Self-tonometry, new monitoring systems provide wealth of data

By Cheryl Guttman Krader

The availability of technologies for ambulatory IOP measurement will usher in a new era in glaucoma patient management.

L. Jay Katz, MD, points out that the enormous amount of data provided by the IOP-monitoring devices is meaningless without physician review and interpretation. (Photo by Stevan Nordstrom)“Clearly IOP measurements are the basis for diagnosing glaucoma and evaluating therapeutic response, and we have a variety of instruments available for determining IOP in the office,” said L. Jay Katz, MD, director of the glaucoma service, Wills Eye Hospital, Philadelphia. “However, access to information beyond IOP spot checks will revolutionize patient care.

Dr. Katz said the technologies will provide better understanding of IOP and patient adherence to topical medications and to make more timely adjustments in therapy while reducing the number of office visits. In addition, these new IOP-monitoring systems will add value by increasing patient involvement in their own care.

Dr. Katz reviewed several devices for ambulatory IOP measurement, none of which have FDA approval.

One of the instruments is a home tonometry device developed by a Finland company, and it is commercially available in countries outside the United States (ICare ONE, icare). It operates on the principle of rebound tonometry, analyzing the motion of a deployed disposable plastic probe as it contacts the eye and bounces back. The output measurement is in mm Hg.

Results from published studies found that about 75% of patients were able to correctly perform self-tonometry with the device and that the data it generated had reasonable agreement with IOP measurements obtained using Goldmann applanation tonometry.  However, the device has some drawbacks.

“The instrument is not inexpensive, the accuracy of its measurements depends on proper positioning, and it cannot be used to measure IOP while the patient is asleep,” Dr. Katz added.

Other devices overcome the latter limitation. One is a contact lens-based system (Triggerfish, Sensimed) with embedded strain gauges to detect corneal curvature changes at the limbus that occur in response to changes in IOP. The device, available outside the United States, records readings for 90 seconds, every 8.5 minutes, providing up to 144 measurements in a 24-hour period. When the patient returns to the physician’s office, the collected data can be transferred wirelessly to a computer via Bluetooth.

“The measurements obtained with the device are not in mm Hg and cannot be converted directly to mm Hg,” said Dr. Katz. “However, studies evaluating its performance have shown that it detects changes that are consistent with circadian IOP patterns, treatment responses, and various maneuvers.”

Implantable devices

Other ambulatory IOP systems being developed are implantable devices that will measure IOP and transmit the readings to external devices via telemetry. Implandata is developing two different versions of its device–one for intraocular placement during cataract surgery (ARGOS-IO) and one for external placement into the subconjunctival or intrascleral space that is intended as a stand-alone procedure (ARGOS-EO).

The system consists of the implanted microsensor and a handheld device that transfers energy to the sensor and receives and stores its data. The system can be programmed to obtain measurements continuously or sporadically at predetermined times as well as to take readings on demand.

Testing conducted in a laboratory setting using cadaver eyes showed the IOP measurements obtained correlated well with manometry readings. A study in living animals showed the expected fluctuations of IOP corresponding with the cardiac cycle and with application of digital pressure to the eye. The device is now under clinical investigation.

AcuMEMS is also developing wireless sensor systems for direct IOP measurement with plans to introduce several models–one that is placed into the anterior chamber during glaucoma surgery (360^AC), a second that would be placed into the capsular bag with the IOL at the time of cataract surgery (360^PC). Results from animal studies demonstrate the feasibility of the implantation procedure and the ability to obtain recordings with the different models.

Dr. Katz acknowledged that the enormous amount of data provided by the IOP-monitoring devices is meaningless without physician review and interpretation. “The companies developing these technologies are very aware of that aspect and are working to establish codes that will allow physicians to be reimbursed for the time they spend evaluating IOP,” he added.