Continuous IOP monitoring devices closer to clinical possibility

Arthur J. Sit

Three approaches for continuous IOP monitoring are being explored to find an ideal device that would accurately measure 24-hour IOP without IOP measurement noise, contact lens intolerance or other issues, Arthur J. Sit, SM, MD, said.

“We may just be getting to the point now with miniaturization, particularly driven by the computer industry, where we could potentially have clinically suitable devices,” Dr. Sit said at Hawaiian Eye 2010.

The combination of a decrease in blood pressure in some patients and increased IOP during sleep could “compromise optic nerve head circulation in susceptible individuals, he said. Continuous IOP measurement could greatly enhance management of glaucoma.

Approaches for measuring continuous IOP are self-tonometry, permanent continuous IOP monitoring and temporary continuous IOP monitoring.

Self-tonometry

Self-tonometry is technically the easiest method of monitoring daily IOP fluctuation, Dr. Sit said. Patients would monitor their IOP over time with easy-to-use devices outside the clinic.

“You can use existing technology — it is adapted for home use — but it’s going to be difficult for some patients to perform these measurements. It can be expensive, depending on the technology,” Dr. Sit said.

Self-tonometry’s use is restricted, however, because it does not allow for nocturnal IOP measurement, he said.

Permanent continuous monitoring

Permanent continuous IOP monitoring could provide for both daytime and nighttime IOP measurement through a self-contained implant. Implants could be accessed remotely with wireless technology, Dr. Sit said. They would be ideal for more advanced glaucoma cases.

“In some ways, it would be an easier technology, because you wouldn’t be measuring the surface, but you’d actually be taking pressure measurements directly inside the eye, subject to less noise and other artifacts,” he said.

The technology was first investigated in 1967 by Collins, who created the bubble tonometer. The tonometer worked by capacitive pressure sensor. The device was never implanted in humans but was advanced technology for its era, Dr. Sit said.

A recent idea, examined by Schnakenberg and colleagues, is an IOL with telemetric IOP monitoring. A digital signal of IOP would be sent via processing on the silicone IOL to a pair of eyeglasses worn externally, according to Dr. Sit.

The digital device could potentially cut down on IOP measurement noise presented with analogue signals.

However, problems still exist with the approach, he said. One issue is where to place permanent IOP-measuring devices in the eye. Another issue is the lack of human testing for many of the devices, with long-term stability a key concern, he said.

Temporary continuous monitoring

Temporary continuous IOP monitoring, the third and final approach for continuous IOP monitoring, could be useful for less advanced glaucoma cases. The devices are typically noninvasive contact lens-based systems that measure pressure externally over a 24- to 48-hour period.

A non-invasive device that monitors daily and nocturnal IOP changes could assist in establishing the effect of IOP fluctuation in glaucoma, which is important to determine when considering implantable devices.

“Whether we could ever achieve regulatory approval without having this information is not clear,” Dr. Sit said.

Temporary devices have potential problems, including corneal thickness variation and noise. Contact lens discomfort or intolerance could also be a problem.

In Europe, the Sensimed Triggerfish system has been available since 2009. It measures change in the radius of curvature with corneal distention.

“They have a contact lens, an antennae system and a little recorder that you can carry around in an ambulatory fashion, and software that will give you pressure readouts throughout the day,” he said. “This has not been validated in clinical trials and it’s not yet available in the U.S.”

The U.S. Food and Drug Administration-approved Pascal Dynamic Contour Tonometer (Ziemer), a digital contact tonometer that is mounted in a slit lamp, measures true IOP not affected by corneal thickness, according to the company. A study has shown that it could potentially be adapted for continuous IOP monitoring. – by Erin L. Boyle

References:

• Chow EY, Chlebowski A, Raghunathan S, et al. A 24-hour continuous IOP monitoring device used for the treatment of glaucoma in humans. Poster presented at: Annual Meeting of the Association for Research in Vision and Ophthalmology; May 5, 2009; Fort Lauderdale, FL.
• Collins CC. Miniature passive pressure transensor for implanting in the eye. IEEE Trans Biomed Eng. 1967;14(2):74-83.
• Frenkel RE, Jaeger M, Kline-Schroeder R, Latina MA, Parel JM, Frenkel MPC. Continuous monitoring of intraocular pressure with an implantable device. Poster presented at: Annual Meeting of the Association for Research in Vision and Ophthalmology; May 5, 2009; Fort Lauderdale, FL.
• Greene ME, Gilman BG. Intraocular pressure measurement with instrumented contact lenses. Invest Ophthalmol.1974;13(4):299-302.
• Lin JC, Chen P-J, Saati S, et al. A minimally invasive wireless intraocular pressure sensor implant. Poster presented at: Annual Meeting of the Association for Research in Vision and Ophthalmology; May 5, 2009; Fort Lauderdale, FL.
• Liu JH, Zhang X, Kripke DF, Weinreb RN. Twenty-four-hour intraocular pressure pattern associated with early glaucomatous changes. Invest Ophthalmol Vis Sci. 2003;44(4):1586-1590.

• Arthur J. Sit, SM, MD, can be reached at the Mayo Clinic College of Medicine, 200 First Street SW, Rochester, MN 55905; 507-266-4918; e-mail: sit.arthur@mayo.edu.