Keywords
diabetic retinopathy - ellipsoid zone - external limiting membrane - retinal pigment epithelium - spectral domain optical coherence tomography
Introduction
Diabetes has become a global burden and the prevalence rates are rising steeply in developing economies.[1] Diabetes is a progressive disorder leading to complications divided into small vessel or microvascular disease and large vessel or macrovascular disease. Microvascular complications affect the innermost layer of the eye—the retina known as diabetic retinopathy (DR). All individuals with diabetes are at risk of developing DR. Understanding of its pathogenesis is essential for developing more effective modalities for treatment. DR is broadly divided into nonproliferative with dot–blot hemorrhages, cotton–wool spots, venous beading or intraretinal microvascular anomalies, and proliferative retinopathy defined by the presence of neovascularization of the optic disc or elsewhere in other parts of retina away from disc ([Fig. 1]). In recent years with the advent of advanced retinal imaging techniques, the understanding of different retinal disorders have become easier. Spectral domain optical coherence tomography (SD-OCT) has emerged as an indispensable tool for in vivo retinal imaging providing layer by layer detailed analysis of retina. Outer retinal layers comprising external limiting membrane (ELM), photoreceptor ellipsoid zone (EZ), and retinal pigment epithelium (RPE) have a very important role to play in different retinal disorders, and their integrity is essential for maintaining visual function of the eye.[2]
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[5] In this article, we have described the role of these outer layers of retina in DR based on SD-OCT.
Fig. 1 (A) Retinal exudates (asterisk) and retinal hemorrhages (triangle), features of nonproliferative diabetic retinopathy. (B) Preretinal hemorrhages (blue arrow), hard exudates (black arrow), venous beading (red arrow), and neovascularization of disc (asterisk), all features of proliferative diabetic retinopathy.
Materials and Methods
Articles with clinical features, pathogenesis, diagnosis, and treatment of DR were thoroughly studied. Articles were searched on PubMed, MEDLINE, and Cochrane Library from 2000 to 2020. Studies focusing on the role of ELM, EZ, and RPE in the pathogenesis of DR based on SD-OCT were included.
Spectral Domain Optical Coherence Tomography: Role in the Assessment of Retinal Morphology
Spectral Domain Optical Coherence Tomography: Role in the Assessment of Retinal Morphology
Optical coherence tomography (OCT) is a noninvasive imaging modality that provides cross-sectional and topographical imaging of the posterior segment of eye ([Fig. 2]). OCT provides ophthalmologists to have a nearly cellular level of resolution of the retina. It utilizes light to image tissue using low coherence interferometry. SD-OCT is currently the most widely used OCT technology. It uses 820- to 880-nm probing light with a scan rate of 52,000 Hz or greater providing excellent imaging of all the layers of retina.[6] Human retina has 10 layers. On OCT cross-sectional imaging, they are broadly classified into inner retinal layers and outer retinal layers. SD-OCT resolves mainly three bands in the outer retina as hyperreflective bands which are of great importance as they are responsible for the retinal functions ([Fig. 3A]).
Fig. 2 Macular analysis (macular cube 512 × 128) done on spectral domain optical coherence tomography. (A) The scanning laser ophthalmoscopy image of the fundus with superimposed color-coded retinal thickness map. (B) The Early Treatment of Diabetic Retinopathy Study grid with values of the retinal thickness (µm) in various quadrants. (C) The cross-sectional images of retinal layers. (D) The topographical image of retinal layers.
Fig. 3 (A) A cross-sectional spectral domain optical coherence tomography (OCT) image showing hyperreflective outer retinal layer bands—external limiting membrane (ELM), ellipsoid zone (EZ), and retinal pigment epithelium (RPE). (B) Spectral domain-OCT cross-sectional image of retina showing disruption of EZ and ELM in a patient of diabetic macular edema.
Outer Layers of Retina
External Limiting Membrane
The innermost hyperreflective band is the ELM, a linear confluence of junctional complexes between Muller cells and photoreceptors.[7] This band typically is thinner and much fainter than the others. The ELM separates the layers of rods and cones from the overlying outer nuclear layer, and is a linear confluence of junctional complexes between Muller cells and photoreceptors. It serves as a barrier against macromolecules.
