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Corneal biomechanics explain variability in results with different tonometers
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A study comparing different methods to measure intraocular pressure in healthy eyes provided new insights into the understanding of how corneal biomechanics affect intermethod variability.
Researchers measured corneal radius of curvature and central corneal thickness (CCT) of 82 eyes of 82 healthy subjects.
IOP was measured using the Rebound Tonometer (Icare), Ocular Response Analyzer (Reichert Ophthalmic Instruments), Tono-Pen (Medtronic Solan) Dynamic Contour Tonometer (Swiss Micro Technology AG) and Goldmann applanation tonometer (AT-900, Haag-Streit AG).
The mean IOP as measured by Dynamic Contour tonometry was significantly higher (1.5 mm Hg) than that of Goldmann applanation tonometry. No significant differences were found with other tonometers, but measurements were not interchangeable because of the wide limits of agreements, ranging between 5.0 mm Hg and 4.2 mm Hg.
Simple linear regression analysis showed a significant association between CCT and both corneal hysteresis (CH) and corneal resistance factor (CRF). CH and CRF were also significantly associated with the measured IOP differences between Goldmann applanation tonometry and Dynamic Contour tonometry, while differences between Goldmann applanation tonometry and Rebound tonometry were independently explained by corneal curvature and CCT.
According to the authors, the combination of CH and CRF may predict the intermethod variability between Goldmann applanation tonometry and other methods, but each tonometer was found to be uniquely influenced by corneal parameters.
“Thus, selection of which tonometer to use in clinical practice for baseline and follow-up intraocular pressure measurements should take this into consideration,” they wrote.
They also suggested that IOP measurements should be corrected for CH in patients with altered corneal biomechanics, such as those with glaucoma, keratoconus and Fuchs dystrophy. – by Michela Cimberle
Disclosure: The authors report no relevant financial disclosures.
Perspective
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Derek MacDonald , OD, FAAO
In the mid-1990s, our clinic purchased an ultrasound pachymeter that came with a handy laminated chart to “correct” IOP based upon central corneal thickness.
That chart soon became a coaster, as Ocular Hypertension Treatment Study investigators confirmed that thin central corneal thickness was a strong and independent risk factor for open angle glaucoma (OAG) irrespective of its influence on applanation tonometry and noting that adjusting IOP did not improve OAG prediction models for individuals with ocular hypertension. Over time, ocular biomechanics, hysteresis, resistance, stress and strain have become part of the glaucoma lexicon.
Notwithstanding acknowledged shortcomings and growing competition, Goldmann applanation tonometry remains the reference standard. Dey and colleagues have demonstrated that other tonometers provide comparable, albeit not interchangeable, measurements and suggest that inter-method differences are primarily attributable to biomechanics.
This study speaks to me about the importance of consistency in the management of glaucoma: ideally, having the same doctor assess the same patient using the same instrumentation (tonometer, automated visual field analyzer, OCT and our own eyeballs) at every appointment. Inter-examiner and inter-instrument differences are well documented and significant. Fortunately, intra-examiner and intra-instrument repeatability tends to be better, and in monitoring for often-subtle disease progression, consistency is critical.
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