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Editors Selection IGR 21-4

Anatomical Structures: Bruch's membrane opening

Comment by Andrew Tatham on:

66747 Does the Location of Bruch's Membrane Opening Change Over Time? Longitudinal Analysis Using San Diego Automated Layer Segmentation Algorithm (SALSA), Belghith A; Bowd C; Medeiros FA et al., Investigative Ophthalmology and Visual Science, 2016; 57: 675-682

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In order to accurately detect glaucomatous changes using optical coherence tomography (OCT), it is important that the measurements obtained are reproducible. Improved reproducibility would enhance our ability to differentiate true progression from background noise due to measurement variability, and improve the accuracy of classification of eyes using normative databases.

It has been proposed that Bruch's membrane opening (BMO) be used as a reference landmark, from which optic nerve head parameters can be estimated. For example, the minimum rim width, which is defined as the minimum distance from BMO to the internal limiting membrane, has been shown to be useful for differentiating healthy and glaucomatous eyes. In this study, Belghith and colleagues examined whether the BMO really is a stable landmark by determining whether its location changes over time.

Improved reproducibility would enhance our ability to differentiate true progression from background noise due to measurement variability, and improve the accuracy of classification of eyes using normative databases

The study included healthy controls, patients with stable glaucoma, and patients with glaucoma and progression on visual fields or stereoscopic optic disc photographs. OCT was performed six-monthly for an average of 3.7 years. Automated software, previously developed by the group (SALSA), was used to identify the BMO in each scan.

Important findings were that the location of the BMO identified by SALSA showed good agreement with manual assessment, and most importantly, the location of the BMO was stable over three to four years of follow-up, in both healthy and glaucomatous eyes. In other words, change in a parameter measured relative to the BMO, such a minimum rim width, is likely to reflect genuine change, rather than change in the position of the BMO alone. This suggests that measurements using BMO as a reference are likely to be anatomically stable and useful for detecting glaucoma progression.

Although the BMO can be identified manually, advantages of SALSA are that it is automated and searches for the BMO in every B scan in an OCT volume scan, unlike manual identification of the BMO, which is limited to examination of a narrow number of radial line scans.

Measurements using BMO as a reference are likely to be anatomically stable and useful for detecting glaucoma progression

The study had some limitations. It did not include eyes with high myopia and had a relatively short follow-up. Also, eyes with advanced glaucoma were not included. It should also be emphasized that for a small number of eyes the difference between the position of the BMO identified manually and by SALSA was relatively high, indicating the continued need to review scans for segmentation errors.

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