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Editors Selection IGR 17-4

Intraoperative Evidence of Outflow: SD-OCT imaging of outflow

Comment by Lawrence Kagemann on:

51884 Spectral domain optical coherence tomography imaging of the aqueous outflow structures in normal participants of the EPIC-Norfolk Eye Study, Day AC; Garway-Heath DF; Broadway DC et al., British Journal of Ophthalmology, 2013; 97: 189-195

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Study of the aqueous humor outflow pathway is experiencing a renaissance as spectral domain optical coherence technology (OCT) has enabled its non-invasive visualization in the living human eye. Here, Day and colleagues present a largely descriptive study applying OCT imaging of the limbus to measurement of trabecular meshwork crosssectional area, Schlemm's canal diameter, and the linear distance from Schwalbe's line to the scleral spur in a cohort of healthy eyes of elderly subjects. The nasal and temporal limbi of 46 eyes OD are examined, finding a significant association between nasal trabecular meshwork cross-sectional area and both age and intraocular pressure (IOP) by multiple linear regression.

In OCT imagery of limbus, the scleral spur is not evident, and its location was assumed to be the posterior-most location within the anterior chamber angle. Similarly, the tissues of the trabecular meshwork cannot be visually discerned from surrounding scleral structures. Straight lines drawn between a three observable landmarks, specifically the bottom of the angle, the end of Descemet's layer, and inner wall of Schlemm's canal, defined the borders of the trabecular meshwork. The cross-sectional measurement of an unseen structure is a limitation of the study, and future validation of the technique against histology is desirable. Ironically, this limitation calls out one of the study's strengths. A major obstacle to being one of the first to perform a measurement is the lack of a gold standard technique for guidance. To that end, the present group wisely utilized a 'training set' of 20 images, which the independent observers first analyzed together to assure agreement between the definition and understanding of landmarks within the limbal OCT scans.

The reappearance of a finding in the nasal but not the temporal quadrant supports future study of regional differences in outflow patterns in the living human eye

This is a pilot study with a limited number of subjects, but a recurrent theme emerging in the literature is importance of the nasal quadrant in outflow. As with previous studies, the nasal outflow pathway, in this instance trabecular meshwork cross-sectional area, appears to be influential. Here, it is significantly associated with IOP. It is still very early, but the reappearance of a finding in the nasal but not the temporal quadrant supports future study of regional differences in outflow patterns in the living human eye.

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