A blood clot in the eye almost always refers to a blockage in one of the retina’s blood vessels, and the most common cause is the same process that leads to heart attacks and strokes: atherosclerotic disease, where fatty deposits in artery walls break loose or trigger clot formation that blocks blood flow. These blockages, called retinal vascular occlusions, come in two broad forms depending on whether an artery or a vein is affected, and the distinction matters because their urgency, treatment, and long-term implications are quite different.
Artery Occlusions Versus Vein Occlusions
The retina has its own dedicated blood supply: a central artery that branches into smaller arteries delivering oxygen, and a network of veins that carries blood back out. A clot can lodge in either system, but the consequences play out differently. A retinal artery occlusion starves tissue of oxygen almost instantly, causing sudden, painless vision loss that can become permanent within minutes. A retinal vein occlusion backs up blood flow, leading to swelling and hemorrhages that damage vision more gradually. Both are linked to aging and cardiovascular risk factors, but artery occlusions are treated as neurological emergencies alongside stroke specialists, while vein occlusions are managed primarily by ophthalmologists over weeks to months.1PubMed Central. Retinal vascular occlusions
Each type can also be “central” or “branch.” A central retinal artery occlusion (CRAO) blocks the main artery feeding the entire retina, while a branch retinal artery occlusion (BRAO) affects only one of its smaller branches, potentially sparing part of the visual field. The same naming applies to vein occlusions: central (CRVO) or branch (BRVO). Branch occlusions tend to be less severe because only a portion of the retina loses its blood supply, but they still require evaluation and treatment.
What Actually Causes the Blockage
In artery occlusions, the usual culprit is an embolus, a small chunk of material that breaks free from a diseased blood vessel elsewhere and travels until it gets stuck. About 55% of central retinal artery occlusions have an identifiable source of embolism, and most of those emboli originate in the carotid arteries in the neck rather than from heart valves.2Eye. Central retinal artery occlusion: a stroke of the eye The clot typically lodges at the narrowest point of the central retinal artery, where it pierces the protective sheath of the optic nerve. A blood clot forming directly at that spot, rather than traveling from elsewhere, is also possible, especially near a structure called the lamina cribrosa.
Vein occlusions have a somewhat different mechanism. Rather than an embolus flying in from a distant artery, the problem usually starts locally. Retinal arteries and veins share a tight connective-tissue sheath where they cross over each other, and as arteries stiffen and thicken with age or disease, they physically compress the vein beneath them. Research comparing the degree of vein narrowing at these crossing points found that people with a history of branch retinal vein occlusion, as well as their unaffected fellow eyes, showed significantly more vein compression at arteriovenous crossings than younger healthy people or age-matched controls.3PubMed Central. Narrowing Ratio of Retinal Veins at Arteriovenous Crossing in Patients With Branch Retinal Vein Occlusion Versus That in Healthy Individuals The compressed vein develops turbulent, sluggish flow, and eventually a clot forms.
Risk Factors You Can and Cannot Control
The risk factors for retinal vascular occlusions overlap heavily with those for heart disease and stroke. High blood pressure, high cholesterol, and diabetes are the big three. A large study of enrollees in a managed care plan found that hypertension alone nearly doubled the hazard of developing a branch retinal vein occlusion compared to people without hypertension. When hypertension had already caused organ damage, the hazard more than doubled. Diabetes by itself did not significantly raise the risk, but diabetes with end-organ damage increased it by about a third.4PubMed Central. Risk Factors Associated with Developing Branch Retinal Vein Occlusion Among Enrollees in a United States Managed Care Plan These same cardiovascular risk factors, including dyslipidemia, diabetes, and systemic hypertension, also raise the risk of retinal artery occlusions, cataracts, and age-related macular degeneration.5European Heart Journal. The eye and the heart
The pattern is clear: controlling blood pressure matters more than any other single intervention for preventing retinal clots, and the severity of your cardiovascular disease predicts the severity of your eye risk. If your doctor has told you that hypertension or diabetes is affecting your kidneys or heart, those same changes are happening in the tiny vessels of your retina.
Less commonly, blood-clotting disorders can play a role, particularly in vein occlusions. Elevated homocysteine levels and antiphospholipid syndrome appear to be genuine causes of central retinal vein occlusion, and problems with the body’s ability to break down clots may also contribute. The more common inherited clotting conditions, however, do not seem to be strong risk factors based on current evidence, though larger studies would help settle the question.6Nature. Central retinal vein occlusion and thrombophilia
Recognizing the Symptoms
A retinal artery occlusion announces itself dramatically. You lose vision in one eye within seconds to minutes, usually without pain. Some people describe it as a curtain dropping over part or all of their visual field. Because the retina is dying from oxygen starvation as you stand there, this is one of the few eye problems that genuinely warrants calling emergency services immediately.
