Glaucoma testing shifting to material properties of cornea

Corneal stiffness and other material properties may be as important as central corneal thickness for assessing IOP.

Issue: December 1, 2006

ByErin L. Boyle

Paradigms in the assessment of IOP are changing rapidly, glaucoma experts say. Until a few years ago, Goldmann tonometry was the standard for measuring intraocular tension. With the publication of the Ocular Hypertension Treatment Study (OHTS) in 2002, the importance of central corneal thickness in determining IOP was brought to light. Now, new instruments that take into account the physical properties of the cornea have appeared, raising the possibility that glaucoma assessment standards will change once again.

“The material properties of the cornea – the viscoelastic and the stiffness properties of the cornea – dwarf the effect of corneal thickness,” said James D. Brandt, MD. “We don’t have great ways to measure this effect yet. But we’re working on it.”

Dr. Brandt, an OSN Glaucoma Section Member, discussed two new technologies for glaucoma diagnosis during a presentation at the OSN New York Symposium: the Ocular Response Analyzer (ORA), from Reichert, which measures corneal hysteresis and other physical properties of the cornea, and the Pascal Dynamic Contour Tonometer (DCT), from Ziemer Ophthalmic Systems, which provides direct transcorneal measurement of IOP and measurement of ocular pulse amplitude.

Richard A. Lewis

Engineering models suggest that the impact of central corneal thickness (CCT) on IOP measurement could be smaller than currently believed, Dr. Brandt said. He cited a study by Liu and Roberts of the impact of corneal biomechanics on IOP measurement, which found that corneal radius of curvature has a “very small” effect, CCT has a “small” effect, and corneal material properties have a “very large” effect. The effect of material properties on IOP varies with CCT, he said.

Tonometry technologies such as the Pascal DCT and the ORA represent a movement toward taking corneal biomechanics into account, Dr. Brandt said.

Richard A. Lewis, MD, also an OSN Glaucoma Section Member, said in a telephone interview that Goldmann tonometry remains the standard for many glaucoma specialists because these newer tools have not yet been widely adopted. While some devices show great promise, none has yet supplanted Goldmann tonometry, he told Ocular Surgery News.

“A problem that a lot of these devices face is the challenge of being adopted by enough people to get consensus,” Dr. Lewis said. “You have a few investigators using these tools, and it becomes an interesting research idea that doesn’t often get clinically applied. We just don’t have studies to get the mass of people to go out and buy another tonometer.”

Goldmann tonometry

According to Dr. Brandt, tonometry began in the 1800s, when “some of the greatest minds in ophthalmology” devised early, pioneering techniques to measure IOP. In the 1950s, Hans Goldmann developed the Goldmann tonometer.

James D. Brandt

Now, about 50 years later, there is increasing evidence that the normal variation in CCT has a clinically significant effect in ocular hypertensive patients. Dr. Brandt said the OHTS showed that CCT is a powerful predictive factor in assessing the risk of progression to glaucoma in patients with ocular hypertension.

“Pachymetry over the past few years has come of age with the recognition that corneal thickness is a very big issue in the accuracy of measuring IOP and the assessment of risk in our glaucoma patients,” Dr. Brandt said. “If you have a patient with a thin cornea and higher pressures, that individual is at much higher risk of developing glaucoma than somebody with the same IOP but a thicker cornea.”

Goldmann tonometry assumes a CCT of 500 µm. Research shows that Goldmann over- or underestimates IOP by as much as 5 mm Hg for every 70 µm of CCT difference from 520 µm, Dr. Brandt said. Goldmann tonometers are also subject to calibration errors, he noted.

Because of the inherent error in Goldmann measurement, he suggested, it is of questionable value to adjust a patient’s Goldmann IOP by using a nomogram based on corneal thickness.

“You are kidding yourself if you try to apply an algorithm to adjust by a few millimeters of mercury the underlying measurement, which is imprecise,” Dr. Brandt said.

New tonometry devices

Recently, new methods of measuring IOP have entered the market. The Pascal DCT, a slit lamp-based instrument, shows promise to be more accurate than Goldmann, Dr. Brandt said. The Pascal DCT measures IOP mostly independent of the effect of corneal thickness and possibly of the viscoelastic properties of the cornea as well, Dr. Brandt said. It also promises to be more effective for measuring IOP in patients who have undergone LASIK.

The ORA from Reichert is the first instrument that attempts to measure the cornea’s viscoelastic properties, Dr. Brandt said. He explained that the device works similarly to noncontact pneumotonometers that employ air puffs, but it measures the cornea in a different way.

“It measures the velocity at which the cornea moves away from the air puff and then how it snaps back,” Dr. Brandt said. “The two velocities give us a measurement of the viscoelastic properties of the cornea, a quality called hysteresis.”

He said the ORA may be a more effective way of assessing IOP because it does not rely on algorithms to adjust results.

Portable pneumotonometer

Dr. Lewis said another consideration in measuring IOP is the position of the patient. Recent studies have shown that IOP tends to be higher when subjects are lying down, so measuring patients in the supine position may be key to achieving the most accurate IOP measurement.

The Model 30 Classic Pneumatonometer from Reichert can measure IOP while the patient is lying down, a feature that a slit-lamp mounted device cannot perform, he noted.

“Perhaps we’re missing the diagnosis of glaucoma or at least the measurement of high pressure because we’re not measuring in the right body position,” Dr. Lewis said. “Perhaps some of these values that allow you to measure in the supine position should be re-evaluated.”

For more information:

James D. Brandt, MD, can be reached at University of California at Davis, Department of Ophthalmology, 4860 Y St., Suite 2400, Sacramento, CA 95817-2307; 916-734-6676; fax: 916-734-6992; e-mail: jdbrandt@ucdavis.edu. Reichert Ophthalmic Instruments (the maker of the ORA) and Swiss Microtechnology (the manufacturer of the Pascal DCT) have placed instruments at UC Davis for ongoing research, and Dr. Brandt has served on their speakers bureaus. Most of the support for his research on corneal thickness has come from the National Eye Institute.

Richard A. Lewis, MD, can be reached at 1515 River Park Drive, Sacramento, CA 95815; 916-649-1515; fax: 916-649-1516; e-mail: rlewiseyemd@yahoo.com. Dr. Lewis has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.

Reichert Ophthalmic Instruments, maker ofOcular Response Analyzer and Model 30 Classic Pneumatonometer, can be reached at 3374 Walden Ave., Depew, NY 14043; 716-686-4500; fax: 716-686-4545. Ziemer Ophthalmic Systems AG, maker of Pascal Dynamic Contour Tonometer, can be reached at Allmendstrasse 11, CH-2562 Port, Switzerland; 41 32 332 70 50; e-mail: innovation@ziemer-ophthalmics.com.

References:

Liu J, Roberts CJ. Influence of corneal biomechanical properties on intraocular pressure measurement: quantitative analysis. J Cataract Refract Surg. 2005;31(1):146-155. Sandhu SS, Chattopadhyay S, et al. Frequency of Goldmann applanation tonometer calibration error checks. J Glaucoma. 2005;14(3):215-218.

Erin L. Boyle is an OSN Staff Writer who covers all aspects of ophthalmology.