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IOP measurement with CCT based correction formulae may have poor results in individuals
Average IOP values measured with the Pascal dynamic contour tonometer were higher than values measured with other methods, according to a retrospective cross-sectional case series.
“If [Pascal dynamic contour tonometer] is the closest measure we have to intracameral IOP, there is a risk of creating clinically significant error after adjustment of [Goldmann applanation tonometry] IOP with [central corneal thickness]-based correction formulae, especially in thicker corneas,” the researchers wrote. “This study suggests that although CCT may be useful in population analyses, CCT-based correction formulae should not be applied to individuals.”
Researchers evaluated the usefulness of CCT-based correction formulae among 289 subjects. They used the Pascal dynamic contour tonometer as the reference standard for IOP measurement.
Subjects attended a specialist glaucoma practice for ophthalmic assessment from February 2007 to August 2009 and sought treatment on a nonacute basis. They were classified into a normal cohort with no glaucoma or a cohort with confirmed glaucomatous optic neuropathy; 59.5% of the subjects were women, mean age was 59.2 years, and 83 subjects (28%) had glaucomatous optic neuropathy.
IOP was measured using Pascal dynamic contour tonometer, Goldmann applanation tonometry and the Ocular Response Analyzer (Reichert). The researchers evaluated discrepancies between readings after stratification into thin, intermediate, and thick CCT groups. The IOP measurements were compared using Bland-Altman analysis.
Goldmann applanation tonometry IOP readings demonstrated poor agreement with Pascal dynamic contour tonometer, the researchers found. However, Goldman-correlated IOP, corneal-compensated IOP and adjustment of Goldmann applanation tonometry IOP with CCT-based formulae resulted in even poorer agreement.
Perspective
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When Goldmann applanation tonometry (GAT) was introduced in the 1950s, corneal thickness was recognized as a potential confounder to IOP measurement. Several formulae were subsequently introduced to “correct” GAT IOP based on central corneal thickness (CCT). The Pascal dynamic contour tonometer (PDCT) is a new digital contact tonometer that claims to be independent of corneal structural properties. Previous studies have shown that measurements from PDCT show good correlation with IOP obtained via anterior chamber cannulation. Park and colleagues have contributed a valuable study that expands our knowledge about the relationship between IOP measured by PDCT, GAT, and CCT-adjusted IOP. In their retrospective case series, adjusting IOP using four different correction formulae resulted in poorer agreement with PDCT IOP than unadjusted GAT IOP. The authors conclude that there is risk of creating clinically significant error after the adjustment of GAT IOP with CCT-based correction formulae. There are compelling reasons for avoiding CCT-based correction algorithms in individual glaucoma patients. First, CCT is only one of several structural properties of the cornea contributing to GAT IOP measurement error. Second, many clinicians approach the management of glaucoma patients by aiming for a percentile reduction of IOP from a pre-treatment level (eg, 30% decrease from baseline). Because CCT is expected to have a similar effect on pre-treatment and post-treatment readings, there is less need to adjust IOP when a proportional lowering of IOP is targeted. Third, prior studies have demonstrated that GAT is not particularly precise nor accurate (especially when measurements are acquired on different devices and when regular calibration is not performed). Adjusting IOP based on CCT implies a level of precision and accuracy that is not present. Finally, it should be recognized that an IOP reading is only a “snapshot” in the life of a glaucoma patient. If an individual patient is shown to have glaucomatous progression based on structural or functional changes, the IOP should be lowered regardless of the errors that may be involved in that measurement.