Ellipsoid Zone
The second band is the boundary between inner and outer segments of the photoreceptors, aligned with the ellipsoid portion of the inner segments, and is termed as EZ.[8] The photoreceptors include an outer segment that absorbs light and converts it into electrical signals and an inner segment that has the metabolic functions of generating energy and proteins.
Retinal Pigment Epithelium
The outermost highly reflective band represents the RPE, Bruch's membrane, and possibly the choriocapillaris.[9]
[10] It is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual cells. The RPE is composed of a single layer of hexagonal cells that are densely packed with pigment granules. The RPE has several functions, namely, light absorption, epithelial transport, spatial ion buffering, visual cycle, phagocytosis, secretion, and immune modulation.
Status of ELM and EZ has been studied in brown Norwegian rats on OCT. It was found that the EZ and ELM disappeared after euthanasia. The origin of the EZ and ELM was found to be related to the biological activities of the photoreceptor cells.[11]
Role of Outer Layers of Retina in Pathogenesis of Diabetic Retinopathy
Role of Outer Layers of Retina in Pathogenesis of Diabetic Retinopathy
Pathogenesis of DR is complex involving multiple molecular pathways occurring due to deranged metabolism in DM. The long-standing hyperglycemia leads to protein glycosylation resulting in the formation of AGEs. AGEs have an impact through their effect on retinal microvasculature, vascular endothelial growth factor (VEGF), intercellular adhesion molecule-1 (ICAM-1), nitrosative and oxidative stress, and vitamin D and calcium metabolism.
AGEs and their receptor activation leads to endothelial dysfunction and release of proangiogenic factors resulting in pericyte apoptosis, vascular inflammation, and angiogenesis damaging the vascular components of the retina.[12]
[13] This leads to leakage and fluid accumulation resulting in macular edema. AGEs increase the expression of VEGF in retinal pericytes and RPE cells.[14] Healthy retina requires interaction between its photoreceptors and the RPE. An intact ELM has been highlighted as a prerequisite for an intact EZ.[15] Diabetic macular edema (DME) is known to be associated with ELM and EZ disruption ([Fig. 3B]).[16]
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ELM comprises attachment of outer process of glial Muller cells to one another and also to inner photoreceptor segments which has been demonstrated in animal model.[18] They revealed that tight junctions existed in the ELM between glial Muller cells and photoreceptors. Occludin was found as a key component of tight junctions. In ELM, occludin was found to be organized between the glial Muller cells and the photoreceptors, thus the ELM should be considered as part of a retinal barrier. In DME, glial Muller cells at the level of ELM are swollen and lose their occludin content. ELM junctions thus could be considered as unique regulatory targets in treatment. VEGF alters tight junctions and promotes vascular permeability in many retinal diseases. Murakami et al highlighted the role of occludin in regulation of endothelial barrier properties.[19]
The EZ serves as a clinical indicator of photoreceptor integrity. Biologically, EZ consists of mitochondria mainly enabling higher levels of energy consumption within the photoreceptors.[20] Mitochondrial dysfunction occurs in the photoreceptors at fovea in DME. The absence of the subfoveal EZ on SD-OCT corresponds to the reduced reflectivity or anatomic absence of the EZ.
Studies demonstrating early retinal damage in experimental diabetes have observed morphological changes in RPE.[21] Oxidative stress has been found to result in lipid peroxidation and oxidation of glycated proteins in the diabetic retina.[22] An association of nitrosative and oxidative stress with retinal photoreceptor EZ and topographical changes in RPE has been demonstrated ([Fig. 4]).[23] Experimental studies have shown interaction of RPE and endothelial cells results in loss of barrier function of RPE which was mediated by VEGF. The balance between VEGF and antiangiogenic factor pigment epithelium-derived factor secreted by RPE is crucial for progression of DR.[24] The VEGF secreted by RPE is essential for maintaining the structural integrity of the outer retina and choriocapillaris.
Fig. 4 (A) A normal single-layer retinal pigment epithelium (RPE) map on spectral domain optical coherence tomography (SD-OCT). It is used to get a three-dimensional topographical view of the RPE layer. (B) The single-layer RPE map on SD-OCT of a patient with diabetic retinopathy shows numerous RPE alterations.
