Diabetic Retinopathy Pathophysiology
Diabetic retinopathy (DR) is a microvascular disorder caused by vision-threatening damage to the retina, a long-term sequela of diabetes mellitus.
DR is the most common microvascular complication in diabetic patients and the leading global cause of vision loss in working middle-aged adults.  The propensity of developing DR is directly proportional to the age of the patient and duration of diabetes as well as with poor glycemic control and hypertension.
For more information on the disease entity, etiology, risk factors, diagnosis, and management, see Diabetic Retinopathy. This article provides detail on the mechanisms and pathological processes involved in DR.
DR can be classified clinically into non-proliferative (NPDR) and proliferative (PDR) forms, according to the presence or absence of retinal neovascularization, and it can present with or without macular edema (DME).
Mechanisms of Diabetic Retinopathy Subtypes
NPDR represents the early stage of DR, with increased vascular permeability and capillary occlusion being the two main observations in retinal vasculature. Based on the severity of retinal vascular lesions, NPDR is categorized into mild, moderate, and severe forms. Lesions vary from microaneurysms, dot and blot hemorrhages, hard exudates, and cotton wool spots to venous beading and intra-retinal microvascular abnormalities (IRMAs).  PDR represents a more advanced stage of DR characterized by the presence of neovascularization. The new abnormal vessels may bleed into the vitreous or cause a tractional retinal detachment, severely impairing vision. DME is characterized by thickening of the macula due to the accumulation of fluid within 500 µm of the center of the macula and it can occur at any stage of DR.  In patients with type I diabetes, PDR is the most prevalent vision-threatening condition. However, the most common cause of vision loss in type II diabetes patients is DME.
Hyperglycemia results in damage to retinal capillaries through the formation of advanced glycation endproducts (AGEs). The resulting endothelial damage compromises capillary walls and results in microaneurysms. Microaneurysms consequently rupture to form hemorrhages deep in the retina, appearing as "dots" on retinal examination, more commonly known as dot and blot hemorrhages. The fundamental pathologic process involved in capillary occlusion is believed to be the result of an activated leukocyte adhering to and damaging the retinal capillary wall, which results in eventual capillary occlusion. This obstruction can cause infarction of the nerve fiber layer, resulting in cotton-wool spots.
Inflammatory cytokines are significantly up-regulated in diabetes, and as a result, chronic inflammation and endothelial damage lead to increased vascular permeability of blood vessels. The pathologic process involved in DME is the resultant fluid leaking into the retina and depositing under the macula. Sediment left behind from this edema leads to waxy, yellow lipid byproducts referred to as hard exudates. Macular edema can occur in NPDR, but it is more common in more severe cases of DR where the increased vascular permeability is more advanced.
Continued ischemia stimulates retinal cells to release pro-angiogenic factors such as VEGF. Such factors stimulate neovascularization to bypass damaged retinal blood vessels. The formation of new blood vessels occurs from existing capillaries as a result of angiogenesis. These blood vessels usually arise in the interface between perfused and non-perfused areas of the retina in retinal neovascularization. They can also originate from the optic disk or iris (neovascularization of the disk/iris). These new vessels are extremely immature, fragile, permeable and bleed very easily, originating severe complications such as vitreous hemorrhage or tractional retinal detachment.
Several pathways are involved in the aforementioned processes.
Hyperglycemia and the regulation of metabolic pathways
Chronic hyperglycemia is the key promotor for the development and progression of DR due to its tissue-damaging effects, as described in the UKPDS and DCCT trials. However, genetic factors may play a role in individual susceptibility to those effects and other clinical factors like hypertension, dyslipidemia and pregnancy have also been implicated. 
Hyperglycemia leads to the activation of alternative pathways of glucose metabolism such as the polyol pathway, advanced glycation endproducts (AGEs) formation, protein kinase C (PKC) activation, hexosamine pathway flux and Poly(ADP-ribose) polymerase activation. The end result of these pathways is the activation of cytokines and growth factors, leading to vascular endothelial dysfunction, increased vascular permeability, and eventual microvascular occlusion. Microvascular occlusion then leads to retinal ischemia, which promotes neovascularization and the formation of IRMAs.
