Diabetes is a condition that can affect vision. This has become a health epidemic due to the increasing number of people with this disease. The result of diabetic changes in the eye do not always cause a noticeable change in vision. Diabetes can lead to anything from a mild blur in one’s vision to severe loss of vision. Remarkably, one cannot rely on their vision as an accurate predictor of one’s disease severity, as blindness can quickly develop in someone with previously normal vision. The fact that symptoms can be so unreliable in diabetic eye disease is the reason it is so important everyone with diabetes be evaluated by an eye doctor at regular intervals. Fortunately, many of the changes that occur in diabetes can be reversed, and as treatment options continue to expand and improve; the extent to which vision loss is permanent or is irreversible from diabetes is decreasing. The instances in which diabetes changes are not reversible can be due to interruption of blood flow to the retina or optic nerve. Since the retina and optic nerve are neural tissue, they do not tolerate lasting interruptions in blood flow.
We perceive our world when our brain processes and interprets images that are detected by the retina and transmitted to our brain via the optic nerve. Damage to any of these three components (retina, optic nerve, brain) leads to a blurred image of the world around us. The optical properties of the eye responsible for focusing light on the retina to form a clear image can also be affected by diabetes. So, even a deficiency of tears that bathe the front of the eye, can also blur vision. Unimpeded light transmission is absolutely essential to excellent vision. To permit light transmission, the eyes must be made of clear, transparent tissues. Light must pass through tears, cornea, aqueous fluid, lens, vitreous, and retina before it is detected. All of these components of the eye are transparent in their normal form. Loss of their transparency leads to vision loss, and is the underlying cause of many different diseases of the eye.
The main culprit of disease progression in diabetes is sustained elevation of blood sugar. Elevated blood sugar leads to modification of proteins and other components inside of blood vessels. These modifications can be damaging to the eye’s circulation. This occurs because when blood sugar levels are high, sugar molecules get tacked on to other components of blood and blood vessel lining. This process creates advanced glycation end-products, where glucose, or sugar, becomes attached to other molecules in the body. This is so important in diabetes that physicians exploit this to monitor one’s diabetes control, by measuring the percentage of a certain protein in the blood that has sugar attached to it (hemoglobin A1c). The higher the percentage, the higher the blood sugar levels have been, and vice versa. One can explain diabetic retinal disease by drawing an analogy to what happens when sugar, fat, and protein react together. The browning of food represents a chemical reaction known as the Maillard reaction, which describes the reaction of amino acids (which comprise proteins) to sugars. The Crème Brule phenomenon that creates a brittle sugar layer on top of dessert, and the browning process that results in a pie crust developing a golden appearance is similar to what happens within small vessels in the retina. When these tiny vessels are bathed in elevated blood sugar levels for many years, they become brittle, close off (capillary drop-out) or break (causing small hemorrhages). The leakage of blood, fluid and cholesterol from these fragile vessels can lead to blurry vision. The photo below depicts a view of some of the changes associated with severe retinopathy. The two photos were taken one week apart, before and after a treatment involving an injection of medicine in the eye that causes a prompt regression of abnormal vessels (arrowheads in the two photos).
One week after one injection of Bevacizumab
The effect of advanced glycation end-products in diabetes can be very detrimental. The importance of transparent tissues to vision cannot be stressed enough. Blood inside the eye abolishes the eye’s transparency, and if severe enough, may require a surgery called a vitrectomy surgery to remove the blood. Earlier treatments target the leakage that occurs from tiny blood vessels. In a retina that is extremely deprived of blood flow, laser surgery may be necessary to decrease the propensity of this retina to cause bleeding. Ultimately, like so many things in medicine, early detection and routine care can save a lot of grief and sight down the road.