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Editors Selection IGR 22-4

Intraocular Pressure: Towards continuous monitoring?

Comment by Kaweh Mansouri on:

54819 Rollable and implantable intraocular pressure sensor for the continuous adaptive management of glaucoma, Piffaretti F; Barrettino D; Orsatti P et al., Conference proceedings : Annual International Conference of the IEEE Engineering in Medicine and Biology Society, 2013; 2013: 3198-3201

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Glaucoma is a 24-h disease and the absence of data on individual patient's circadian IOP characteristics constitutes a gaping deficit in glaucoma management. Yet, for years, it seemed that medical technology was plunging ahead, while tonometry was standing still. This has changed in recent years, with temporary (contact-lens based) and permanent (intraocular) sensors being developed. The latest attempt is by Piffaretti and Barettino from the University of Applied Sciences of Southern Switzerland who present the proof of principle of a novel implantable device for continuous 24-h IOP monitoring.

Their technology is based on a 1 x 1 mm piezo-resistive pressure sensor with a absolute pressure range of 0-1.5 bars (1 mbar = 0.75 mmHg). The challenge here is to keep the sensor as small as possible while maintaining high sensitivity (1.3 mbar). Radiofrequency identification (RFID) is used to wirelessly transfer data via electromagnetic fields and power the sensor in return. The energy range is limited to 20 mm, which equates to having a close outside reader, for example embedded in spectacles. These elements are set on a printed circuit board that can be rolled up and inserted through a 1.4-2.5 mm, as in standard cataract surgery. Finally, with all implantable sensors, the biocompatibility and hermeticity of the coating is an issue. For this purpose, Parylene C, an FDA-approved material, was used. The technology is very similar to an earlier one that has been used in rabbit eyes,¹ with the notable exception that it is rollable and can be inserted into the eye through a smaller opening.

Of interest, the authors used only off-the-shelf components for their device, which may partially explain the low yield (50%) and sensitivity (60%) of the device. The quality of the rollable circuit board is critical (and may have been suboptimal here), as the sensing elements can be damaged during the insertion process. Seen from the cost angle, however, the industrial availability of the components implies low production costs. These are important shortcomings that need to be addressed prior to investigation in humans.

The ideal 24-h IOP monitor would reunite the following features: (1) well-tolerated and safe; (2) accurate; (3) highfrequency of measurements (> 40 Hz) to detect ultra short events; and (4) affordable. Currently, none of the devices in use or under investigation fulfills more than two of these.^2,3 Once one or several devices have been validated for clinical use, bigger questions will arise. The most challenging will be how to best deal with the data avalanche that continuous monitoring provides. If, as empirical experience suggests, for a given patient IOP ranges (e.g.) from 15 to 25 mmHg over a 24-h period, how will the clinician know which value to use?

After the Pascal DTC, the Triggerfish contact lens sensor, the heirs to Dr. Hans Goldmann continue their quest for improved IOP monitoring. Numerous other groups are actively working in this challenging field, which raises the prospect that the (cumbersome) acronym AIOPM (ambulatory IOP monitoring) may soon find its way into our daily practice.

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