Corneal arcus linked to ocular hypertension, thinner corneas

Tin Aung

Corneal arcus was found to be independently associated with higher IOP and lower central corneal thickness, which could affect the accuracy of IOP measurements, according to a study.

“There may be changes in the biomechanical properties of the cornea in eyes with corneal arcus, which may bias the measurement of IOP,” Tin Aung, MBBS, MMed(Ophth), FRCS(Ed), FRCOphth, FAMS, PhD(Lond), lead author of the Singapore-based study, told Ocular Surgery News.

Corneal arcus was defined in the study as a “gray-white or yellow opacity located near the periphery of the cornea, but separated from the limbus by a clear zone.” Because its effect on the structure and function of the cornea is not understood, corneal arcus could contribute to falsely low or high IOP readings, the study authors said.

According to Dr. Aung, corneal arcus is associated with cardiovascular risk factors such as hyperlipidemia, but the study showed that arcus is associated with a higher IOP, independent of systolic and diastolic blood pressure.

The population-based cross-sectional study was published in Archives of Ophthalmology.

Study parameters

Researchers looked at a randomly selected group of 3,280 Malay participants aged 40 to 80 years living in Singapore. Of the participants, 3,015 could be evaluated based on study criteria, 1,447 men and 1,568 women with a mean age of 57.9 years.

The study excluded patients with uveitis, secondary glaucoma, pseudoexfoliation syndrome, a history of any ocular surgery, primary angle-closure glaucoma, current use of ocular hypotensive medications or conditions that could interfere with IOP measurement.

Study participants underwent systemic examinations, which included blood pressure measurements and non-fasting blood work to determine glucose, creatinine, glycated hemoglobin and serum lipid levels.

Researchers also measured IOP, central corneal thickness (CCT) and corneal curvature radius. Anterior segment abnormalities, including corneal arcus, were evaluated using a slit lamp biomicroscope.

A total of 1,747 patients (57.9%) had corneal arcus in their right eyes.

Study outcomes

According to the authors, IOP was significantly higher — 1.14 mm Hg after adjusting for age, sex and systemic factors — in eyes with corneal arcus than in eyes without the condition.

However, CCT was found to be 2.8 µm thinner in eyes with corneal arcus, which remained statistically significant even after adjusting for age, sex and systemic factors.

“The mean [Goldmann applanation tonometry]-measured IOP was significantly higher in eyes with corneal arcus. This could not be explained by CCT differences, as the mean CCT was significantly lower in these eyes, nor could it be explained by CCR or axial length differences,” the authors said.

Another significant finding was that 3.2% of patients with corneal arcus had ocular hypertension as opposed to only 1.8% of patients without the condition. In contrast, no correlation was found between corneal arcus and the incidence of primary open-angle glaucoma.

“This may be related to the finding that participants with corneal arcus had a higher mean IOP than those without,” Dr. Aung said. “However, due to the sample size, we did not have many patients with primary open-angle glaucoma.”

The authors noted a potential selection bias in excluding participants who were on ocular hypotensive medication or who had a history of laser or surgical treatments for glaucoma. These patients could have increased the mean IOP with higher baseline IOP readings.

Additional study limitations included its cross-sectional nature, as well as its failure to assess corneal arcus severity. Dr. Aung said further studies are being planned to confirm the results.

“It will be interesting to know whether participants with corneal arcus have a higher glaucoma incidence in the future, and this will address the question about the clinical significance of the association between corneal arcus and higher IOP,” he said. – by Courtney Preston

Reference:

• Wu R, Wong TY, Saw SM, Cajucom-Uy H, Rosman M, Aung T. Effect of corneal arcus on central corneal thickness, intraocular pressure and primary open-angle glaucoma. Arch Ophthalmol. 2010;128(11):1455-1461.

• Tin Aung, MBBS, MMed(Ophth), FRCS(Ed), FRCOphth, FAMS, PhD(Lond), is Senior Consultant and Head, Glaucoma Service, Singapore National Eye Centre; Deputy Director, Singapore Eye Research Institute; Associate Professor, Dept of Ophthalmology, Yong Loo Lin School of Medicine, National University of Singapore. He can be reached at Singapore National Eye Centre, 11 Third Hospital Avenue, Singapore 168751; 65-62277255; fax: 65-62263395; e-mail: aung_tin@yahoo.co.uk.
• Disclosure: No products or companies are mentioned that would require financial disclosure.

The authors of this paper describe another association between corneal characteristics and IOP. They found that the patient with corneal arcus had a slightly higher IOP (average of 1 mm Hg higher) and had a slightly thinner cornea (3 µm thinner). This paper gives us time to reflect on the fact that the IOP is not the most important measurement to follow when trying to detect glaucoma in patients. Careful analysis of the nerve morphology and function over time is much more important. An interesting side comment on corneal arcus is that during clear cornea cataract surgery in patients with dense arcus, the cornea is softer and appears to seal less well, more often requiring sutures, than in patients with less arcus.

– David R. Hardten, MD
OSN Cornea/External Disease Section Editor
Disclosure: No products or companies are mentioned that would require financial disclosure.

The importance of IOP as both a risk factor and a treatment parameter for glaucoma is well-established. In a population-based study of over 3,200 people, the authors have identified corneal arcus as having a significant influence for both the measurement of IOP and as an independent risk factor for the presence of ocular hypertension but not glaucoma. This has been the first report to document the influence of a very common age-related finding on corneal biomechanics. At this time, clinicians should be aware of the effect of arcus on IOP measurement. As study into the exact nature of its effect on corneal biomechanics is revealed, further clinical recommendations may be warranted.

– Douglas J. Rhee, MD
OSN Glaucoma Board Member
Disclosure: No products or companies are mentioned that would require financial disclosure.