Anterior Segment Cataract
Intraocular Pressure Contact Lenses – Suitable for Everyday Use Yet? Christoph Faschinger and Georg Mossböck Medical Doctors, University Eye Clinic, Medical University Graz
Abstract
The importance of intraocular pressure (IOP) as a relevant and treatable risk factor for the development or progression of glaucoma is evident. In managing this disease, to take only a single or a few measurements of this crucial parameter is definitely suboptimal. Technical research has developed contact lenses with embedded structures that are able to measure the IOP or surrogates over a longer period of time. One such system measures changes of the peripheral corneal curvature by means of strain gauges, and another measures the IOP by means of a piezoresistant membrane. Comfort is increased by wireless data transmission and miniaturisation of devices that do not influence visual acuity or daily/nightly activities. The safety and tolerability of the Triggerfish® contact lens is good and proved by several clinical observational studies. Missing are studies that confirm the reliability and reproducibility of the results. So far, no device can be recommended for everyday use in an ophthalmological office.
Keywords Intraocular pressure, continuous monitoring, contact lenses
Disclosure: The authors have no conflicts of interest to declare. Both authors received financial support from Pfizer, Allergan and MSD to hold lectures at different symposia. Acknowledgement: Triggerfish® was sponsored by Pfizer Ophthalmics. Received: 26 August 2011 Accepted: 9 October 2011 Citation: European Ophthalmic Review, 2011;5(2):136–8 Correspondence: Christoph Faschinger, Universitäts-Augenklinik, Auenbruggerplatz 4, A-8036 Graz, Austria. E:
christoph.faschinger@medunigraz.at
Glaucoma is defined as a chronic, progressive optic neuropathy with loss of retinal ganglion cells and their nerve fibres. One of the most important and the only modifiable risk factor is intraocular pressure (IOP), which gets higher than the (unknown) individual tolerance level (of the optic disc). IOP is not a static value during 24 hours. The fluctuations are of varying amounts in healthy and glaucomatous eyes.1
IOP is subject to a
chronobiological pattern and is influenced by changes of head position, emotions, sports activities or the playing of wind instruments.
The gold standard for measuring IOP is Goldmann applanation tonometry, usually used once during the daytime, which gives information over a period of about five seconds. In patients with progressive glaucoma or in patients with advanced glaucoma a so-called diurnal curve is performed. This means three to five IOP measurements are taken as a minimum within office hours from Monday to Friday. Does this provide enough information about IOP? There is only a 60 % chance to identify peak IOPs of individuals between 08:00 and 16:00.2 Almost nothing is known about IOP during sleeping hours (at least one-third of a day) except a few data from sleep-laboratory studies.3,4 These sophisticated examinations showed that the lowest IOP occurred in the final wake measurement in the light, and the peak IOP occurred in the final measurement in the dark. An increased amount of information is possible by repeated self-tonometry (only when awake), permanent continuous monitoring (inside the eye with an intraocular lens) or temporary continuous monitoring (contact lens).
External Intraocular Pressure Measurement by a Contact Lens
When the distensibility of rabbit eyes was measured using circumference gauges, changes were found with an increase in
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IOP. Distensibility was slightly higher in the sclera than in the cornea over the range 30–90 mmHg. The change in the angle where the cornea joins the sclera was about 0.020–0.016 radians per mmHg over the range 10–45 mmHg.6
Greene and Gilman were the first to
use strain gauges embedded in customised contact lenses in rabbit eyes.7
Strain gauges are made of specific metals (e.g., platinum) or silicon and a strain induces an increase of the resistance (in Ohms) measured by a Wheatstone bridge. The miniaturisation of microchips and progress in radiofrequency transmission have helped technicians to optimise the devices.
Triggerfish
It is a silicone contact lens with a diameter of 14.4 mm and contains two active strain gauges made out of platinum–titanium (wire loops of 7 μm diameter), one of diameter 11.5 mm, two passive strain gauges for temperature compensation, one antenna (gold, 30 μm) and one microprocessor (50 μm thick) (Figure 1). The contact lens is coated with a hydrophilic substance for better tolerability. Power and data transfer work telemetrically and are wireless from and to an antenna, worn as a ring around the orbit (Figure 2). This antenna is connected to a small recorder, which switches off automatically after 24 hours. The measurements take place in intervals of 8.5 minutes for a period of 1.5 minutes (older software) or in intervals of 5.0 minutes for one of 1.0 minute (new software), which results in 144 or 240 measurements, respectively. The result is a profile in arbitrary units, because the contact lens measures only the change of the curvature of the peripheral cornea caused by changes of the IOP and not the IOP per se. No nomogram exists
Leonardi et al. developed the idea of a contact lens with embedded strain gauges and called their product Triggerfish (Sensimed, Switzerland).8,9
© TOUCH BRIEFINGS 2011
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