Anatomical correlates to the bands seen in the outer retina by optical coherence tomography: literature review and model

Abstract

Purpose: To evaluate the validity of commonly used anatomical designations for the four hyperreflective outer retinal bands seen in current-generation optical coherence tomography, a scale model of outer retinal morphology was created using published information for direct comparison with optical coherence tomography scans.

Methods: Articles and books concerning histology of the outer retina from 1900 until 2009 were evaluated, and data were used to create a scale model drawing. Boundaries between outer retinal tissue compartments described by the model were compared with intensity variations of representative spectral-domain optical coherence tomography scans using longitudinal reflectance profiles to determine the region of origin of the hyperreflective outer retinal bands.

Results: This analysis showed a high likelihood that the spectral-domain optical coherence tomography bands attributed to the external limiting membrane (the first, innermost band) and to the retinal pigment epithelium (the fourth, outermost band) are correctly attributed. Comparative analysis showed that the second band, often attributed to the boundary between inner and outer segments of the photoreceptors, actually aligns with the ellipsoid portion of the inner segments. The third band corresponded to an ensheathment of the cone outer segments by apical processes of the retinal pigment epithelium in a structure known as the contact cylinder.

Conclusion: Anatomical attributions and subsequent pathophysiologic assessments pertaining to the second and third outer retinal hyperreflective bands may not be correct. This analysis has identified testable hypotheses for the actual correlates of the second and third bands. Nonretinal pigment epithelium contributions to the fourth band (e.g., Bruch membrane) remain to be determined.

Fig. 1

Drawing of the outer hyperreflective bands overlaid on a representative spectral-domain OCT scan of a normal macula. Each of the bands has had different anatomical correlations proposed over time, with varying designations by research group and time period. Typical attributions are as follows: Band 1, the ELM; Band 2, the boundary between the ISs and OSs of the photoreceptors; Band 3, the OS tips or Verhoeff membrane; Band 4, the RPE, possibly including Bruch membrane and the choriocapillaris.

Fig. 2

Alterations in band appearance caused by logarithmic transformation. A. A gray scale sine wave shows equivalent width bands of light and dark, with a gradual transition between. A log transform of (A) results in (C). This type of transformation is commonly done to display information from OCT scans. B. The longitudinal intensity profile through the image shown in (A) reveals the expected sine wave pattern. D. The longitudinal density profile taken through (C) reveals a distortion of gray scale values and broadening of the bright bands induced by the log transform.

Fig. 3

A. Lower magnification and (B) high magnification in two parts. A scale drawing of the outer retina showing cones in the central fovea (A, low magnification). In (B) (higher magnification) the Muller cells form junctional complexes with the photoreceptors that when viewed in aggregate are called the ELM. In reality, it is not a membrane, limiting or otherwise. The foveal cones are narrow and cylindrical, like rods. The inner portion of the IS is called the myoid and the outermost division is the ellipsoid, which contain numerous thin mitochondria. Extending over the proximal OS are fine cytoplasmic extensions called the calycal processes. The RPE has small apical extensions called microvilli. The OS continue to the RPE and are enveloped in specialized apical processes forming a contact cylinder (left cone). A small gap (the supracone space) is present between the outermost part of the OS and the RPE, seen in the cross-sectional view (right). Retinal pigment epithelial cells have junctional complexes (drawn larger than scale for clarity) formed with neighboring RPE cells. The confluence of these complexes as seen by light microscopy is called, in a manner analogous to the ELM, Verhoeff membrane.

Fig. 4

A. Lower magnification and (B) high magnification in two parts. IS, perifoveal cone. Cone IS widen with greater eccentricity from the foveal center. Mitochondria fill approximately 75% of the ellipsoid volume and account for the enormous oxygen use by photoreceptors. Like foveal cones, perifoveal cones have small cytoplasmic extensions called the calycal processes that extend over the innermost OSs. The ELM designates junctional complexes named for their appearance by light microscopy. Outer segments of cones outside of the central macula stop well short of the underlying RPE cells. Specialized apical extensions arise from the RPE to encase the outer one third of OS length, recognized here by the faint obscuration of the OS disks. These ensheathing apical processes have cellular machinery not ordinarily found in microvilli such as ribosomes and mitochondria. Shed OS are moved back to the RPE cell body for phagocytosis through the supracone space. The RPE apical junctional processes are labeled according to their appearance on light microscopy, Verhoeff membrane.

Fig. 5

A. Lower magnification and (B) higher magnification in two parts. Rods are present outside of the central fovea. The rod IS is composed of the myoid and ellipsoid sections. Both rod IS width and mitochondrial packing density are much less than cones in the perifovea. The cytoplasmic extensions of the calycal process are longer for rods than cones. The OS tips are surrounded by fine apical extensions, analogous to the contact cylinder of cones (rod at left). Rods extend down to contact the RPE cells in a cross-sectional view (rod at right).

Fig. 6

A. A representative OCT image of the fovea showing the outer bands. In the central fovea, the third band appears to blend in with the fourth band. The highlighted box is a 10-pixel-wide region used to create the LRP. The dashed lines show the agreement between the hyperreflective bands and the LRP. These dashed lines extend to a drawing scaled to coincide with the peak of the band attributed to the ELM (Band 1) and the junctional complexes between the Muller cells and the photoreceptors in the drawing. The dashed lines represent the full-width at half maximum point of the LRP curve for the ELM. The second band is centered over and encompasses the ellipsoid section of the photoreceptors. Note that the local nadir of the LRP colocalizes near the IS/OS boundary (arrow). The outer hyperreflective band, formed by the overlap of the third and fourth bands corresponds to the region extending from the contact cylinders to slightly sclerad to Bruch membrane. B. These relationships are shown in a magnified version in the inset at right.

Fig. 7

A. A representative OCT of the perifoveal region 2 mm temporal to the center of the fovea showing the outer bands. The highlighted box is a region 10 pixels wide that was used to create the LRP. The dashed lines from the OCT through the LRP were continued to the drawing. The first band of the OCT represents the ELM and was aligned with the junctional complexes between the Muller cells and the photoreceptors. The dashed lines represent the full-width at half maximum point of the LRP curve for the ELM. The second band corresponded very closely with the ellipsoid section of the IS. The third band encompassed the region of the cone OS/contact cylinder region. The fourth band was aligned to the RPE with the peak of the LRP corresponding to the inner one third of the RPE cells, which is where the melanosomes are more commonly found. As opposed to the comparison involving the fovea, the outer portion of the fourth band does not extend past Bruch membrane in this region. Note the boundary between the IS and OS is near the lowest local reflectivity in the LRP (arrow). B. These relationships are shown in a magnified version in the inset at right.

Fig. 8

Near the nerve, there are isolated patches of short, wide cones seen in otherwise normal donor eyes. Because of the displacement of the contact cylinder anteriorly, it is possible for more than one level to be observed. The rods are not shown in this drawing.