Metabolic damage in DM can lead to chronic low-grade inflammation, loss of retinal endothelial cells and insufficient vascular repair. Many studies have shown that inflammatory factors in the vitreous and aqueous humour are related to the progression of DR. The inflammatory process induces a series of complex molecular and cellular signalling pathways that change the physiological response of the affected eye tissues, thereby producing an "inflammatory" phenotype. Once released, these cytokines can spread inflammatory reactions in the retina, resulting in increased leukocyte infiltration and direct or indirect damage to retinal nerve cells6. In the PDR phase, ischaemic and hypoxic retinal cells release VEGF, driving the formation of neovascularisation, which reaches the anterior retinal space or vitreous cavity, producing various inflammatory factors and further triggering the inflammatory cascade. In recent years, it has been found that anti-VEGF treatment has a positive effect on the efficacy of DR. The level of VEGF in vitreous fluid and fibrous vascular tissue is increased in PDR patients. Therefore, after preoperative anti-VEGF treatment, the regression of vascular components of fibrous tissue is conducive to the segmentation and layering of the membrane, thereby reducing the adhesion to the retina and separating it from the retina. In addition, the haemodynamic changes of retinal circulation, such as the contraction and flow reduction of new blood vessels, minimize the possibility of intraoperative haemorrhage, shorten the time of blood reabsorption after PPV, reduce the incidence of vitreous haemorrhage recurrence, and further improve BCVA. As a new member of the anti-VEGF drug family, conbercept has been shown to be an effective surgical adjuvant and is currently mainly used in China.
The clinical operation indexes in this study showed that compared with the control group, the operation time of the anti-VEGF group was shortened, the number of electrocoagulation bleeding events was reduced, and the oedema in the macular area was reduced. The difference was statistically significant (P < 0.05). We speculate that anti-VEGF drugs can reduce the pulling of neovascularisation membrane on the retina, thus making it easier for the surgeon to peel off the neovascularisation membrane. Additionally, the operation area and the operation plane will not be blocked by fresh blood clots, the retinal blood vessels are less likely to be damaged during the stratification process, and the reduction of intraoperative bleeding provides the surgeon with a better surgical field of vision. Thus, the operation time is shortened, and the use of surgical instruments is reduced. Mao et al2 also confirmed that IVC before PPV can reduce the chance of intraoperative bleeding, reduce the use of electrocoagulation to control bleeding, and shorten the operation time, which is conducive to treating PDR. Our results are also similar to those of Wang Ping et al. Therefore, reducing retinal neovascularisation in PDR patients and controlling its proliferation process is the key to successful treatment. In this study, there was no significant difference between the two groups in the formation of iatrogenic retinal tears, the need for gas/silicone oil tamponade, postoperative visual acuity improvement, postoperative haemorrhage, or postoperative high intraocular pressure (P > 0.05). This finding may be due to the insufficient number of cases. Previous studies have confirmed that IVC can reduce the occurrence of recurrent vitreous haemorrhage, make neovascularisation subside and reduce vascular leakage, thus reducing postoperative haemorrhage and macular oedema, preventing retinal function reduction, and improving the patient's visual acuity. Previous studies have suggested that IVC can cause a temporary elevation of intraocular pressure, while other studies have suggested that there is no significant correlation between the number of anti-VEGF drug injections and the elevation of intraocular pressure. In this study, although there was high IOP in the two groups within 7 days after surgery, the IOP was reduced to the normal level through the use of antihypertensive drugs, and no neovascular glaucoma occurred.
VEGF enhances the expression of plasma plasminogen activator by activating some key genes of endothelial cells and then deforms the extracellular matrix, increases the permeability of the vascular wall, and leads to neovascularisation. VEGF can also bind to VEGFR-1 as a chemical inducer of proinflammatory cells, leading to the chemotactic recruitment of monocytes and macrophages. Activated macrophages express VEGFR-1, secrete PDGF and participate in inflammation, forming a positive feedback loop, further leading to increased vascular permeability and neovascularisation. In this study, the lower the VEGF level was, the lighter the intraoperative haemorrhage was; intraoperative retinal electrocoagulation use was also reduced. These results suggest that anti-VEGF drugs can inhibit the generation, leakage and rupture of neovascularisation by reducing the VEGF level in the eye and reducing the traction of oedema and abnormal fibre proliferation on the retinal membrane, thereby reducing intraoperative haemorrhage and improving the patient's vision. However, anti-VEGF drugs also have problems such as short maintenance times and lack of patient response. After several anti-VEGF drug treatments, some patients still progress from NPDR to PDR. The study found that compared with the normal control group, the potential of aqueous humour angiogenesis in PDR patients was increased, but it was not affected by VEGF level and anti-VEGF drug treatment. In addition to affecting VEGF, other cytokines may also play a role in the occurrence and development of PDR or DME, and its pathogenesis is multifactorial. This conclusion is consistent with our findings. We found that the higher the level of IL-8 was, the longer the vitrectomy times and the more severe the intraoperative bleeding were. The higher the level of FGF was, the more intraoperative retinal electrocoagulation was required, and the lower the incidence of postoperative macular oedema was. Analyzing this statistical result, we believe that anti-VEGF drugs may affect the surgical prognosis by affecting the levels of IL-8 and FGF in the aqueous humour.
