pressure procedures

Alternative IOP Measurements

A look at the non-GAT methods

Pinakin Gunvant Davey, O.D., Ph.D., F.A.A.O. Pomona, Calif.

Goldmann applanation tonometry (GAT) remains the current clinical standard in obtaining intraocular pressure (IOP) measurements. That said, several ophthalmic device manufacturers offer tonometers that obtain IOP measurements in a different fashion than GAT. The reason: Recent research reveals various physiological parameters, such as central corneal thickness (CCT), corneal curvature, corneal hydration, rigidity, surgery and age-related changes to the cornea, can influence GAT's readings.^1-6

Here's a look at the tonometers that provide alternative IOP measurements. (For a full list of tonometers and blood flow analyzers, see OM's 2012 Diagnostic Instrument Buying Guide, at www.optometricmanagement.co m/articleviewer.aspx?articleID=1 07253.)

Compensated IOP and…

The Ocular Response Analyzer (ORA), from Reichert Technologies, uses a bi-directional applanation process to identify the cornea's biomechanical properties, such as elasticity and thickness, and, thus, decrease their influence on the IOP measurement, the company says.⁷ This IOP measurement is known as the corneal compensated IOP (IOPcc).

Additionally the device provides Goldmann-correlated IOP (IOPg), corneal hysteresis (CH) and corneal resistance factor (CRF) measurements. The IOPg strongly corresponds with the GAT IOP, Reichert Technologies says. The IOPg is available so you can compare it with the IOPcc and determine the corneal biomechanical property-effect on this measurement.

CH is the difference in applanation values between the pressure exerted by the device's jet of air and the jet of air pressure on the cornea while it returns to its original curve. This makes it the direct measure of the corneal biomechanical properties, the company says. Research shows CH is affected in conditions such as keratoconus, Fuch's dystrophy and glaucoma.^8-10 Research reveals glaucoma patients, for instance, have a significantly lower CH, and that CH is an independent risk factor for disease progression.^11,12 Furthermore, it can be predictive of laterality of asymmetry in glaucomatous patients and correlated with the compliance of the optic nerve head to elevated IOP.^12,13

The CRF measurement provides data on the cornea's elastic properties when the jet of air pressures the cornea, the company says.¹⁴ The exact role and use of this parameter is under investigation. However, this parameter varies closely as a function of change in IOP.

The 7CR Auto tonometer, also from Reichert Technologies, provides the IOPcc and the IOPg alone.¹⁵

Non-contact tonometers

Non-contact (or air-puff) tonometers (NCT) acquire IOP via a jet of air to applanate the cornea. Once initiated, the jet of air force increases until the cornea is applanated over a predetermined area. The NCT then translates the applanation force into multiple IOP measurements and averages them into one measurement. The reason: The NCT measurements are instantaneous, and IOP can vary with each cardiac cycle, as the choroid fills and empties (ocular pulsations).¹⁶

Cornea contour matching

The PASCAL Dynamic Contour Tonometer (DCT), from Zeimer Group, employs a concave tip with a miniature pressure sensor that contour-matches to the cornea. Once the concave tip contacts the eye, the tear film and corneal interface elicits the cornea to conform to the tip, freeing the cornea from its radial tensions. This, in turn, enables the direct measurement of IOP as if the cornea were not present, the company says.¹⁷

Additionally, the DCT provides ocular pulse amplitude (OPA), which is the difference in systolic and diastolic IOP. This is indicative of the overall blood flow to the eye. Specifically, OPA is reduced in patients who have NTG or primary open-angle glaucoma (POAG) when compared with healthy patients or those who have ocular hypertension.^18-20 Also, an increased OPA in glaucoma patients appears to correlate with less severe disease. Conversely, a small OPA is linked with moderate-to-severe glaucoma-caused visual field defects. Thus, it may serve as an additional parameter when evaluating glaucoma patients, Zeimer Group says.^19,21

Ocular blood flow

Research shows pulsatile ocular blood flow (OBF), like the OPA, may provide information regarding the eye's vascular network, and thus, the presence of glaucoma or other retinal pathologies.^19,22 Specifically, studies demonstrate OBF is compromised in NTG patients both with and without visual field loss.²³

The Ocular Blood Flow Analyzer, (OBFA) from Paradigm Medical Industries, Inc., takes this into account. The device records pulsatile IOP, aiding in the identification of both low glaucoma and NTG suspects, the company says. In addition, it provides pulse amplitude, rate and volume, ciliary choroidal blood perfusion, net blood in-flow speed, inflow duration ratio, systolic and diastolic cycles and standard deviation percentiles. Also, its small applanation probe enables the IOP measurement to be less influenced by CCT variations, Paradigm Medical Industries Inc. says.²⁴

Micro strain gauge technology and…

Lightweight, portable, battery-powered hand-held contact applanation tonometers that contain LCD screens provide IOP via micro strain gauge technology, among other forms of applied science. Micro strain gauge technology measures small corneal anomalies.

The Tono-Pen XL and Tono-Pen AVIA, both from Reichert Technologies, also include software to ensure measurements are acquired only during applanation while they obtain IOP data at 500 samples per second, the company says. These devices then average four and 10 independent readings, respectively, and combine them with a statistical confidence index to arrive at the IOP measurement. (The Tono-Pen AVIA doesn't require daily calibration.)^25,26

The Accupen Handheld Applanation Tonometer, from Accutome, averages nine independent readings and includes an IOP-correction calculator to adjust IOP based on CCT, and gravityoffset technology, so less device calibration is required, the company says.²⁷

These “handy” tonometers are ideal for mobility-challenged patients. They work at any angle.

Rebound technology

The iCare hand-held tonometer, from iCare, uses the rebound measuring principle.²⁸

Specifically, the device contains two coils coaxial to a magnetized probe shaft that lightly touches the cornea. The device's induction-based coil system then measures the probe's deceleration upon corneal touch. If the deceleration is more rapid, the patient's IOP is high, if the deceleration is slow, the IOP is low, the company says.

Transpalpebral technology

The Diaton hand-held tonometer, from Bicom, Inc., obtains IOP measurements transpalpebrally.²⁹

Specifically, the device calculates the eyeball's resistance and converts it into an IOP measurement after averaging six measurements, the company says.

Eyelid IOP measurements are ideal for scarred or extremely irregular or prosthetic corneas.

In closing…

The non-GAT devices provide an alternative means of obtaining IOP measurements with additional data that can enhance disease management. OM

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Dr. Davey is an associate professor of optometry at Western University of Health Sciences in Pomona, Calif. His research is focused on improving glaucoma management. He has authored numerous related articles. E-mail him at pdavey@westernu.edu, or send comments to optometricmanagement@gmail.com.