Acta Ophthalmologica 2014

Assessment of the TriggerfishTM contact lens sensor for measurement of intraocular pressure variations Gordana Sunaric-Megevand,1 Peter Leuenberger2 and Paul-Rolf Preußner3 1

Clinical Research Center Memorial A. de Rothschild, Geneva, Switzerland; 2 Centre Ophtalmologique de Rive, Geneva, Switzerland; 3University Eye Hospital, Mainz, Germany doi: 10.1111/aos.12455

Dear Editor, n a pilot investigation in two of our own eyes, we tried to assess the TriggerfishTM (Sensimed, Switzerland) recordings in comparison with artificially increased intraocular pressure (IOP) controlled by Goldmann applanation tonometry (GAT) measurements. Increased intraocular pressure was increased by a ring pressing against the eye through the lids, with either opened or closed eyes. The corresponding device has been described and assessed earlier (Preußner & Duran 1996). The force of the ring pressing to the eye is proportional to the gravitational force of little weights pulling on a lever. The IOP increase caused by this device can be measured by GAT. It depends on individual parameters such as eye size and lid rigidity and therefore needs an individual calibration for each eye. We performed two measurement series, one in the closed eye of GSM and one in the open eye of PRP (Fig. 1). First, IOP was stepwise increased by increasing the weights on the device in steps of 8 g from 0 to 32 g and decreased again the same way, with TgriggerfishTM recordings at each step. Second, the same IOP elevation procedure was performed, but without the TriggerfishTM and GAT measurements instead at each step (Fig. 1). The GAT IOP values and the TriggerfishTM recordings were compared with each other after subtraction of the lowest values of either measurement (lower image of Fig. 1). All measurements were taken in the presence of a Sensimed representative.

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Fig. 1. Measurement with artificial increased intraocular pressure (IOP) elevation. Upper subimage: IOP steps produced by the IOP elevation device and measured by Goldmann applanation tonometry (GAT) are shown for the corresponding weights pulling on the lever of the device (red: data of GSM, blue: data of PRP). Lower subimage: IOP as preset by the elevation device (horizontal axis) after subtraction of the lowest value of each sample (GSM or PRP). Vertical axis: TriggerfishTM recordings (arbitrary units) after subtraction of the lowest value of the corresponding sample.

The correlation coefficient between GAT and TriggerfishTM IOP differences of GSM of 0.28 is statistically not significantly different from zero (p = 0.46). The correlation coefficient of the corresponding data of PRP of 0.19 is statistically not significantly different from zero (p = 0.62). This investigation was performed to quantify the correlation between TriggerfishTM recordings and artificially increased IOP values measured by GAT. Such a correlation could not be found. The contact lens-supported IOP measuring device TriggerfishTM uses the change in corneal curvature as a measure of change in IOP (Leonardi et al. 2004). The absence of a correlation between the artificially increased IOP and the corresponding TriggerfishTM data found in the present study implies either that the IOP and the corneal curvature are not correlated, or that

the TriggerfishTM output is not correlated with the corneal curvature. The former possibility is consistent with the observations by Preußner & Duran (1996) that the artificially increased IOP does not significantly change the eye’s refraction. It is also consistent with the findings of Lam & Douthwaite (1997) that artificially increased IOP from head-down posture did not affect corneal curvature as well as with the findings of Hjortdal et al. (1996) where pressure-induced deformation of normal and excimer laser-ablated human corneas were not significant. On the other hand, a lack of a correlation between IOP and the corneal curvature radius in the present study contradicts the findings of Leonardi et al. (2004). Their experiments with enucleated porcine eyes demonstrated a linear relationship between corneal radius (R) variations and IOP variations of approximately oR/ o(IOP) = 3 lm/mmHg.

