The limitations of our present study include small sample size and the semiquantitative nature of the analysis. Although OCT images are affected by axial length,
17,18 the OCT device used in the present study was unable to provide magnification correction. Although eyes with axial length >27 mm were excluded, many myopic eyes with relatively long axial lengths remained. As already mentioned, tissue reflectance varies across different scans and individuals; hence we did not quantify the reflectance intensity of each image. However, Ashimatey et al.
5 observed a strong correlation between entire or hemifield en face RNFL reflectance abnormalities and cpRNFL thinning by using a slab analysis, wherein the slab thickness was adjusted to minimize the putative glial alteration reflectance on visualizing RNFL reflectance in patients with early glaucomatous change with a mean MD of −3.3 dB. As stated above, no apparent glial alteration reflectance was observed in the present study. However, as indicated by Ashimatey et al.,
5 a fixed slab thickness for the entire macular region analysis might have missed reflective abnormalities in the deeper layer and underestimated the correlation between thickness and reflectivity, particularly at the temporal region. In addition, the directional reflectivity of the nerve fiber bundles may affect the variability of the reflectivity.
10 Further refinement is required to quantify the reflectivity and analyze the correlation between the degree of reflectivity and visual field damage. Lastly, we could not perform direct comparison between macular RNFL (mRNFL) thickness map and en face slab images with our data because the OCT system used in this study does not provide mRNFL thickness maps, and it was technically difficult to create the map by ourselves. As shown in
Figure 2, the findings of en face slab images and averaged value of mRNFL thickness in each sector were correlated. Thus, similar results can be produced by using the mRNFL thickness map, as long as the segmentation is performing properly. But mRNFL per A-scan was too thin to be properly and consistently segmented and thus was practically highly challenging to measure. This is the major reason why the current version of Cirrus OCT does not report the mRNFL thickness. In clear comparison, it is quite straightforward to obtain en face slab images, which reflect both thickness and intensity information. If a more advanced device such as Cirrus HD-OCT model 5000, adaptive optics SLO, or swept-source OCT were used, as reported by Hood et al.,
4 the comparison between mRNFL thickness and the slab image analysis in terms of the structure–function correlation in the advanced stage of glaucoma could be carried out. Further analysis is required to compare the ability of the mRNFL thickness map and en face slab image to detect residual functioning RGC axon bundles.