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22,46 The increase in RNFL thickness resolves in about 1.5 months. In the acute phase of optic neuritis, the peripapillary RNFL thickens as a result of edema, and a hyporeflective space develops in the subretinal space. OCT has emerged as a potential metric for axonal loss in patients with MS. Since the first quantitative OCT study of RNFL thickness in MS was published in 1999, 45 there has been rapidly growing interest in various applications of OCT in the diagnosis and management of patients with multiple sclerosis associated optic neuritis (MSON) and multiple sclerosis without optic neuritis (MSNON). Myron Yanoff MD, in Ophthalmology, 2019 Multiple Sclerosis 36 The fourth hyperreflective outer retinal band is attributed to the RPE, with potential contribution from Bruch's membrane and choriocapillaris, with abundant experimental and clinical evidence supporting this designation. 33 This band typically merges with the fourth band in the central fovea, and this is explained by a greater height of the contact cylinder of the cones and RPE outside the fovea. 16,37 This third band appears to correspond to the contact cylinder between the RPE apical process and the external portion of the cone outer segment, and has been suggested to be called the interdigitation zone by the recent consensus meeting. 33,34 The third band is referred to as either the OS tips or as Verhoeff membrane.
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The second of the four bands has been commonly ascribed to the boundary between the IS/OS photoreceptors, but a recent consensus meeting suggested that this band correlates with the inner segment ellipsoid zone (EZ), although this interpretation is not universally accepted. The nomenclature for the middle two bands has much less supportive evidence. This band is typically thinner and fainter than the others. 29,30,36 The innermost band has been attributed to the external limiting membrane (ELM). There is discordance between different authors regarding which anatomic structure correlates with each band.
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Outside the central fovea, commercially available SD-OCT instruments typically resolve four bands in the outer retina. 35 The retinal vessels may sometimes be seen on OCT images as circular hyperreflective structures located in the inner retina, with a vertical shadow or reduced reflectivity extending into deeper layers. A recent study demonstrated that the incidence of the light beam could affect the appearance of Henle fiber layer by OCT, resulting in a thin hyperreflective layer corresponding to the photoreceptor synapses or a thicker hyperreflective layer corresponding to photoreceptor axonal extensions enveloped by the outer cytoplasm of Müller cells ( Fig. Within the retina, the retinal nerve fiber layer and the plexiform layers (both inner and outer) are seen as hyperreflective while the ganglion cell layer and the nuclear layers (both inner and outer) are relatively hyporeflective. In some patients, the posterior hyaloid can be seen above the ILM as a hyperreflective layer. The first detected layer in most OCT scans is the ILM that appears as a hyperreflective layer at the vitreoretinal interface. 2,30Īlthough the interpretation of features of the retina, which can be defined for our purpose to span from the ILM to the outer segments of the photoreceptors appears to correlate well with histology, the OCT features of the outer retina are less well understood and remain a topic of discussion ( Fig. Therefore, one-to-one correspondence of histology with OCT images cannot be expected. The angle of incidence of the light, motion artifacts, speckled noise, and image contrast can affect the axial resolution of the retinal imaging. 30 When light travels through the retinal tissue it can be reflected, scattered, or absorbed, and this creates the multilayered pattern of the retina. 29 However, care must be taken when making assumptions about these correlations because histologic sections require fixation and exogenous staining to produce contrast within tissue, and this can introduce artifacts, while OCT relies on intrinsic differences in tissue optical properties to produce image contrast. With the increase in the axial resolution of the new SD-OCT instruments (5–8 µm) and the ultrahigh-resolution OCT (2 µm), it has become possible to correlate OCT images accurately with histologic features of the retina. The OCT image closely approximates the histologic appearance of the macula and, for this reason, it has been referred to as an in vivo optical biopsy. Schachat MD, in Ryan's Retina, 2018 Normal Macular Anatomy