August 01, 2006
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Classify corneas simply as average, thin or thick

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Murray Fingeret, OD [photo]
Murray Fingeret

Years ago, when I was in optometry school at what was then called the Massachusetts College of Optometry (now New England College of Optometry), Goldmann applanation tonometers (GAT) were not present on every biomicroscope in the general clinic. The main clinic, located at that time in Kenmore Square, was set within an old residential building with two exam rooms at each end of the floor. Each room had a slit lamp, and each floor also had one or two high-quality slit lamps that contained a GAT. Students were offered a choice of performing tonometry using either a non-contact tonometer (air puff) or GAT.

Nearly every student chose to perform GAT, in part because a drug had to be instilled (years ago this was a novelty) as well as the perception that GAT was a more accurate measurement. GAT was viewed as a precise Swiss tool with the number appearing on the dial believed to be the actual intraocular pressure (IOP). Noncontact tonometry was seen as an estimate, while GAT was viewed as an accurate measurement.

For years, this mindset dictated how clinicians viewed GAT. To disparage GAT was almost heresy, yet information has been available for 30 years indicating researchers were aware of some of its problems.

It took the advent of refractive surgery to bring the relationship between corneal thickness and IOP to light, and the Ocular Hypertension Treatment Study (OHTS) to illustrate the importance of measuring central corneal thickness (CCT) in individuals at risk of developing glaucoma. These recent events led to increased scrutiny of GAT, and now this measurement tool is not considered as accurate as once thought.

For years, we recognized the limitations of a single IOP measurement in attempting to diagnose or manage ocular hypertension or glaucoma, but more importantly, aside from when and how often the measurement was taken, the tool itself is now seen as old and out of date. GAT is a 50-year-old mechanical instrument — outdated compared to recent technological developments.

Correcting IOP for corneal thickness

The concept of correcting the IOP based upon corneal thickness became a hot topic not long after the publication of OHTS results and the recognition that corneal thickness affects IOP measurements.

In 1975, Ehlers showed a relationship of 5 mm Hg for every 70-µm difference from the base corneal thickness level. Whitacre, in work from the Rotterdam Eye study that was published in 1997, showed 2 mm Hg correlated with a 100-µm change in CCT from the base level, while Doughty and Zaman in 2000 published a meta-analysis that showed 2.5 mm Hg correlated with a 50-µm difference from the base CCT. Each of these correction factors had proponents, and the use of algorithms to correct for the IOP based upon CCT became popular.

The Doughty correction factor was most commonly used, because it was based upon many previous studies. Cards were distributed by the manufacturers of pachymeters that used one of these correction factors. In addition, these algorithms were installed in some pachymeters to allow tonometry readings to be automatically converted based upon corneal thickness.

The concept of correcting IOP based upon corneal thickness grew, and many clinicians started to record “corrected” IOPs in their charts. Case discussions would often lead to the proverbial question: “Are the IOPs corrected for corneal thickness?”

More and more people believed that we could accurately correct the IOP based upon CCT. The correction cards, developed as a marketing tool, morphed into something practitioners believed had clinical value.

Do not correct IOP; predict risk

Dr. Fingeret's guide to corneal thickness classification
Very thin <510 µm
Thin <535 µm
Average 540 µm to 560 µm
Thick >565 µm
Very thick >600 µm

Source: Fingeret M

I am a strong proponent that every patient with ocular hypertension or glaucoma should have his or her corneal thickness measured. I simply do not believe that we can accurately correct the IOP based upon corneal thickness for an individual patient. Rather, pachymetry readings allow us to assess an ocular hypertensive’s risk for developing glaucoma if using the new risk calculators. Pachymetry readings are also important for individuals with glaucoma, because data appear to show that thinner corneas are at greater risk of developing worse forms of the disease.

So, what are the problems with correcting IOP based upon corneal thickness? First, if we are going to correct the IOP, what algorithm do we use? We have at least three — all different and each published in a peer-reviewed journal. Do we use Doughty’s work, which in essence splits the difference between the others? In addition, while each correction factor is linear, Orssengo and Pye showed that the impact is not linear, but rather more significant as the cornea gets thinner or the IOP gets higher.

Surprisingly, even when we correct for IOP using these different models, it does not replace corneal thickness in the OHTS multivariate model. Do we not have the proper corrective factor, or does a thin cornea represent some other anatomic property within the eye, such as a thin lamina, that is associated with a more easily damaged optic nerve? To this last issue, there are few data to support a thin lamina being associated with a thin cornea. While this concept makes sense, I await evidence that this is truly the case.

Several factors affect IOP measurement

The measure of IOP using GAT is imprecise. Tonometry readings are an estimate, with a great deal of noise. This concept gets confused when studies, such as the Baltimore Eye Survey, definitively show that the level of IOP correlates with the prevalence of glaucoma. These data are based upon thousands of measurements, and much of the noise is filtered out. For an individual patient, when we are dealing with a single measurement, the signal-to-noise ratio is high.

