Diabetes damages the eyes primarily by weakening the tiny blood vessels in the retina, the light-sensitive tissue at the back of your eye. Over time, high blood sugar triggers a chain of vascular damage that can lead to vision loss, and roughly 1 in 4 Americans over 40 with diabetes already have some degree of retinal disease. But the retina isn’t the only structure at risk. Diabetes also accelerates cataract formation and can cause a dangerous type of glaucoma.
What High Blood Sugar Does to Retinal Blood Vessels
The retina is fed by a dense network of microscopic capillaries. Sustained high blood sugar damages the cells lining those capillaries and kills off pericytes, the support cells that wrap around each vessel to keep it stable. As the body tries to repair this damage, the capillary walls thicken and the channels narrow, which reduces blood flow and starves the retina of oxygen.
This sets up a vicious cycle. Oxygen-starved tissue signals for more repair, the vessel walls thicken further, and tiny bulges called microaneurysms form along weakened sections. Eventually some capillaries close off entirely. When enough of the retina loses its blood supply, the eye responds by growing new blood vessels to compensate. That sounds helpful, but these replacement vessels are fragile and poorly formed. They leak blood and fluid into the eye, scar easily, and can pull the retina out of position. This progression from early vessel damage to abnormal new vessel growth is what ophthalmologists mean when they describe diabetic retinopathy moving from a nonproliferative to a proliferative stage.
Chronic inflammation accelerates every step. Inflammatory signals cause pericytes to die faster and loosen the tight junctions between endothelial cells, making vessel walls more permeable. The longer blood sugar stays elevated, the more damage accumulates.
How Vision Gets Blurry: Macular Edema
The macula is the small central zone of the retina responsible for sharp, detailed vision. When damaged retinal vessels become leaky, fluid seeps into the macula and causes it to swell. This condition, diabetic macular edema, is one of the most common reasons people with diabetes notice their vision deteriorating.
A protein called VEGF (vascular endothelial growth factor) is a central driver. In a healthy eye, small amounts of VEGF help maintain normal blood supply. But in a diabetic eye, oxygen-starved retinal cells overproduce VEGF, which makes blood vessel walls far more permeable than they should be. VEGF also triggers the growth of those fragile new vessels mentioned above, compounding the problem. The result is fluid accumulation right where you need the sharpest focus, leading to blurred or distorted central vision.
Diabetes and Cataracts
People with diabetes develop cataracts earlier and more frequently than those without. The mechanism is distinct from what happens in the retina. When blood sugar is high, excess glucose enters the lens of the eye and gets converted into a sugar alcohol called sorbitol. Unlike glucose, sorbitol doesn’t pass easily out of the lens. It accumulates, along with fructose, and both sugars draw water into the lens through osmotic pressure. The lens fibers swell, their internal structure warps, and the lens gradually becomes cloudy. This is the same clouding that causes age-related cataracts, but diabetes fast-tracks the process.
How Diabetes Causes Glaucoma
Diabetes raises the risk of a particularly aggressive form of glaucoma called neovascular glaucoma. It develops as a late complication of proliferative diabetic retinopathy. When the retina is severely oxygen-deprived, the flood of VEGF doesn’t just trigger new vessel growth in the retina. Abnormal blood vessels also sprout on the iris and in the drainage angle at the front of the eye. Fibrous tissue grows alongside these vessels and physically blocks the trabecular meshwork, the eye’s main drain for fluid. With the drain blocked, pressure inside the eye climbs sharply, compressing the optic nerve and causing rapid, sometimes irreversible vision loss.
Symptoms to Watch For
Early diabetic eye damage often produces no symptoms at all, which is why screening exams matter so much. As damage progresses, the signs that typically appear include:
- Floaters: spots or dark strings drifting across your field of vision, caused by blood leaking into the gel that fills the eye
- Blurred vision: often from macular swelling, and sometimes fluctuating day to day as blood sugar levels shift
- Dark or empty patches: areas in your visual field where damaged retina is no longer sending a signal
- Sudden vision changes: a rapid onset of blurriness, spottiness, or haziness that warrants immediate attention
Because the early stages are silent, you can have significant retinal damage before you notice anything wrong. By the time floaters or blurred vision appear, the disease has often already progressed beyond its mildest form.
Who Is Most at Risk
In the United States, an estimated 9.6 million people with diabetes had diabetic retinopathy as of 2021, representing about 26 percent of all people with diabetes. Of those, 1.84 million had vision-threatening disease. The risk is not evenly distributed. Black Americans face vision-threatening retinopathy at a rate of 8.7 percent, compared to 7.1 percent for Hispanic Americans and 3.6 percent for white Americans. These disparities likely reflect differences in access to care, rates of uncontrolled blood sugar, and other systemic health factors.
Both type 1 and type 2 diabetes carry risk. Duration matters: the longer you’ve had diabetes, the higher the likelihood of retinal changes. Poorly controlled blood pressure and high cholesterol further accelerate damage because they independently stress the same small blood vessels that high glucose is already weakening.
How Diabetic Eye Disease Is Treated
The most significant advance in treatment over the past two decades is anti-VEGF therapy. These are injections delivered directly into the eye that block the protein driving abnormal vessel growth and leakage. Anti-VEGF treatment stabilizes vision in about 9 out of 10 people who receive it, and roughly 1 in 3 see meaningful improvement. The injections are typically given on a regular schedule, sometimes monthly at first, then less frequently as the eye responds.
Laser treatment remains an option, particularly for proliferative retinopathy. It works by sealing leaking vessels or deliberately destroying small areas of peripheral retina to reduce the eye’s overall demand for oxygen, which in turn reduces VEGF production. For advanced cases where blood has filled the eye or the retina has detached, surgery to remove the gel inside the eye (vitrectomy) can restore some vision.
Cataracts caused by diabetes are treated the same way as any cataract, with surgical lens replacement. Neovascular glaucoma is harder to manage and typically requires a combination of anti-VEGF injections to halt new vessel growth, laser therapy, and sometimes surgical implants to help fluid drain from the eye.
Screening Schedules That Catch Problems Early
The American Diabetes Association recommends different timelines depending on your type of diabetes. If you have type 2, you should get a dilated eye exam at the time of diagnosis, because type 2 often goes undetected for years and retinal damage may already be underway. If you have type 1, the first comprehensive dilated exam should happen within five years of diagnosis.
After that initial exam, the schedule depends on what your eye doctor finds. If there’s no sign of retinopathy and your blood sugar is well controlled, screening every one to two years is generally sufficient. If any level of retinopathy is present, annual exams become the minimum. More advanced or progressing disease may require visits every few months.
Protecting Your Vision Long Term
Blood sugar control is the single most effective way to slow or prevent diabetic eye disease. Every sustained reduction in your average blood sugar, measured by HbA1c, lowers the risk of retinopathy developing or worsening. Keeping blood pressure and cholesterol in check provides additional protection, since all three factors converge on the same vulnerable blood vessels in the retina.
One important nuance: if your blood sugar has been very high for a long time, bringing it down too rapidly can temporarily worsen retinopathy. This phenomenon, called early worsening, is usually mild and resolves, but it’s a reason your doctor may lower your blood sugar gradually rather than all at once, and it’s another reason to keep up with eye exams during periods of changing diabetes management.