What is Diabetic Retinopathy?
Diabetic retinopathy is a complication of diabetes and a leading cause of blindness. "Retinopathy" is the medical term for damage to the tiny blood vessels (capillaries) that nourish the retina, the tissue at the back of the eye that captures light and relays information to your brain. These blood vessels are often affected by the high blood sugar levels associated with diabetes. Nearly half of people with known diabetes have some degree of diabetic retinopathy. Initially, most people with diabetic retinopathy experience only mild vision problems. But the condition can worsen and threaten your vision.
Topographic and cellular organization of the retina
The retina is a transparent film of neural tissue between the retinal pigmented epithelium and the vitreous body. Normal vision depends on intact cell-cell communication between neuronal, glial, microglial, vascular, and pigmented epithelial cells of the retina. The fundamental function of the retina is to capture photons, convert the photochemical energy into electrical energy, integrate the resulting action potentials, and transmit them to the occipital lobe of the brain where they are deciphered and interpreted into recognizable images. The retina is partitioned from the systemic circulation by the blood-retinal and blood-aqueous barriers, and receives its nutritional supply from the retinal and choroidal circulations, and perhaps from the ciliary body by diffusion through the vitreous gel.
The retina must operate under a wide range of light intensities, from bright light conditions to near darkness, so photoreceptor sensitivity varies over nine orders of magnitude to adapt rapidly to a wide range of environments. The retina is organized into the central macular region which operates in moderate to bright light to subserve detailed resolution or acuity and color perception, and the extra-macular retina which operates in dim light conditions to detect motion and facilitate peripheral vision.
Retinal physiology and its vulnerability to diabetes
The unique retinal structure imparts special physiologic requirements compared to other parts of the nervous system because of the essential requirement for transparency, and these requirements may contribute to its susceptibility to diabetes. For example, retinal axons are not ensheathed by myelin, because myelin is opaque and blocks light transmission. Unmyelinated nerves require more energy to maintain membrane potentials than myelinated axons. Second, the density of blood vessels that would absorb light is relatively low, so the oxygen tension of the inner retina is relatively hypoxic. Third, the inner retina possesses relatively few mitochondria that contain light-absorbing heme-based cytochrome proteins of the electron transport chain. The inner retina, which is less enriched in mitochondria, heavily relies on glycolysis, a less efficient means of generating ATP than oxidative phosphorylation which predominates in the outer retina. In spite of this sparse vascularity and low pO2, the retina has one of the highest metabolic demands of any tissue.
Signs and symptoms
In the early, most treatable stages of diabetic retinopathy, you usually experience no visual symptoms or pain. The disease can even progress to an advanced stage without any noticeable change in your vision. Symptoms of diabetic retinopathy may include:
- "Spiders," "cobwebs" or tiny specks floating in your vision
- Dark streaks or a red film that blocks vision
- Vision loss or blurred vision
- A dark or empty spot in the center of your vision
- Poor night vision
- Difficulty adjusting from bright light to dim light
Diabetes affects the circulatory system of the retina. The earliest phase of the disease is known as background diabetic retinopathy. This phase is also called a nonproliferative diabetic retinopathy (NPDR) type. In this phase, the arteries in the retina become weakened and leak, forming small, dot-like hemorrhages. These leaking vessels often lead to swelling or edema in the retina and decreased vision. As background diabetic retinopathy progresses, other signs of damage appear. These include swelling or beading of some of the larger retinal veins and patches of swollen nerve fibers, which are called cotton-wool spots because they look like fluffy wisps of cotton.
The next stage is known as proliferative diabetic retinopathy (PDR). In this stage, circulation problems cause areas of the retina to become oxygen-deprived or ischemic. New, fragile, vessels develop as the circulatory system attempts to maintain adequate oxygen levels within the retina. This is called neovascularization. Unfortunately, these delicate vessels hemorrhage easily. Blood may leak into the retina and vitreous, causing spots or floaters, along with decreased vision. This abnormal growth generally follows the widespread closing of capillaries in the retina. The condition can cause vision loss affecting both your central and peripheral vision. The new blood vessels may leak blood into the vitreous, which clouds or even blocks your vision. Other complications include detachment of the retina due to scar tissue formation (traction retinal detachment) and a form of glaucoma associated with the growth of abnormal blood vessels on the iris, the colored portion of the eye surrounding the pupil (neovascular glaucoma).
Blurred vision can be brought on by rapid fluctuations in blood sugar. Prolonged periods of elevated blood sugar cause sugar and its breakdown products to accumulate in the lens. This accumulation sucks up water and makes the lens swell, resulting in nearsightedness — meaning distant objects appear blurry. The nearsightedness subsides once your blood sugar is brought under steady control. Blurred vision can also be caused by macular swelling (edema), regardless of your blood sugar level. This is cause for greater concern because macular edema often develops in people with diabetic retinopathy. The swelling may fluctuate during the day, making your vision get better or worse. If blood vessels in your eye are hemorrhaging, you might notice spots floating in your field of vision. These small spots are often followed within a few days or weeks by larger spots or clouds, which are caused by more marked hemorrhaging.