Intraocular Pressure Contact Lenses – Suitable for Everyday Use Yet?
to convert these measurements. Artefacts caused by blinking or very bright light are filtered.
The tolerability and the safety of the contact lenses were good in patients with ocular hypertension and glaucoma. None of our patients (18 eyes),10,11 series12
but two out of 15 patients in Mansouri and Shaarawy´s
discontinued the recording over 24 hours (one patient had severe dry eye, the monitoring of another was interrupted after 17 hours). These authors described ‘significant’ and ‘important’ fluctuations in two of the 24-hour profiles despite different scales on the y-axis and without a definition of a fluctuation – one or more points of measurements? The authors concluded that this technology could possibly lead to improved care of glaucoma patients. Good tolerability and functionality were confirmed also by De Smedt et al.13
These
authors examined 10 healthy volunteers to evaluate the tolerability and comfort by a comfort score and the reliability of signal transmission over a 24-hour period. There was no increase in corneal thickness, but there was a low shift to myopia. Immediately after removal of the Triggerfish the visual acuity was reduced. In seven out of the 10 subjects, the Triggerfish was decentred in the vertical axis, which may be the result of a too-tight fit.
Two Other Approaches
The same method of indirect measurement of the IOP by recording changes in the peripheral cornea of pig eyes and of one human volunteer (for two hours) was used by a Spanish group.14
Instead of
metallic strain gauges they used a sensor film based on a conducting all-organic nanocomposite bilayer film. The device is a prototype and still wire connected and showed good correlations between changes of resistance and IOP in rapid IOP changes.
The third group, from the University of California, Davis, used polydimethylsiloxane elastomers and modified the material with powdered silver in a predetermined pattern to create conductive wires.15
Usefulness of Results?
There are still no clinical data as to the reproducibility of these results. Leonardi et al.9
carried out their laboratory work to prove
the functionality of the device on enucleated porcine eyes; they increased the IOP in rapid shots and obtained a good correlation with the transmitted profile of the contact lens sensor. However, does this reflect reality? Do we have very short and rapid changes in our IOP? Yes, we do, but rarely. We do it in the Valsalva manoeuvre, when playing wind instruments or in yoga exercises with the head upside down. Is this contact lens able to measure changes at the peripheral cornea that result from very slow diurnal, chronobiological changes of the IOP? How much influence does the thickness or the viscosity of the cornea have? There are no data on correlation between corneal hysteresis and Triggerfish profiles. How high is the dependence on a perfect fitting of the contact lens to avoid the lens slipping off the limbal region? We tried to repeat Leonardi´s experiments and used enucleated human eyes instead of porcine eyes. We could not produce a corresponding profile when we increased the IOP of the globe stepwise for 10 minutes. We obtained good correlations only when we increased the IOP rapidly over seconds. In situ examinations differ totally from those of enucleated eyes because of adaptive changes that influence IOP, such as IOP-dependent outflow, ciliary body volume, etc., which are seen with longer term fluctuations but
EUROPEAN OPHTHALMIC REVIEW
This has a very thin (7 μm) peripheral strain gauge made of a platinum–titanium wire 11.5 mm in length, which measures the changes of the peripheral corneal curvature that result from changes of the intraocular pressure. The three-wire antenna is a golden and rectangular microchip. Within the antenna is the passive strain gauge. The overall diameter of the silicon contact lens is 14.4 mm.
Figure 2: Triggerfish Contact Lens in the Left Eye Figure 1: Triggerfish Contact Lens
So far, the patterns are located centrally and therefore disturb vision. No animal or human trials are published.
Around the orbit a loop antenna is glued at the skin for wireless data and power transmission. It is connected by wire to a recorder worn around the belly.
are not encountered in short, rapid changes caused by Valsalva, posture, etc. Additionally, we checked habitual (vertical, horizontal, head down) changes of IOP and obtained no correlations with the IOP.16
In summary, in our opinion, the Triggerfish is not yet ready for everyday use in an ophthalmological office. Several controlled examinations are needed before we can interpret effectively the profile obtained by the wireless contact lens sensor. We have to know what noise there is and what artefacts there are. We also have to
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