Tonometer modified for practice

January 15, 2010

A new auto tonometer with corneal response technology features corneal compensated IOP, a pressure measurement significantly less affected by the cornea than with other methods of tonometry.

The device incorporates the bidirectional applanation technology used in a non-contact applanation tonometer (Ocular Response Analyzer [ORA], Reichert) to quantify the biomechanical properties of the cornea and determine IOPcc. Reichert formally introduced the tonometer in December. It is intended to help clinical practitioners with diagnostic and treatment decisions, whereas the ORA is primarily used in research settings and in specific settings of glaucoma and cornea subspecialty care.

Using non-contact technology, the tonometer simultaneously provides an IOP measurement similar to that obtained with the Goldmann applanation tonometer (GAT) and other current tonometers (referred to as Goldmann-correlated IOP or IOPg), as well as the IOPcc, a pressure that is independent of biomechanical artifacts such as corneal viscoelastic properties and thickness.

Information on two groups

The combination of measurements provided by the new tonometer potentially could provide important information about two groups of patients, Dr. Radcliffe said.

"One is patients with corneal disease, where you're worried that the corneal properties might interfere with pressure measurements, and the other is glaucoma patients, where corneal properties may be masking elevated IOP," he said.

Dr. Radcliffe cited a recent study by Tetsuya Morita, MD, et al.1 investigating IOP measurement values in eyes with normal-tension glaucoma (NTG) using GAT, the ORA, and a third tonometer that also is said to be little affected by corneal biomechanical properties. Dr. Morita found that the IOPcc was higher than pressure measured by GAT or the Pascal dynamic contour tonometer in patients with NTG, while there was no significant difference between the measurements in normal eyes.

"In patients with NTG, it is likely that we are underestimating the true IOP and that the higher pressure in these patients is at least partially masked by corneal biomechanical properties," Dr. Radcliffe added.

He added that corneal hysteresis, which is one of the biomechanical properties accounted for by IOPcc, is a risk factor for glaucoma, and incorporation of this component helps provide a more complete picture of the IOP values.

"This hasn't been evaluated yet, but it's my hope that by using IOPcc, not only will we be better able to identify patients with elevated pressure by unmasking the corneal properties, but we'll be better able to assess the effects of our therapy by measuring the pressure more accurately rather than relying on GAT, which measures pressure mixed in with confounding variables," Dr. Radcliffe said.

Using an ORA in his practice, Dr. Radcliffe has found a high correlation between IOPg and traditional Goldmann tonometry.

"I would feel pretty comfortable if I had to move away from GAT to using the Goldmann-correlated pressure," he said. "I find the ability to know the IOPcc a very important part of my practice, and I'm imagining a day when I stop checking GAT and am just using IOPg and IOPcc to follow my patients."

There are other advantages to using a non-contact system-for example, not having to anesthetize the cornea or apply fluorescein to test the pressure, Dr. Radcliffe said. Also, research has shown that many GATs are calibrated improperly.2,3

"Not only are we using an instrument that even on its best days has flaws, but most of us aren't even keeping it calibrated properly," he added. "Contrast that with a non-contact instrument that really doesn't even need calibration."

There is tremendous interest in corneal biomechanics and its effect on ocular diseases, Dr. Radcliffe added. "It's only a matter of time before we're seeing more IOPcc measurements in clinical practice," he said.

References

1. Morita T, Shoji N, Kamiya K, Hagishima M, Fujimura F, Shimizu K. Intraocular pressure measured by dynamic contour tonometer and ocular response analyzer in normal tension glaucoma. Graefes Arch Clin Exp Ophthalmol. 2009 Nov 20. [Epub ahead of print]

2. Choudhari NS, George R, Baskaran M, Vijaya L, Dudeja N. Measurement of Goldmann applanation tonometer calibration error. Ophthalmology. 2009;116:3-8.

3. Sandhu SS, Chattopadhyay S, Amariotakis GA, Skarmoutsos F, Birch MK, Ray-Chaudhuri N. The accuracy of continued clinical use of goldmann applanation tonometers with known calibration errors. Ophthalmology. 2009;116:9-13.