The histologic definition for what constitutes the OPL is well established. Stephen Polyak, the renowned twentieth century neuroanatomist, defined the OPL as a trilaminar structure consisting of Henle’s layer (“fibrous expansions of rod and cone cells”), “rod spherules and larger cone pedicles,” and “outer expansions of bipolars and all expansions of horizontal cells...” Curcio et al, in the first graphic representation and thickness database of normal macular and chorioretinal layers, defined the OPL layers as the OPL Henle, OPL pedicles, and OPL synaptic, now felt to be better described as the OPL Henle, OPL terminal (spherules and pedicles), and OPL dendritic (processes of bipolar and horizontal cells) layers (written communication July 16, 2020).
Correlating OPL histology with OCT findings remains a work in progress since current OCT technology does not allow for routine separation of the OPL layers from the ONL. In 2014, The International Nomenclature for Optical Coherence Tomography Panel (IN•OCT) suggested the current widely accepted terminology for OCT retinal anatomic landmarks. Their hyperreflective band 7 was defined as the “dendritic outer plexiform layer.” The inner half of their hyporeflective band 8 was felt to represent Henle’s nerve fiber layer and the outer half the ONL. Implied but not explicitly mentioned in this publication, is that Henle’s layer is the non-dendritic portion of the OPL. The nuanced description of their hyperreflective band 7 representing the synaptic portions of the photoreceptors with the horizontal and bipolar cells, is generally lost in clinical use since their Figures 1 and 2 called this the “Outer Plexiform Layer.”
Most of our eyes exhibited the well-known phenomenon of a thickened, diffusely hyperreflective OPL, what Ouyang et all termed the bright OPL phenotype. We also noticed a common, but rarely previously discussed, OPL bifurcation overlying retinal detachment. We found that the OPL split along the steeper, shallower and within both portions of equally shallowly detached retina. Splitting was noted with an angle of detachment as low as 10 degrees (case 8) and as high as 57 degrees (case 2). Similarly, OPL splitting was absent with an angle as low as 12 degrees and as high as 38 degrees (case 6). In addition, splitting was found nasally, temporally and on both sides of the detachment. In other eyes with detachment there was no OPL splitting. We expected that the split OPL should be seen within a given angle of detachment, since others have suggested that the OPL is best visualized when the incident OCT beam is most perpendicular to it.[3,4] However, we saw this finding regardless of location (nasal vs temporal) or angle of detachment. We are not sure why this was the case.
Our cases confirmed prior observations that the height of the hyperreflective paracentral OPL in attached retina was thinner and the outer nuclear layer thicker than the region between the area of apparent OPL splitting or hyperreflectivity within detached retina. Curcio et al provided retinal and choroidal layer thicknesses for the central and parafoveal macula. Mean central foveal thickness for the OPL synaptic, OPL pedicles, OPL Henle, and ONL (ONL rods plus ONL cones) were 0, 0, 22, and 34 µm, respectively. Mean thickness 1mm nasal to the macular center for the OPL synaptic, OPL pedicles, OPL Henle, and ONL (ONL rods plus ONL cones) were 6, 7, 29, and 27 µm, respectively. Mean thickness 1mm temporal to the macular center for the OPL synaptic, OPL pedicles, OPL Henle, and ONL (ONL rods plus ONL cones) were 7, 6, 55, and 32 µm, respectively. OPL Henle in the paracentral macula is thus about 70% thicker than the ONL. Current OCT technology does not allow for us to accurately measure the height of these bands into µm resolution, but the relative heights of these regions in our eyes were most consistent with Curcio’s measurements within the area of detachment (Figure XIV). Each of our patients’ altered OPL, whether it was a bump, bifurcation, or solid hyperreflective band, all had the same height approximating its true anatomic thickness. Thus, altering the angle of the OPL with the incident OCT laser beam, either by manipulating the beam’s location within the patient’s pupil or by nature presenting us with a retinal detachment, may allow for better separation of these layers and a truer correlation with anatomic measurements.
We therefore suggest a revised OCT OPL and ONL classification scheme applicable only when the normal retinal anatomy is altered either by the OCT technician or overlying retinal detachment (Figure XV). The innermost hyperreflective band, the OPL Dendritic Zone, includes the synaptic and terminal subzones of horizontal and bipolar cell processes and terminals of the rods and cones. The anatomic thickness of this region is only 13-14 µm, and it is unclear what components contribute to its hyperreflectivity. The thickened hyperreflective region at the inflection from attached to detached retina, as well as the hyporeflective band within detachment, is the OPL Henle’s Zone. This area may also appear as a solid hyperreflective band. The border between Henle’s and the ONL is the outermost hyperreflective band, followed by the hyporeflective ONL. We prefer to call these bands, zones, to emphasize that these are heterogeneous cellular structures.
Our retina image teaching collection, built from patients seen at our practice, is composed of images selected for their quality and how well each exemplifies and teaches something about each entity. No attempt has been made over the years to include every example of every patient we have examined. Similarly, when retrospectively looking through this collection for images with OPL splitting, we made no attempt to include every example we found. Thus, we can make no conclusion regarding how often these findings are present with retinal detachment.
In summary, we expand on prior descriptions of splitting of the OPL within a macular detachment (bifurcation sign) and a thickened bump within the OPL at the inflection from attached to detached retina (bump sign). These findings were common in our practice’s retinal image teaching collection and were also found unmentioned in many published references dealing with retinal detachment and subretinal fluid. As previously described, the appearance of what we currently call the OPL and ONL on OCT changes depending on the angle of the incident OCT laser with the retinal microstructures. However, contrary to prior explanations for this phenomenon, the varied OPL phenotypes inexplicably appear to be independent of the incident angle of the illuminating OCT laser. This altered appearance may give a clearer picture of the true anatomy.