Diabetic macular oedema (DMO) occurs in up to 2% of patients with diabetic retinopathy. It is caused by disruption of the blood-retinal barrier at the posterior pole. Progression to DMO is associated with duration of disease, poor glycaemic control and the need for insulin in type 2 diabetes. Sustained hyperglycaemia causes a multifactorial cascade with increased vascular permeability, altered ocular blood flow, hypoxia and inflammation. Vascular endothelial growth factor-A (VEGF-A), which is stimulated by microvascular ischaemia, contributes to the inflammatory process and, in particular, increased permeability and subsequent oedema.