Pathophysiology of Retina in Type II Diabetes Mellitus: Conventional Diagnostic Parameters Versus Emerging Roles of Growth factors and Cytokines

Abstract

Until today, type-II diabetes mellitus remains one of the most devastating metabolic disorder affecting millions of people around the world. It is expected that the number of diabetic patients world rise to 70 million by 2030. Diabetic retinopathy is one of the three very significant microvascular complications of progressing diabetes that leads ultimately to blindness. Sustained hyperglycemia causes generation of advanced glycation end products thereby forming reactive oxygen species. The resulting stress causes retina to become hypoxic and anemic. As a result, traumatized retinal tissue induces a number of cytokines and growth factors to promote neovascularization in order to supply oxygen to the failing retina. Tortuous growth of blood vessels however impairs the vision, and at times hemorrhagic retina is the complication that appears due to rupturing of fragile vessels. On fundoscopy, retinal artery microaneurysms, dilated veins, hard exudates, edematous retina exhibit in non-proliferative retinopathy. Further worsening and advancement of retinopathic damage leads to proliferative retinopathy characterized by appearance of cotton wool spots, hard exudates and marked neovascularization. Factors like obesity, hypertension, and elevated random and fasting plasma sugar, raised cholesterol level, hyperlipidmia, and serum creatinine contributing to diabetes are very well known risk factors. Situation in Pakistan is no different from the rest of the world. According to relatively recent estimates the prevalence of diabetic retinopathy is 4-5 million. The current study was designed to determine the specific parameters, viz. serum and vitreous vascular endothelial growth factor (VEGF), interleukin-6 (IL-6), and leptin in diabetic retinopathic patients. Serum levels were also compared with diabetic but non- retinopathic patients and normal healthy subjects. Determination of all major conventional risk factors and complete fundus examination were also carried out to correlate changes in these parameters with the specific parameters. Over 2000 male and female patients of median age of 50 years ranging between 37-65 years were screened in the outpatients departments of four main hospitals, Khyber Teaching Hospital, Hayatabad Medical Complex and Lady Reading Hospital, located in Peshawar city and Al-Shifa Eye Trust Hospital located in Rawalpindi city. Patients with confirmed type-II diabetes mellitus (338) were selected, and patients with complications otherwise were excluded. The duration of the disease and retinopathy was 5-20 years. Normal healthy subjects (39; age range: 35-61; median age 53) were also included in the study to get comparisons with the diseased patients. Standard methods were followed to determine the body mass index (BMI), fasting (FBSS) and post prandial plasma glucose (RBS), glycated hemoglobin (HbA1c), cholesterol, triglycerides (TG), high-density lipoproteins (HDL), low-density lipoproteins (LDL), serum creatinine, urine creatInine and urinary protein. Commercial kits were used to determine the serum parameters and IL-6, leptin and VEGF concentrations. For obvious reasons, vitreous concentrations of IL-6, leptin and VEGF could not be determined in normal subjects and DNR patients. Data were analyzed statistically to determine correlations between predicator variables with those of specific variables, and differences between males and females were done by one way analysis of variance (ANOVA). Combined analysis was also done to get population estimates. The results demonstrated significantly higher (P < 0.001) concentrations of serum IL- 6 (70%), leptin (64%) and VEGF (55%) in DNR, NPDR and PDR patients. Vitreous IL-6, leptin and VEGF concentrations were alarmingly increased (100%, 93% and 100% respectively P < 0.001) in NPDR and PDR patients. For conventional parameters significantly (P < 0.001) elevated BMI, RBS, FBS, HbA1c, TG, LDL, serum and urine creatinine and urinary protein concentrations were found in DR, NPDR and PDR patients. Values of these parameters were remarkably low (P < 0.001) in normal subjects. All parameters were affected linearly with the severity of the disease. Accordingly highest levels were found in PDR patients. Serum cholesterol concentrations were well in the range. HDL concentrations were significantly reduced (P < 0.001) in DNR patients, NPDR and PDR patients; but group comparisons showed slightly greater levels of HDL in NPDR and PDR patients than the DNR patients. TC/HDL ratio and LDL/HDL ratio were also increased in NPDR and PDR patients. Separate male and female comparisons did not show any significant differences with combined male and female analysis demonstrating that the disease prevalence is irrespective of gender; however a small female predisposition is evident from the data. Most importantly, since all of the above patients were being treated with oral hypoglycemic and several PDR patients had already underwent laser photocoagulation, elevated concentrations of specific and conventional parameters raise questions about the therapy. Of 338 diabetic patients following were the frequencies of non-retinopathy and retinopathy: DNR (11 %), NPDR (31.95 %) and PDR (56.80 %). Gender-wise, 38 % (129) were males consisting of 12% DNR patients, 36% NPDR patients, and 52% PDR patients. Of 209 female patients, 11% were DNR patients, 29 % were NPDR patients, and 60% were PDR patients. The study points out that IL-6, leptin and VEGF can be significant diagnostic factors in clinical settings to predict the probability of retinopathy. They also demonstrated correlations, positive or negative, with some conventional parameters. Alarmingly elevated levels of these factors indicate them to be independent risk factors. Although most conventional parameters can be controlled via intensive treatment but the chain of events that hyperglycemia induces for the first time initiates the vicious cycle of biochemical changes that cannot be controlled with routine therapies and ultimately lead to failure of retina culminating in certain cases into complete blindness. Associated outcomes were obesity, dyslipidemia and microalbiminuria. The study suggests that ophthalmologists and diabetologists working in the clinical set ups should emphasize on the determination of serum IL-6, VEGF and leptin levels in patients presenting with diabetes and retinal problems to reach an early diagnosis about the severity of the disease and future affliction with retinopathy. This may help for an earlier decision to proceed for invasive therapies like the application of antibodies injections against VEGF and IL-6. Currently, for unknown reasons the role of leptin could not be ascertained. Further detailed studies from around different geographic regions of Pakistan and analyses of even newer retinopathy promoting and inhibiting factors are definitely required to get a more comprehensive data from this region of the world.