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Editors Selection IGR 12-2

Structural Examination Methods: RNFL defect patterns by Cirrus OCT

Comment by Marco Vizzeri on:

26996 Retinal Nerve Fiber Layer Imaging with Spectral-Domain Optical Coherence Tomography Pattern of RNFL Defects in Glaucoma., Leung CK; Choi N; Weinreb RN et al., Ophthalmology, 2010; 117: 2337-2344

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Along with the excitement derived by utilizing new technology, comes the dilemma of whether clinicians truly benefit from its use. Cirrus HD-OCT, for example, now provides an RNFL thickness map over a 6 x 6 mm² area around the optic disc, not available with previous OCT technology. Using the deviation map constructed from the thickness map with reference to the built-in normative database, Leung et al. (1716) evaluated the spatial distribution and area of RNFL defects in glaucoma. The deviation map is composed of 50 x 50 superpixels (one superpixel = 4 x 4 pixels) labeled in yellow or red if the RNFL thickness in that location falls below the 95 or 99 percentile range of the normative database, respectively. By examining the angular width, area and distribution of red superpixels within the map, the authors provide an interesting description of the patterns of RNFL defects in glaucoma, as detected by Cirrus. As expected, RNFL thickness loss was more evident in the inferotemporal and superotemporal sectors and tended to be more widely diffuse in advanced glaucoma. It would have been important to compare these findings with the results obtained from the circle scan that is now automatically centered on the optic disc for evaluating sectoral and clock hour RNFL thicknesses. In fact, similar conclusions may be reached by assessing the RNFL thickness along the circle scan, although one specific pattern suggests the presence of RNFL defects close to the disc margins, in a region that would have been missed using the conventional circle scan diameter of 3.4 mm. The authors do not confirm if these RNFL defects also can be seen on red-free photographs or whether they correlate with visual field abnormalities. A newly introduced parameter describing the proportion of normal RNFL area within the deviation map failed to demonstrate a better diagnostic accuracy for detecting glaucoma than the average RNFL thickness calculated using the circle scan. Additional cross-sectional as well as longitudinal studies are required to determine how the RNFL thickness map can effectively improve clinicians' ability in glaucoma diagnosis and follow-up.

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