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Editors Selection IGR 15-2

Anatomical Structures: Schlemm's canal

Comment by Lawrence Kagemann on:

53106 Spectral-domain optical coherence tomographic assessment of Schlemm's canal in Chinese subjects with primary open-angle glaucoma, Hong J; Xu J; Wei A et al., Ophthalmology, 2013; 120: 709-715

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The superior resolution of spectral domain optical coherence technology (OCT) allows non-invasive quantification of aqueous humor outflow structure morphology in the living human eye. Here, Hong and colleagues used the excellent visualization of the anterior segment provided by the RTVue OCT to compare Schlemm's canal cross-sectional area in health and disease. They found that Schlemm's canal cross-sectional area is reduced in a cohort of Chinese subjects with primary open angle glaucoma compared to a cohort of age-matched controls. Further, Schlemm's canal cross-sectional area pooled for all subjects was significantly negatively correlated with intraocular pressure. The measurements performed by the authors, as they are, support their conclusions. Unfortunately, a single scan was used to represent Schlemm's canal. Instead of characterizing Schlemm's canal size in a meaningful way, the authors actually compared the largest observable Schlemm's canal cross-sectional area that a scan fell upon between the two groups. There are two problems with this approach: (1) A single scan cannot guarantee identification of the largest existing structure, but only what is captured as the tissue is surveyed in live scan mode; and (2) it is possible to find any desired Schlemm's canal cross-sectional area with a single scan by surveying the tissue, and trying to land on a desired size. This introduces the potential for measurement bias. Experience in our lab shows that Schlemm's canal cross-sectional area has a mean value of ~4,000 square micrometers, areas are approximately normally distribution with and standard deviation of ~1,500 square micrometers, and occasional values exist in the ranges reported in Hong's study. The values reported herein are larger than 90% of cross-sectional area values found in a thoroughly sampled Schlemm's canal segment. The differences in health and disease found herein may represent true differences that occur with disease, but the author's scan technique does not permit the measurements to be interpreted as representative of Schlemm's canal cross-sectional area. Nevertheless, studies such as this are needed to transition aqueous humor outflow structure morphology imaging into the clinical management of glaucoma.

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