Significance of central corneal thickness on the management of glaucoma

In a new study, researchers at Columbia University Medical Center demonstrated the significance of central corneal thickness (CCT) on the clinical management of patients with glaucoma and those suspected to have glaucoma.

While confirming previous research about the relevance of CCT in glaucoma management, this study represents one of the first attempts to determine exactly how great an impact CCT has on a patient’s intraocular pressure (IOP), fluid build-up inside the eye that is a glaucoma risk factor. Results found that CCT affected more than half of the patients in the study.

“We were astounded to find that so many of the glaucoma patients in our study needed an IOP adjustment, based on their CCT measurement,” said James C. Tsai, M.D., the study’s senior author. Dr. Tsai is associate professor of ophthalmology and chief, division of glaucoma, at the Edward S. Harkness Eye Institute at the Columbia University Medical Center. “While more research needs to be done to further determine the effects of CCT on clinical management and consequent long-term outcomes, it’s clear that CCT should be considered when making glaucoma treatment decisions.”

Glaucoma is a group of diseases that can damage the eye's optic nerve and result in vision loss and blindness. Similar to cholesterol readings for heart disease, measurements of a patient’s IOP are a cornerstone of glaucoma treatment – with the clinical goal to get as low as possible to reduce the risk of long-term disease progression and vision loss.

However, if a patient also has an abnormal CCT, research has shown that the true IOP level (measured in millimeters of mercury – Hg) may be masked, which may result in under-treatment or over-treatment of the glaucoma. Patients with thinner corneas tend to have underestimated IOPs (i.e., their true IOP is higher than the measured value); whereas those with thicker corneas tend to have overestimated IOPs (i.e., their true IOP is lower than the measured value).

The researchers tested a linear correction scale that was used to guide treatment management decisions for all patients with glaucoma or suspected glaucoma seen in a two-month period (n=188). Based on whether a patient’s CCT was found to be thinner or thicker than normal (defined in the study as 545 µm), the linear scale added or subtracted 2.5 mm Hg to the IOP for every 50 µm difference in CCT from 545 µm. Corrected IOP = Measured IOP – (CCT-545/50 X 2.5 mm Hg).

• Measurement significant adjustments were defined as IOP corrections of 1.5 mm Hg or greater (in either direction).
• Any CCT-associated IOP adjustments of 3.0 mm Hg or greater (in either direction) were designated as outcomes significant.

Results of the linear scale found that 55.9 percent (105 patients) had at least a measurement significant adjustment to their IOP, with 35.6 percent (67) having adjustments between 1.5 and 3.0 mm Hg and 20.2 percent (38) having an outcomes significant correction (=3.0 mm Hg in either direction). A mathematical formula was used for comparison and was found to have similar results.

The study was published in Archives of Ophthalmology.