A blood clot in your eye forms when a vein inside the retina becomes blocked, a condition called retinal vein occlusion (RVO). It affects an estimated 16 million people worldwide and is one of the most common retinal vascular diseases. The clot itself typically develops because of changes in the blood vessel walls, not because a clot travels to your eye from somewhere else in your body.
How the Clot Actually Forms
The retina, the light-sensitive tissue lining the back of your eye, has its own network of tiny arteries and veins. These blood vessels run very close together, and in some spots they share a tight sheath of tissue. When an artery in that space stiffens or thickens (a process called arteriosclerosis), it can physically compress the neighboring vein. That compression slows blood flow, damages the inner lining of the vein wall, and eventually triggers a clot to form right at that site.
Once the vein is blocked, blood backs up behind the clot. Pressure builds inside the smaller vessels feeding into it, and fluid starts leaking into the surrounding retinal tissue. This causes swelling, particularly in the macula, the central part of the retina responsible for sharp, detailed vision. The swelling disrupts the barrier that normally keeps fluid out of the retinal tissue, and the process feeds on itself: as tissue swells, oxygen delivery drops further, white blood cells stick to vessel walls, inflammation increases, and blood flow slows even more.
Two Types of Retinal Vein Occlusion
The location of the blockage determines the type and severity. A branch retinal vein occlusion (BRVO) happens when a smaller branch vein is blocked, typically at a point where an artery crosses over it. This affects only part of your visual field. A central retinal vein occlusion (CRVO) is more serious. It occurs when the main vein draining the entire retina is blocked, usually just behind the point where the optic nerve exits the eye. CRVO tends to cause more widespread damage and greater vision loss.
The Biggest Risk Factors
High blood pressure is the single strongest risk factor. A large meta-analysis found that hypertension doubles the risk of developing a retinal vein occlusion. This makes sense mechanically: chronically elevated blood pressure accelerates the stiffening of retinal arteries, which then compress the veins next to them.
Beyond blood pressure, other significant risk factors include a history of heart attack, a history of stroke, elevated cholesterol, and diabetes. Glaucoma also raises risk because increased pressure inside the eye can further compress retinal blood vessels. Smoking contributes by damaging vessel walls and promoting clot formation. Most people who develop RVO are over 50, and the risk increases with age as arteries naturally lose flexibility.
Blood Clots in Younger People
When a retinal vein or artery occlusion occurs in someone under 50, the cause is often different. Research on young patients with retinal artery blockages found that 68% had an underlying blood-clotting disorder, compared to about 18% in a matched control group. Conditions like Factor V Leiden (a genetic mutation that makes blood clot more easily) and elevated levels of certain clotting proteins were strongly associated with these events. A personal or family history of blood clots elsewhere in the body was also a significant predictor. If you develop a retinal vein occlusion at a younger age, your doctor will likely test for these inherited clotting conditions.
What It Feels Like
The hallmark symptom is painless vision loss in one eye. It can happen suddenly, or it may develop gradually over hours to days. Your vision might become blurry, or you might notice a dark area or blind spot in part of your visual field. With BRVO, you may lose vision in just one quadrant. With CRVO, the effect is more diffuse.
Pain or pressure in the affected eye can occur, but this typically signals a more severe case. Many people first notice something is wrong when they cover their unaffected eye and realize the other one isn’t seeing clearly. The condition almost always affects only one eye at a time.
How Doctors Confirm It
An eye doctor can often identify a retinal vein occlusion during a dilated eye exam by seeing characteristic signs like swollen veins, hemorrhages, and fluid leakage in the retina. Two imaging tests help confirm the diagnosis and guide treatment decisions.
Optical coherence tomography (OCT) uses light waves to create a cross-sectional image of the retina. It measures exactly how much swelling is present in the macula and checks whether the light-sensing cell layers are still intact. These measurements often determine whether and when treatment is needed.
Fluorescein angiography involves injecting a fluorescent dye into a vein in your arm, then photographing it as it flows through the retinal blood vessels. This reveals exactly where blood flow is blocked, how much of the retina has lost its blood supply, and whether abnormal new blood vessels are growing. Doctors use this to distinguish between milder cases and ischemic (oxygen-starved) cases, which carry a higher risk of complications.
What Happens Without Treatment
The blocked vein itself doesn’t “unblock” in most cases. The immediate concern is macular edema, the fluid buildup in the center of the retina that causes the most vision loss. Left untreated, this swelling can permanently damage the light-sensing cells in the macula.
In severe cases where large areas of the retina lose blood supply, the eye responds by growing new, fragile blood vessels. These abnormal vessels can bleed into the eye or grow into the drainage system that controls eye pressure, leading to a painful and sight-threatening form of glaucoma.
How It’s Treated
The standard first-line treatment targets the swelling in the macula using injections of medication directly into the eye. These drugs block a protein that drives fluid leakage and abnormal blood vessel growth. The injections are given in a clinic setting after numbing drops are applied. While the idea of an eye injection sounds alarming, most patients describe mild discomfort rather than significant pain.
A 36-month study of 527 eyes treated with these injections found an average improvement of about 10 letters on a standard eye chart. Thirty-seven percent of patients achieved good functional vision (70 letters or better on the chart, roughly equivalent to 20/40). Patients who completed the full three years of follow-up averaged 18 injections over 26 visits, with a slightly better outcome of 12 letters gained. The treatment doesn’t cure the underlying blockage, but it manages the swelling that causes vision loss.
Not everyone responds equally. Some people regain near-normal vision, while about 30% still had significantly reduced vision (35 letters or fewer) at the three-year mark. How well you respond depends partly on how much damage occurred before treatment started and how much of the retina lost its blood supply.
Reducing Your Risk
Because the underlying cause is almost always vascular, the same habits that protect your heart also protect the blood vessels in your eyes. Keeping blood pressure well controlled is the single most impactful thing you can do. Managing cholesterol, blood sugar (if you have diabetes), and maintaining a healthy weight all reduce the arterial stiffening that compresses retinal veins. Regular exercise, not smoking, and routine eye exams round out the practical steps that lower your odds of developing a clot in your eye.
If you’ve already had a retinal vein occlusion in one eye, these same measures help protect the other eye. Your doctor may also monitor your eye pressure more closely, since elevated intraocular pressure is an independent risk factor.