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Glaucoma and tonometry

Currently, there is no commercially available method to monitor intra-ocular pressure (IOP) over prolonged periods of time. According to researchers and clinicians a device with this capability would be a major asset in the diagnosis and treatment of glaucoma. The World Health Organisation reports that glaucoma affects 70 million people worldwide.¹ This disease is characterised by a gradual and irreversible loss of peripheral visual field because of optic nerve pinching. Depending on the type of glaucoma, treatment may include medication (in the form of eye drops) or surgery to lower the pressure in the eye and prevent further damage to the optic nerve. As yet, there is no cure for glaucoma; however, early diagnosis and continuing treatment can preserve eyesight.

The most widespread method for measuring the IOP is applanation tonometry. This method is routinely used by ophthalmologists and uses a probe brought into contact with the eye to locally flatten the cornea. In applanation tonometry, the IOP is deduced from the resistance of the cornea to applanation. To perform this measurement, the patient has to attend the ophthalmologist’s office and anaesthesia of the cornea is necessary prior to measurement.

The most important limitation of applanation tonometry is that it cannot measure the variation of the IOP over time. Detailed knowledge of the behaviour of the IOP in glaucoma patients during the day is of paramount importance, because increased IOP and wide IOP variations are considered to be major risk factors for the progression of glaucoma. The sensing contact lens presented in this article defines a new approach to IOP measurement. It allows IOP to be monitored for prolonged periods, regardless of the patient position and activity, and it is noninvasive. This product is currently under development under the Healthy Aims project, which is funded by the European Union.²

The sensing contact lens

Figure 1: (click to enlarge) Photo of the sensing contact lens in its wired version.

The method chosen for measuring the evolution of the IOP in the sensing contact lens is indirect and correlates the spherical deformation of the eyeball with the changes in IOP. It has been proven by previous studies that a change of IOP of 1 mm of mercury (Hg) causes the radius of curvature of the central cornea of the human eye to change by approximately 3 µm over a typical radius of 7.8 mm. To measure this change, a soft contact lens was designed in which microfabricated strain gauges were inserted in a Weatstone bridge configuration. This allows high-precision measurements to be made and compensates for thermal drift. The sensor is made from two active strain gauges, which are placed circumferentially, and two passive strain gauges for thermal com- pensation, which are placed radially. The sensor is stimulated by a DC current and gives an output voltage that is proportional to the strain and consequently to the IOP variation. Figure 1 shows this soft contact lens and indicates the location of the strain gauges. The sensing contact lens is not an absolute pressure sensor, it only measures the IOP changes.

Verification of the measurement principle

Figure 2: (click to enlarge) Recording of the sensing contact lens placed on an enucleated porcine eye.

Before the start of trials on humans, the assessment of the function of the sensing contact lens was tested on enucleated porcine eyes, because porcine eyes have dimensions that are similar to human eyes.³ To do this, the eye is cannulated with a needle placed in its posterior chamber and connected to a syringe pump to induce controlled variations in IOP. The measurement of IOP is made simultaneously by a pressure sensor and by the sensing contact lens, which allows both signals to be compared. Six enucleated porcine eyes were stimulated by applying ramps of increasing and decreasing IOP, ranging from absolute values of pressure of 17–29 mm of Hg. In all cases, the voltage variations measured with the contact lens correlated well with the IOP, as measured with the pressure sensor. Figure 2 shows a four-minute recording of the pressure sensor and the output signal of the sensing contact lens.

First trials on humans

Figure 3: Sensing contact lens placed on the patient’s eye.

The sensing contact lens with wires was tested on healthy patients at Fribourg Cantonal Hospital (Fribourg, Switzerland). Seven patients used the contact lens device for 40–80 min. The sterilised sensing contact lens was positioned on the patients’ eyes by an ophthalmologist so that the wire from which the signal was recorded did not cause any major discomfort to the patients (Figure 3) and did not affect eye movement. Various signals of interest were recorded during the trials, in particular a signal that could be correlated to the heart rate was measured on some patients (Figure 4); this was identified as being the ocular pulsation. The heartbeats induce a small variation of the IOP (the amplitude of this signal is known to be of approximately 2 mm of Hg in healthy patients), which results in a small corneal deformation that was measured by the sensing contact lens. The amplitude of the changes in IOP in patients suffering from glaucoma have an amplitude that is approximately 10 times higher than the ocular pulsation, which clearly shows the capabilities of the sensor for glaucoma detection and diagnostic.

Future wireless developments

Figure 4: (click to enlarge) Ocular pulsation recorded from the contact lens placed on a human eye.

Further developments are required to bring the sensing contact lens to the market. If there is no major problem in wearing a contact lens with a wire coming out of it for one or two hours, it is evident that having a contact lens that works in a wireless way would be much more comfortable for the patient and would probably also lead to better and more reliable signals. A wireless version of the contact lens would, in particular, allow patients to have a normal activity while recording the evolution of their IOP during the whole day.

Figure 5: (click to enlarge) Photomontage of the wireless sensing contact lens currently being developed.

Figure 5 shows the concept of wireless sensing contact lens that the group is trying to bring to the market under the Healthy Aims project. As in the prototype with wires, the sensors are strain gauges, but a telemetry microchip is added inside the contact lens, as well as an antenna. The antenna will allow the transfer of energy from a pair of glasses worn by the patient, in which an emitter coil, electronics and a battery will be embedded. The antenna will also be used to transmit the data measured by the strain gauges to the glasses for storage. To do all these operations, a telemetry microchip is required inside the contact lens. The integration of all elements into the contact lens without blocking the visual field is, of course, a major challenge.

Dr Arnaud Bertsch is Senior Scientist, Matteo Leonardi is a Ph.D. student and Professor Philippe Renaud is the Head of the Microsystems Laboratory at the Swiss Federal Institute of Technology, EPFL, STI-LMIS4, BM 3.124 – Station 17, CH-1015 Lausanne, Switzerland, tel. +41 21 693 66 06, e-mail: arnaud.bertsch@epfl.ch, http://lmis4.epfl.ch.