**3.4 Immune cell activation**

Histological evaluation of blood vessels, early in the course of DR, have shown increased interaction between leukocytes and endothelial cells. This phenomenon called leukostasis is characterised by an adherence of monocytes and neutrophils to the endothelial lining of the blood vessels. This leads to blockage of the thin retinal capillaries and areas of retinal non-perfusion [17].

### **3.5 Retinal pigment epithelium and choroid**

The changes are not only localised to the inner retinal layers but also extend to the RPE and the choroid compromising the outer blood retinal barrier. RPE

dysfunction and leakage from the choriocapillaris is noted leading to outer retinal edema and impaired clearance of fluid [18].

Choroidal atrophy has been noted in cases of long standing DR, with a diminished choroidal thickness noted on OCT evaluation. This thinning of the choroid have been linked to HbA1c levels and may lead to choroidal neovascular membranes or intra-choroidal microvascular abnormalities [19].
