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Artur V Cideciyan, Alexander Sumaroka, Samuel G Jacobson; Hypotheses about the Identity of Outer Retinal Microstructure in OCT Cross-sections and Quantitative Measurements in Normal Subjects. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3397.
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Cross-sectional imaging of the human retina by OCT produces layers of differing backscatter intensity. Correspondence between these OCT layers and histologically definable structures have been evolving over the last two decades as the axial, lateral and temporal resolution of the OCT systems improve, and with increasing observations in molecularly-clarified retinal pathology. Here we present our current interpretation of outer retinal layers obtained with a commercial OCT instrument.
OCT was performed (RTVue-100; Optovue Inc) in normal subjects (n=22; ages 8-62) with un-averaged line scans. Dense raster scans were performed at the fovea in a subset.
There were at least four hyperscattering bands distal (choroidal) to the hyposcattering band commonly agreed to correspond to the outer nuclear layer. In proximal to distal order, Band 1 is thought to be at the outer limiting membrane (OLM) and Band 2 at or near the ellipsoid region of inner segments (ISe). The distance between Bands 1 and 2 would be expected to be proportional to the photoreceptor inner segment length and it varied from 33.5±2.19 um (mean±sd) at the foveola to 24.0±1.94 and 22.2±2.22 um at 4 mm superior and inferior retina, respectively. Band 3 is thought to be near the interface between cone outer segment tips and the RPE contact cylinder, and the distance between Bands 2 and 3 would be expected to be proportional to the cone photoreceptor outer segment length, which varied from 38.1±2.77 um at the foveola to 14.8±3.32 and 10.2±3.27 um at 4 mm superior and inferior retina, respectively. Band 4 had two distinct sub-peaks across the retina except for the foveola. The proximal subpeak Band 4a is hypothesized to correspond to the interdigitation of rod outer segment tips with RPE apical processes, and the distal subpeak Band 4b is hypothesized to correspond to the interface near basal RPE and Bruch’s membrane. The missing Band 4a at the foveola would correspond to the rod-free zone. The thickness between Band 2 and Band 4a would be proportional to rod outer segment length and it ranged from 47.7±2.85 um outside the foveola to 33.1±2.21 um and 31.4±2.23 um at 4 mm superior and inferior retina, respectively.
Hypotheses about outer retinal structural identities need to be tested in concert with results in molecularly-clarified diseases primarily affecting rods or cones.
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