A retinal vein occlusion tends to come on somewhat less abruptly. You might notice blurred or distorted vision in one eye that worsens over hours or days. The blurriness comes from fluid and blood leaking out of the congested veins into the retina, causing swelling at the macula, the part of the retina responsible for sharp central vision. Vein occlusions are not as immediately catastrophic as artery occlusions, but they still need prompt evaluation because the swelling and lack of blood flow can cause lasting damage if left untreated.
Neither type causes the dramatic red blotch on the white of the eye that many people picture when they hear “blood clot in the eye.” That visible red patch, called a subconjunctival hemorrhage, is a broken blood vessel on the eye’s surface and is almost always harmless. Retinal vascular occlusions happen deep inside the eye where you cannot see them from the outside. The only visible sign is what happens to your vision.
How Doctors Diagnose It
An ophthalmologist can often diagnose a retinal vascular occlusion during a standard dilated eye exam. In an artery occlusion, the retina appears pale and swollen because it has lost its blood supply. In a vein occlusion, the retina shows scattered hemorrhages and engorged, tortuous veins. But confirming the diagnosis and assessing the damage usually requires imaging.
Fluorescein angiography, where a dye is injected into a vein in your arm and photographed as it flows through the retinal blood vessels, has been the standard for decades. It reveals exactly where blood flow is blocked or leaking. More recently, optical coherence tomography angiography (OCTA) has become an important tool because it can map the retinal blood vessels in fine detail without requiring a dye injection.7PubMed Central. Optical Coherence Tomography Angiography in Retinal Diseases OCTA provides a detailed view of the retinal vasculature and can pick up areas where capillaries have dropped out or abnormal new blood vessels are starting to grow, both of which influence treatment decisions.8PubMed Central. Optical coherence tomography angiography In some cases, OCTA can reveal abnormal vessel growth more clearly than traditional angiography.9PubMed Central. En-face optical coherence tomography angiography of neovascularization elsewhere in hemicentral retinal vein occlusion
Treating a Retinal Artery Occlusion
A retinal artery occlusion is a medical emergency, and the American Heart Association has called for systems of care that prioritize early recognition and rapid triage to emergency treatment.10PubMed. Management of Central Retinal Artery Occlusion: A Scientific Statement From the American Heart Association The challenge is that the retina is exquisitely sensitive to oxygen deprivation. Animal studies suggest that if the blockage can be cleared within about 97 minutes, full recovery of vision is possible, with partial recovery observed up to about four hours.2Eye. Central retinal artery occlusion: a stroke of the eye That window is extremely tight, and most people do not reach specialized care in time.
Historically, doctors have tried a grab bag of techniques: ocular massage to physically dislodge the clot, breathing high-concentration carbon dioxide mixtures to dilate the arteries, draining fluid from the front of the eye to lower pressure, and various vasodilator drugs. None of these “standard” approaches have been shown to definitively change the outcome. More recently, researchers have explored clot-busting drugs (thrombolytics) delivered either through a vein in the arm or directly into the ophthalmic artery via catheter. While some case series reported dramatic vision recovery, randomized controlled trials have been disappointing. Direct catheter delivery of thrombolytics carries a risk of bleeding in the brain, and intravenous tissue plasminogen activator was not shown to be effective when given within 24 hours of symptom onset. The consistent lesson from these trials is that treatment probably needs to happen within about six hours to have a chance of working.11PubMed Central. Treatment options for central retinal artery occlusion
Despite the uncertainty about which specific treatment works best, the current consensus is that intravenous thrombolysis may be effective and that patients need urgent screening and treatment of cardiovascular risk factors.10PubMed. Management of Central Retinal Artery Occlusion: A Scientific Statement From the American Heart Association In practice, this means the ideal response to sudden painless vision loss is the same as for a stroke: get to an emergency department as fast as possible.
Treating a Retinal Vein Occlusion
Vein occlusions are managed very differently. Because the primary problem is not oxygen starvation but rather swelling and fluid leakage from congested veins, treatment focuses on controlling the resulting macular edema and preventing complications like abnormal new blood vessel growth.
The mainstay of treatment today is injections of anti-VEGF drugs directly into the eye. VEGF (vascular endothelial growth factor) is a protein the retina releases when it is deprived of adequate blood flow, and it drives both fluid leakage and the growth of fragile new blood vessels that can bleed. In trials of ranibizumab, one of the most commonly used anti-VEGF agents, patients with branch retinal vein occlusion gained an average of about 17 to 18 letters on a vision chart at six months, compared to about seven letters in the untreated group.12Asia-Pacific Journal of Ophthalmology. Update on the Use of Anti-VEGF Intravitreal Therapies for Retinal Vein Occlusion These injections typically need to be repeated every month or so initially, then less frequently as the edema comes under control. Many patients require ongoing injections for a year or longer.
