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Central corneal thickness (CCT) bears an inverse relationship with the risk of developing glaucoma. The risk of glaucoma progression, however, seems to be independent of CCT. New tonometry technology will allow for more accurate risk stratification of patients.
As more is learned about the nuances of the relationship between glaucoma and CCT, evolving tonometry technology may offer a new paradigm that allows for more accurate risk stratification, he added.
The Ocular Hypertension Treatment Study (OHTS) showed that collective use of individual risk factors could provide the clinician with a greater appreciation for the effect of these factors on outcome and that having a thin central cornea was associated with the greatest risk of developing glaucoma, Dr. Herndon said.
Dr. Herndon and colleagues studied CCT as a risk factor for advanced glaucoma damage and published their results several years ago (Arch Ophthalmol. 2004; 122:17-21). Approximately 200 patients were evaluated retrospectively to determine whether CCT was related to glaucoma severity at initial presentation to a glaucoma specialist.
Predictor, outcome variables
They analyzed predictor variables, including several that have been associated with glaucoma, such as age, race, family history, and IOP. They also analyzed outcome variables, such as Advanced Glaucoma Intervention Study score, mean deviation of visual field, vertical and horizontal cup-to-disk ratio, and number of glaucoma medications.
"We found that having a thin cornea predicted for a much greater degree of glaucoma damage when we looked at all of the outcome variables," Dr. Herndon said. "This increased association was independent of the IOP. Of all the predictor variables, CCT had the most profound effect on the degree of glaucoma damage. This was true in univariate and multivariate modeling."
He also said that Goldmann applanation tonometry (GAT), generally considered the gold standard, is flawed. "The problem is that all IOP measurements are indirect values of the true IOP. It is not practical or possible to get the right manometric measures on our patients," Dr. Herndon said.
In addition, corneal properties such as CCT, hydration, biomechanics, and radius of curvature may interfere with the accuracy of tonometry with the Goldmann method, he said.
But new instruments have been designed that can remove these corneal influences from the accuracy equation, Dr. Herndon said. One (Pascal Dynamic Contour Tonometer, Zeimer Ophthalmics) is a third-generation digital device that uses the principle of contour matching instead of applanation to measure IOP.
Dr. Herndon cited one study comparing IOP measurements obtained by GAT, contour-matching tonometry (CMT), pneumotonometry (PTG), and intracameral manometry. IOP was measured in 16 freshly enucleated human cadaver eyes. The CMT values measured 0.58 mm Hg higher than true IOP, GAT measured 4.01 mm Hg lower than true IOP, and PTG measured 5.09 mm Hg lower than true IOP. The authors concluded that measurement with the CMT device provided IOP values significantly closer to true manometric levels than either GAT or PTG.
The CMT device also produces a measurement called ocular pulse amplitude (OPA), the difference in IOP between systole and diastole.
"This may be a marker for overall ocular rigidity," Dr. Herndon said. "It varies not only with the amount of blood transported to the eye during systole but also depends on choroidal vascular rigidity as well as scleral rigidity. We are currently studying the clinical utility of OPA in glaucoma patients."