CCT appears less reliable than corneal elasticity for glaucoma risk

Central corneal thickness should not be the only measurement because different devices and techniques can produce different results.

Issue: August 10, 2008

ByErin L. Boyle

Central corneal thickness remains an important risk factor for glaucoma, but research shows that the measurement could be affected by other factors and might not be as accurate as measuring corneal elasticity, a physician said.

Odette Callender

Odette Callender, MD, said landmark studies, such as the Ocular Hypertension Treatment Study (OHTS), European Glaucoma Prevention Study and Barbados Eye Study, have found that central corneal thickness (CCT) is one risk factor for the disease.

However, CCT should not be the only or most important measurement of risk, Dr. Callender said. Corneal hysteresis appears to provide more reliable measurement, but the method of measurement is not as widely available.

The Ocular Response Analyzer (Reichert) device measures the corneal elasticity or stiffness, providing physicians with an accurate way to measure hysteresis and IOP, she said. It uses an air puff to measure the corneal response time from applanation to recovery.

“Right now, primarily, we’re going to be using corneal thickness as the surrogate for that information,” Dr. Callender said at the Hawaiian Eye 2008 meeting. “I think it is an important measurement that we use. It has been shown to be a risk factor, not just for development of glaucoma, but also for the severity of the disease. This may give you some guidance as to what to do with your patient.”

Theories

Dr. Callender said there are two primary theories as to why CCT is a risk factor for glaucoma. Corneal thickness could affect accurate pressure measurement, with thinner corneas causing underestimated results and thicker corneas causing overestimated results. Another theory is that the measurement could be evidence of an underlying pathology that increases risk of developing the disease.

There is no consensus regarding CCT’s overall effect on IOP or an exact measurement number that indicates a risk for all patients, she said. The OHTS found that 555 µm or less of corneal thickness was an effective threshold measurement for glaucoma risk. Anything less could indicate an increased risk of glaucoma, but the average population’s thickness measures 535 µm to 545 µm.

“If your patient’s corneal thickness is in the less than 555 µm [range], they may just be normal, not necessarily at risk for developing glaucoma,” Dr. Callender said. “If you’re going to use corneal thickness, you want to categorize it as thick, thin, average, but don’t really focus on the exact number.”

Measurement factors

Dr. Callender cautioned physicians to be especially careful in the method of measuring CCT and other factors related to the measurement. Everyday actions, such as a patient wearing a contact lens, can affect the measurement’s accuracy.

Different measurement devices and techniques can produce different results, she said. Studies have shown that ultrasound pachymetry produces thicker measurements than optical pachymetry but often provides thinner results than the Orbscan system (Bausch & Lomb).

Corneal drying during pachymetry additionally can affect results, Dr. Callender said, especially in longer applications. A study by Dayanir and colleagues found that the cornea was thinned by 3% at 60 seconds of drying. Refractive surgery can also affect corneal thickness measurements, as well as biomechanics.

Diurnal variation

Different studies have disagreed on whether diurnal variation can affect measurements, Dr. Callender said. Shah and colleagues found no correlation between diurnal IOP variation and diurnal variation of ultrasound CCT, at a rate of less than 1%. Their study examined measurements over a 24-hour period of 28 open-angle glaucoma patients.

In contrast, Harper and Fogagnolo and colleagues found significant differences in diurnal variations in two separate studies, Dr. Callender said. In Harper’s study of eight normal patients in a 48-hour period, there was a 7.2% difference in diurnal measurements of corneal thickness.

In Fogagnolo’s study of 30 open-angle glaucoma patients over a 24-hour period, patients had circadian fluctuations of 16.5 µm.

“There’s also a question of long-term variation,” Dr. Callender said. “Wickham and colleagues did measurements 85 days apart, approximately, and noticed that all the second readings were increased. On the other hand, Weizer and colleagues did measurements about 8 years apart, and all the second measurements were decreased. So again, there’s no clear consensus.”

Corneal hysteresis

Although the cornea’s central thickness measurement is still important in determining risk of glaucoma, studies are increasingly showing the importance of corneal elasticity.

“Corneal elasticity … is independent of corneal thickness and IOP and, as we’re learning, it seems to be more important that central corneal thickness,” Dr. Callender said.

She used the example of two individuals who have the same pachymetry readings. If one patient has a stiff cornea and the other patient has a soft cornea, their IOP measurements are affected in different ways, regardless of pachymetry results, she said.

Dr. Callender cited a study by Liu and Roberts that did model testing of the effect of CCT, corneal curvature and stiffness with Goldmann tonometry. They found that corneal thickness predicted IOP readings by a 2.7 mm Hg difference in pressure, whereas corneal curvature accounted for 1.76 mm Hg. Corneal elasticity, however, affected IOP measurements by 17.26 mm Hg in their study.

“If you look at glaucoma patients, they tend to have a lower corneal hysteresis or a softer cornea, and they also have a much wider range,” she said.

For more information:

Odette Callender, MD, a clinical assistant professor at the University of Maryland School of Medicine, can be reached at the Wilmington Veterans Administration Hospital, Medical Center 1601, Kirkwood Highway, Wilmington, DE 19805; 302-994-2511, ext. 4661; fax: 302-633-5358; e-mail: odette.callender@va.gov. Dr. Callender has no direct financial interest in the products discussed in this article, nor is she a paid consultant for any companies mentioned.

Bausch & Lomb, maker of the Orbscan system, can be reached at One Bausch & Lomb Place, Rochester, NY 14604-2701; 585-338-6000; fax: 585-338-6007; Web site: www.bausch.com. Reichert Ophthalmic Instruments, maker of the Ocular Response Analyzer, can be reached at 3362 Walden Ave., Depew, NY 14043; 716-686-4500; fax: 716-686-4545; Web site: www.reichert.com.

References:

Dayanir V, Sakarya R, et al. Effect of corneal drying on central corneal thickness. J Glaucoma. 2004;13:6-8.

Fogagnolo P, Rossetti L, Mazzolani F, Orzalesi N. Circadian variations in central corneal thickness and intraocular pressure in patients with glaucoma. Br J Ophthalmol. 2006;90:24-28.

Harper CL, Boulton ME, et al. Diurnal variations in human corneal thickness. Br J Ophthalmol. 1996;80:1068-1072. (Erratum in: Br J Ophthalmol. 1997;81:175.)

Liu J, Roberts CJ. Influence of corneal biomechanical properties on intraocular pressure measurement: Quantitative analysis. J Cataract Refract Surg. 2005;31:146-155.

Shah S, Spedding C, et al. Assessment of the diurnal variation in central corneal thickness and intraocular pressure for patients with suspected glaucoma. Ophthalmology. 2000;107:1191-1193.

Weizer JS, Stinnett SS, Herndon LW. Longitudinal changes in central corneal thickness and their relation to glaucoma status: An 8-year follow up study. Br J Ophthalmol. 2006; 90:732-736.

Wickham L, Edmunds B, Murdoch IE. Central corneal thickness: Will one measurement suffice? Ophthalmology. 2005;112:225-228.

Erin L. Boyle is an OSN Staff Writer who covers all aspects of ophthalmology.