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Take the Next Step in Tonometry

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A new instrument is helping eyecare practitioners understand how
corneal biomechanics influence the accuracy of IOP measurements.


Over the past 100 years, new diagnostic instruments have changed how we measure IOP. Before the invention of the ophthalmoscope, eyecare practitioners checked for elevated IOP by pressing the patient’s eye with their fingers to see if it felt hard.

The ophthalmoscope was followed by the Schiotz and the Goldmann tonometers in the 1960s and noncontact tonometers (NCTs) in the 1970s. These instruments are still widely used to measure IOP in clinical settings, but a new instrument is taking its place beside these classic tonometers. The Reichert Ophthalmic Response Analyzer (ORA) has added a new dimension to tonometry. In addition to providing more accurate IOP readings, this instrument measures new corneal properties and reveals the relationship between corneal structure and IOP.

To understand the significance of this technology, we need to review tonometer basics.

Drawbacks of Traditional Tonometry

Measuring IOP is much different from measuring pressure in an automobile tire. A tire gauge measures pressure by opening a valve stem and releasing air. As the air escapes, it pushes against a calibrated dial that displays the tire’s pressure. Since eyes don’t have valve stems, we must evaluate IOP either directly by inserting a small needle connected to a pressure gauge into the anterior chamber or indirectly by measuring the force needed to deform the cornea.

Early IOP-measuring techniques were similar to judging pressure in a football: Press the ball to see how hard it feels. One such inaccurate method used a series of calibrated weights and an inkpad to see how a known weight flattened the cornea. This device demonstrates a problem that has plagued all tonometers. To measure true IOP with an indentation or applanation tonometer, you have to subtract the amount of force needed to indent the cornea from the total measurement, either by referring to calibration tables or by using instruments that automatically compensate for this corneal force.
Accuracy and the Average Cornea

Goldmann applanation is the benchmark against which all tonometers are measured, but it isn’t infallible. Obtaining accurate IOP readings with this device requires a skilled operator and periodic recalibration. However, the greatest source of inaccuracy is more fundamental.

The Goldmann tonometer was designed to measure IOP in the “average” eye characterized by typical corneal thickness, curvature and ocular rigidity. For example, Dr.Goldmann decided to use a 3.06-mm applanation diameter because mathematical models showed this is the size at which ocular rigidity in the average cornea is canceled out by tear-generated attractive forces around the edges of the probe. The resulting measurement is IOP.

As we now know, no eye is truly average, especially in the degree of corneal stiffness. The design of noncontact tonometers (NCTs) may make them less vulnerable to operator error and less likely to cause aqueous massage, but they’re still not perfect.

Since NCTs are calibrated with the Goldmann tonometer, they’re also affected by corneal stiffness and may underor overestimate IOP in patients with abnormal or compromised corneas.
The first widely accepted tonometer was the Schiotz, a portable instrument that indents the cornea with a weighted probe. This instrument uses a calibration scale to transform the degree of indentation for a given weight to IOP. The Schiotz is portable, which makes it suitable for use outside the office, but its drawbacks include the need for topical anesthesia and the possibility of false low readings caused by aqueous massage after several measurements.

Corneal structure is another factor that influences IOP readings taken with the Schiotz tonometer. Indenting a rigid cornea requires more force than indenting a softer one, but the Schiotz has no way to account for variation among corneas. Patients with unusually stiff corneas tend to have false high readings, whereas patients with unusually soft corneas may have apparently normal IOPs, even if they have glaucoma.

Corneal stiffness also affects the accuracy of IOPs measured with Goldmann and noncontact tonometry. The Goldmann tonometer was calibrated for an average cornea, and the NCT is calibrated against the Goldmann. (See “Accuracy and the Average Cornea” on page 2 for more information.) Studies show, and the International Organization for Standardization accepts, that NCTs measure IOP as accurately as Goldmann tonometers do, but both devices share the same weakness when used to document elevated IOPs in patients with “non-average” corneas.

Compensating for Corneal Thickness

Recent studies show that many patients who have IOP measurements between 20 mm Hg and 30 mm Hg never develop glaucomatous field defects, whereas some patients with normal IOP measurements do. This finding suggests that Goldmann and NCT readings are falsely elevated or depressed by stiffer or softer than usual corneas.

Intuitively, we’d expect thicker corneas to be stiffer than thinner corneas, differences we might compensate for by consulting tables or factoring in central corneal thickness (CCT). However, compensation tables often disagree with each other and a lack of correlation between CCT and IOP can lead clinicians to correct IOP by the wrong magnitude or in the wrong direction. As a result, we can’t rely on CCT to correct Goldmann or NCT readings for unusual corneas. (See “How Important Is Central Corneal Thickness” on page 3 for more information.)
Pre- and post-LASIK corneal-compensated IOP (IOPcc ) values change
very little when measured with the Ocular Response Analyzer (ORA).


The difference between the pressure needed to achieve inward and
outward applanation produces a measurement called corneal hysteresis.