The Polyol Pathway
Excess glucose is metabolized via the polyol pathway to sorbitol. Sorbitol is impermeable to cellular membranes, accumulating inside the cell and inducing osmotic damage.  It can also be metabolized to fructose and subsequently to fructose-3-phosphate and deoxyglucosone, both of which are strong glycolyzing agents and lead to the deposition of AGEs. In addition, upregulation of the polyol pathway results in a reduced availability of NADPH, thereby enhancing the sensitivity of affected cells to oxidative stress.
Due to the high availability of glucose, AGEs formation is markedly increased in diabetic patients. AGEs have the capacity to cross-link proteins which alters their structure and function, affecting basement membranes, cellular receptors, and blood vessel wall components. Moreover, AGEs receptors activation induces prooxidant and pro-inflammatory cascades, thus exacerbating oxidative stress and leukocyte adhesion. The accumulation of AGEs has also been correlated to pericyte loss.
An increase in glycolysis activity also occurs during hyperglycemic episodes, elevating the synthesis of diacylglycerol (DAG) which in turn activates the PKC pathway. PKC activates the mitogen-activated protein kinase (MAPK) factors, leading to enhanced expression of stress-related proteins and mediators of vascular function such as c-Jun kinases and heat shock proteins. In particular, the PKC-β isoform increases VEGF expression. PKC activation also drives over-expression of NADPH oxidase and NFκB in vascular cells, exacerbating oxidative stress and inflammation.
Hexosamine Pathway Flux
In the hexosamine pathway, fructose-6-phosphate (F6P) is converted into uridine-5-diphospho-N-acetylgalactosamine (UDP-GlcNAc). O-GlcNAc transferase (OGT) catalyzes the addition of GlcNAc to serine and threonine residues at phosphorylation sites on SP1, upregulating its transcriptional activity and consequently the expression of transforming growth factor beta (TGFβ) and plasminogen activator inhibitor-1 (PAI-1) in vascular cells. The glycosylation of RNA polymerase-II transcription factors by OGT and UDP-GlcNAc affects the expression of multiple factors involved in DR pathophysiology, representing a key regulatory mechanism of glucose-responsive gene transcription.
Poly(ADP-Ribose) Polymerase Activation
Hyperglycemia-induced oxidative stress correlates to increased poly(ADP-ribose) polymerase (PARP) activation. The formation of ROS leads to NAD+ depletion and inhibition of glyceraldehyde phosphate dehydrogenase (GAPDH) through the depletion of the enzyme’s catalytic cofactor and PARP-mediated ribosylation. In conjunction, these molecular mechanisms contribute to DNA damage and endothelial cell dysfunction in diabetic blood vessels.
Several signaling pathways can be altered by having hyperglycemia in different tissues, which produces oxidative stress. Hyperglycemia activates a particular pathway involving diacylglycerol (DAG), the activation of protein kinase C (PKC), and the NADPH-oxidase system. This particular signaling pathway is involved in the control of angiogenesis, oxidative stress, and cell death.
Increasing evidence points to inflammation as a key factor in the pathogenesis of DR, although the exact molecular mechanisms are not well understood. The simultaneous course of multiple metabolic pathways, such as oxidative stress, AGEs, and increased VEGF expression all likely contribute to the inflammatory response. Chronic low-grade inflammation is a key driver of capillary occlusion and hypoxia that reinforces VEGF expression and concomitant hallmark vascular abnormalities of DR.
Inflammatory cytokines such as tumor necrosis factor alpha (TNF-α), interleukin 6 (IL-6), IL-8 and IL-1 were significantly up-regulated in diabetic patients, and their expression level is correlated with the severity of DR.  Leukostasis has been associated with occlusion of retinal microvasculature and is correlated with endothelium damage and BRB impairment in diabetic rats, contributing to endothelial cell loss and breakdown of BRB. 
Retinal glial cell dysfunction is also presumed to be involved in inflammation in DR. Under hyperglycemic stress, microglia activation increases secretion of TNF-α, IL-6, MCP-1 and VEGF. Numerous studies show that inflammation inhibition by using anti-inflammatory drugs such as intravitreal triamcinolone acetonide and NSAIDs like nepafenac reduces VEGF expression and vascular permeability, inhibits retinal cell death, diminishes leukostasis, and ultimately improves visual acuity.  
