Tonometer enables IOP measurement independent of corneal thickness

Clinicians can eliminate corneal biomechanical properties to obtain more accurate IOP readings.

In part five of a series from the OSN Technology and Equipment Workshop featured at Hawaiian Eye 2010, David Taylor, product manager of Advanced Diagnostic Devices at Reichert Technologies, explained the indications and function of Reichert’s 7CR Auto Tonometer, which uses bidirectional applanation to measure corneal biomechanical properties and eliminate them to more accurately measure IOP.

“The accuracy of current tonometers is influenced by corneal properties,” Mr. Taylor said. “And this reduces the clinical validity of the number that you get from that device because it’s contaminated by the cornea. You’re not interested in the cornea. You are interested in the pressure.”

The 7CR is designed to provide accurate IOP measurements in all patients, but it has advantages in measuring IOP in patients with normal tension glaucoma, primary open-angle glaucoma, Fuchs’ dystrophy, corneal edema, keratoconus or abnormal corneal biomechanics. It is also useful in patients after LASIK or refractive surgery.

Simultaneous IOP measurements

The 7CR enables simultaneous measurement of corneal-compensated IOP (IOPcc) and Goldmann-correlated IOP.

“Goldmann tonometry is the gold standard. It has been for a long time,” Mr. Taylor said. “But it’s a flawed standard, and it’s coming under increasingly more fire for its accuracy.”

Flaws in the Goldmann applanation tonometry measurement stem from inadequate ability to determine corneal biomechanical properties in the mid-1950s, when the Goldmann tonometer was developed. IOPcc measurement is intended to compensate for those corneal properties that could not be quantified until recent advances, Mr. Taylor said.

“It is important to note that the IOPcc measurement is still correlated to match Goldmann on average, except it doesn’t have the contamination built into it that a Goldmann tonometer does, so it fixes the errors in the Goldmann system,” he said. “We didn’t want to throw that scale away by giving you a new number. … Let’s just fix the errors in the determination of the numbers we are currently using.”

The new tonometer shines a beam of infrared light onto the cornea and monitors the reflected light with a photo detector. Dynamic bidirectional applanation involves the use of an air pulse to rapidly move the cornea inward and then allow it to return outward to its original configuration. This process enables quantification of corneal biomechanical properties and, therefore, a more accurate assessment of the IOP simultaneously.

Corneal thickness and IOP

Some pachymeters have IOP correction algorithms based on central corneal thickness. However, Mr. Taylor said that IOP measurement based on corneal thickness is an outmoded technique.

“These formulas have been scientifically and mathematically discredited,” he said.

The correlation between corneal thickness and measured IOP is only valid for observing population data and cannot be extrapolated to individual eyes.

“If you have a population of 200 eyes or 500 eyes and you see this relationship between thin and thick corneas and you draw that regression line, it’s only telling you something about the relationship of these parameters in that population,” he said. “So if you’re measuring corneal thickness, don’t use it to adjust the pressure. Just use it to know what the corneal thickness is, as thin central corneal thickness has been shown to be an independent risk factor for conversion to glaucoma.

“While it has been shown that thinner corneas tend to give lower Goldmann readings and thicker corneas tend to give higher readings, it is the scatter in this relationship that makes corneal thickness adjustment formulas mathematically impossible to work,” Mr. Taylor said. “And this is an oversimplification, but of course it’s not thickness — it’s resistance or strength characteristics that affect the accuracy of a tonometer.”

Corneal hysteresis

The measurement of the difference between the inward and outward peak applanation in the bidirectional applanation process is termed corneal hysteresis and is related to corneal viscoelastic properties.

“The cornea is a viscoelastic system, like a strut assembly on a car,” Mr. Taylor said. “It has material characteristics of both elastic and viscous components.”

The corneal hysteresis measurement is useful in diagnosing keratoconus and assessing biomechanical properties of the cornea prior to refractive surgery, Mr. Taylor said.

“It’s also been shown to be independently predictive of progression in glaucoma,” he said. “But beyond that, we can use this biomechanical tissue property information to correct the intraocular pressure or to give you a pressure that’s unaffected by these properties in the first place.”

Mr. Taylor discussed the results of a study showing that IOPcc measurement enabled investigators to distinguish normal tension glaucoma eyes from normal eyes.

“I think that could be very important to help detect these patients moving forward,” Mr. Taylor said. – by Matt Hasson

• David Taylor can be reached at Reichert Technologies, 3362 Walden Ave., Depew, NY 14043; 716-686-4551; fax: 716-686-4545; e-mail: dtaylor@reichert.com.