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Editors Selection IGR 10-2
Anatomical Structures: Iris dynamics and angle-closure risk
Comment by Shan Lin on:
56333 Comparison of dynamic changes in anterior ocular structures examined with anterior segment optical coherence tomography in a cohort of various origins, Seager FE; Jefferys JL; Quigley HA, Investigative Ophthalmology and Visual Science, 2014; 55: 1672-1683
Seager et al.1 reported the results of anterior segment optical coherence tomography imaging among groups with different geographical origin, seen within a clinic setting. Diagnoses ranged from normal to primary open-angle glaucoma to various forms of angle closure (suspects, PAC, AACG and PACG). The total number of subjects was 267, including whites, African-Americans, Chinese, Koreans, Indian (Asian), and Others. The main findings were that whites and African-Americans had a greater iris area at baseline than Chinese and Koreans; however, the Chinese race was the only one in which less loss of iris area per mm of pupil dilation (going from light to dark conditions) was associated with greater risk for an angle closure diagnosis. The authors speculate that this dynamic factor is one of the predisposing reasons that Chinese may be at higher risk for angle closure.
Overall this paper presents interesting findings, which add to the literature on anatomic reasons for angle closure variability among races. There are some aspects worthy of comment and further consideration. For example, in the present paper, 'baseline' for iris parameters is under light conditions. In most studies, the 'baseline' values for iris and angle measurements are in the dark. This makes physiological sense in that dark situations are when angle closure risk is higher. Their use of parameters in the light is likely the reason for discrepancies with prior studies.2,3
The findings related to the dynamic change in the iris going from light conditions to the dark seem to be very relevant to understanding why Chinese have particularly high risk. Dr. Quigley is to be congratulated in proposing early on that the anatomic conditions leading to angle closure are more related to dynamic rather than static features of the anterior segment. In support of their findings, our group's study comparing ethnic Chinese and whites found that Chinese have greater thickening of the iris at the angle when going from light to dark.4 Also, Chinese have greater narrowing of the angle than whites in that transition, which was associated with iris thickening only in the Chinese group.5
References
- Seager FE, Jefferys JL, Quigley HA. Comparison of dynamic changes in anterior ocular structures examined with anterior segment optical coherence tomography in a cohort of various origins. Investigative Ophthalmology and Visual Science 2014; 55: 1672-1683.
- Wang D, Huang G, He M, Wu L, Lin S. Comparison of anterior ocular segment biometry features and related factors among American Caucasians, American Chinese and mainland Chinese. Clin Experiment Ophthalmol. 2012 Aug;40(6):542-9.
- Wang D, He M, Wu L, Yaplee S, Singh K, Lin S. Differences in iris structural measurements among American Caucasians, American Chinese and mainland Chinese. Clin Experiment Ophthalmol. 2012;40:162-169.
- Wang D, He M, Wu L, Kao A, Peckmezci M, Lin S. Dark-light change of iris parameters and related factors among American Caucasians, American Chinese and mainland Chinese. Curr Eye Res. 2012 Jul;37(7):599-605.
- Wang D, Chiu C, He MG, Wu L, Kao A, and Lin S. Differences in baseline dark and dark-light changes of anterior chamber angle parameters among whites and ethnic Chinese. Invest Ophthalmol Vis Sci. 2011 Dec 9;52(13):9404-9410.
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