Easy-to-use tonometer hits consumer, professional markets

The phosphene tonometer allows patients to monitor IOP at home.

TORONTO — A researcher here set out to develop a phosphene-based tonometer that would simplify measurement of intraocular pressure (IOP) by eye care professionals. The device is so easy to use, though, that it may allow glaucoma patients to monitor their own IOPs at home.

The Proview Eye Pressure Monitor is manufactured and marketed by Bausch & Lomb. It is a hand-held, phosphene pressure tonometer that potentially can be used by almost anyone wanting to determine an IOP. Medical personnel could use the device in emergency, operating-room and other hospital settings. Eye care practitioners will certainly find the system beneficial in both office and outreach settings as it does not require corneal applanation or anesthesia. Patients could even measure their own eye pressure at home.

Filling a need

The tonometer’s inventor, Bernard B. Fresco, MSc, OD, in practice here, saw the need for a more clinically friendly method for determining IOP. “Although I originally meant it to be primarily a tool for clinicians, by circumstance the tonometer happened to be so easy to use, the focus has become more on home-use,” commented Dr. Fresco, who has a master’s degree in physiological optics from the University of Houston.

First introduced as the Fresco Phosphene Tonometer at the Association for Research in Vision and Ophthalmology annual meeting in 1997, the device is scheduled to be launched in the United States by Bausch & Lomb this month.

The simple utility of the tonometer comes from the entoptic phenomenon called a pressure phosphene. Phosphene (Greek for “to show a light”) is the sensation of light produced by mechanical or electrical stimulation of the peripheral or central optic pathway. To determine IOP, the spring compression device is gently applied to the superonasal portion of the patient’s partially closed eyelid while the patient is instructed to look down at his or her corresponding elbow. With gentle pressure the phosphene is generally seen as a circular glow opposite the area where the pressure was applied (corresponding to the inferotemporal visual field). As soon as the patient perceives the phosphene, the probe is removed and IOP is determined by reading the mm Hg scale on the device.

To determine IOP, the spring compression device is gently applied to the supero-nasal portion of the patient’s partially closed eyelid while the patient is instructed to look down at his or her corresponding elbow.	Patient self testing with the Proview Eye Pressure Monitor.

Reliable and consistent, so far

In an article published in Ophthalmology in 1998, Dr. Fresco showed the phosphene tonometer to correlate with Goldmann tonometry within 0.3 mm Hg. In a controlled study of 100 consecutive patients, Dr. Fresco found that 51% of the measurements made with the phosphene tonometer and Goldmann tonometer were within ±1 mm Hg of each other and 74.9% were within ±2 mm Hg. The average deviation among the 192 eyes tested was 1.8 mm Hg.

Because the Proview tonometer depends on patients “seeing” the phosphene, it is not indicated in patients with no usable visual field, Dr. Fresco explained. He has also found that more pressure is needed before a phosphene can be seen by patients with limited vision. “This may yield falsely high pressures. But if a ‘corrective factor’ is determined with Goldmann tonometry, the device is still practical for these patients,” he said.

Dr. Fresco has presented his phosphene tonometer at several meetings and gained Food and Drug Administration and Health Canada approval in 2000.

“I’ve gotten very promising results with the tonometer, but I’m the first to say these findings still need to be further studied in multicenter trials,” he told Ocular Surgery News. “Bausch & Lomb will be working with leading clinicians to evaluate the full scope of the device.”

More study needed

Allan Kolker, MD, director of the Glaucoma Institute and clinical professor of ophthalmology at Washington University, St. Louis, is among those now evaluating the tonometer. In a study of 50 patients, Dr. Kolker found that the average pressure measurements obtained with the Proview tonometer closely approximated those of Goldmann tonometry but it is not a perfect match.

“I am somewhat concerned that the scatter was greater than I’d like. I also don’t know how much the learning curve is contributing to this scatter question,” Dr. Kolker said. He stressed, however, that his examination involved a short protocol and that further study is warranted.

Dr. Kolker is optimistic about Proview. “It’s potentially great for patients you can’t otherwise get good results for with Goldmann tonometry. As well, it will provide a tremendous security for patients who can check their own pressures at home,” he said.

Intriguing gadget

Dr. Kolker contends the device could be useful for patients with visual field changes and normal IOPs, a condition that presents a treatment dilemma for glaucoma specialists and leads to the tedious practice of in-office or in-hospital diurnal pressure monitoring for patients. The Proview tonometer will also be helpful for patients with corneal transplants, keratoconus, eyelid squeezing habits, infections, corneal abrasions or ulcers, and post-Lasik corneas. All of this, of course, is dependent on its accuracy, said Kolker.

“It’s an intriguing little gadget that will have many uses if its accuracy and reliability versus Goldmann tonometry are confirmed. We also need to confirm our ability to properly explain to patients accurate operation of the tonometer,” Dr. Kolker said.

Bausch & Lomb intends to make the Proview Eye Pressure Monitor available to consumers without a prescription for less than $100.

For Your Information:

• Bernard B. Fresco, MSc, OD, FAAO, can be reached at 40 St. Clair Avenue West, No. 303, Toronto, Canada M4T 1M9; (416) 922-2020; fax: (416) 922-2020; e-mail: bernyfresco@aol.com. Dr. Fresco has a direct financial interest in the product mentioned in this article. He is a paid consultant for Bausch & Lomb. Allan Kolker, MD, can be reached at 633 Emerson Road, St. Louis, MO 63141; (314) 872-9357; fax: (314) 994-3258.

• For information about the Proview Eye Pressure Monitor, contact Bausch & Lomb Pharmaceuticals, Tampa, FL, Proview product manager; (800) 227-1427, ext. 7656.