**4. Disorganization of the retinal inner layers for diabetic macular edema prediction**

Diabetic macular edema (DME) is one of the most vision-threatening manifestations of DR, affecting almost 30% of patients with a duration of diabetes mellitus of more than 20 years [96].

Elevated levels of vascular endothelial growth factor (VEGF) are a major contributor to retinal microvascular dysfunction and the development of DME. VEGF interferes with tight junctions of the vascular endothelium, leading to a breakdown of the blood retinal barrier and consequently leakage into the retinal tissue [97]. Therefore, repetitive intraocular injections of anti-VEGF agents are a first-line therapy among the currently available treatments for DME. These injections have demonstrated efficiency in reducing macular thickness and improving best-corrected visual acuity (BCVA) [98]. However, while beneficial for some patients, others do not respond to intraocular drug injections. Furthermore, the resolution of DME may not be followed by a recovery in visual function. To date, no reliable methods exist to determine which individuals with DME will or will not respond to available treatments. The implementation of predictive biomarkers would guarantee an efficient therapeutic selection to identify patients with a limited prognosis of visual recovery despite ongoing 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 hyperreflective foci [105, 106].

of DRIL by prospectively collecting data on the visual outcome of patients with DME, with additional studies to clarify the histologic equivalent accompanying the appearance of DRIL

Potential Imaging Biomarkers in the Development and Progression of Diabetic Retinopathy

http://dx.doi.org/10.5772/intechopen.71747

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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

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,

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

Signs of neurodegeneration were not visible in fundus examination in the era of the ETDRS. Therefore, these changes did not contribute to the characterization or diagnosis of the disease. However, retinal neurodegeneration has widely been accepted as part of DR over the last

These abnormalities in retinal neural tissue lead to well-studied functional changes that typically precede the clinical diagnosis of DR, and in some cases occur even prior to the diagnosis

**5. Diabetic retinopathy as a neurodegenerative disease**

pericytes and are intimately associated with retinal neurons [115].

considered a combined neuro-vascular degeneration [113].

**5.2. Retinal neurodegeneration**

decades.

which controls the flux of fluids and metabolites into the retinal tissue [116].

in SD-OCT [103].

**5.1. The neurovascular unit**

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

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 ganglion cells.

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 of DRIL by prospectively collecting data on the visual outcome of patients with DME, with additional studies to clarify the histologic equivalent accompanying the appearance of DRIL in SD-OCT [103].
