Cotton wool spots are small areas of yellowish white coloration in the retina. They occur because of swelling of the surface layer of the retina, which consists of nerve fibers. This swelling almost always occurs because the blood supply to that area has been impaired and in the absence of normal blood flow through the retinal vessels the nerve fibers are injured in a particular location resulting in swelling and the appearance of a “cotton wool spot. ” The most common causes of cotton wool spots are diseases, which affect the retina such as diabetes and high blood pressure. Often cotton wool spots will disappear on their own, but new ones may occur because the underlying condition may continue to cause blood flow problems. Most often the cotton wool spots themselves do not cause visual difficulties, but the condition which led to the cotton wool spots can cause retinal damage and the best treatment is to address the disease that caused the cotton wool spots initially.
The earliest clinically recognizable hallmark of diabetic retinopathy is the microaneurysm. These are small round dark red dots on the retinal surface (not arising from visible vessels) that are by definition less than the diameter of the major optic veins as they cross the optic disc.
They increase in number as the degree of retinal involvement progresses. Increasing numbers of microaneurysms are associated with capillary occlusion (visible on fluorescein angiography) leading to retinal ischemia (lack of oxygen) and progression of retinopathy.
As the degree of retinopathy advances retinal hemorrhages become evident. These may have a variable shape sometimes resembling bundles of straw but they may also be round or flame shaped.
They indicate an increasingly ischemia (loss of oxygen) retina such that when they appear in large numbers may be a feature of non-proliferative retinopathy. As their numbers increase the retinal vessels becomes more damaged and leaky leading to exudation of fluid, lipid and proteins.
Exudates represent accumulations of lipid and protein. They are typically bright, reflective, white or cream colored lesions seen on the retina. They indicate increased vessel permeability and an associated risk of retinal edema (swelling). If this occurs on the macula (macular edema) vision may be lost.
Although not sight threatening in themselves they are a marker of fluid accumulation in the retina and if they are seen close to the macula center are considered sight threatening lesions. They are commonly seen in association with microaneurysms which themselves have increased leakage such that the classical lesion is a circular ring of exudates with several microaneurysms at its center. Focal laser treatment to the microaneurysms may prevent worsening of retinal swelling.
New Vessels or Neovascularization
These represent Proliferative retinopathy. These new vessels that grow tend to be poor quality and leak or rupture, thus causing blindness. They are named according to their origin in the retina as new vessels on the disc or new vessels elsewhere.