Iris thickness, area associated with risk of primary angle-closure glaucoma

Research links OCT iris measurements to narrow angles as new potential risk factors for primary angle-closure disease.

Iris thickness, curvature and area were found to be independently associated with narrow angles, and were especially prevalent in women and patients 60 years old and older, leading to a possible explanation for the higher prevalence of primary angle-closure glaucoma for those populations, a study found.

Tin Aung

“Increased iris thickness and cross-sectional area [representing volume] are associated with angle closure risk,” lead author of the Singapore-based study, Tin Aung, MD, told Ocular Surgery News. “Identification of this new factor may help identify subjects at risk for the disease. It is possible that among people with narrow angles, people with thicker irises may progress to angle closure glaucoma.”

The cross-sectional, community-based study was published in Ophthalmology in January of 2010. The study looked at iris parameters because research has shown that the iris appears to be part of the pathogenesis of angle closure.

In addition, Asian women and people over 60 are also considered at higher risk, but current physiologic risk factors for primary angle-closure glaucoma (PACG) have not yet completely accounted for those subpopulations, the study authors said.

Study parameters

The study looked at a group of 2,047 participants selected from a government-run polyclinic in Singapore. Of those participants, 1,465 eyes met the study criteria and could have iris parameters analyzed. Of those, 1,319 were Chinese (71.6%) and 793 were female (54.1%). Mean age was 62.7 years.

Patients were excluded from the study mostly because of “problems identifying the scleral spur,” a step that is required when using the iris measurement software according to the authors, which was also considered a limitation of the study itself.

Those patients included in the study underwent gonioscopy and anterior segment ocular coherence tomography (AS-OCT) under dark conditions. Using customized software, horizontal AS-OCT scans measured for iris curvature, iris area and iris thickness at 750 µm and 2,000 µm from the scleral spur.

A total of 315 patients had angle closure on gonioscopy, based on standardized criteria.

Study results

Dr. Aung said that results show the association between iris parameters and angle closure risk. He and his colleagues have continued to analyze data.

“We have another paper in press in [the] British Journal of Ophthalmology confirming that increased iris thickness is associated with angle closure glaucoma in another study hospital-based population,” he said.

In the Ophthalmology study, for patients with angle closure on gonioscopy, mean iris curvature and iris thickness measurements were greater for subjects with angle closure than without.

Female patients had significantly higher multivariate adjusted odds ratio for all iris parameters. In addition, women with increased iris area had a nearly five times greater association with narrow angles than men.

For age results in the study, patients aged 60 years and older also had significantly higher odds ratio for iris area and iris thickness.

Dr. Aung said the take-home message of the study is that “a novel risk factor for angle closure has been identified – that of the iris thickness and area.”

“The use of anterior segment OCT has allowed the measurement of iris thickness and area,” he said. “Without this technology, this was not possible before.” – by Erin L. Boyle

References:

• Alsbirk PH. Anatomical risk factors of angle-closure glaucoma: a 10-year study of limbal and axial anterior chamber depths in a risk population. [in Danish] Ugeskr Laeger. 1994;156:5117–5121.
• Congdon NG, Youlin Q, Quigley H, et al. Biometry and primary angle-closure glaucoma among Chinese, white, and black populations. Ophthalmology. 1997;104:1489–1495. Abstract
• Lavanya R, Wong TY, Friedman DS, et al. Determinants of angle closure in older Singaporeans. Arch Ophthalmol. 2008;126:686–691.
• Wang B, Sakata LM, Friedman DS, Chan YH, He M, Lavanya R, Wong TY, Aung T. Quantitative iris parameters and association with narrow angles. Ophthalmology. 2010;117(1):11-17.
• Wang N, Wu H, Fan Z. Primary angle closure glaucoma in Chinese and Western populations. Chin Med J (Engl). 2002;115:1706–1715.

• 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.