Corneal hysteresis better predicts defect severity

January 1, 2011

Lower corneal hysteresis is associated with worse glaucomatous damage independent of its effect on IOP measurement, and may be the best indicator of the eye with worse visual field loss.

New York-Lower corneal hysteresis (CH) is associated with worse glaucomatous damage independent of its effect on IOP measurement, and may be the best indicator of the eye with worse visual field loss.

In addition, asymmetric, treated primary open-angle glaucoma (POAG) is associated with asymmetry of non-contact applanation tonometry (Ocular Response Analyzer [ORA], Reichert) parameters, but not with corneal thickness or Goldmann applanation tonometry (GAT), according to C. Gustavo De Moraes, MD.

"CH is a new parameter that is associated with the corneal biomechanical properties," said Dr. De Moraes, research assistant professor of ophthalmology, New York University, New York. "Various studies have shown that central corneal thickness (CCT) is a risk factor for glaucoma onset and progression."

"However, CCT may not be the best parameter to describe corneal biomechanical properties and explain [their] association with glaucoma progression," he added. "Our study aimed to investigate whether there was an association between CH and glaucoma severity and whether CH was a better predictor of disease severity than CCT and Goldmann IOP in patients [with treated glaucoma]."

For this prospective, cross-sectional study, Dr. De Moraes and colleagues measured ORA parameters in 117 patients with POAG who had asymmetric visual field damage. The ORA measures CH and corneal resistance factor (CRF), two corneal biomechanical parameters. It also provides a measure of IOP (IOPcc) corrected for these parameters.

Asymmetry was defined as a five-point difference between the eyes using the Advanced Glaucoma Intervention Study (AGIS) scoring system. Patients were excluded if they had had previous intraocular or refractive surgery, ocular comorbidities-including macular dystrophies, diabetic retinopathy, age-related macular degeneration, neuro-ophthalmologic disorders-or conditions with a known or anticipated effect on ORA measurements (including diabetes, rigid contact lens use, keratoconus, and corneal scarring).

Mean age of patients was 62 years. In all, 61.5% had unilateral visual field defects, and 38.5% had asymmetric visual field defects. Mean follow-up was 4.4 years.

As expected, eyes with worse visual field damage had mean AGIS scores significantly higher than their fellow "better" eyes (8.1 versus 1.0, respectively; p < 0.001), and their mean CH was significantly lower (8.2 versus 8.9 mm Hg; p < 0.001).

Median ORA-corrected IOP was higher in the worse eyes (IOPcc 17.4 versus 16.9 mm Hg; p < 0.001). Worse eyes had a slightly lower mean CRF (p = 0.04), and more myopic mean spherical equivalent (p = 0.02).

No statistically significant differences in CCT or GAT were seen between eyes (p = 0.32).

Upon multivariate analysis, only CH remained associated with the worse eye (OR: 25.9, 95% CI: 10.1 to 66.5). ROC curves demonstrated that only CH and IOPcc had a discriminative ability for the eye with worse visual field damage (AUC: 0.82 and 0.70, respectively).

Increased risk

"In a patient [with treated glaucoma], the eye with lower CH measurement is most likely the one with worse glaucomatous visual field damage," Dr. De Moraes said. "Therefore, eyes with low CH may be at increased risk of glaucoma progression, even though we did not perform a longitudinal study investigating the role of CH as a predictor of progression. Eyes with low CH may require more aggressive therapy."

The authors noted their results show that the GAT not only underestimates true IOP, but may also underestimate asymmetry in IOP in patients with asymmetric POAG.

"It is well-known based on the current literature that the transcorneal pressure provided by GAT does not reflect the true IOP, that is, the pressure that is responsible for mechanical stress to the optic disc and retinal nerve fiber layer in glaucoma," Dr. De Moraes said. "Instead, GAT provides just an estimation of the IOP. Recent studies that employed the ORA to estimate the true IOP have suggested that this modality of tonometry may do it better than GAT.

"Among our findings, we reported that there was no difference in GAT measurements in patients with asymmetric, treated glaucoma, that is, GAT-IOP measurements were statistically similar between eyes despite asymmetric visual field damage," he continued. "Assuming that ORA-IOP measurements better estimate the true IOP, our study shows that even though clinicians may assume that the IOP is satisfactory in patients with asymmetric damage, the true IOP may actually be higher and not ideal in the eye with worse visual field damage."

Practicing ophthalmologists may not yet find practical applications for these results because, noted Dr. De Moraes: "This was a cross-sectional study that did not investigate the predictive role of CH and the effect of treatment on progression, hence we do not have sufficient evidence that would allow clinicians to make any treatment decisions or changes at this point based solely on our study."

He added, however: "When evaluating a glaucoma patient who presents significant asymmetry in CH values, the eye with lower CH needs closer surveillance because it may be at increased risk of visual field deterioration due to underestimation of the true IOP."

fyiC. Gustavo De Moraes, MD
Dr. De Moraes has no financial disclosures.
This study was supported by the James Cox Chambers Research Fund of the New York Glaucoma Research Institute.