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Goldmann tonometry ‘less accurate’ than previously thought, surgeon says
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BARCELONA, Spain — Measuring intraocular pressure by Goldmann applanation tonometry can be a “major risk factor” for glaucoma, James D. Brandt, MD, said during his presentation here at the International Glaucoma Symposium. Goldmann tonometry is commonly used to both screen and classify patients with glaucoma and results are used to set therapeutic targets and to evaluate response to treatment, he said.
“In the last few years, pachymetry has really come of age as a major aspect to our understanding of glaucoma diagnosis and management,” Dr. Brandt said. “And over the last decade or so there has been the recognition that Goldmann is a lot less accurate than we previously recognized.”
The accuracy of Goldmann tonometry is known to be affected by central corneal thickness (CCT), Dr. Brandt explained, but the magnitude of this effect has been controversial.
The Ocular Hypertension Study (OHTS) found CCT to be a major risk factor in determining which subjects would develop glaucoma among those who were already had ocular hypertension. Dr. Brandt noted the “confounding effect of CCT on measuring IOP” is greater than researchers previously thought.
“Variability in corneal thickness is diagnostically important in patients with ocular hypertension, normal tension glaucoma and almost certainly in refractive surgery patients,” Dr. Brandt said. “Corneal thickness varies quite a bit among the general population.”
Corneal thickness differs among racial lines as well, with black patients having thinner corneas than their white counterparts, Dr. Brandt said.
Several algorithms exist to correct for corneal thickness, he said. If those algorithms are corrected for IOP, more than half the participants in the OHTS would not have qualified to be in the study.
“We were clearly misclassifying a significant portion of these patients based on their IOP,” Dr. Brandt said.
Further, Dr. Brandt said when he compared patients’ response to beta blockers in the OHTS study, there was a 1 to 1.5-mm Hg difference in response based on corneal thickness. In the Lumigan (bimatoprost) phase 3 study, Dr. Brandt said, at 3 months post treatment, there was a 3-mm Hg difference between patients with thick corneas and patients with thin corneas. Dr. Brandt suggests that cannot be overlooked in future IOP trials. Non-responders may simply be patients with thick corneas, Dr. Brandt said.