Retinal VEGF induces ICAM-1 expression leading to leukostasis and breakdown of blood–retinal barrier.[25] Jain et al demonstrated the correlation of VEGF and ICAM-1 on ELM and EZ disruption[15] ([Fig. 4]). AGEs trigger oxidative stress generation resulting in inflammatory and thrombogenic reactions. Mitochondrial superoxide generation plays an important role in the formation and accumulation of AGEs under diabetic conditions.[26] Sharma et al showed nitric oxide and oxidative stress in DR are associated with disruption of ELM and EZ.[23] Increased serum antimyeloperoxidase antibody levels have been found to be associated with ELM and EZ disruption in DR.[27] Deranged metabolism in DM leads to compromised nutritional status. Vitamin D deficiency has been found to be associated with disruption ELM and EZ.[28] Vitamin D also plays an important role in regulating calcium homeostasis. Increased serum ionized calcium induces retinal photoreceptor apoptosis resulting in ELM and EZ disruption in DR.[29]
Effect of Outer Layers of Retina on Visual Function in Diabetic Retinopathy
Effect of Outer Layers of Retina on Visual Function in Diabetic Retinopathy
The ELM and EZ integrity are essential for the maintenance of normal vision. Many cross-sectional or longitudinal studies have shown the clinical importance of both the layers in DME. The length of the disrupted or absent EZ has been associated with visual impairment.[30] DME is known to be associated with disruption of ELM and EZ, which in turn affects visual acuity (VA). An increase in VEGF and ICAM-1 levels correlates with increased macular thickness (central subfield thickness), sequential disruption of ELM and EZ, and an increase in severity of DR.[15] Accordingly, disruption of ELM and EZ has been graded as per our published classification: grade 0: no disruption of ELM and EZ; grade 1: ELM disrupted, EZ intact; and grade 2: both ELM and EZ disrupted. This a physician-friendly grading system with an excellent reproducibility and is an important predictor of disease severity and visual outcome. EZ disruption was found to be significantly associated with the severity of retinopathy. RPE which forms the outer retinal barrier in turn maintains nutrition and normal physiology of photoreceptors and inner retinal layers. The RPE alterations have been reported to be associated with the severity of DR and decrease in VA ([Fig. 4B]).[31]
Effects of Therapeutic Agents of Diabetic Retinopathy on Outer Layers of Retina
Effects of Therapeutic Agents of Diabetic Retinopathy on Outer Layers of Retina
Anti-VEGFs are considered as the first-line treatment for DME. Administration of intravitreal anti-VEGF agents has been found to be associated with improvement in VA. Restoration of the foveal photoreceptors occurs following administration of intravitreal ranibizumab, a Food and Drug Administration–approved anti-VEGF agent in DME has been reported.[20] Improvement in photoreceptor integrity takes place after second and third doses of ranibizumab with improvement in VA and color vision. A larger foveal photoreceptor microstructure defect is associated with lower VA. Patients with larger foveal photoreceptor microstructure defects at baseline had lesser VA improvements.[32] The improvement in EZ defect size is dependent on the pattern of DME on SD-OCT.[33]
De et al discovered the mechanism of ELM and EZ restoration after anti-VEGF therapy in DME.[34] Anti-VEGF therapy led to restoration of barrier effect of ELM. The ELM was established as a retinal structural barrier and was found to restore first followed by EZ restoration. Decrease in logMAR VA was more pronounced in patients associated with restoration of ELM and EZ.
An increase in VEGF results in sequential ELM and EZ disruption on SD-OCT. An intact ELM is a prerequisite for an intact EZ in DME. Anti-VEGF therapy leads to restoration of barrier effect of ELM. The ELM restores first, followed by EZ restoration.
Newer anti-AGEs agents and their receptor blockers are being developed which have a positive effect on maintaining the health of RPE.[35]
Conclusion
In DR, to understand the complex disease process, the knowledge of outer layers of retina is essential. They play an important role in pathogenesis of DR at both structural and molecular levels. Thus, the integrity of ELM, EZ, and RPE is essential for maintaining visual function of retina in DR.
