**5.1. The neurovascular unit**

therapeutic actions, where early visual disability support instead of burdensome treatment schedules may be warranted. SD-OCT provides high-resolution imaging of the retinal structure and allows insight into the pathogenesis of DME *in vivo*. Central retinal thickness (CRT) measured with OCT is commonly used in the evaluation and management of DME. However, CRT only explains 27% of the variation in visual acuity [99]. Various other OCT measures have been studied, but none of these measures has been consistently demonstrated to account for visual outcomes in patients with DME, and most of these studies were conducted retrospectively in mixed treatment cohorts. Examples of these measures include the integrity of the ellipsoid zone (EZ) (formerly described as the inner segment/outer segment photoreceptor junction) [100, 101], the integrity of the external limiting membrane [101, 102], the visibility of the cone outer segment tips (COST) [103], as well as the presence of subretinal fluid [104] and

Furthermore, intraretinal cystoid fluid has been named as a predictor of poor response to anti-VEGF treatment in a prospective study [101], as well as in two post hoc analyses [107, 108] in large datasets of patients with DME using a machine-learning approach. Recently, disorganization of the retinal inner layers (DRIL) has been suggested to be a valid predictive biomarker for visual outcomes in patients with DME. DRIL was defined as the inability to distinguish boundaries between any two of the inner retinal layers (including the ganglion cell-inner plexiform layer (GCIPL) complex, the inner nuclear layer, and the outer plexiform layer) in >50% of the foveal 1-mm zone [103]. DRIL in the central millimeter is strongly associated with visual acuity in eyes with center-involving DME. Resolving DRIL seemed to be a good indicator of subsequent visual improvement [109]. In addition, the presence and extent of DRIL before treatment are correlated with BCVA outcomes to anti-VEGF therapy after the loading dose of ranibizumab in treatment naive patients with DME [101]. Similarly, patients with DME showed gain in visual acuity if DRIL resolved compared with non-resolvers, whose visual acuity worsened. This correlation between DRIL and visual acuity could not be substantiated for eyes with macular edema due to other causes [110]. Additionally, it is well known that approximately 55% of patients with DME have co-existent macular capillary nonperfusion [111], which may be masked angiographically by leakage from the edema. Macular capillary non-perfusion hinders efficient transport of oxygen and nutrients to the inner retinal layers, which in turn compromises inner retinal integrity and may therefore lead to the appearance of DRIL in OCT scans. This concept has been substantiated by a recent study reporting 84.4% sensitivity and 100% specificity of DRIL in detecting angiographic evidence

The exact mechanisms of DRIL affecting VA have yet to be determined, but their correlation in eyes with DME is plausible as DRIL may represent an interruption in anatomic structures within these inner retinal layers including axons and nuclei of bipolar, amacrine, and/or horizontal cells, and therefore a disruption in the visual pathway from photoreceptors to retinal

These data suggest that DRIL is a robust biomarker of visual acuity in eyes with present or resolved DME, correlating better with visual acuity than other OCT measures including CRT. Future multicenter longitudinal studies have to validate the predictive potential

hyperreflective foci [105, 106].

20 Early Events in Diabetic Retinopathy and Intervention Strategies

of capillary non-perfusion in the macula [112].

ganglion cells.

Fundoscopic clinical examination of patients with DR reveals pathognomonic features including hard exudates, hemorrhages, microaneurysms, and cotton wool spots. However, it does not reveal the complex organization of the neurosensory retina. Similar to other tissues throughout the central nervous system, neurons, glia, microglia, and blood vessels are organized into neurovascular units that work interdependently in close coordination in the retina [113].

The complex interconnections in the neurovascular unit prompted early anatomists to call this tissue the retina, literally a network of cells [114]. The capillary networks of the inner retina are in close contact with neurons of the inner nuclear and ganglion cell layer. These capillaries consist of a basal lamina with a single layer of adherent endothelial cells surrounded by pericytes, glial, and microglial cells on the external surface. Microglia interact directly with retinal pericytes and are intimately associated with retinal neurons [115].

This intimate physical contact and functional integration are essential for vision and facilitate physiologic adaptation in response to varying conditions. Neuronal activity evokes localized reactions including vasodilation and increased blood flow to meet the energy demands of neuronal signal transduction and transmission [114]. In addition to the coordination of metabolic demand, close signaling interdependence manifests itself in the blood-retinal barrier, which controls the flux of fluids and metabolites into the retinal tissue [116].

The diabetic environment causes the neurovascular unit to disintegrate both in early and late DR with the physiology of the neurovascular unit being similarly altered as it is in diseases of the brain such as stroke [117], Alzheimer's, and Parkinson's diseases [118]. Although DR has traditionally been considered merely a microvascular diabetic complication, recent studies support the concept that retinal neurodegeneration precedes and contributes to the formation of microvascular abnormalities in DR. These findings suggest that DR should at least be considered a combined neuro-vascular degeneration [113].