Vascular abnormalities and angiogenesis pathways
Hyperglycemia causes pericyte loss, apoptosis of endothelial cells and thickening of the basement membrane, which collectively contribute to the impairment of the BRB. Since pericytes are responsible for providing structural support for capillaries, their loss leads to microaneurysm formation. Furthermore, pronounced loss of pericytes and endothelial cells results in capillary occlusion and ischemia. Retinal ischemia/hypoxia leads to upregulation of VEGF through activation of hypoxia-inducible factor 1 (HIF-1).
Neural retina cells are also affected in DR pathophysiology. In fact, retinal neurodegeneration is an early event during the progression of DR that may even precede vascular apoptosis. Upregulation of pro-apoptotic molecules has been detected in retinal neurons in diabetic animals and humans.   Oxidative stress seems to be involved in the activation of these pathways. Pro-apoptotic mitochondrial proteins such as cytochrome c and apoptosis-inducing factor (AIF) were also found to be significantly increased, implicating mitochondrial dysfunction in retinal degeneration In diabetic patients, inner retinal thinning was detected with no DR or minimal DR. This highlights the sensitivity of neuronal cell types to apoptotic stimuli such as oxidative stress and mitochondrial dysfunction. Therefore, neuroprotective agents may play a role in preventing retinal neurodegeneration in early stages of DR. Neuronal and vascular cells interact with each other to regulate blood flow in the retina via an autonomic independent mechanism. New evidence shows that this interaction is uncoupled in DR .
The pathophysiology of DR is fascinating and complex, with many mechanisms that need further study. DR treatment is an economic burden due to the number of patients affected and the cost of anti-VEGF therapies. Therefore, filling the gaps in the landscape of DR pathophysiology is of the utmost importance for a better understanding of the disease.
- Shukla UV, Tripathy K. Diabetic Retinopathy. [Updated 2021 Feb 14]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK560805/
- Wong, T.Y., et al., Guidelines on Diabetic Eye Care: The International Council of Ophthalmology Recommendations for Screening, Follow-up, Referral, and Treatment Based on Resource Settings. Ophthalmology, 2018. 125(10): p. 1608-1622.
- Solomon, S.D., et al., Diabetic Retinopathy: A Position Statement by the American Diabetes Association. Diabetes Care, 2017. 40(3): p. 412-418.
- Diabetic Retinopathy
- Wang, W. and A.C.Y. Lo, Diabetic Retinopathy: Pathophysiology and Treatments. Int J Mol Sci, 2018. 19(6).
- Romero-Aroca, P., et al., Diabetic Macular Edema Pathophysiology: Vasogenic versus Inflammatory. J Diabetes Res, 2016. 2016: p. 2156273.
- Cheung, N., P. Mitchell, and T.Y. Wong, Diabetic retinopathy. Lancet, 2010. 376(9735): p. 124-36.
- Fu X, Gens JS, Glazier JA, Burns SA, Gast TJ. Progression of Diabetic Capillary Occlusion: A Model. PLoS Comput Biol. 2016;12(6):e1004932. Published 2016 Jun 14. doi:10.1371/journal.pcbi.1004932
- Chung YR, Kim YH, Ha SJ, et al. Role of Inflammation in Classification of Diabetic Macular Edema by Optical Coherence Tomography. J Diabetes Res. 2019;2019:8164250. Published 2019 Dec 20. doi:10.1155/2019/8164250
- Vislisel J, Oetting T. Diabetic Retinopathy: from one medical student to another. EyeRounds.org. Sept. 1, 2010; Available from: EyeRounds.org/ tutorials/diabetic-retinopathy-med-students/
- Falcão, M., Diabetic Retinopathy: Understanding Pathologic Angiogenesis and Exploring its Treatment Options. The Open Circulation & Vascular Journal, 2012. 3: p. 30-42.
- Intensive blood-glucose control with sulphonylureas or insulin compared with conventional treatment and risk of complications in patients with type 2 diabetes (UKPDS 33). UK Prospective Diabetes Study (UKPDS) Group. Lancet. 1998 Sep 12;352(9131):837-53. Erratum in: Lancet 1999 Aug 14;354(9178):602. PMID: 9742976.
