Should IOP be adjusted for corneal thickness alone?
While the Goldmann applanation tonometer is a very accurate instrument for measuring IOP compared with previous instruments, ophthalmologists now know that central corneal thickness (CCT) is much more variable than was believed when the instrument was developed.
It is generally accepted that the Goldmann applanation tonometer gives a correct value for the pressure when CCT is approximately 520 μm. When the thickness is below normal, the applanation reading is too low, and when it is above normal, the reading is too high, with a 70-μm change in CCT corresponding to a difference of about 5 mm Hg of IOP, Dr. Herndon said.
"Those with ocular hypertension had significantly thicker corneas than normal, and fully 24% of subjects had corneal thicknesses greater than 600 μm," said Dr. Herndon, associate professor of ophthalmology, Duke University Eye Center, Durham, NC.
Results of OHTS showed that the risk of developing primary open-angle glaucoma (POAG) was greatest among participants with the thinnest central corneas (Gordon et al. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open angle glaucoma. Arch Ophthalmol 2002;120:714-720.) In addition, patients with a CCT of 555 μm or lower had a threefold greater risk of developing POAG than participants with a CCT higher than 588 μm.
Since publication of the OHTS data, many other researchers have also studied the relationship between CCT and glaucoma. In one recent study published last fall,1 the authors looked at predictive models for 5-year risk of conversion to glaucoma, derived from OHTS results. Twenty-five percent of the 126 subjects in the longitudinal study developed glaucoma over follow-up.
"A scoring system was used to estimate the 5-year risk by adding up six predictive factors," Dr. Herndon said. "CCT carries the greatest weight on this conversion table. There's almost perfect agreement between the predictive risk as estimated from the point system and those estimated directly from the Cox regression equations."
One of the seminal papers on this topic, published in 1999,2 looked at determination of true IOP by using mathematical models to assess the influence of CCT and corneal radius of curvature.
"They showed that adjusting one's corneal radius of curvature would have very little influence on the applanating IOP," Dr. Herndon said. "However, varying one's corneal thickness would have a significant impact on the true IOP.
"There have been numerous published studies that have shown the importance of CCT in glaucoma," he concluded. "Risk models show that CCT is the most important factor."
Dr. Brandt began his portion of the debate by explaining that his opinion had changed with recent research findings and that he no longer supports adjusting pressure measurements for CCT in individual patients.