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Review Glaucoma Contact Lens Sensor Triggerfish – What Do We Know? Christoph Faschinger Deputy of the Head of the Department of Ophthalmology Medical University, Graz, Austria M ultiple measurements of the intraocular pressure (IOP) at different times during the day and at night provide information on the peak and trough values, and the number and magnitude of short- and long-term fluctuations. Goldmann applanation tonometry (GAT) is generally used only once or a few times during office hours. The ideal would be a safe, well-tolerated, and valid 24- hour measurement. The contact lens sensor Triggerfish ® (SENSIMED AG, Lausanne, Switzerland) is claimed to measure changes in corneal curvature induced by changes in IOP. After animal experiments, multiple studies in healthy and glaucoma patients were performed, in some cases revealing good correlation with tonometer values, in others poor or no correlation at all. The interpretation of the 24-hour profiles is not standardised, and there is no agreement upon definition of a fluctuation. Comparability between mmHg and an electrically measured value cannot be established. Despite the fact that safety and tolerability of the device are good, the absence of definitive validation of the results means that the product cannot yet be recommended for daily clinic or office use. This product is also no longer marketed as providing ’24- hour IOP profile’ or ‘continuous monitoring of IOP fluctuation’, but now rather as recording a ’24-hour profile of ocular dimensional changes’. Keywords Intraocular pressure, sensor contact lens, strain gauge, validity of “pressure” profile Disclosure: Christoph Faschinger received honoraria for lectures from Allergan and AMO. Editorial assistance was provided by Sven Obholzer, MD in translating work. No funding was received in the publication of this article. This study involves a review of the literature and did not involve any studies with human or animal subjects performed by any of the authors. Open Access: This article is published under the Creative Commons Attribution Noncommercial License, which permits any non-commercial use, distribution, adaptation and reproduction provided the original author(s) and source are given appropriate credit. Received: 25 July 2016 Accepted: 1 August 2016 Citation: European Ophthalmic Review, 2016;10(2):113–6 Corresponding Author: Christoph Faschinger, Auenbruggerplatz 4, A-8036 Graz, Austria. E: The main risk factor for glaucoma damage is an intraocular pressure (IOP) above an undeterminable threshold value for an individual patient. The IOP is not constant but is subject to multiple physiological and pathological fluctuations. These can vary in magnitude and duration, for example, the pulsation of retinal vessels, Valsalva manoeuvre, changes from upright to supine position, and circadian rhythm. The reference standard to determine IOP is Goldmann applanation tonometry (GAT), which is usually performed only once and seldom multiple times during office hours. This only gives information of a few seconds out of 86,000 seconds in a day, and as such decreases the value of information gained. Measurements in sleep laboratories are cumbersome. In addition to this there is still critical discussion about weather fluctuations pose an additional independent risk factor for glaucoma progression. 1,2 Multiple IOP measurements take time and the repeated topical anaesthetic used with GAT may damage the cornea. An alternative would be multiple or alternating IOP measurements with non-contact or other tonometers, which however are not the reference standard. Similar to non- invasive 24-hour electrocardiogram (continuous) or blood pressure measurements (intermittent), continuous IOP measurement would perhaps provide additional important information. This idea was seemingly realised with the contact lens sensor (CLS) Triggerfish ® (SENSIMED AG, Lausanne, Switzerland). This medical device received CE certification in 2008 and American Food and Drug Administration (FDA) approval in 2016. The review describes the current state of knowledge about this medical device, especially discussing the question whether an application in daily clinical or office use for diagnosing and therapeutic monitoring of glaucoma patients is necessary and/or recommended. Contact lens sensor Triggerfish – product description The theory published by Leonardi et al., was, that IOP could be measured indirectly by a strain gauge embedded in a contact lens. 3 The basis for this theory was that the corneal curvature in the limbal area (diameter of the strain gauge at 11.5 mm) changed with variation in IOP, and that this could be measured. Two circular strain gauges made from platinum-titanium (7 µm) and a loop antenna made from gold (30 µm) for data transfer and a microchip (50 µm) for wireless data management were embedded in a silicone contact lens (diameter 14.2 mm), through a complicated manufacturing process. For better patient tolerance it was coated with an oxygen plasma. Power is supplied by the recorder and an antenna which is stuck around the orbit. Measurements are taken at 5 minute intervals for 30 seconds, within these 30 seconds 10 times with 300 datasets. The gained data are transferred wirelessly via this antenna to a recorder which is worn around the waist. During the 24-hour measurement period spectacles with metal frames must be avoided, but all other normal daily activities should be performed and documented in a diary. This is with the exception of activities that include moisture and water (e.g. swimming, showering etc.) and TOU CH MED ICA L MEDIA 113