IOP SENSOR

Studies conducted so far with a noninvasive contact lens-based system (TriggerfishR, Sensimed) designed to provide continuous IOP-related monitoring demonstrate it is safe, welltolerated and reproducibly records 24-hour patterns. The contact lens sensor (CLS) measures IOP indirectly by detecting three-dimensional changes in eye shape near the corneoscleral junction, and it records the fluctuations in units of millivolts for which there is no calibration for direct conversion to mmHg.

While the device is generating excitement for its potential to bring new understanding about the pathophysiology of glaucoma and the development of personalised approaches to disease management, research is still focusing on the basic issue of how to interpret the data it provides. “The CLS is speaking to us about IOP in a new language, but first we need to decipher the alphabet,” said Rene Goedkoop MD, chief medical officer, Sensimed, Lausanne, Switzerland, speaking at the 2013 ARVO meeting in Seattle. “Elucidation of the relationship between the CLS measurements and IOP is an important, but complicated issue because the data cannot be obtained concurrently in the same eye. Furthermore, we know that tonometry itself is relatively unreliable and has a lot of variability.”

As of May, 2013, more than 630 patients had been entered into clinical trials using the CLS and 26 studies were ongoing. Researchers aiming to validate the device have conducted studies to determine whether it can capture patterns that are known for IOP and to see if its measurements can be related to tonometrydetermined IOP. At ARVO 2013, Ungaro and colleagues from Parma, Italy, reported on using the CLS in healthy subjects undergoing a water drinking test to provoke increased IOP. A similar study enrolling patients with primary open-angle glaucoma (POAG) washed out from treatment was reported by Mansouri et al. at the American Glaucoma Society 2013 meeting.

In both investigations, the CLS output correlated with IOP measured by Goldmann applanation tonometry (GAT) in the contralateral eye. Both studies found that the peak measurement occurred later using the CLS than with GAT. Other investigations at ARVO 2013 found that changes in the CLS signal were not related to changes in IOP measured by tonometry. In a multicentre study including POAG patients who underwent continuous CLS monitoring in one eye for 3-24 hours, results from regression modeling showed a low correlation between the CLS and tonometry readings (GAT and iCare Pro) in fellow eyes and in the same eyes after removing the CLS.

“The low correlation between these techniques may be due to a difference in measurement methodology and/or location (central vs. peripheral cornea), or to some unknown reasons. We believe that better understanding is needed before this device can be applied to patient care and in research studies,” said Milko Iliev, MD, professor of ophthalmology, consultant and head of the glaucoma service, University of Bern, Switzerland.

The CLS is also being used to profile IOP patterns in a number of treatment intervention studies that are actively accruing patients, and a registry capturing pre- and post-intervention 24-hour curves is active at 10 sites and will be expanded to more than 25 sites worldwide. At ARVO 2013, investigators from the Singapore National Eye Centre reported their experience using the CLS sensor to study circadian IOP in newly diagnosed primary angle closure glaucoma treated by laser peripheral iridotomy.

In France, Nordmann et al. are studying POAG patients treated by selective laser trabeculoplasty (SLT). Data available from the first three patients showed dramatic post-SLT decreases in night-time IOP in two patients and no change in the third.

“These data have to be interpreted with caution given the low number of subjects in the two aforementioned studies,” Dr Goedkoop said. Further studies are warranted, which is why the CLS is also being used to profile IOP-related patterns in a number of active interventional treatment studies and a registry capturing pre- and post-intervention 24-hour curves that is active at 10 sites and will be expanded to more than 25 sites worldwide, he concluded.