In addition, correcting for IOP based upon any of these correction charts indicates a level of precision that does not exist. Many factors affect the IOP measurement including technique, instrument calibration, corneal curvature, corneal hydration, corneal thickness and corneal rigidity.

Liu and Roberts looked at the impact of corneal curvature, corneal thickness and Young’s modulus (a measure of stiffness) on IOP measurements. Changes seen clinically related to corneal curvature would account for approximately a 2-mm Hg difference in IOP readings. With respect to CCT, changes commonly seen would account for about a 3-mm Hg difference in IOP, while in analyzing Young’s modulus, commonly seen changes could account for differences up to 17 mm Hg. Stiffness, a biomechanical term we are just now recognizing as important, has the greatest clinical impact on IOP measurement. In addition, the impact of Young’s modulus depends upon the CCT.

Correlate corneal thickness to risk

I would propose that instead of thinking about “corrected IOPs,” the clinician should be aware of his or her patient’s CCT and classify it as being an average, thin or thick. From here, a thin cornea has a greater risk. This is similar to how we think of optic discs as being small, average or large in our evaluation of the optic nerve. The question is “what is a thin, average or thick” cornea? There are no evidence-based data for this so I will provide my thoughts.

I consider a thin cornea as being less than 535 µm and a very thin cornea as being less than 510 µm. An average cornea is between 540 µm and 560 µm. A thick cornea is 565 µm or more, with a very thick cornea being greater than 600 µm.

Only recently have we come to recognize the limitations of GAT. New ways to measure IOP are being introduced and must undergo the test of time. Still, a quick fix is to not use a simple card to create a precise reading when it does not exist.

For more information:
  • Murray Fingeret, OD, is chief of the optometry section at the Department of Veterans’ Affairs Medical Center in Brooklyn and Saint Albans, N.Y., and a professor at SUNY College of Optometry. He is also a member of the Primary Care Optometry News Editorial Board. He may be contacted at St. Albans VA Hospital, Linden Blvd. and 179th St., St. Albans, NY 11425; (718) 298-8498; fax: (516) 569-3566; e-mail: murrayf@optonline.net.
Suggested Reading
  • Brandt JD, Beiser JA, Gordon MO, Kass MA. Central corneal thickness in the ocular hypertension treatment study (OHTS). Ophthalmology. 2001;108:1779-1788.
  • Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach. Surv Ophthalmol. 2000;44:367-408.
  • Ehlers N, Bramsen T, Sperling S. Applanation tonometry and central corneal thickness. Acta Ophthalmol. 1975 (Copehn);53:34-43.
  • 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.
  • Herndon LW, Weizer JS, Stinnett SS. Central corneal thickness as a risk factor for advanced glaucoma damage. Arch Ophthalmol. 2004;122:17-21.
  • Jonas JB, Holbach L. Central corneal thickness and thickness of the lamina cribrosa in human eyes. Invest Ophthalmol Vis Sci. 2005;46:1275-1279.
  • Jonas JB, Stroux A, Velten I, et al. Central corneal thickness correlated with glaucoma damage and rate of progression. Invest Ophthalmol Vis Sci. 2005;46:1269-1274.
  • Liu J, Roberts CJ. Influence of corneal biomechanical properties on intraocular pressure measurement- quantitative analysis. J Cataract Refract Surg. 2005;31:146-155.
  • Mederios FA, Sample PA, Zangwill LM. Corneal thickness as a risk factor for visual field loss in patients with pre-perimetric glaucomatous optic neuropathy. Am J Ophthalmol. 2003;136:805-813.
  • Orssengo GJ, Pye DC. Determination of the true intraocular pressure and modulus of elasticity of the human cornea in vivo. Bull Mathematical Biol. 1999;61:551-572.
  • Sandhu SS, Chattopadhyay S, Birch MK, Ray-Chaudhuri N, et al. Frequency of Goldmann Applanation tonometer calibration error checks. J Glaucoma. 2005;14:215-218.
  • Shimmyo M, Ross AJ, Moy A, et al. Intraocular pressure, Goldmann applanation tension, corneal thickness an corneal curvature in Caucasians, Asians, Hispanics and African-Americans. Am J Ophthalmol. 2003;136:603-613.
  • Sommer A, Tielsch JM, Katz J, et al. Relationship between intraocular pressure and primary open angle glaucoma among white and black Americans. The Baltimore Eye Survey. Arch Ophthalmol. 1991 Aug;109(8):1090-1095.
  • Whitacre MM, Stein RA, Hassanein K. The effect of corneal thickness on applanation tonometry. Am J Ophthalmol. 1993;115:592-596.
  • Wolfs RC, Klaver CC, Vingerling JR, et al. Distribution of central corneal thickness and its association with intraocular pressure: the Rotterdam Eye Study. Am J Ophthalmol. 1997;123:767-772.