Measuring IOP: the Corneal Factor

Applanation tonometry has been the gold standard for determining IOP for almost 50 years. This approach is less likely to be influenced by variables such as scleral rigidity, which can significantly affect measured values obtained by Schiotz tonometry.¹ Goldmann discussed the influence of variations in central corneal thickness (CCT) on IOP measured by applanation in his landmark article.² He believed, however, that significant variations in CCT occurred only rarely. After an optical pachymeter became commercially available, investigators found a positive correlation between CCT and IOP measured by applanation.³ Moreover, the Ocular Hypertension Treatment Study prospectively demonstrated that thinner CCT predicts ocular hypertensive patients' progression to primary open-angle glaucoma (POAG).⁴ This article describes current thinking on the role of corneal biomechanics in measuring IOP and provides several tips for obtaining successful IOP readings today.

BIOMECHANICS
Corneal biomechanics, the physical characteristics of corneal tissue, may affect the accuracy of IOP applanation measurements as much as, or even more than, CCT.⁵ Goldmann applanation tonometry calculates the IOP by flattening the cornea without accounting for corneal properties in this measurement. A stiff cornea, however, requires greater force to applanate than a soft one, leading to an overestimation of the IOP. Biological variability likely encompasses both thick, soft corneas and thin, stiff corneas. Theoretically, the accuracy of the measurement is predominantly influenced by biomechanical properties rather than CCT.⁵ This idea is illustrated in the pathologic case of Fuchs' corneal dystrophy in which edema results in a thick, soft cornea. Goldmann IOP tends to generate low readings despite very thick corneas.

Researchers are studying new parameters that can affect the accuracy of IOP measurements such as corneal hysteresis, which is a measure of corneal viscoelasticity or a time-dependent response to an air puff. Currently, the only instrument that can measure corneal hysteresis is the Ocular Response Analyzer (Reichert, Inc., Depew, NY), which uses an air puff to generate an inward applanation event as well as an outward applanation event as the cornea recovers its original shape.⁶ The pressure at which the first event occurs (P1) is greater than the pressure at which the second applanation event occurs (P2) due to the absorption or dissipation of energy, which characterizes the viscoelastic nature of the cornea. Data from both events are used to calculate corneal hysteresis (denoted on the Ocular Response Analyzer as CH), the corneal resistance factor (denoted as CRF), the Goldmann-correlated IOP (denoted as IOPg), and a “corneal compensated” IOP (denoted as IOPcc) that is less affected by corneal properties.

Corneal hysteresis is the difference between P1 and P2, and the Goldmann-correlated IOP is the average of the two pressures. The formulae for the corneal-compensated IOP and corneal resistance factor were empirically determined based on linear combinations of P1 and P2 such that the corneal-compensated IOP did not correlate with corneal factors but the corneal resistance factor did.⁷ Both corneal hysteresis and the corneal resistance factor are influenced by the viscous and elastic properties of the cornea. The corneal resistance factor, however, is weighted such that it has a stronger correlation with CCT and may reflect the elastic component to a greater degree.

The Ocular Response Analyzer also measures CCT. Two recent articles have shed further light on the measurements that are obtained with the device.^8,9

Another approach is to find a way to measure IOP that is unaffected by the cornea's thickness or biomechanical properties. The Pascal Dynamic Contour Tonometer (Pascal DCT; Ziemer Ophthalmic Systems AG, Port, Switzerland) is a digital device that uses the principle of contour matching to measure IOP as well as ocular pulse amplitude.¹⁰ In a recent, prospective, in vivo clinical study, IOP measured by the Pascal DCT was within 1 mm Hg of the intracameral pressure measured manometrically with a reference pressure sensor over a wide range of pressures.¹¹ In addition, there was no correlation between the measured pressure and either the CCT or the corneal curvature.

The Pascal DCT could be the future gold standard for measuring IOP. Even if this turns out to be the case, however, it will take a number of years for the device to replace the Goldmann tonometer. For one thing, practitioners are accustomed to using the latter instrument. Second, the Pascal DCT costs more than $5,000 per unit, so making the switch requires a substantial monetary investment, especially if one plans to have the device in multiple lanes. Third, because the Pascal DCT's readings may differ from the Goldmann readings, a switch to a new set of standards based on ongoing research will be required; IOP numbers will mean something different than the figures to which practitioners have become accustomed. For now, clinicians are likely to continue using Goldmann tonometry and to continue trying to understand the interaction between CCT, biomechanical properties, and the measurement of IOP.