- Diabetes Control and Complications Trial Research Group, Nathan DM, Genuth S, Lachin J, Cleary P, Crofford O, Davis M, Rand L, Siebert C. The effect of intensive treatment of diabetes on the development and progression of long-term complications in insulin-dependent diabetes mellitus. N Engl J Med. 1993 Sep 30;329(14):977-86. doi: 10.1056/NEJM199309303291401. PMID: 8366922.
- Yau, J.W., et al., Global prevalence and major risk factors of diabetic retinopathy. Diabetes Care, 2012. 35(3): p. 556-64.
- Whitehead, M., et al., Diabetic retinopathy: a complex pathophysiology requiring novel therapeutic strategies. Expert Opin Biol Ther, 2018. 18(12): p. 1257-1270.
- Szwergold, B.S., F. Kappler, and T.R. Brown, Identification of fructose 3-phosphate in the lens of diabetic rats. Science, 1990. 247(4941): p. 451-4.
- Gabbay, K.H., Hyperglycemia, polyol metabolism, and complications of diabetes mellitus. Annu Rev Med, 1975. 26: p. 521-36.
- Gabbay, K.H., The sorbitol pathway and the complications of diabetes. N Engl J Med, 1973. 288(16): p. 831-6.
- Barnett, P.A., et al., The effect of oxidation on sorbitol pathway kinetics. Diabetes, 1986. 35(4): p. 426-32.
- Peppa, M., J. Uribarri, and H. Vlassara, Glucose, Advanced Glycation End Products, and Diabetes Complications: What Is New and What Works. Clinical Diabetes, 2003. 21(4): p. 186.
- Hammes, H.P., et al., Aminoguanidine treatment inhibits the development of experimental diabetic retinopathy. Proc Natl Acad Sci U S A, 1991. 88(24): p. 11555-8.
- Stitt, A., et al., The AGE inhibitor pyridoxamine inhibits development of retinopathy in experimental diabetes. Diabetes, 2002. 51(9): p. 2826-32.
- Wang, Q.J., PKD at the crossroads of DAG and PKC signaling. Trends Pharmacol Sci, 2006. 27(6): p. 317-23.
- Rosse, C., et al., PKC and the control of localized signal dynamics. Nat Rev Mol Cell Biol, 2010. 11(2): p. 103-12.
- Xia, P., et al., Characterization of the mechanism for the chronic activation of diacylglycerol-protein kinase C pathway in diabetes and hypergalactosemia. Diabetes, 1994. 43(9): p. 1122-9.
- Koya, D. and G.L. King, Protein kinase C activation and the development of diabetic complications. Diabetes, 1998. 47(6): p. 859-66.
- Nerlich, A.G., et al., Expression of glutamine:fructose-6-phosphate amidotransferase in human tissues: evidence for high variability and distinct regulation in diabetes. Diabetes, 1998. 47(2): p. 170-8.
- Wells, L., K. Vosseller, and G.W. Hart, Glycosylation of nucleocytoplasmic proteins: signal transduction and O-GlcNAc. Science, 2001. 291(5512): p. 2376-8.
- Hart, G.W., Dynamic O-linked glycosylation of nuclear and cytoskeletal proteins. Annu Rev Biochem, 1997. 66: p. 315-35.
- Kiss, L. and C. Szabó, The pathogenesis of diabetic complications: the role of DNA injury and poly(ADP-ribose) polymerase activation in peroxynitrite-mediated cytotoxicity. Memórias do Instituto Oswaldo Cruz, 2005. 100: p. 29-37.
- Obrosova, I.G. and U.A. Julius, Role for poly(ADP-ribose) polymerase activation in diabetic nephropathy, neuropathy and retinopathy. Curr Vasc Pharmacol, 2005. 3(3): p. 267-83.
