APR 2013

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

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60 EW GLAUCOMA April 2013 February 2011 Continuous IOP monitoring is almost here B ecause there is no established method of home tonometry, we are limited to in-office measurement of IOP. Accordingly, we get perhaps three or four IOP measurements each year. From these measurements, we must determine if our patients need to be treated or if their treatment is effectively lowering their IOP. Once those Goldmann tonometry mires are aligned, we observe IOP for no more than a second or so before recording it in the medical record. There are more than 31.5 million seconds in a year, so our sampling rate is four in 31.5 million —one in eight million. Think of it this way: There are eight million people in the state of Virginia—if we wanted to know how Virginians felt about climate change, would we be content to call one Virginia resident at random and let her speak for the whole state? Continuous IOP monitoring remains one of the holy grails of glaucoma. And it may be within our grasp. A new contact lens based device called the Triggerfish (Sensimed AG, Lausanne, Switzerland) can noninvasively record the IOP pattern for up to 24 hours at a time. "The Triggerfish measures the radial deformation of the eye shape near the corneoscleral junction due to changes in IOP and ocular volume," said Rene Goedkoop, MD, chief medical officer, Sensimed. The new device is approved for use in many international markets and is in the process of garnering U.S. approval. Changes in eye shape cause tiny deformations of the device, which records these changes in mV that do not directly convert to mm Hg, he said. Measurements are collected for 30 seconds every five minutes during a full 24-hour period. The data are transmitted wirelessly to a receiver secured around the periorbital skin, which in turn is wired to a small data storage unit the size of a smartphone. Kaweh Mansouri, MD, University of Geneva, Switzerland, has significant experience with the device. "Twenty-four hour IOP monitoring can provide us with critical clinical information that we currently have no access to outside of a research sleep laboratory," he said. Dr. Mansouri explained that while the Triggerfish's output is not by Tony Realini, MD Sensimed Triggerfish is a noninvasive, soft hydrophilic contact lens used for monitoring IOP up to 24 hours. Source: René Goedkoop, MD in mm Hg, the curve shape is very informative of circadian IOP patterns. Validating this has been challenging, though, because with the device in place, IOP cannot be checked by any other means so physicians cannot independently establish IOP. As a step toward validating the device and establishing its clinical value, he recently conducted an innovative study in which he monitored patients' IOP during the water-drinking test. "After consuming 1 liter of water, IOP is known to rise over the next 30 to 60 minutes before normalizing," Dr. Mansouri said. "This is thought to be due to an increase in episcleral venous pressure and possibly also to transient choroidal expansion." If the Triggerfish data is indicative of IOP, he said, then the device's output should demonstrate IOP changes following the water-drinking test. His patients wore the Triggerfish in one eye and underwent serial IOP measurements using Goldmann applanation tonometry in the fellow eye. "In fact, in a majority of patients, we did see increased signal from the Triggerfish in parallel with increased IOP by Goldmann tonometry in the fellow eye," Dr. Mansouri said. Interestingly, however, the timing of the peak IOP was 30 minutes by Goldmann tonometry and 60 minutes by Triggerfish. "Also, normalization of IOP took longer by Triggerfish than by Goldmann." A similar provocative test was conducted by Dr. Goedkoop, who monitored Triggerfish output with patients first in the sitting position and then in the supine position, which is also known to raise IOP. Subjects in this study underwent Goldmann IOP monitoring in the fellow eye. He reported that the Triggerfish signal increased after changing from sitting to supine and normalized again after returning to the sitting position. Like in Dr. Mansouri's study, the time to peak IOP was later with Triggerfish compared to Goldmann and also took longer to normalize. "This may be due to differences in measurement technique and effects of corneoscleral biomechanical properties," Dr. Goedkoop said. These data show that Triggerfish output reflects the expected IOP rise from both the water-drinking test and positional change. The latter is particularly important because as Dr. Mansouri said, "IOP is highest when the ophthalmologist is sleeping. Triggerfish gives us something else we can't get right now: IOP outside of typical doctors' office hours." Because the device obviously has more components than a typical vision-correcting contact lens, there are potential tolerability issues with its use for 24 hours, particularly in patients who are not accustomed to wearing contact lenses. Katrin Lorenz, MD, University of Mainz, Germany, conducted a post hoc analysis of data pooled from five Triggerfish studies in which comfort was scored using a standardized comfort survey. "The comfort level as scored by subjects was constant during the 24hour wearing period and was similar for repeated exposure one week later and was similar between the various study populations," she said. Corneal swelling is also a potential side effect of contact lens wear and may be particularly relevant to a device that works by detecting changes in the shape of the cornea. Florentina Freiberg, MD, Julius Maximilian University, Würzburg, Germany, evaluated changes in central corneal thickness (CCT) in the same pooled data set used by Dr. Lorenz, which included four 24hour studies and one nine-hour overnight study. She reported that CCT was higher than baseline first thing in the morning among those in the nine-hour study, but among the patients who wore the Triggerfish for a full 24 hours, CCT had normalized by the end of the full circadian cycle. This is similar to the patterns typically seen with vision-correcting soft contact lenses and is not unique to the Triggerfish. The Triggerfish contact lens sensor may turn out to be an important clinical tool in the diagnosis and management of glaucoma. According to Dr. Goedkoop, the manufacturer has submitted the necessary regulatory documentation to the FDA requesting its approval and is awaiting a determination. EW Editors' note: Dr. Realini is associate professor of ophthalmology, West Virginia University, Morgantown, W.V. He can be contacted at realinia@wvuhealthcare.com. Drs. Goedkoop, Lorenz, and Mansouri have financial interests with Sensimed AG. Contact information Freiberg: f.freiberg@augenklinik.uni-wuerzburg.de Goedkoop: rene_goedkoop@sensimed.ch Lorenz: katrin.lorenz@unimedizin-mainz.de Mansouri: kawehm@yahoo.com

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