Macular Vessels Density Before and After Panretinal Photocoagulation in Patients With Proliferative Diabetic Retinopathy

The global prevalence of diabetes mellitus in 2019 is estimated to be 9.3% (463 million people).1 Diabetic retinopathy (DR) is microangiopathy characterized by capillary non- perfusion, microaneurysms (MAs), and retinal ischemia. 2It may cause many complications, such as diabetic macular edema (DME) and diabetic macular ischemia (DMI).3 Capillary ischemia decreases the nutrition of the retina and causes hypoxia which results in increased level of vascular endothelial growth factor (VEGF), which promotes angiogenic responses causing both neovascularization (proliferative diabetic retinopathy, PDR) and vascular permeability (macular edema).

Panretinal photocoagulation (PRP) by argon or diode laser is the standard treatment for proliferative diabetic retinopathy. It improves oxygenation to the ischemic retina. Destruction of the highly active photoreceptor cells is the suggested mechanism of action for PRP. Subsequently, production of vascular endothelial growth factor (VEGF), the key player in neovascularization process, is reduced leading to regression of new vessels.5

PDR eyes have an overall lower blood flow than normal or non-PDR eyes, parallel to the higher level of retinal ischemia and disease severity. With regression of these neovascular and shunt vessels following PRP, normalization of flow in the macula may reverse the ischemia and decrease the stimulus for new blood vessel formation. Closure of intraretinal microvascular abnormalities and neovascularization would theoretically increase overall resistance to flow and, combined with the constriction of the large vessel in response to increased oxygen in the inner retina, collectively decrease the overall blood flow.6 While most previous studies have explored the large vessel effects of PRP, the development of ocular coherence tomography angiography (OCTA) allowed the study of microvascular retinal changes in a detailed manner. It is a non-invasive modality that allows vascular mapping with high speed and quality and promotes visualization of vascular system in different retinal and choroidal levels. Several investigations have demonstrated the competence of OCTA in the quantification of microvascular density, choroidal flow area, and foveal avascular zone (FAZ) area in diabetic patients.