IL-8 belongs to the C-X-C α A subfamily of proinflammatory chemokines with chemotactic activity on neutrophils, monocytes and lymphocytes. Hyperglycaemia in patients at the early stage of DR leads to retinal microvascular hypoxia. NF-κB is activated under hypoxia, increasing the expression of IL-8, and the increase in IL-8 concentration is thought to contribute to the development of neovascularisation. In the DR proliferation phase, IL-8 can open the connection between vascular endothelial cells and pigment epithelial cells; chemotactic immune cells degrade the Bruch’s membrane and promote the expansion of the proliferative membrane in the eye and the further formation of the neovascular membrane, aggravating the disease. IL-8 can also be produced by various active inflammatory cells (such as monocytes/macrophages and neutrophils) and other cell types (such as endothelial cells and glial cells). It has been found that the levels of IL-6 and IL-8 in the vitreous fluid of DR patients can induce an increase in VEGF secretion, which is conducive to neovascularisation. According to our research findings, the higher the level of IL-8 is, the longer the vitrectomy times are, and the more severe the intraoperative bleeding will be. It is speculated that anti-VEGF drugs can reduce the level of VEGF and thus indirectly affect the level of IL-8, affecting the surgical prognosis. After the injection of anti-VEGF drugs, in this study, the level of IL-8 in the aqueous humour increased. It is speculated that this effect may be due to the compensatory increase in IL-8 after VEGF is suppressed. This effect may also be caused by a series of inflammatory reactions caused by anterior chamber puncture16, consistent with the study of Fortooghian et al. It has been reported that under hypoxic conditions, IL-8 can increase the expression of VEGF in retinal pigment epithelial cells, and VEGF synthesized by glial cells jointly activates neovascularisation. IL-8 can stimulate fibroblasts and monocytes to produce VEGF and can also promote the formation of vascular neovascularisation independent of VEGF. Anti-VEGF drugs reverse the neovascularisation of the retina and iris by reducing VEGF levels, induce the apoptosis of vascular endothelial cells and occlusion or disappearance of new blood vessels, increase the number of pericytes, promote angiogenesis and vascular maturation to reduce fundus exudation and bleeding, and maintain the stability of blood-retinal barrier.
FGF is a polypeptide that can promote the growth of fibroblasts and has several isomers. Once bFGF binds to its receptor, it can promote the division and proliferation of vascular endothelial cells and fibroblasts, subsequently leading to the formation of new blood vessels. FGF can also upregulate the production of VEGF and synergistically stimulate the expression of PDGF and TGF, thus promoting neovascularisation. We found that the lower the level of FGF was, the lower the requirement was for intraoperative retinal electrocoagulation. This finding may be because FGF is related to neovascular membrane fibrosis. After IVC, anti-VEGF drugs can significantly reduce the content of VEGF in the aqueous humour without increasing the expression of bFGF, thus reducing the number of intraoperative haemorrhages and electrocoagulation. We also found that the higher the level of FGF in the aqueous humour is, the lighter the macular oedema is one week after the operation. This finding may be due to the sudden decrease in VEGF levels one week after the operation in addition to the influence of a variety of inflammatory cytokines, which caused a compensatory increase in bFGF levels. However, because the patient had a vitreous haemorrhage before the operation, we only evaluated macular oedema after the operation. This conclusion indicates that the IVC may reduce macular oedema after surgery.
Many inflammatory cytokines involved in DR are closely related to pathophysiology, and it is expected that at least some of these biomarkers will be used in routine clinical practice in the future based on extensive basic research. At present, anti-inflammatory drugs and inflammatory molecular inhibitors can be used alone or in combination with VEGF inhibitors to treat DR. Although anti-VEGF treatment is still the preferred treatment method for DR, this approach cannot effectively control the inflammatory components that cause retinal tissue damage. Here, we found that anti-VEGF drugs may improve surgical prognosis by affecting some inflammatory factors, providing a new perspective for the future treatment of DR patients. After IVC in DR patients, the levels of VEGF, IL-8 and FGF in the aqueous humour can be reduced, the vitrectomy time can be shortened, and the intraoperative haemorrhage, retinal electrocoagulation times and postoperative macular oedema can be reduced. Therefore, we believe that conbercept may shorten the vitrectomy time, reduce intraoperative haemorrhage, and reduce the use of endodiathermy by affecting the levels of VEGF, IL-8 and FGF in the aqueous humour of DR patients, thus improving the surgical efficacy and prognosis.
The limitations of this study should also be noted: the cytokines we tested were in aqueous humour samples, not vitreous samples. Generally, the concentration of cytokines in the vitreous is high, which can better reflect the situation of diabetic retinopathy. Second, to exclude the influence of neovascularisation on the surgical effect, we only collected data from patients with diabetic retinopathy with vitreous haemorrhage. The number of clinical cases is small. Many cytokines are involved in diabetic retinopathy. Although the research results are statistically significant, a larger number of samples are still needed for further analysis. Finally, due to the lack of follow-up after the operation, the long-term prognosis of the use of anti-VEGF drugs could not be observed; thus, the follow-up time could be extended to further explore the long-term prognosis of patients treated with anti-VEGF drugs.