Diabetic retinopathy - Causes of diabetic retinopathy

Diabetic retinopathy is caused by high blood sugar levels damaging the network of tiny blood vessels that supply blood to your retina.

The retina is the light-sensitive layer of nerve cells at the back of your eye. It converts light into electrical signals, which are sent to the brain through the optic nerve. The brain interprets these signals into the images you see.

The retina, like all parts of the body, needs a constant supply of blood, which flows to the retina through a network of tiny blood vessels.

Over many years, the blood vessels can be damaged by high blood sugar (glucose) levels that may be present in people with poorly controlled diabetes.

How diabetic retinopathy progresses

During the initial stages of retinopathy, the damage is limited to tiny bulges (microaneurysms) in the blood vessel walls. Although these can leak blood and fluid, they do not usually affect your vision.

This can damage the blood vessels that supply the most sensitive part of the retina, called the macula, which distinguishes colours and focuses your eyes for tasks such as reading and writing.

If fluid leaks into the macula, it can cause swelling (macular oedema), leading to some loss of vision. You may not be able to see objects clearly at a distance, or see things in fine detail, such as small print in a document.

When retinopathy reaches more advanced stages, some of the blood vessels that supply your retina will become blocked. In an attempt to restore the blood supply, new blood vessels will start to form.

However, they are unstable and prone to bleeding, which can cause blurred and patchy vision.

Over time, this bleeding can lead to scar tissue forming, which can pull your retina out of position. This is known as retinal detachment, and can lead to a darkening of vision, floaters and, if left untreated, blindness.

Risk factors

Several factors increase your risk of developing diabetic retinopathy. These are described below.

Length of time you have had diabetes

The longer you have had diabetes, the greater your chance of developing retinopathy. 

About 90% of people with type 1 diabetes will have some degree of retinopathy after 10 years of having diabetic symptoms.

For people with type 2 diabetes who do not need to take insulin, about 67% will have some degree of retinopathy after 10 years of having diabetic symptoms.

For people with type 2 diabetes who need to take insulin, about 79% will have some degree of retinopathy after 10 years of having diabetic symptoms.

Blood glucose level

If you have diabetes and your blood glucose level is high, you have a higher risk of developing retinopathy.

Blood glucose levels are measured using the HbA1C test. HbA1C is a form of haemoglobin and is the oxygen-carrying substance that is found in red blood cells and has glucose attached to it.

Small changes in the levels of HbA1C can significantly affect your risk of developing retinopathy. For example, if you have an HbA1C level of 8% (64 mmol/mol), you are 40% more likely to develop retinopathy than someone with an HbA1C level of 7% (53 mmol/mol). 

Learn more about HbA1C units (PDF, 56kb).

High blood pressure

If you have diabetes and high blood pressure, you have a higher risk of developing advanced retinopathy.

Therefore, taking steps to prevent high blood pressure, such as giving up smoking and cutting down on salt in your diet, can help reduce your risk of developing retinopathy.

Read more about preventing high blood pressure.