- Volpe, C.M.O., Villar-Delfino, P.H., dos Anjos, P.M.F. et al. Cellular death, reactive oxygen species (ROS) and diabetic complications. Cell Death Dis 9, 119 (2018). https://doi.org/10.1038/s41419-017-0135-z
- Rübsam A, Parikh S, Fort PE. Role of Inflammation in Diabetic Retinopathy. Int J Mol Sci. 2018;19(4):942. Published 2018 Mar 22. doi:10.3390/ijms19040942
- Koleva-Georgieva, D.N., N.P. Sivkova, and D. Terzieva, Serum inflammatory cytokines IL-1beta, IL-6, TNF-alpha and VEGF have influence on the development of diabetic retinopathy. Folia Med (Plovdiv), 2011. 53(2): p. 44-50.
- Boss, J.D., et al., Assessment of Neurotrophins and Inflammatory Mediators in Vitreous of Patients With Diabetic Retinopathy. Invest Ophthalmol Vis Sci, 2017. 58(12): p. 5594-5603.
- Schroder, S., W. Palinski, and G.W. Schmid-Schonbein, Activated monocytes and granulocytes, capillary nonperfusion, and neovascularization in diabetic retinopathy. Am J Pathol, 1991. 139(1): p. 81-100.
- Joussen, A.M., et al., Suppression of Fas-FasL-induced endothelial cell apoptosis prevents diabetic blood-retinal barrier breakdown in a model of streptozotocin-induced diabetes. Faseb j, 2003. 17(1): p. 76-8.
- Abcouwer, S.F., Muller Cell-Microglia Cross Talk Drives Neuroinflammation in Diabetic Retinopathy. Diabetes, 2017. 66(2): p. 261-263.
- Kern, T.S., et al., Topical administration of nepafenac inhibits diabetes-induced retinal microvascular disease and underlying abnormalities of retinal metabolism and physiology. Diabetes, 2007. 56(2): p. 373-9.
- Kuppermann, B.D., et al., Randomized controlled study of an intravitreous dexamethasone drug delivery system in patients with persistent macular edema. Arch Ophthalmol, 2007. 125(3): p. 309-17.
- Gillies, M.C., et al., Intravitreal triamcinolone for refractory diabetic macular edema: two-year results of a double-masked, placebo-controlled, randomized clinical trial. Ophthalmology, 2006. 113(9): p. 1533-8.
- Feldman-Billard, S., E. Larger, and P. Massin, Early worsening of diabetic retinopathy after rapid improvement of blood glucose control in patients with diabetes. Diabetes Metab, 2018. 44(1): p. 4-14.
- Schmidt-Erfurth, U., et al., Artificial intelligence in retina. Prog Retin Eye Res, 2018. 67: p. 1-29.
- Bressler, N.M., R.W. Beck, and F.L. Ferris, 3rd, Panretinal photocoagulation for proliferative diabetic retinopathy. N Engl J Med, 2011. 365(16): p. 1520-6.
- Kowluru, R.A. and P. Koppolu, Diabetes-induced activation of caspase-3 in retina: effect of antioxidant therapy. Free Radic Res, 2002. 36(9): p. 993-9.
- Podesta, F., et al., Bax is increased in the retina of diabetic subjects and is associated with pericyte apoptosis in vivo and in vitro. Am J Pathol, 2000. 156(3): p. 1025-32.
- Abu-El-Asrar, A.M., et al., Expression of apoptosis markers in the retinas of human subjects with diabetes. Invest Ophthalmol Vis Sci, 2004. 45(8): p. 2760-6.
- Sasaki, M., et al., Neurodegenerative influence of oxidative stress in the retina of a murine model of diabetes. Diabetologia, 2010. 53(5): p. 971-9.
- Sohn, E.H., et al., Retinal neurodegeneration may precede microvascular changes characteristic of diabetic retinopathy in diabetes mellitus. Proc Natl Acad Sci U S A, 2016. 113(19): p. E2655-64.
- van Dijk, H.W., et al., Selective loss of inner retinal layer thickness in type 1 diabetic patients with minimal diabetic retinopathy. Invest Ophthalmol Vis Sci, 2009. 50(7): p. 3404-9.
- Barber, A.J. and B. Baccouche, Neurodegeneration in diabetic retinopathy: Potential for novel therapies. Vision Res, 2017. 139: p. 82-92.
- Simo, R., A.W. Stitt, and T.W. Gardner, Neurodegeneration in diabetic retinopathy: does it really matter? Diabetologia, 2018. 61(9): p. 1902-1912.