Parallelism for quantitative image analysis of photoreceptor-retinal pigment epithelium complex alterations in diabetic macular edema.
PURPOSE
To propose a new method to quantitatively and comprehensively evaluate photoreceptor-RPE complex alterations on spectral-domain optical coherence tomography (SD-OCT) imaging in eyes with diabetic macular edema (DME).


METHODS
Spectral-domain OCT images from a consecutive series of 90 eyes in 79 patients with DME and 30 healthy eyes in 30 volunteers were analyzed retrospectively. The subfoveal area covering the photoreceptor layers was skeletonized and the orientation of the segmented lines was termed "parallelism," which reflects image complexity. Photoreceptor layer status at the fovea was categorized by graders, including continuity of the external limiting membrane (ELM) line, inner segment ellipsoid line, and the presence of hyperreflective foci in the outer retinal layers. The relationships among parallelism, visual acuity, and photoreceptor layer status were evaluated.


RESULTS
Parallelism was significantly lower in eyes with DME than in normal eyes (P < 0.0001), and correlated strongly with visual acuity in eyes with DME (R = -0.592; P < 0.0001). Eyes with an intact inner segment ellipsoid line or ELM line had significantly better visual acuity (VA) and higher parallelism than eyes with a discontinuous or absent inner segment ellipsoid line or ELM line. Parallelism was significantly higher (P < 0.0001) and logMAR VA significantly better (P < 0.0001) in the group without hyperreflective foci in the outer retinal layers than in the group with hyperreflective foci in the outer retinal layers.


CONCLUSIONS
Parallelism has the potential to reflect structural changes of the photoreceptor layers in DME.