Pressure detector

16 August 2010 | By David Wilson

Glaucoma, the second most common cause of blindness worldwide, is an asymptomatic, progressive and irreversible disease that leads to blindness unless treatment is provided.

The disease causes an increase in pressure inside the eye. When the pressure is too high, damage occurs to the optic nerve, which can cause blind spots at the outer edges of the field of vision. As the damage worsens, the visual field can shrink, leading to tunnel vision or loss of central vision.

To determine the intraocular pressure (IOP) of the aqueous humour inside the eye, doctors use a tonometer. Currently, however, the only way to obtain data on the dynamic behaviour of the IOP is to repeat such measurements several times a day.

However, such infrequent measurements may fail to detect an elevated IOP because the pressure of the eye varies during the day and often peaks during sleep. As a result, the disease is often diagnosed only after damage to the optic nerve has occurred, and the disease keeps progressing in many patients due to inadequate treatment.

Lausanne-based Sensimed has developed a solution to the problem. Called Triggerfish, it will enable doctors to continuously monitor the IOP of patients at risk of glaucoma over a period of 24 hours, enabling them to diagnose the disease more effectively and tailor the treatment to patients more appropriately.

According to Matteo Leonardi, who co-founded Sensimed with Sacha Cerbonico, Triggerfish uses a smart disposable contact lens that continually monitors the pressure of the eye.

A circular antenna taped around the eye not only powers the electronic components in the contact lens, but also acts to receive the data from the lens. The antenna is connected to a battery-powered recorder the size of an iPod through a thin flexible cable, which stores the information until it can be wirelessly uploaded to an ophthalmologist’s computer at the end of the monitoring session.

To monitor the pressure of the eye, the soft hydrophilic silicone disposable contact lens has within it a micro electro-mechanical strain-gauge sensor made from a sandwich of polyamide and platinum that runs around the circumference of the lens. When the strain gauge bends following a change in the pressure of the cornea, it stretches, and the change in its resistance is then measured by a custom-built, application-specific integrated circuit, which then encodes and transmits the data via an antenna to the receiver around the eye.

A circular antenna taped around the eye not only powers the electronic components in the contact lens, but also acts to receive the data from the lens

Because the lens is powered via the received radio waves, it does not need to be connected to a battery and the embedded components are positioned in the lens in such a way that they do not interfere with the patient’s vision. The lens is fitted by the ophthalmologist and when the patient returns the next day the ophthalmologist removes the lens and receiver, obtaining a complete record of IOP changes over the preceding 24 hours.

Although the new system does not measure the pressure of the cornea directly, it does monitor pressure fluctuations. According to Leonardi, this is a key parameter for ophthalmologists who are concerned about how pressure fluctuates in a patient suffering from glaucoma. The system allows them to determine how many pressure peaks occur and how long they last. Armed with this knowledge, they can make the proper diagnosis and provide more personalised medication.

Moreover, since the doctor will always take an IOP measurement at the beginning and at the end of the 24-hour monitoring session with the usual tonometer, he or she will always know the initial and final pressures in the eye, which can then be related to the data obtained from Triggerfish.

The Sensimed Triggerfish, which has already obtained the CE mark, is undergoing trials and is commercially available in selected parts of the world. ’The device is easy to use and has facilitated and improved patient care substantially,’ claimed Dr Kaweh Mansouri, who has been using the Triggerfish at the University Hospital in Geneva.

The key facts to take away from this article

» Glaucoma is often only diagnosed after nerve damage has occurred
» Infrequent measurement may fail to detect an elevated IOP
» Triggerfish monitors IOP continuously over a period of 24 hours
» The system can enhance diagnosis and enable more suitable treatment