Details of Glaucomatous Damage Are Better Seen on OCT En Face Images Than on OCT Retinal Nerve Fiber Layer Thickness Maps.

Spectral-domain optical coherence tomography (SDOCT) is routinely used to detect glaucomatous damage to the retinal nerve fiber layer (RNFL). For clinical use, circumpapillary images are obtained with circle scans and/or derived from cube (volume) scans of the disc. To detect glaucomatous damage, most commonly, the thickness of the circumpapillary RNFL (cpRNFL) is compared with values from healthy controls. Less commonly, analysis of RNFL thickness for clinical diagnosis includes the larger area of the peripapillary retina encompassed by the cube scan. Both approaches require accurate segmentation of the RNFL boundaries. Although OCT scans provide valuable information about cpRNFL thickness, the lateral resolution of commercially available OCT instruments is limited to ∼15 to 20 μm. Adaptive optics applied to either OCT (AO-OCT) or to scanning laser ophthalmoscopy (AO-SLO) provide better lateral resolution and enable the visualization of individual axon bundles.1–7 These bundles are less reflective and/or missing from the RNFL in eyes with glaucoma.4–7 Recently, Hood et al.7 showed that locations with equally thinned cpRNFL, as measured with OCT, had different patterns of RNFL bundle loss when imaged with AO-SLO. For example, in some patients, these regions were devoid of RNFL bundles, whereas in others, bundles were present. Figure 1 provides an example from their study. In Figure 1A, a montage of AO-SLO images is superimposed on a fundus photograph. Two regions of abnormal RNFL bundles can be seen within the red borders. The abnormal region in the inferior disc appears totally devoid of bundles. On the other hand, in the superior region, there are two bundles (yellow arrow) within a region that otherwise appears devoid of bundles. Interestingly, the corresponding region on the OCT image (yellow arrow in Fig. 1B) appears more reflective (i.e., more intense). Of course, without the AO-SLO images, this small variation in reflectivity on the OCT would be difficult to interpret. In any case, there are local variations in reflectance intensity in regions of the OCT scan with approximately the same RNFL thickness. Figure 1 (A) For patient P1, AO-SLO images superimposed on a fundus photograph. The regions within the red borders are largely devoid of RNFL bundles. The green rectangle encompasses a region shown as an en face image in Figure 3A. (B) The temporal half (white ... There is evidence from studies of humans, as well as of animal models, that glaucomatous damage can affect RNFL reflectivity, as well as RNFL thickness (Fortune B, et al. IOVS 2014;55:ARVO E-Abstract 2644 and Refs. 7–11). In a nonhuman primate model, Fortune et al. (Fortune B, et al. IOVS 2014;55:ARVO E-Abstract 2644 and Refs. 12–14) demonstrated that thickness and reflectivity carry different information and that OCT en face imaging can be used to detect these differences (Fortune B, et al. IOVS 2014;55:ARVO E-Abstract 2644). In humans, Van der Schoot et al.9 and Vermeer et al.10 developed en face image maps based on the RNFL reflectance intensity normalized to the RPE reflectance intensity. They suggested that these RNFL attenuation coefficient maps were superior to the familiar two-dimensional map of RNFL thickness. In fact, their results are consistent with the hypothesis that the internal structure of RNFL axon bundles is altered in glaucoma and, as a consequence, so too are the optical (scattering) properties.8,11 However, as they pointed out, their method requires accurate determination of the boundaries of both the RNFL and RPE layers. In practice, this can create a problem as the automated algorithms designed to accomplish this task often make errors. To compare the details of local glaucomatous damage seen on OCT en face images to those seen on OCT RNFL thickness maps, we analyzed wide-field, swept-source (ss)OCT scans of patients for whom peripapillary AO-SLO images,7 as in Figure 1A, were available. In particular, following Fortune et al. (Fortune B, et al. IOVS 2014;55:ARVO E-Abstract 2644), we derived projection images by averaging the reflectance intensity of a fixed axial portion of each A-scan, thus creating an image of a “slab” with fixed thickness below the vitreous/inner limiting membrane (V/ILM). The V/ILM boundary segmented by automated algorithms shows the best precision, as well as best agreement with human experts, compared with the other segmented boundaries.15,16 We found that there were local variations in RNFL bundles present on AO-SLO images (e.g., the preserved bundles in the superior disc in Fig. 1A) that could be seen with OCT en face imaging. These details were not seen with typical RNFL thickness maps.