MAKING THE MOST OF GOLDMANN TONOMETRY
Until an alternative to Goldmann applanation tonometry becomes popular, here are a few strategies that can help you to get the best outcomes using the familiar technology.

Do Not Rely on Conversion Tables
In the past few years, many physicians have come to rely on conversion tables that use various algorithms to adjust the IOP measured with Goldmann tonometry based on CCT. The available formulae are not consistent, however, indicating that factors other than CCT are influencing the measurement. A growing understanding of corneal biomechanics suggests that this IOP “correction” based on CCT may not be reliable.⁵ It may produce a more accurate IOP reading for some patients, but there is no way to know which patients those are. In fact, an adjustment based on CCT might lead you to “correct” the pressure downward, when the true IOP is actually greater than the measured pressure, such as with Fuchs' corneal dystrophy. One 580-µm cornea may be altering the IOP reading very differently than another, depending on the biomechanical properties of the individual corneas.

Ask the Patient to Blink Right up Until You Measure the CCT
If you anesthetize the eye, patients tend to lose their blink response, and it only takes 15 to 30 seconds for a cornea to begin drying out, causing it to become thinner.

Measure the CCT Before Performing Gonioscopy
Because the contact lens you place on the eye to perform gonioscopy will slightly alter the cornea, perform gonioscopy after you have taken corneal measurements.

Think of CCT as Falling Into Three Categories
Although corneal biomechanics may make it difficult to be certain of the accuracy of Goldmann measurements, simply noting that a cornea is especially thin, average, or thick is useful information. A CCT of less than 500 µm merits concern, because the Ocular Hypertension Treatment Study showed that an eye with a thinner cornea is more likely to convert to glaucoma over time (this finding can be at least partly attributed to the fact that a thinner cornea requires less force to flatten, making Goldmann readings underestimate the true pressure).

If a cornea is thicker than 600 µm, the patient is probably in less danger. Thicker corneas tend to produce artificially higher IOP readings, although there is a great deal of variability, and the literature suggests that these individuals have a better prognosis.⁴ Of course, you still need to follow these patients expectantly, but you can be fairly sure that their true IOP is lower than what you are reading off the machine.

A Patient's Relative Change in IOP Is Useful Information
Despite the fact that corneal biomechanical properties in a given patient may shift Goldmann readings higher or lower than the actual IOP, you can still use these measurements to gauge your treatment and its effectiveness. For example, if a patient's IOP measures 24 mm Hg using Goldmann tonometry, you probably need to strive for a significant reduction in pressure on the order of 25% or 30%, based on the patient's presentation of glaucomatous damage. Moreover, you can judge the effectiveness of the treatment in terms of the relative drop in IOP, whether or not the numbers are absolute.

One CCT Measurement May Not Be Good for Life
Currently, most ophthalmologists only measure a patient's CCT once. Aside from the possibility of short-term changes in thickness caused by the cornea's hydration status, edema, or diurnal variation, data published in the British Journal of Ophthalmology showed that corneas can thin over time. CCT measurements were taken 8 years apart and were found to have thinned 17 to 23 µm, a statistically significant change.¹² Of course, statistical and clinical significance are not necessarily the same; this level of change might not alter your recommendations. On the other hand, people can have glaucoma for decades, so it is probably worth measuring CCT again when considerable time has passed.

You should certainly repeat measurements of CCT if the patient has had any kind of surgical intervention such as refractive surgery. CCT is even more unreliable as a correction factor for IOP if the cornea has been surgically modified, because both the CCT and the corneal biomechanics have likely been altered. In one study,¹³ applanation tonometry produced both lower and higher IOP measurements postoperatively versus preoperatively, despite all subjects' having reduced corneal thickness and curvature due to refractive surgery for myopic correction. The change in CCT did not predict the change in measured IOP. Although the average measured postoperative IOP was lower than the preoperative average, the magnitude of the average difference was low, and the variability in response was high.

Measure in the Center
Most ophthalmologists know that measurements of IOP or corneal thickness need to be centered on the cornea, with the probe held perpendicular to the anterior corneal surface, in order to be accurate.

View IOP in the Context of Other Measures
Measuring a patient's IOP is important, but clinicians must consider many other factors as well: the optic nerve; the visual field; and scans with advanced imaging technologies. IOP is just one aspect of the patient's overall situation. Use all the information at your disposal when determining the best treatment option for each patient.