Other factors, such as corneal curvature, tear film thickness, astigmatism, hydration and differences in internal composition also affect corneal stiffness. The difference between the pressure needed to achieve inward and outward applanation produces a measurement called corneal hysteresis. These factors suggest we need a new way to measure corneal stiffness that’s independent of other influences.
The ORA appears to meet this criterion.

More Accurate IOP Readings

I use “stiffness” to describe how corneal tissue resists deformation during applanation, but this is an oversimplification. We can compare the cornea’s reaction to tonometry with that of a coil spring to a strut in a car’s suspension system. This elastic-viscous combination reacts very differently depending on the speed at which a car hits a bump. The car’s strut compresses slowly and smoothly when it hits a mild bump at lower speeds, but more quickly and irregularly when it hits a deep pothole at faster speeds.

Like the car strut, the cornea reacts differently to different types of pressure. Slow, gradual applanation forces, as in Goldmann tonometry, produce even inward and outward pressures. However, rapid applanation forces, as in noncontact tonometry, produce higher inward than outward pressures. The ORA is similar to a NCT in this respect, but provides more details of how the cornea responds to a brief applanation air pulse.

First, the ORA emits an air pulse that pushes the cornea to applanation and beyond to concavity. As the air pulse declines, the cornea rebounds and passes through applanation again as it returns to convexity. Inward and outward applanation points are represented by the left and right peaks in the applanation signal. (See “ORA Mechanics” on page 4 for a more detailed description.)

The second applanation always occurs at a lower pressure due to viscous damping in the cornea. The difference between inward and outward IOP readings depends on the biomechanical properties of the cornea being measured. When taking these readings, the ORA creates a signature curve of the cornea’s dynamic response to IOP measurement. Such a signature can be obtained only by rapidly deforming the cornea and measuring its response to this action. The whole process takes 20 milliseconds with the ORA.

Averaging inward and outward applanation pressures produces a better Goldmann-correlated IOP, but the result is still influenced by the biomechanical properties of the cornea.

The ORA circumvents this problem by using a specific algorithm to calculate IOP from the inward and outward applanation pressures. The result is a measurement called corneal-compensated IOP (IOPcc).

The difference in accuracy between Goldmann or NCT and IOPcc readings is apparent in pre- and postoperative IOPs of LASIK patients. Artificially low Goldmann and NCT IOPs are well-documented in post-LASIK patients because the procedure changes corneal thickness and composition. However, pre- and post-LASIK IOP values are consistent when they’re measured with the ORA. (See “IOPcc Pre- and Post-LASIK” on page 4 for a comparison.)

Corneal-compensated IOP is more accurate than Goldmann or NCT IOPs in patients who have CCTs ranging between 450 µm and 650 µm. The correlation of ORA IOPcc measurements and CCT across this wide range is essentially zero (R2 0.0006), indicating that ORA IOPcc measurements are not sensitive to changes in corneal thickness.

Quantifying Corneal Properties

Unlike Goldmann and noncontact tonometry, which only measure IOP, the ORA provides information about dynamic corneal properties such as corneal hysteresis (CH) and corneal resistance factor (CRF).

Hysteresis is the difference between the amount of pressure needed to produce inward and outward applanation during evaluation with the ORA. Studies show that CH is independent of IOP and unique to each cornea at the time of measurement. An eye measured with the ORA has the same CH, unless some force or process changes the basic structure of the cornea. (See “LASIK Changes Corneal Hysteresis” on page 5 for an example.)

Patients with primary open-angle glaucoma (POAG) and normal-tension
glaucoma (NTG) have lower-than-average corneal hysteresis values
spread over a wider range.

The second biomechanical property measured by the ORA is CRF, which is related to what I call corneal stiffness. Studies show that CH and CRF are good indicators of corneal disorders, such as keratoconus and Fuchs’ corneal dystrophy. (See “Keratoconus Changes Corneal Hysteresis” on page 5 for an example). The average CH of a normal cornea is about 12 mm Hg, whereas the CH of a cornea with keratoconus or Fuchs’ corneal dystrophy is about 9 mm Hg. CRF also is lower in patients with these disorders, averaging about 7 mm Hg, compared with about 12 mm Hg in normal eyes.

Corneal hysteresis and CRF values also may help estimate a patient’s risk of developing glaucoma. Recent studies show that primary open-angle glaucoma (POAG) and normal-tension glaucoma (NTG) patients tend to have CH and CRF values of 9 mm Hg, compared with about 12 mm Hg in normal corneas. In addition to providing more accurate IOP readings, the ORA provides two independent risk factors for glaucoma and indicators for other anterior segment disorders.

A third potential clinical application for CH and CRF values is preoperative LASIK evaluations. CH and CRF measurements may be better than CCT for identifying patients who are likely to develop post- LASIK ectasia.

More Than a Tonometer


The ORA uses refined noncontact tonometry techniques to measure IOPcc, as well as CH and CRF, two new dynamic properties.

