Dr. Allingham, professor of ophthalmology and chief of the glaucoma service, at the Duke University Eye Center, was motivated to create a device that would be practical for glaucoma therapy in settings such as developing countries where conventional medical and surgical treatments are often unavailable or unaffordable.

Dr. Allingham discovered the need for a modified valve while conducting field research on glaucoma genetics in Ghana and working in local clinics. The prevalence of glaucoma in that country is high, and few patients have money for pressure-lowering drugs. In addition, the scarcity of medical resources means that most cases of glaucoma are not identified until an advanced stage, when it is typical to see patients who have lost vision in one eye and have damage to the other.

"Medical treatment was just not a good option," Dr. Allingham said. However, surgery was not a much better choice, as trabeculectomy was too time-consuming and labor-intensive. The procedure also required frequent follow-up, patients needed expensive and scarce medication, the surgery often failed, and is prone to infections in that environment.

Under these circumstances, Dr. Allingham concluded that a surgical implant may be the best form of therapy for these advanced cases.

"But the implants we were using have a tendency to fail over time due to capsular fibrosis," he said. "We needed an implant that would give a good postoperative pressure without the need for additional medical or surgical treatment. Furthermore, the follow-up had to be very brief, allowing the patient to have surgery, take a course of medication, then go home, rather than come back repeatedly.

"That's the backdrop of why we were considering this project. It's really to address the major glaucoma issues that occur commonly in the developing world," he continued.

In this scenario, a more suitable implant was needed because of two major issues. First, according to Dr. Allingham, the most frequent reason current implants fail is the development of a very dense capsule around the aqueous humor reservoir. Antimetabolites have not been effective in addressing this problem with implants.

Another issue was the extent of manipulation required to control the flow of fluid through the tube in valveless implants.

"One has to restrict the fluid flow so you maintain an acceptable eye pressure in the early postoperative period," he said.