GLAUCOMA MONITORING

Continuous intraocular pressure (IOP) monitoring would provide researchers and clinicians with information that could help to elucidate the pathophysiology of the disease and revolutionise its treatment. That dream is now close to realisation, reported Kaweh Mansouri MD, MPH at the World Ophthalmology Congress in Tokyo.

“It is no longer a dream to think that we have access or will soon have access to methods that allow us to monitor IOP continuously for at least 24 hours. These are taking two directions – monitor devices implanted directly in the eye requiring invasive surgery, or temporary contact lens-based approaches,” said Dr Mansouri, a consultant ophthalmologist at the Glaucoma Sector, Geneva University Hospitals, Geneva, Switzerland.

He contrasted the current situation of a glaucoma specialist with that of a diabetes specialist. A diabetes specialist may see a patient only four times a year, but is able to take a standard measurement to help determine how well the patient is controlled. A glaucoma specialist may also see a patient four times in a year, but relies on a single ‘snapshot’ IOP reading, knowing that that measure can fluctuate quite a bit over time.

“You take one measurement that can fluctuate between 15 mmHg and 21 mmHg. Is this patient well controlled or not? It is hard to say. You now also might have access to hospitalisation that can give you a series of 24-hour diurnal measurements, but these are cumbersome and can also fluctuate. We know from research that we miss peak pressures in as many as two of three patients.”

Implantable Device

One approach to continuous IOP monitoring that has been tested in animals and is now entering clinical trials is an implantable device with pressure sensors that were borrowed from the automobile industry. The advantage of this approach would be that it provides round-the-clock measurements. However, the downside is that it requires invasive surgery via a 6.0mm limbal incision for implantation of the device.

Researchers at the University of Alabama, US have developed an implantable system that wirelessly records 500 measurements of IOP per second. The sealed telemetric sensor implant has been tested in non-human primates for two-and-a-half years. Such a system could be very useful in that it provides real IOP measurements over long periods of time. However, there are still significant concerns regarding the safety of that approach. If the device drifts, surgery could be required. Moreover, there could be the potential of leakage of toxic compounds from the sealed implant, Dr Mansouri noted.

An entirely different approach involves the use of non-invasive contact lens-based IOP monitoring. Dr Mansouri has been involved in clinical trials with the Triggerfish sensor from the Swiss company Sensimed. The Triggerfish, approved in the EU but not in the US, is a soft contact lens embedded with strain gauges in the periphery that can measure radial deformation of the eye near the corneoscleral junction due to changes in IOP and ocular volume. It includes an antennae that transmits data to an external recorder. It takes measurements every five minutes. This information can then be downloaded by the clinician in the office.

Since the device does not measure actual IOP, its developers are working to devise a system to convert the strain gauge changes with IOP equivalents. In sleep lab and other clinical studies the measurements provided by the device do appear to correspond to IOP changes. “We are measuring volume changes rather than the intraocular pressure that everyone is familiar with. The changes are relative rather than absolute. The biggest challenge will be how to interpret the data. We are going from looking at two seconds of data to 60,000 seconds of measurements. We are going from a problem of small data to a problem of big data,” Dr Mansouri observed.

Kaweh Mansouri: kawehm@yahoo.com