Over time, high blood glucose levels caused by diabetes can lead to damage of the small, specialised blood vessels in the retina at the back of the eye. The vessels become weaker and may leak clear fluid and/or become blocked. This is called non-proliferative diabetic retinopathy and normally does not affect vision.
Leakage of fluid from damaged retinal blood vessels can result in swelling of the retina (edema) and disrupt its normal function. If this swelling is in the central macular area, (causing diabetic macular edema or DME), it can cause progressive loss of detailed, central vision and even legal blindness, although it won’t cause ‘black’ or total blindness.
Diabetic macular edema is the most common cause of vision loss in people with diabetic retinopathy. It frequently affects both eyes at the same time.
Blockage of small retinal blood vessels can disrupt macular function but more importantly, can cause potentially blinding proliferative diabetic retinopathy (PDR). If sufficient blockage occurs, there is a reduction in the supply of oxygen (ischaemia) to the retina. Nature tries to fix this by growing new blood vessels from the retina. Unfortunately, these new vessels are abnormal and very fragile. They grow forward into the central gel cavity of the eye where they tend to break and bleed into the gel cavity, obstructing vision.
The process is progressive and without any symptoms until the vessels bleed. These vessels then scar and may pull on the retina, possibly causing the retina to detach. If proliferative diabetic retinopathy is not treated early, it can result in total (‘black’) blindness.
With time, the majority of people with diabetes will develop retinopathy, however the severity of disease is greatly influenced by how well diabetes is controlled.