With time, diabetes affects the blood vessels of the retina. It leads to impaired circulation and thickening of the vessel walls, causing them to leak. In the earliest phase of the disease, small dot-like haemorrhages may be visible on the retina, but vision is not threatened and the patient may not have any symptoms. This is known as “background diabetic retinopathy”.
If leakage from blood vessels progresses, the retina becomes impaired further and visual symptoms develop. This in turn can lead to the central macular area of the retina becoming swollen. This appearance is described medically as macular oedema and this phase is known as “diabetic maculopathy”. If left untreated, it may permanently damage central vision. Maculopathy is common in Type 2 diabetes, which is the form of the disease usually occurring later in life.
The later stage of the condition is known as “proliferative diabetic retinopathy”, in which there is a serious danger of haemorrhage into the vitreous gel in the eye leading to rapid and extensive vision loss. The cause of proliferation is inadequate oxygenation of the tissues of the retina, sometimes referred to as “ischaemia”. This loss of function of the existing retinal vessels leads to the release of a chemical from the surrounding tissue called Vascular Endothelial Growth Factor (VEGF). This stimulates the formation of new vessels, or neovascularisation, in an attempt to supply the deprived tissues. These new vessels are extremely weak and leak blood into the retina causing visual problems.
This type of retinopathy can occur in Type 1 and Type 2 diabetes and, without adequate screening, may go unnoticed until severe haemorrhage occurs.
The most serious form of diabetic retinopathy occurs when proliferation and bleeding result in scarring between the retina and the gel within the eye. Contracting scar tissue can pull the retina away from the eye wall, resulting in permanent blindness.