Emerging Insights into Pathogenesis

Diabetic retinopathy (DR) is a frequent complication of diabetes mellitus (DM). Persistent hyperglycemia leads to the activation of multiple cellular pathways involved in the pathogenesis of DR, resulting in increased inflammation, oxidative stress, and vascular dysfunction. DR has been considered a "chronic, low-grade inflammatory disease of the retina". However, an increasing body of evidence suggests that inflammation and neurodegeneration both occur in human diabetes even before the development of clinical signs of DR. One of the first signs of inflammation in DM is the activation of retinal glia cells (RGC). RGC include microglia and macroglial cells (Müller cells and astrocytes). Activated RGC release cytotoxic substances responsible for the recruitment of leukocytes, blood-retinal barrier breakdown, direct glial dysfunction, and neuronal cell death. Recently, many inflammatory mediators, growth factors, and other molecules have been investigated in human vitreous and aqueous humor samples to better understand, in vivo, the multiple pathways and mechanisms leading to the pathogenesis of DR and its complications, such as diabetic macular edema, with potential for more tailored treatment. Moreover, non-invasive techniques, like optical coherence tomography, have allowed to detect new findings in the retinal layers, such as the hyperreflective intraretinal spots, which have been hypothesized to represent an in vivo marker of microglial activation, and early neural cell loss, confirming the hypothesis that neurodegeneration occurs early both in type 1 and 2 diabetes. These new emerging insights foster a better understanding of the pathogenesis of DR, which can no longer be considered as a pure retinal vascular complication of DM.