Eyeworld

EW GLAUCOMA 74 February 2017 by Stefanie Petrou Binder, MD, EyeWorld Contributing Writer New technologies blaze a blue trail through the aqueous pathway W ith the rise of micro- invasive glaucoma surgery (MIGS) devic- es, finding the means to visualize a patient's aqueous outflow pathway can heighten device success to reduce IOP by shunting aqueous to the right places. The ongoing develop- ment of outflow imaging techniques is key to enhancing MIGS outcomes, and new investigations are showing promise. Two specialists spoke to EyeWorld about how far we have come in "finding outflow." 3-D and 360 degrees Ocular coherence tomography (OCT) has been instrumental in visualizing the aqueous humor out- flow network, particularly for more proximal parts of the collecting system like Schlemm's canal and the immediate connecting vasculature. In a study that combined structural and functional spectral domain OCT imaging, Schlemm's canal was accu- rately identified and Doppler signals assessed from aqueous outflow in Schlemm's canal, the collector channels, and the scleral veins. It used the noninvasive technique in both eyes of 21 healthy individuals and one eye in each of three people with glaucoma. 1 Speaking with EyeWorld on some of the challenges he encountered using OCT to image outflow, Larry Kagemann, PhD, FARVO, adjunct associate professor, Department of Ophthalmology, New York University Langone Medical Center, and biomedical engineer, Diagnostic and Surgical Devices Branch, Food and Drug Administra- tion (FDA), explained, "One of the challenges is that we are looking at something that provides a very weak signal. There is not a lot of contrast between the Doppler that we are Charting the trabecular pathways O ne of the major gaps in our under- standing of glaucoma is outflow. What is the basic defect in the outflow system that leads to elevated intra- ocular pressure despite a seemingly "open" angle? One might assume that such a basic question would have been answered long ago. Yet the answer eludes us. The MIGS approaches all try to improve outflow. But we can't test them to demonstrate how they work and whether or not they truly achieve better outflow. Furthermore, we can't tell the difference in outflow from operations that are very successful in reducing pres- sure versus operations where the pressure doesn't change. Our lack of outflow understanding raises the question: Is it possible to fix something that we don't understand? Eighteen years ago, my wife, Mary Lynch, MD, and I pitched an idea we had for a trabecular bypass device—the EyePass—to a company that was run by cardiologists. They immediately got the concept, but then they asked how it would affect outflow. They were surprised that we didn't have a deeper knowledge of how such a device would affect outflow dynamics. But they also remembered that cardiology only got serious about angiography after the early development of coronary stents. The treatment technology demanded a better diagnostic approach. We are fortunate to have Alex Huang, MD, and Larry Kagemann, PhD, describe their approaches to characterizing ocular drainage. They, along with Drs. Murray Johnstone, Haiyan Gong, and Carol Torres, are doing the most insightful work in this area. The experience in cardiology has been that the diagnostic test—angi- ography—not only helped improve and guide the development of better coronary stents, but it has become a critical—and profitable—examination in its own right. A similar outcome would be anticipated for an outflow "angiogram." Now that major ophthalmic companies have invested more than $2 billion in MIGS technology, we can expect a bigger industry commitment to solving the outflow mystery. Reay Brown, MD, Glaucoma editor Glaucoma editor's corner of the world Aqueous angiography was performed on a post-mortem left eye of a 79-year-old female with fluorescein. S: superior; T: temporal; I: inferior; N: nasal; arrow: presence of perilimbal angiographic signal; arrowhead: absence of perilimbal angiographic signal; asterisk: distal angiographic signal Source: Alex Huang, MD

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