Diabetic retinopathy is caused by prolonged high blood sugar damaging the tiny blood vessels in the retina, the light-sensitive tissue at the back of your eye. The damage unfolds gradually, often over years, through a chain of events that starts at the cellular level and can eventually lead to vision loss. Globally, blindness from this condition increased by 326% between 1990 and 2021, making it one of the fastest-growing causes of preventable vision loss worldwide.
How High Blood Sugar Damages Retinal Blood Vessels
The retina is fed by a dense network of capillaries, the smallest blood vessels in your body. These capillaries have a unique feature: they don’t leak. Their walls are sealed with tight junctions between cells, forming what’s called the blood-retinal barrier. Chronic high blood sugar breaks down this barrier through several overlapping processes.
First, excess glucose thickens the walls of retinal capillaries. The basement membrane, a thin structural layer that wraps around each capillary, accumulates extra proteins like collagen, laminin, and fibronectin. In animal studies, these changes appear as early as eight weeks after diabetes onset. The thickened walls become stiffer and less able to deliver oxygen and nutrients efficiently.
Second, high glucose kills pericytes, the supportive cells that wrap around capillaries and help regulate blood flow. Pericytes are especially abundant in the retina, and their loss is one of the earliest and most distinctive signs of diabetic retinopathy. Without them, capillary walls weaken, bulge outward into tiny balloon-like pouches called microaneurysms, and eventually break down altogether.
There’s also a chemical pathway that compounds the damage. When too much glucose floods a cell, an enzyme called aldose reductase converts the excess into sorbitol, a sugar alcohol the cell can’t easily get rid of. Sorbitol accumulates, drawing water into the cell through osmotic pressure and causing it to swell and malfunction. This process is particularly harmful to the delicate cells lining retinal capillaries.
From Leaky Vessels to New, Fragile Ones
As capillary walls deteriorate, they begin to leak fluid, fats, and blood into the surrounding retinal tissue. Healthy retinal capillaries never do this. The retina also has no lymphatic drainage system, so leaked fluid has nowhere to go. It simply accumulates, causing swelling.
When enough capillaries are damaged or blocked, patches of the retina become starved of oxygen. The retina responds by releasing a signaling protein called VEGF (vascular endothelial growth factor), which tells the body to grow new blood vessels to restore the oxygen supply. In theory, this sounds helpful. In practice, it’s catastrophic. The new vessels are fragile, poorly formed, and grow in the wrong places. They bleed easily, leak constantly, and can pull on the retina as scar tissue forms around them, potentially detaching it from the back of the eye.
Stages of Retinopathy
Diabetic retinopathy progresses through two main phases: non-proliferative and proliferative. The non-proliferative phase is further divided into mild, moderate, and severe stages based on how much visible damage is present in the retina.
- Mild non-proliferative: A few microaneurysms are present. You likely have no symptoms at this point.
- Moderate non-proliferative: More microaneurysms appear alongside small hemorrhages and early signs of vein distortion.
- Severe non-proliferative: Widespread hemorrhages, significant vein distortion, and abnormal vessel changes are present across multiple areas of the retina. The risk of progressing to the proliferative stage is high.
- Proliferative diabetic retinopathy: New, abnormal blood vessels grow on the retina, the optic disc, or the iris. These vessels can bleed into the gel-like fluid filling the eye or cause the retina to detach, both of which threaten vision.
How Macular Edema Causes Vision Loss
The macula is the small central area of the retina responsible for sharp, detailed vision. When damaged retinal vessels leak fluid into this region, it swells, a condition called diabetic macular edema. This can happen at any stage of retinopathy and is one of the most common reasons people with diabetes lose functional vision.
If the swelling doesn’t involve the very center of the macula, you may not notice anything at first. Only very observant people pick up on subtle blank spots in their peripheral central vision at this stage. Once the center is affected, symptoms develop over weeks to months: gradual blurring, faded colors, difficulty adjusting between bright and dim lighting, and distortion where straight lines appear wavy. The longer fluid sits in the macula, the more permanent the damage becomes.
Blood Sugar Control Is the Strongest Risk Factor
The single most powerful predictor of diabetic retinopathy is how well blood sugar has been controlled over time, measured by HbA1c. A large Swedish population study found that for every 1 percentage point increase in HbA1c, the odds of developing any retinopathy rose by 51% over 8 to 10 years of follow-up. Over 16 to 20 years, that same 1-point increase nearly tripled the risk of proliferative retinopathy.
Duration of diabetes matters too. The longer you’ve had diabetes, the more cumulative exposure your retinal vessels have had to high glucose. Type 1 and type 2 diabetes both cause retinopathy through the same mechanisms, though the timeline differs because type 2 often goes undiagnosed for years before detection.
High Blood Pressure Accelerates the Damage
Hypertension and diabetes frequently coexist, and high blood pressure independently speeds up retinopathy progression. Elevated blood pressure puts extra mechanical stress on already-weakened retinal capillaries, worsening leakage and vessel damage. Treating hypertension to a goal below 150/85 mmHg has been shown to reduce the rate of retinopathy worsening by 34% over 7.5 years and lower the risk of significant vision loss by 47%.
Interestingly, pushing blood pressure even lower doesn’t seem to help much more. A major trial comparing a systolic goal below 120 mmHg versus below 140 mmHg found no significant difference in retinopathy progression rates. Moderate, consistent blood pressure control appears to be what matters most.
Pregnancy Can Worsen Existing Retinopathy
For women who already have diabetic retinopathy before becoming pregnant, pregnancy carries a significant risk of worsening. Retinopathy progresses in 50% to 70% of these cases. The reason is partly mechanical: pregnancy increases cardiac output and blood volume while decreasing resistance in blood vessels, creating a high-flow state that puts additional shear stress on fragile retinal capillaries. Current guidelines recommend an eye exam before pregnancy, during the first trimester, and potentially every trimester depending on the severity of existing retinopathy.
Screening Timelines for Type 1 and Type 2
Because retinopathy develops silently, screening is essential. The recommended schedules differ by diabetes type. If you have type 1 diabetes, your first comprehensive dilated eye exam should happen five years after diagnosis, since retinopathy rarely develops before that point. If you have type 2 diabetes, you should get an eye exam at the time of diagnosis, because the disease may have been present for years before it was caught.
After the initial exam, annual screening is standard for anyone without retinopathy or with only mild changes. If your blood sugar is well-controlled and consecutive exams are normal, screening every one to two years may be sufficient. If any retinopathy is detected, annual exams become the minimum, with more frequent monitoring if the disease is progressing. Retinal photographs can supplement these exams but don’t replace the full dilated evaluation.