Loss of vision in diabetes occurs largely through blood vessel damage. With high blood sugars, damage can occur to blood vessels throughout the body in three ways:
Leakage: high blood sugars cause damage to individual cells and later shows up as damage to structures like capillaries, the smallest of blood vessels. Endothelial cells, which form a very smooth surface on the inner walls of blood vessels, and helper cells, called pericytes, are especially damaged by excess sugar. These cells lose the electrical charge normally found at their surface due to inactivation of mineral transporting enzymes and to depletion of energy resources in the high blood sugar environment. As damage progresses, the blood vessel walls starts to become porous, letting proteins and other materials leak out abnormally.
Blood vessel blockage: high blood sugars cause partial and total blockages within existing blood vessels. Blockage of capillaries is found in background retinopathy, but a more serious form of blockage to arterioles occurs in preproliferative and proliferative retinopathy. Arteriolar blockage slows the delivery of oxygen and other nutrients that are required to maintain cell health. The oxygen deficit in turn can trigger release of growth factors.
Abnormal growth: as blood vessels become blocked and oxygen deprivation begins, excess growth factors start to be released to promote the growth of new blood vessel, or neovascularization. Among these various growth factors, one called vascular endothelial growth factor or VEGF, is found in the endothelial cells lining these blood vessels. Retinal blood vessels have three times as many receptors for VEGF as vessels elsewhere, and the oxygen deficit dramatically raises VEGF levels. VEGF is believed to play a major role in triggering neovascularization, and in playing a role in vascular leakage. The laser treatments used to treat proliferative diabetic retinopathy work in part by lowering levels of growth factors like VEGF. Antibodies to various growth factors are also beginning to appear in research designed to stop the growth of cancers, and some of these are likely to work as well in proliferative retinopathy.