Central corneal thickness a marker of susceptibility?

Results from a case study comparing inter-eye central corneal thickness (CCT) in patients with asymmetric glaucoma show a positive association between more advanced glaucomatous cupping and a thinner central cornea, according to Shawn J. Khan, MD, FRCSC.

"We believe thinner central corneas may be an indicator of glaucomatous disease severity and susceptibility in a subset of glaucoma patients," said Dr. Khan, attending surgeon, W. K. Kellogg Eye Center, University of Michigan, Ann Arbor.

"These patients may be predisposed to more advanced glaucomatous optic neuropathy due to ocular anatomic abnormalities that are completely independent of the biomechanical effect of CCT on IOP, and our findings raise the question of whether a thinner central cornea is an important biological indicator of glaucoma in susceptible eyes."

"This study was designed to test the hypothesis that in a subset of patients with asymmetric glaucoma, the eye with more advanced glaucomatous cupping would have a thinner central cornea," Dr. Khan said.

It enrolled 43 patients with asymmetric glaucoma and a control group consisting of patients without glaucoma. Asymmetric glaucoma was defined as an inter-eye difference in cup/disc ratio (CDR) >0.2. The controls were required to have a CDR <0.8 in both eyes and an inter-eye difference in CDR of <0.2. Patients with any corneal edema, previous corneal or refractive surgery, intraocular surgery within the past 4 months, neovascularization, previous cyclodestructive procedures, prior glaucoma surgery with corneal evidence of hypotony, chronic angle-closure glaucoma, posterior polymorphous dystrophy, iridocorneal endothelial (ICE) syndrome, or monocular status were excluded from the study. CCT measurement was performed in all eyes in a standardized manner using ultrasound pachymetry.

Among the cases of asymmetric glaucoma, the eyes with the worse glaucoma (larger CDR) had a mean CCT that was 14.0 μm thinner than the fellow eyes (p <0.0001). In 36 (86%) of the 43 patients, the eye with the worse glaucoma was the eye with the thinner cornea. In contrast, there was only a 0.2-μm inter-eye difference in CCT in the controls when comparing the right and left eyes.

Discussing the research, at the annual meeting of the American Academy of Ophthalmology, Jody R. Piltz-Seymour, MD, director of the glaucoma service, Scheie Eye Institute, University of Pennsylvania, raised a concern that the study results could not be used to support the hypothesis regarding CCT and glaucoma biosusceptibility since the study did not control for IOP effects.

"Even if there was no significant difference in IOP between the eyes with more glaucomatous damage and thinner corneas, we know there is a difference because IOP in eyes with thinner corneas is higher than the measured value. Therefore, until we can remove IOP from the model as a confounding variable, all we can say from the findings of this study is that eyes with more advanced disease had thinner corneas," she said.

James Brandt, MD, professor of ophthalmology and director of the glaucoma service, University of California-Davis, pointed out that the analysis also failed to consider the possible confounding influence of glaucoma medication use with respect to number of medications and duration.

"Some studies suggest longer duration of use of medications may accelerate thinning, and presumably the more severely damaged eyes would have been treated with medications longer. So the thinner corneas seen in the eyes with more advanced disease may reflect an effect of many years of use of topical medications," he said.