New tonometer provides error-corrected measurement
In the past 20 years, a number of new tonometers have become available. With several of these new alternatives, the benefits offered have mostly related to portability and convenience.
The Pascal dynamic contour tonometer (DCT; Ziemer Ophthalmic Systems) and the Ocular Response Analyzer (Reichert Technologies) offer greater IOP measurement accuracy, but have not been widely adapted to everyday clinical practice on a large scale.
Introduced in the early 1950s by Hans Goldmann, Goldmann applanation tonometry (GAT) has remained the “gold standard” of measurement for IOP. Numerous significant errors in GAT IOP measurements have been identified and are mostly related to patient variability in corneal thickness, rigidity, curvature and corneal tear film adhesion. GAT theory assumes the cornea to be an infinitely thin membrane requiring no force to applanate except that produced by the IOP. Much of the GAT IOP measurement error is due to the inaccuracy of this assumption.
Given the limited measurement technologies available 65 years ago, it should be no surprise that, independent of its “gold standard” status, GAT has been clearly shown to be less than perfect. Collectively, GAT errors are significantly affected by the geometric properties of corneal thickness and corneal curvature. Variable material properties of the cornea such as Young’s and shear moduli of elasticity both significantly affect the applanating force of the cornea, adding to GAT
The Ocular Hypertension Treatment Study highlighted many problems associated with GAT errors, noting that pressure readings tend to be overestimated in thick corneas and underestimated in thin corneas, leading to a misdiagnosis of glaucoma (Kass et al.; McCafferty, Schwiegerling et al.). Central corneal thickness (CCT) correction is an incomplete correction for total GAT error and its use without other corrections has questionable utility. The effects of LASIK surgery have been shown to render GAT IOP measurements inaccurate (McCafferty, Schwiegerling et al.). The fact that approximately 15 million Americans have had LASIK surgery combined with the reality that few clinical sites have a reliable method of IOP assessment in these individuals should alarm us all.
Modification of GAT
Recently approved by the FDA, the Correcting Applanation Tonometry Surface (CATS) prism is a long overdue, economical and welcomed modification of the GAT that optimizes the corneal applanating surface of the flat GAT prism.
Clinical application of the CATS prism, including the force to pressure conversion, was designed to be unchanged from the GAT prism. A modified Goldmann or CATS prism design was optimized to minimize the IOP measurement error due to corneal thickness, stiffness, curvature and tear film adhesion. Mathematical modeling with finite element analysis and manometric IOP-referenced cadaver eyes were used to optimize and validate the design. The optimized CATS prism was designed to measure the same pressure as a GAT prism in corneas with measured average corneal properties in corneal thickness, curvature, rigidity and tear film. However, 50% of the patient population do not have average corneas (Liu et al.; Whitacre et al.; Kass et al.; McCafferty, Schwiegerling et al.).
Five scientific studies demonstrated the clinical performance of the CATS prism compared with the GAT prism (McCafferty, Enikov et al.; McCafferty, Lim et al. 2016; McCafferty, Lim et al. 2017; McCafferty, Levine et al. 2017; McCafferty, Levine et al. 2018). The peer-reviewed publications included a direct comparison with the existing GAT prism; human cadaver eye intracameral pressure transducer comparison of the CATS and GAT; an actual intracamerally placed pressure transducer comparison of the CATS and GAT in live human eyes; and tear-film hydrostatic study. All five studies confirmed our original mathematical modeling design predictions developed in an NIH SBIR phase 1 grant.
Additionally, subgroups of pediatric patients and postrefractive (LASIK, PRK, small incision lenticule extraction and RK) surgery patients have significantly improved IOP measurements. These patients are included in the 50% of the population benefiting from clinically significant improved IOP accuracy over the GAT prism. Further testing in these populations with the CATS tonometer is required to examine the prism’s full potential.
Clinicians today almost universally have the capability to measure IOP with a GAT, and a majority consider it the “gold standard” and most accurate measurement of IOP in a normal person. GAT errors are well known to most clinicians, and current clinical practice does not correct for most corneal biomechanical errors. Ultimately, the consequence of the systematic error in Goldmann applanation can be underestimation of IOP in our clinics, which may result in late or misdiagnosis of ocular hypertension and glaucoma. Consequences of Goldmann error can be even more dramatic when considering the reality that more than 15 million Americans have had LASIK.
Conveniently and economically, the CATS tonometer demonstrates the capacity to substantially correct for these inaccuracies and can provide a single error-corrected measurement without additional measurements or calculations, with reduced interpretation error.
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Elliot M. Kirstein OD, FAAO, is the director at Harper’s Point Eye Associates, Ohio. He can be reached at: email@example.com.
Disclosure: Kirstein reports he is a consultant to Ocuflow; has received research support from Diopsys; and is on the speakers bureau of Alcon, Haag-Streit, Optovue and Reichert.