Acta Ophthalmologica 2014

This seeming contradiction has a simple physical explanation. When the volume inside a sphere consisting of an elastic skin and filled with an incompressible fluid-like water is increased, the pressure necessarily increases as well. This implies an increase of the curvature radius as well, which thus can be used as a measure of the pressure increase inside the sphere. But this model oversimplifies the situation of a human eye. The IOP cannot only be increased by an increasing volume, but also by changing the eye’s geometry. In this case, the corneal curvature radius may not be a simple function of the IOP, and in fact, application of the device to artificially increase the IOP decreases the eye’s volume instead of increasing it. Deformations of the eye that change the IOP can be expected with all eye- and lidmovements. Such movements increase the IOP with deviations of the eye from an ideal sphere and decrease the IOP when the eye becomes more spherical. Any such movements may also have an impact on corneal curvature, potentially eliminating the correlation between corneal curvature and IOP that is seen in an isolated (enucleated) eye. This correlation also becomes invalid when the rigidity of the envelope of the eye (cornea and sclera) is not uniform but varies with location. In the case of a very thin and highly elastic sclera or cornea as seen in myopia or keratoconus, the normally increasing corneal radius with increasing IOP may even decrease because the higher elasticity in a certain area causes a corneal ectasia. Finally, the relationship between IOP and corneal curvature also depends on changes of corneal hydration (Hjortdal & Koch Jensen 1995). An initial increase in corneal radius from IOP elevation can later decrease because of decreased corneal water uptake. Generally, the relationship between IOP and corneal curvature is a complicated function that depends on the thickness and elasticity of cornea and sclera, on other anatomical structure influencing the moving eye’s geometry in the orbit, and on corneal hydration which is not stable in time. Thus, corneal radius cannot be simply used as a measure of IOP, and there are likely situations where corneal radius does not change at all despite changing IOP.

There are two plausible assumptions in the present investigation which, however, have not been verified in the study: the artificially increased IOP values are virtually the same or at least highly correlated with and without the TriggerfishTM lens in place, and the function of the TriggerfishTM is not systematically disturbed by the device for artificially increasing the IOP. The first assumption can be supported by the high reproducibility of the increased IOP values already shown in the earlier paper of Preußner & Duran (1996). With respect to the TriggerfishTM function, there are theoretically two possible modes of interference or disturbance, electromagnetic and mechanic. Electromagnetic interference is very unlikely because the lever with the ring pressing on the eye originally manufactured from stainless steel has been replaced by PMMA of the same geometry for the present investigation. This widely excludes any electromagnetic interference with the TriggerfishTM electronics. Mechanical disturbance is unlikely because the inner ring radius of 22 mm is sufficiently large to avoid direct mechanical interference with the contact lens. In addition, such a mechanical interference should produce a systematic offset of the TriggerfishTM results rather than a random scattering of the data. An indirect mechanical interference by changing the eye’s geometry, however, is very similar to a corresponding influence from lid- or eye-movements occurring also in normal life conditions. In summary, under the conditions of our study design, the TriggerfishTM was not able to record induced IOP variations and did not show any correlation to Goldmann tonometric values. For ethical reasons, we discontinued further measurements.

pressure on the central corneal curvature. Ophthalmic Physiol Opt 17: 18–24. Leonardi M, Leuenberger P, Bertsch A & Renaud P (2004): Steps toward non-invasive intraocular pressure monitoring with a sensing contact lens. Invest Ophthalmol Vis Sci 45: 3113–3117. Preußner PR & Duran A (1996): New device for artificially increased intraocular pressure. Graefes Arch Clin Exp Ophthalmol 234: 683–687.

Correspondence: Gordana Sunaric-Megevand, MD, FEBO Clinical Research Center Memorial A. de Rothschild 44 bis Ave Krieg CH 1208 Geneva Switzerland Tel: +41223474980 Fax: +41223474981 Email: [email protected]

Comparison of intraocular pressure in glaucoma subjects treated with Xalatan® versus generic latanoprost Patrick Egan,1 Alon Harris,1 Brent Siesky,1 Leslie Abrams-Tobe,1 Austin L. Gerber,1 Joshua Park,1 Steven Holland,1 Nathaniel J. Kim1 and Ingrida Januleviciene2 1 Department of Ophthalmology, Indiana School of Medicine, Indianapolis, Indiana, USA; 2Eye Clinic, Lithuanian University of Health Sciences, Kaunas, Lithuania

doi: 10.1111/aos.12321

Editor,

References Hjortdal J & Koch Jensen P (1995): In vitro measurement of corneal strain, thickness and curvature using digital image processing. Acta Ophthalmol Scand 73: 5–11. Hjortdal J, B€ ohm A, Kohlhaa M, Olsen H, Lerche R, Ehlers N & Draeger J (1996): Mechanical stability of the cornea after radial keratotomy and radial photorefractive keratectomy. J Refract Surg 12: 459– 466. Lam AKC & Douthwaite WA (1997): The effect of an artificially elevated intraocular

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any intra-ocular pressure (IOP)-lowering ophthalmic drugs, including prostaglandin analogues, beta-blockers, alpha agonists and CA inhibitors, are now available in generic versions. Such ophthalmic generic drugs have raised concerns regarding their decreased efficacy compared to that of the branded innovator drug (Stewart et al. 2002; Narayanaswamy et al. 2007). This report analyses a pilot study that examines the efficacy of Xalatanâ, a prostaglandin analogue eye drop, relative to that of generic

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Assessment of the Triggerfish contact lens sensor for measurement of intraocular pressure variations.

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