DR is one of the most significant and severe ocular complications in diabetes and is the primary cause of visual disturbance and vision loss among working-age people [11]. In addition to its negative impact on vision, DR can negatively affect the mental health of patients, resulting in anxiety, depression, and reduced overall well-being [19]. DR can contribute to emotional distress and reduce patient independence, impairing the quality of life in these patients [20]. Moreover, potential job and income loss due to DR-related vision impairment, on the one hand, and the huge costs of DR treatment, on the other hand, can pose a substantial economic burden on patients, their families, and society [21]. Careful understanding of contributing factors to DR is integral to its prevention and management. Thus, the present study aimed to identify DR prevalence and risk factors in a group of type 2 diabetic patients presenting to Beijing Huairou Hospital in China.

The present study found a prevalence rate of 16.8% for DR in patients with type 2 diabetes. The national prevalence of DR in China is yet to be reported. However, our finding corroborates those of previous regional studies in China by Pan et al. [13] and Cui et al. [16], reporting a prevalence rate of 18 and 18.2 for DR in type 2 diabetic patients. DR prevalence in patients with type 2 diabetes in other Chinese studies ranges from much lower (8.1% in Cui et al.‘s study [22]) to much higher (43.1% in Xie et al.’s study [23]) than our finding, indicating a large discrepancy in the incidence of this event among various regions of China. This inconsistency in DR prevalence is also evident in the studies conducted in other countries. For instance, the results of a recent systematic review and meta-analysis reported a 22.27% global prevalence of DR. The highest rate was 35.9% for Africa, and the lowest rate was 13.37% for South and Central America [9]. The observed discrepancy among studies is partly due to demographic, socioeconomic, and environmental factors, as well as methodological factors, including differences in the study population, diagnostic criteria, sample size, sampling method, and variations in data collection methods. However, the regional prevalence rate of DR may reflect the diabetic burden and its inadequate management, inequality in accessing health care, lack of awareness of diabetes and its complications, and lack or deficiency in DR prevention and management plans in that region.

NPDR is the most common form of diabetic retinopathy [24]. In this regard, most DR cases in the present study were of NPDR type, and PDR constituted a much lower proportion of the cases (NPDR and PDR were 94.1% and 5.9% of all cases, respectively).

NPDR is an earlier stage of the disease characterized by alterations in retinal blood vessels, including microaneurysm, hemorrhage, and exudate. It can occur in many diabetic patients before progressing to a more severe proliferative stage. On the other hand, PDR is a more advanced stage of disease characterized by abnormal growth of blood vessels in the retina and occurs in a lower percentage of diabetic patients [25].

Diabetic maculopathy develops as DR affects the macula, the central region of the retina responsible for sharp central vision. The leakage of macular blood vessels can cause swelling, known as macular edema, which distorts vision and can lead to permanent vision loss. International Clinical Disease Severity Scale criteria were utilized in this study to classify DR into NPDR and PDR types [18, 26]. In this classification system, mild NPDR is characterized by the presence of a few microaneurysms, while moderate NDPR is defined as the presence of microaneurysms, intraretinal hemorrhages, or venous beading. Severe NDPR is determined based on the 4:2:1 rule by any of the following findings: hemorrhages in all four quadrants of the retina, venous beading in two or more quadrants, or intraretinal microvascular abnormalities in one or more quadrants. PDR is defined by the presence of neovascularization at the disc, the retina, the iris, the angle, vitreous hemorrhage, or tractional retinal detachment [18]. DR screening results are categorized into non-referable and referable groups. Diabetic patients with no or mild DR fall into the non-referable category, meaning they do not need immediate referral to an ophthalmologist. These patients will likely be able to continue with regular follow-up appointments with their optometrist or ophthalmologist. Referable DR refers to a situation in which the ocular condition has progressed to an extent (moderate, severe, or proliferative DR) that warrants referral to an ophthalmologist for further evaluation and management.

Concerning the risk factors for DR, the results of the present study suggested that diabetes duration, FPG, HbA1c, hypertension, hyperlipidemia, diabetic nephropathy, UA, ACR, and insulin therapy were independently associated with DR.

The results of this study verified the generally recognized independent risk factors of DR such as diabetic duration, FPG, and HbAlc [27]. It has been suggested that hyperglycemia leads to systemic microvascular lesions, increased vascular permeability, and leakage of intravascular liquid components into tissues. These alterations cause retinal edema, ischemia, and neovascularization, contributing to retinopathy and dysfunction [25].

The present study found no significant difference in BMI between the DR and non-DR groups, which contradicts the results of previous studies. Some studies have reported that DR is more likely to occur in people with lower BMI [28], while others suggested that high BMI is related to DR [12]. These inconsistencies are likely due to differences in research design, the size, and the nature of the study population. It seems that further research is required to establish the association between BMI and DR.

In consistent with previous studies [12, 29], the present study indicated that diabetic patients with hypertension were more prone to DR than those with normal blood pressure. Consistently, a study by Liu et al. [30] found a correlation between DR and elevated SBP in diabetic patients. Hypertension can cause further damage to retinal blood vessels, which are at risk due to diabetes, increasing the risk of diabetic retinopathy. Hypertension can also cause fluid leakage and swelling in the retina, exacerbating the diabetic-induced damage [31].

Our findings also demonstrated that hyperlipidemia was independently related to DR. Inconsistencies exist in the literature regarding the relationship between high serum lipid levels and DR.

For instance, Klein et al. [32] found a relationship between retinopathy and higher serum levels of total and LDL cholesterol in diabetic patients. In the study by Rema et al. [33], diabetic patients with DR had significantly higher levels of triglyceride compared to healthy people. In contrast, the results of Wong et al. [34] showed no significant association between DR and triglyceride, LDL cholesterol, and HDL cholesterol in diabetic patients. The underlying mechanism of the relationship between hyperlipidemia and DR in diabetic patients is yet to be discovered. It is thought that hyperlipidemia, in a hyperglycemic milieu, may play a role in blood vessel damage in the retina by inducing inflammation, oxidative stress, and mitochondrial damage [35, 36]. These results suggest that along with glycemic control, rigorous monitoring and control of blood pressure and lipid profile in diabetic patients can be beneficial in preventing or delaying DR.

Our results demonstrated insulin therapy as an independent risk factor for DR in type 2 diabetic patients. In contrast to our finding, Cepeda-Nieto et al. [37] reported an inverse relationship between insulin therapy and DR in patients with type 2 diabetes and cited insulin therapy as a protective factor for DR in diabetic patients. However, our result was consistent with a growing body of literature. A study by Jingi et al. [38] found that compared to oral hypoglycemic drugs, insulin use had a direct significant relationship with DR. Studies by Raman et al. [14], Javadi et al. [39], and Thomas et al. [40] identified insulin therapy as an independent factor that significantly increases the risk of DR development. It is thought that exogenous insulin can act synergistically with vascular endothelial growth factor (VEGF) expressed by the ischemic retina, which triggers vascular proliferation and aggravates DR [41].

Moreover, the present study found that higher serum levels of UA and ACR, as well as coexisting diabetic nephropathy, were significantly and independently associated with DR development in diabetic patients. Concomitance and association of diabetic retinopathy and nephropathy in diabetic patients is well-established. On the other hand, elevated serum levels of UA [42] and ACR [43] are known indicators of early diabetic nephropathy in type 2 diabetic patients. Hence, paying close attention to the changes in these indicators may be helpful in early diagnosis of DR.