Diabetic retinopathy (DR) is the leading cause of legal blindness in patients aged 20 – 65 years in Western countries. This is a disease of the retina – the film at the back of the eye responsible for detecting light.
From the time of diagnosis of Type 2 (late onset, or non-insulin dependant diabetes) it is not common to develop retinopathy within the first 5 years. However, as Type 2 diabetes can be present many years before diagnosis, about 5% of people will already have diabetic eye disease at the time of diagnosis.
Diabetes causes retinopathy by damaging the small blood vessels, narrowing them down and making blood flow sluggish. As a result the retina does not get enough oxygen, known as retinal hypoxia and this causes ischaemia (cell death). When the retina is starved of oxygen it releases chemicals to promote the growth of new blood vessels in an attempt to get more oxygen. Some of these chemicals are called vascular endothelial growth factors (VEGF) – remember this for later.
Even smaller blood vessels in the eye, called capillaries become weakened by diabetes. There is a reduction in the number cells, pericytes, supporting these small vessels. Without these pericytes acting as a "corset", the capillaries bulge, forming micro-aneurysms and they become leaky. The leakage can be just a fluid that causing swelling (oedema), or blood which obviously means bleeding in the eye.
It has been shown in many studies that poor control of blood sugars leads to a higher risk of retinopathy. Is that piece of cake or chocolate worth the risk to your vision? There have been many studies looking at other risk factors for DR, many with varying conclusions. Factors researched include obesity, hypertension, smoking and alcohol. So, after reading many studies I still do not feel I can say for certain that any one factor will independently increase your risk of severe DR and visual loss. What I can say is that being overweight, having high blood pressure or indulging in certain vices will impact negatively on your overall health by increasing the risk of heart disease and stroke. It won't matter what your eyes are like if your heart stops!
Diabetic retinopathy has different classifications depending on the severity.
1. Background disease: this is characterized by micro-aneurysms, small areas of bleeding and oedema. The fluid may result in the deposition of fats within the retina, these are called hard exudates.
With these changes your eyes need yearly review.
2. Pre-proliferative disease: these changes are seen when there is not enough oxygen to the retina, resulting in ischaemia. This is seen as "cotton-wool spots" in the eye, areas of the retina that look white and fluffy, hence the name. The veins begin to alter appearance – looking like a string of beads or very loopy. New blood vessels open up between arteries and veins, in an effort to bypass the parts of the eye with no oxygen.
When these changes are seen your doctor may start to see you every few months rather than yearly.
3. Proliferative diabetic retinopathy: the hallmark of this stage is the growth of new blood vessels, and is thought to occur when the diabetes has caused over a quarter of the retina to be lacking in oxygen. These abnormal vessels can cause bleeding and scarring within the eye. This severe type of DR affects one in ten type 2 diabetics – a lot of people when you consider that one million people have diabetes in Australia. If you developed Type 1 diabetes (early onset or insulin dependant diabetes) the chances of having this form of DR are 60% within 30 years.
Diabetic maculopathy describes the condition where bleeding and fluid leak occur in the central part of the retina, called the macula. The macula is responsible for fine detail vision. Obviously even a small amount of distortion in this part of your vision can be very noticeable and disabling.
There are several tests that can be done to determine the extent of the damage within the eye. The most common is called a flurescein angiogram. This involves injection of dye into a vein on your arm. The dye travels within the blood stream throughout the body. This includes the blood vessels in the back of the eye. As described earlier, the main problem lies in the blood vessels of the eye – small aneurysms or leaking. Thus by taking photos which show the flow of dye, we can detect if there are any blood vessel problems and where they are exactly. Another test called the OCT is like a high powered ultrasound scan of the eye, that allow us to see if there is fluid leaking from vessels in the deep layers of the eye. Both of these tests allow doctors to determine if you need treatment on the eyes.
Laser therapy is frequently the treatment of choice. This is done in the clinic, while you are awake, sitting at a slit lamp machine. (See equipment). Most people do not feel any significant pain with this treatment, and for those who do we can give numbing injections near the eye. The laser therapy can be used in one of two ways. Firstly, leaking blood vessels can be isolated by laser burns to stop the leaks. This is called focal or grid laser, and is used in maculopathy. Secondly, if the peripheral retina is not receiving enough oxygen, that area produces chemicals that stimulate damaging new blood vessels to grow. These irreversibly damaged areas of the retina can be burnt with laser to stop the release of these chemicals. This is called pan retinal photocoagulation (PRP) and is used in patients with proliferative DR. It may take several visits to complete a full course of this treatment.
There are several different injections that are now being used in DR. These involve an injection into the white part of your eye, so the drug is delivered inside the eye near to where the damage is being done. One is called triamcinolone, a steroid to reduce swelling. The others are VEGF inhibitors, there are several brands, but the most commonly used in DR are Avastin and Lucentis. By blocking the VEGF chemicals that signal new vessel growth and promote leaking from the vessels, the aim is to stop or slow down the damage caused by proliferative DR. These injections are still being researched and I plan to make a whole new blog entry discussing some of the latest research on these medications. This is more of an overview article, so keep your eyes on these pages for more details later.