Corneal-compensated IOP is more accurate than IOP measured with Goldmann applanation and noncontact tonometry. In addition, CH and CRF measurements go beyond those currently used to determine corneal thickness, shape and curvature because they show total dynamic corneal responses as derived from the interaction of corneal thickness, hydration, curvature, astigmatism and microscopic integrity.

Lower-than-average CH and CRF readings can indicate corneal abnormalities associated with keratoconus and Fuchs’ corneal dystrophy; can identify patients with POAG and NTG; and may screen LASIK candidates more effectively than CCT.

The ORA began as a means to more accurately measure IOP, but has introduced a new way to assess the health of the anterior segment, especially as eyecare practitioners find new clinical applications for this technology.
How Important Is Central Corneal Thickness?

An early FDA requirement — that the center of ophthalmic lenses had to be at least 2 mm thick to reduce breakage — and its subsequent revision is a good example of the poor relationship between static “thickness,” dynamic “stiffness” and structural performance.

The minimum thickness criterion made sense when most spectacle lenses were made of simple glass, but it became obsolete as plastic, high-index polycarbon, photochromic and progressive-addition lenses became common. Eventually, the FDA replaced static thickness measurements with a more dynamic drop-ball test to evaluate spectacle lens stiffness.

To apply this protocol change to measuring IOP, we now know that static central corneal thickness (CCT) measurements aren’t correlated with the cornea’s complex, dynamic response to noncontact tonometry and can’t be used reliably to correct Goldmann IOP results.

Corneal response to tonometry depends on thickness, curvature, astigmatism, hydration, internal tissue composition and the interaction between these factors, so it’s not sur prising that CCT can’t correct Goldmann or noncontact tonometry measurements of nonstandard corneas.
Unlike Goldmann tonometry, which is a slow, static process, only a fast, noncontact tonometry-type process can measure the dynamic ocular response of a patient’s cornea. The Ocular Response Analyzer (ORA) uses such a system to produce unique dynamic corneal signatures.

In addition to providing standard, Goldmann-correlated IOP readings, this device provides cornea-compensated IOPs for nonstandard corneas and new measurements that can help clinicians screen for certain corneal diseases and identify eligible LASIK candidates.

The Ocular Response Analyzer (ORA) uses a bidirectional applanation process to measure IOP, corneal hysteresis and corneal resistance factor. As the emitter focuses a beam of light on the cornea, the cornea’s convex shape disperses the beam, reflecting light over a wide angle (A). The air pump depresses the cornea, changing the intensity of the reflected light until it reaches applanation and focuses the reflected light on the light detector (B).

The amount of force it takes to achieve light convergence is the inward applanation pressure.
The pump continues to depress the cornea until it becomes concave, once again dispersing the reflected light over a wide angle (C). The cornea passes through applanation a second time as the air pulse weakens and the cornea returns to its original convex shape.

OcularResponse Analyzer Mechanics

The Ocular Response Analyzer (ORA) uses a bidirectional applanation process to measure IOP, corneal hysteresis and corneal resistance factor. As the emitter focuses a beam of light on the cornea, the cornea’s convex shape disperses the beam, reflecting light over a wide angle (A). The air pump depresses the cornea,
changing the intensity of the reflected light until it reaches applanation and focuses the reflected light on the light detector (B). The amount of force it takes to achieve light convergence is the inward applanation pressure.

The pump continues to depress the cornea until it becomes concave, once again dispersing the reflected light over a wide angle (C). The cornea passes through applanation a second time as the air pulse weakens and the cornea returns to its original convex shape.

Abnormal Corneas and LASIK
Jay S. Pepose, M.D., Ph.D., Chesterfield, Mo.



Recently, I used the Ocular Response Analyzer (ORA) to evaluate a 30-year-old refractive surgery candidate with low myopic astigmatism. He wore soft toric contact lenses in the past. He had no family history of keratoconus , but he had some allergies and admitted he frequently rubbed his eyes.

Atlas Placido-disk topography of the left eye was norm al, but topography of the right eye suggested inferior steepening (left). The ORA waveform showed lower corneal hysteresis in the right eye (7.9 mm Hg) (right), than in the left eye. Based on these findings, I decided he wasn’t a good LASIK candidate. These results also show the ORA was a useful diagnostic adjunct to corneal topography for detecting forme fruste keratoconus in this patient.
Dr. Pepose is a noted cataract, cornea and vision correction subspecialist. He owns his own practice in Chesterfield, Mo.

Pre- LASIK (top) and post-LASIK Ocular Response Analyzer (ORA) waveforms in the same eye. Corneal hysteresis (CH) is lower in post-LASIK eyes due to flap creation and laser ablation, which change the viscoelastic characteristics of the cornea. Ocular Response Analyzer (ORA) waveforms for a normal eye (top) and an eye with advanced keratoconus (bottom). Note the lower corneal hysteresis value (3.21) and changes in the overall waveform pattern in the keratoconus eye, especially in the second applanation event (P2).
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