INTRAOCULAR PRESSURE MONITORING

While continuous intraocular pressure monitoring is not a new idea, the rapid pace of technological change in recent years means that 24-hour IOP measurement is now a feasible goal and will likely have a major impact on clinical management of glaucoma in the near future, according to a study presented here. “Continuous IOP monitoring has been around for at least 50 years and people have been probably dreaming about it for even longer than that,†Arthur J Sit MD told delegates attending the World Glaucoma Congress. “Permanent and temporary monitoring are two different paradigms that will likely have different but complementary uses. Advances in technology are coming very rapidly with some of the first devices for 24-hour IOP monitoring already on the market and when we fully integrate these into our clinical practices I think this will have a major impact on how we treat our glaucoma patients,†he added.

Dr Sit, associate professor of ophthalmology at the Mayo Clinic in Rochester, Minnesota, said that there is now a growing body of evidence highlighting the importance of round-the-clock IOP monitoring in glaucoma patients.

“Several studies have now shown that IOP tends to fluctuate during the day and jumps up significantly at night-time. What is particularly interesting about this pattern is that the elevation of IOP at night-time corresponds to a decrease in systemic blood pressure at night that occurs in most individuals. It is certainly possible that exaggerated nocturnal IOP peaks combined with exaggerated systemic blood pressure dips may compromise optic nerve head perfusion in certain susceptible individuals and increase the risk of glaucoma development and progression,†he said.

Dr Sit identified several different approaches to measuring 24-hour IOP, including sleep laboratories, self-tonometry, temporary continuous IOP measurement and permanent continuous IOP measurement.

While self-tonometry is technically the easiest method of monitoring daily IOP fluctuation, enabling patients to monitor their IOP over time with devices outside the clinic, it does not allow for nocturnal IOP measurement, said Dr Sit.

Focusing his talk on the latter two approaches, Dr Sit said that temporary continuous IOP monitoring, which could be useful for less advanced glaucoma cases, has a long history dating back to David Maurice’s indentation tonometer in 1958. Although a clear advance on previous efforts to developed automated recording of IOP, Maurice’s technology was not suitable for widespread clinical use, he said.

More recent evolutions in this field have proved more promising, said Dr Sit, citing the work of Leonardi et al in using soft contact lenses to detect changes in the radius of curvature of the cornea with IOP.

More studies needed

Dr Sit noted that this technology has been integrated into a commercial device, the Triggerfish system (Sensimed AG), available in Europe and undergoing FDA trials in the US. The Triggerfish sensor is described as a single-use device with passive and active strain gauges embedded in a silicone contact lens to monitor fluctuations in the radius of curvature of the cornea. The output signal is sent wirelessly to an adhesive antenna worn around the eye, which is connected to a portable recorder through a thin flexible wire.

Dr Sit said that more studies needed to be done on the calibration and validation of such systems before they could become a routine part of glaucoma management.

In terms of permanent continuous monitoring of IOP, suitable for more advanced glaucoma cases, this concept also has a long history, said Dr Sit, referring to the pioneering research of Dr Collins and his “bubble tonometer’’ in 1967.

“He developed a capacitive pressure sensor that could be implanted into the eye. The circuit of the sensor consisted of a pair of parallel spiral coils encased within a gas-filled plastic pill, and the idea was that the distance between the two coils would change according to pressure fluctuations. It was very cumbersome to use, however, and its use never extended beyond rabbits,†said Dr Sit.

More recent technology developed by Schnakenberg and colleagues holds greater promise for continuous IOP monitoring, said Dr Sit. “The technology is basically an IOL with integrated pressure sensors that are combined with a microchip to digitally transmit IOP data. This approach should overcome a lot of the signal problems that are associated with other types of devices and there is a lot of active research going on in this particular field,†he said.

In terms of how the data derived from the latest generation of monitoring devices might improve day-to-day management of glaucoma patients, Dr Sit said it would potentially enable more targeted treatments for individual patients. “We now know that latanoprost has a significant pressure lowering effect over the entire 24-hour period while timolol has good effect in the day but not at night. Similarly, brimonidine given three times a day has good efficacy during the day but not at night. If we know that a patient is experiencing IOP peaks during the night, we can alter the therapeutic regimen to take account of that factor and hopefully reduce the risk of disease progression,†he said.

In the future, Dr Sit said that such data could also be potentially combined with active implants capable of delivering IOP lowering drugs directly into the eye over sustained periods of time.