A 'selfie' that might save your sight. An implanted sensor could help people with glaucoma monitor the pressure in their eyes using a smartphone camera.
The second biggest cause of blindness after cataracts, glaucoma occurs when fluid builds up in the eye. This raises the pressure, damaging the optic nerve. Accurate pressure readings are crucial for giving the right treatment, but one-off measurements during check-ups produce variable results and can be misleading.
Vision care providers routinely monitor intraocular pressure with tonometers during eye examinations as well as observing the optic nerve. As glaucoma progresses there are visible changes in the shape and morphology of the optic nerve. The increased pressure damages the nerve fibers in the glaucomatous eye resulting in a 'silent' loss of peripheral vision, which is largely asymptomatic.
In some patients the intraocular pressure is read as normal during a one-time measurement, while the elevations occur between visits. Some patients are examined several times during a day (a diurnal intraocular pressure test), and in some patients the IOP increases only during the night or early morning hours.
Yossi Mandel at Bar-Ilan University in Ramat Gan, Israel, and his colleagues have developed a pressure sensor which can be inserted into the eye during surgery to provide easy, regular monitoring from home.
A few millimetres in length, the sensor can be embedded into the synthetic lenses used to replace the natural lenses of people with cataracts. It works like a miniature barometer, and contains a fluid column that rises with eye pressure. The level can be read at any time using a smartphone camera fitted with a special optical adapter. Software then analyses the image and calculates the reading.
"Continuous monitoring is a clear unmet need in glaucoma," says Francesca Cordeiro, a glaucoma researcher at University College London.
Several eye pressure probes have been developed since the early 1990s. These include a contact lens made by Sensimed, based in Lausanne, Switzerland, that can estimate eye pressure based on how the cornea bends. Sensimed's sensor is the only wearable pressure device that is commercially available, but it is intended for short-term use only. Mandel says his sensor could be installed in a patient's eye for many years.
Implanting the synthetic lens that contains the sensor would require invasive surgery, something only suitable for patients who need new lenses anyway due to cataracts.
"Ultimately, only a small proportion of glaucoma patients could benefit from this sensor," says Cordeiro. Developing a standalone sensor that could be implanted in front of the iris could solve this problem, suggests Mandel.
Summary and Conclusion:
Glaucoma is the second most common cause of blindness in the world. It is a multifactorial disease with several risk factors, of which intraocular pressure (IOP) is a primary contributing factor. IOP measurements are used for glaucoma diagnosis and patient monitoring. IOP has wide diurnal fluctuation and is dependent on body posture, so the occasional measurements done by the eye care expert in the clinic can be misleading. Here we show that microfluidic principles can be used to develop an implantable sensor that has a limit of detection of 1 mm Hg, high sensitivity and excellent reproducibility. This device has a simple optical interface that enables IOP to be read with a smartphone camera. This sensor, with its ease of fabrication and simple design, as well as its allowance for IOP home monitoring, offers a promising approach for better care of patients with glaucoma.
