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Corneal thickness can affect tonometry readings
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We can divide the tests that comprise our ocular examination into different categories. First are subjective tests that require patient involvement. These tests include visual acuity measurement or subjective refraction. Another category is tests that have us view a situation and interpret our observations. Biomicroscopy or fundus and optic nerve evaluation are examples. A third group of tests involves actual measurements in which data are obtained. Examples for this group include keratometry, lensometry and tonometry. For this category, if the instrument is properly calibrated, there is no reason to believe the reading is not accurate.
Measuring the intraocular pressure (IOP) with Goldmann tonometry appears similar to the use of a ruler to measure the length of a particular object. While it may be cumbersome at times to obtain a particular reading, there does not appear to be a reason to question the measurement’s accuracy. How can a ruler’s gradations not be precise?
Recently, a series of articles have been published that illustrate with Goldmann tonometry occasions in which measurement errors may occur. Specifically, corneal thickness appears to be an important variable that can affect the IOP measured. If the cornea being measured is either substantially thinner or thicker than average, the reading may be lower or higher than it is in actuality.
Labeling patients
Individuals may then be labeled as having normal tension glaucoma if the cornea is very thin, the IOP measured is within the normal range and signs of glaucoma (nerve damage, field loss) are present. Another situation may have individuals labeled as ocular hypertensive if their corneas are thicker than average, their IOPs are elevated and full visual fields and healthy optic nerves are present.
Thick corneas may affect tonometric findings, leading to elevated IOP measurements that are higher than what would be measured with manometry. The opposite has also been found; that is, thinner corneas are associated with reduced IOP readings.
Copt and colleagues recently performed Goldmann tonometry and ultrasound pachymetry on 22 patients diagnosed with normal tension glaucoma (NTG), 49 patients with primary open angle glaucoma (POAG), 44 individuals with ocular hypertension (OHTN) and 18 normal control subjects (Copt RP, Thomas R, Mermoud A. Corneal thickness in ocular hypertension, primary open-angle glaucoma, and normal tension glaucoma. Arch Ophthalmol. 1999;117:14-16).
The central corneal thickness (CCT) was then used to correct for thickness effects on the IOP and to reclassify patients diagnosed with either ocular hypertension or glaucoma. The authors found little difference in the CCT between controls (552 ± 35 µm) and patients with POAG (543 ± 35 µm). The CCT was reduced (521 ± 31 µm) in the patients labeled as having NTG and elevated in individuals with OHTN (583 ± 34 µm). In correcting for corneal thickness using a conversion factor, the authors calculated that 31% of patients with NTG should be reclassified as having POAG, and 56% of individuals with OHTN should be reclassified as being normal.
These CCT measurement results are similar to a study by Morad and colleagues, who evaluated 21 patients with NTG and compared them with 25 patients with POAG and 27 age-matched controls (Morad Y, Sharon E, Hefetz L, Nemet P. Corneal thickness and curvature in normal-tension glaucoma. Am J Ophthalmol. 1998;125:164-168).
Corneal thickness measurements were obtained with an ultrasonic pachymeter, corneal curvature measurements with a keratometer and IOP using Goldmann tonometry. CCT was found to be 521 ± 37 µm in individuals with NTG, 556 ± 35 µm in patients with POAG and 555 ± 34 µm in the control group. The corneal curvature measurements were similar in each group.
Tonometry pitfalls
These studies are a few of a group published over the past decade that illustrate pitfalls with Goldmann tonometry (Brubaker RF. Tonometry and corneal thickness editorial. Arch Ophthalmol. 1999;117:104-105). While Goldmann tonometry is usually dependable, there are potential measurement errors. The concern is: What should one do? Should we perform pachymetry on all individuals who exhibit elevated IOP readings (OHTN) without findings of glaucoma or those with glaucomatous findings and low IOP (NTG)? The accompanying question is whether a conversion factor exists that could reliably allow us to correct the IOP measurements for CCT.
Ehlers and colleagues’ data implied that Goldmann tonometry was accurate when the CCT was 520 µm, this being their average thickness of the normal cornea (Ehlers N, Bramsen T, Sperling S. Applanation tonometry and central corneal thickness. Acta Ophthalmol. 1975;53:34-43). They calculated that a 10-µm deviation led to a 0.7 mm Hg error in IOP measurement.
Whitacre and colleagues found a different correction factor, 0.18 to 0.23 mm Hg per 10-µm deviation (Whitacre MM, Stein RA, Hassanein K. The effect of corneal thickness on applanation tonometry. Am J Ophthalmol. 1993;115:592-596).
The Rotterdam study found a correction factor to be 0.19 mm Hg per 10 µm (Wolts RC, Klaver CC, Vingerling JR, et al. Distribution of central corneal thickness and its association with intraocular pressure. The Rotterdam Study. Am J Ophthalmol. 1997;123:767-772).
Thus, a dilemma exists, because the Whitacre and Rotterdam studies found corneal thickness to have little clinical impact on the accuracy of Goldmann tonometry, while Ehlers’ data implied that it can be clinically relevant. The thinnest corneal thickness for the Rotterdam study was 420 µm, so this 100-µm difference would equate to about a 2-mm measurement error. The thickest cornea in the Rotterdam study was 620 µm, also leading to about a 2-mm error on the high side. But if Ehlers’ conversion table is used, the 100-µm difference would lead to a 7-mm Hg difference in IOP, which could easily change the diagnostic category from one being normal to ocular hypertensive or open angle to normal tension glaucoma.
Accuracy of tonometry
Forty years ago, Goldmann and Schmidt, in their landmark work, recognized that scleral rigidity and corneal thickness may limit the accuracy of Goldmann tonometry. They calculated that a thin cornea with little tear film opposition may have a maximum measurement error of 2.25 mm Hg. This is within the recognized limits for IOP measurements and close to the work of Whitacre. But if the work of Ehlers and Copt is accurate, then potential measurement errors are greater than initially expected and need to be considered whenever we manage ocular hypertension. Just as significantly, as more of our patients undergo refractive surgery with the end result being a markedly thinned cornea, we may have measurement errors induced that will impact how we diagnose and manage glaucoma for this growing group of patients in the years to come.
Just like IOP, there is a wide range of corneal thickness measurements in individuals, both normal and those with glaucoma. One important question is whether a difference from the so-called 520-µm median CCT may cause large, clinically relevant measurement errors in IOP. Will individuals be inappropriately labeled as having ocular hypertension because they have thick corneas and not because they have elevated IOP? Will individuals with elevated IOP be found to have “normal IOP” because their corneas are unusually thin?
Unfortunately, the answers are not available at this time, because several studies offer conflicting results. But, if Copt’s data prove accurate, pachymetry may need to be added to our battery of tests as we work up individuals for glaucoma.