HEADED IN THE RIGHT DIRECTION
Researchers are now investigating the possibility that a thick or thin cornea may indicate something about the back of the eye. It has been noted that blacks have thinner corneas than whites and often have more advanced glaucoma.^14,15 It is therefore possible that a thinner cornea may indicate a thinner or more susceptible lamina cribrosa or optic nerve. Other research has found that ocular hypertensive patients with thin corneas have thinner nerve fiber layers than ocular hypertensive patients with thick corneas and healthy control subjects.¹⁶ There may be an anatomical correlation, but it has not been clearly established yet.

Clinicians have learned a lot about the interaction of CCT and glaucoma during the past 30 years, and one can hope that it will not take another 3 decades to resolve the remaining issues. In any case, you know enough right now to be of real help to your patients, and the future looks even brighter as research reveals more about the influence of corneal biomechanics on the measurement of IOP. 

Leon W. Herndon, MD, is Associate Professor of Ophthalmology at Duke University Eye Center in Durham, North Carolina. He acknowledged no financial interest in the products or companies mentioned herein. Dr. Herndon may be reached at (919) 684-6622; leon.herndon@duke.edu.

Cynthia Roberts, PhD, is Associate Professor of Ophthalmology and Biomedical Engineering at The Ohio State University in Columbus. She is a consultant to Reichert, Inc., and the Ziemer Group. Dr. Roberts may be reached at (614) 292-1831; roberts.8@osu.edu.

1. Drance SM. The coefficient of scleral rigidity in normal and glaucomatous eyes. Arch Ophthalmol. 1960;63:668-674.
2. Goldmann H, Schmidt T. Uber applanationstonometrie. Ophthalmologica. 1957;134:221-242.
3. Hansen FK, Ehlers N. Elevated tonometer readings caused by a thick cornea. Acta Ophthalmol. 1971;49:775-778.
4. Gordon MO, Beiser JA, Brandt JD, et al. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open-angle glaucoma. Arch Ophthalmol. 2002;120:714-720.
5. Liu J, Roberts CJ. Influence of corneal biomechanical properties on intraocular pressure measurement: quantitative analysis. J Cataract Refract Surg. 2005;31:146-155.
6. Luce DA. Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. J Cataract Refract Surg. 2005;31:156-162.
7. Luce D. Methodology for cornea compensated IOP and corneal resistance factor for the Reichert Ocular Response Analyzer. Poster presented at: The ARVO Annual Meeting; May 1, 2006; Fort Lauderdale, FL.
8. Congdon NG, Broman AT, Bandeen-Roche K, et al. Central corneal thickness and corneal hysteresis associated with glaucoma damage. Am J Ophthalmol. 2006;141:868-875.
9. Medeiros FA, Weinreb RN. Evaluation of the influence of corneal biomechanical properties on the intraocular pressure measurements using the ocular response analyzer. J Glaucoma. 2006;15:364-370.
10. Kanngiesser HE, Kniestedt C, Robert YCA. Dynamic contour tonometry: presentation of a new tonometer. J Glaucoma. 2005;14:344-350.
11. Weber A, Boehm AG, Spoerl E, Pillunat LE. The effect of corneal thickness, corneal curvature and axial length on IOP measurements taken by Pascal dynamic contour tonometry. Poster presented at: The ARVO Annual Meeting; May 3, 2006; Fort Lauderdale, FL.
12. Weizer JS, Stinnett SS, Herndon LW. Longitudinal changes in central corneal thickness and their relation to glaucoma status: an 8 year follow up study. Br J Ophthalmol. 2006;90:732-736.
13. Pepose JS, Feigenbaum SK, Qazi MA, et al. Changes in corneal biomechanics and intraocular pressure following LASIK using static, dynamic, and noncontact tonometry. Am J Ophthalmol. 2007;143:39-47.
14. LaRosa FA, Gross RL, Orengo-Nania S. Central corneal thickness of Caucasians and African Americans in glaucomatous and nonglaucomatous populations. Arch Ophthalmol. 2001;119:23-27.
15. Shimmyo M, Ross AJ, Moy A, et al. Intraocular pressure, Goldmann applanation tension, corneal thickness, and corneal curvature in Caucasians, Asians, Hispanics, and African Americans. Am J Ophthalmol. 2003;136:603-613.
16. Henderson PA, Medeiros FA, Zangwill LM, et al. Relationship between central corneal thickness and retinal nerve fiber layer thickness in ocular hypertensive patients. Ophthalmology. 2005;112:251-256.