Redefining the Limit of the Outer Retina in Optical Coherence Tomography Scans

Abstract

A highly reflective layer seen in retinal optical coherence tomography (OCT) has been believed to correspond to the choriocapillaris (CHC) and retinal pigment epithelium (RPE). On gray-scale scans of OCT-2000, and on Stratus OCT, this layer by the outer retinal limit can be resolved into 2 distinct laminae. We analyzed these 2 laminae in normal and abnormal maculae to infer their anatomic correlate. Retrospective study. Analysis of macular OCT scans was performed in 44 patients using OCT-2000, and in 39 patients using Stratus OCT. Thirty of these patients had no ocular disease, and their OCT was normal. The other 53 patients had several macular diseases of different etiologies. Both color and gray-scale images were analyzed. Macular OCT scans showed a double laminae at the level where the retina interfaces the RPE in normal subjects using both OCT-2000 and Stratus OCT. In 2-dimensional scans, this laminar structure appears as a double line. It is best distinguished on the Stratus OCT and gray-scale images of OCT-2000. This double line consisted of a thin inner line and a thicker outer line. Similar analysis in patients with macular pathology showed a discernible double line at the retina/RPE interface in at least part of the scan. However, in patients with macular hole, the area corresponding to the absent retina showed only a single line. The inner line component appeared to follow the contour of the retina. This phenomenon was also seen in eyes with neurosensory detachment secondary to central serous chorioretinopathy and other etiologies. In contrast, in macular pathologies where the outer retina did not lose contiguity with the RPE, such as in lamellar macular hole and in cystoid macular edema, the double line persisted. Software for retinal thickness measurements regularly place the outer limit of the retina at the internal aspect of the inner line, probably underestimating the retinal thickness by about 24 to 34 mum. A double laminar structure at the outer retina/RPE/CHC interface can be consistently distinguished on commercially available OCT of normal eyes. In eyes with macular pathology, OCT analysis of the inner lamina leads us to conclude it is most likely part of the neurosensory retina and not part of the RPE/CHC complex as previously thought.