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

Dear Editor, I n a pilot investigation in two of our own eyes, we tried to assess the Triggerfish (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 Tgriggerfish recordings at each step. Second, the same IOP elevation procedure was performed, but without the Triggerfish and GAT measurements instead at each step (Fig. 1). The GAT IOP values and the Triggerfish 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. The correlation coefficient between GAT and Triggerfish 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 Triggerfish recordings and artificially increased IOP values measured by GAT. Such a correlation could not be found. The contact lens-supported IOP measuring device Triggerfish 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 Triggerfish data found in the present study implies either that the IOP and the corneal curvature are not correlated, or that the Triggerfish 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. 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: Triggerfish recordings (arbitrary units) after subtraction of the lowest value of the corresponding sample.