For people who do not respond well to anti-VEGF drugs or who have other considerations, corticosteroid implants are an alternative. A dexamethasone implant placed inside the eye slowly releases medication over several months, reducing macular edema effectively. Studies have found these implants are generally safe and well tolerated, leading to both measurable improvements in retinal swelling and meaningful visual rehabilitation.13PubMed Central. Evaluation of the effectiveness and safety of glucocorticoids intravitreal implant therapy in macular edema due to retinal vein occlusion The main side effect to watch for is a rise in eye pressure. In one study, about a third of patients experienced a clinically significant increase in intraocular pressure, though most managed it with eye drops and fewer than 2% needed surgery for glaucoma.14Retina. EFFICACY AND SAFETY OF TWO OR MORE DEXAMETHASONE INTRAVITREAL IMPLANT INJECTIONS FOR TREATMENT OF MACULAR EDEMA RELATED TO RETINAL VEIN OCCLUSION (SHASTA STUDY)
Laser therapy used to be the standard treatment for branch vein occlusions before anti-VEGF drugs arrived. Grid laser applied to the macula reduces edema, and scatter laser applied to the peripheral retina can prevent or treat abnormal new vessel growth. Laser still has a role, particularly for managing complications like retinal neovascularization or vitreous hemorrhage, and vitrectomy surgery may be needed if bleeding inside the eye does not clear on its own.15PubMed. Branch retinal vein occlusion: treatment modalities: an update of the literature
The Stroke Connection
One of the most important things to understand about a retinal artery occlusion is that it is, in a very real sense, a stroke of the eye. The retina is brain tissue, and the blood supply that feeds it branches from the same arteries that feed the brain. A clot that blocks a retinal artery may be a warning that a larger, more devastating stroke is imminent.
A systematic review and meta-analysis found that 30% of patients with acute central retinal artery occlusion and 25% of those with branch retinal artery occlusion already had evidence of acute brain ischemia on MRI, even if they had no neurological symptoms they noticed.16PubMed Central. Risk of acute stroke in patients with retinal artery occlusion: a systematic review and meta-analysis Those are strikingly high rates. A population-based study estimated the risk of symptomatic ischemic stroke in the 15 days before and after a CRAO at about 2.2%, which, while lower than the figures from specialized centers, still supports urgent neurological evaluation.17PubMed Central. Stroke Risk before and after Central Retinal Artery Occlusion: A Population-based Analysis
This is why the workup for a retinal artery occlusion extends far beyond the eye. Doctors will typically order imaging of the carotid arteries (ultrasound or CT angiography), cardiac monitoring to look for irregular heart rhythms like atrial fibrillation, and brain imaging to check for silent strokes. These evaluations are not optional extras; they can reveal life-threatening problems that need immediate intervention.
Retinal vein occlusions carry a weaker stroke association but still signal underlying cardiovascular disease that deserves attention. If you are diagnosed with any type of retinal vascular occlusion, expect your doctor to check or adjust your blood pressure, cholesterol, and blood sugar management as part of the treatment plan.
When It Happens to Younger People
Most retinal vascular occlusions occur in people over 50, and the typical patient profile looks a lot like the typical heart attack patient: older, hypertensive, maybe diabetic. But these clots can and do happen in younger people, and when they do, the underlying causes tend to be different.
A study of 27 patients who developed retinal artery obstruction before age 30 found no clinical evidence of the atherosclerotic disease that drives the problem in older adults. Instead, about a third had a history of migraine, and coagulation abnormalities were also common. Other identified causes included trauma, sickle cell disease, cardiac disorders, oral contraceptive use, pregnancy, lupus, and intravenous drug use. Some had eye-specific findings like elevated eye pressure, buried drusen of the optic nerve, or a congenital arterial loop near the optic disc.18Ophthalmology. Retinal Arterial Obstruction in Children and Young Adults The visual prognosis for these younger patients was roughly the same as for older patients, but the causes required a completely different workup.
If you are under 50 and experience a retinal vascular occlusion, your doctors will likely investigate clotting disorders, autoimmune conditions, and cardiac abnormalities rather than focusing primarily on atherosclerosis. Women of reproductive age may be evaluated for antiphospholipid syndrome and asked about hormonal contraceptive use. People with sickle cell trait or disease may be at elevated risk and should be aware that sudden vision changes warrant immediate attention.
What Recovery Looks Like
Recovery from a retinal vascular occlusion depends heavily on the type and how quickly treatment began. Artery occlusions have the worst visual prognosis because the retina tolerates so little oxygen deprivation. Many patients with a central retinal artery occlusion are left with severely impaired vision in the affected eye, often counting-fingers or worse. Experimental techniques like using a laser to physically break up a visible clot inside the retinal artery have shown promise in small series, with one study reporting that about 89% of treated patients gained meaningful improvement in visual acuity, with an average improvement of roughly five lines on a standard eye chart.19RETINA. RESTORATION OF RETINAL BLOOD FLOW VIA TRANSLUMENAL Nd:YAG EMBOLYSIS/EMBOLECTOMY (TYL/E) FOR CENTRAL AND BRANCH RETINAL ARTERY OCCLUSION However, these are small case series, not large randomized trials, and the technique is not widely available.
Vein occlusions generally have a better outlook, especially branch vein occlusions. Many patients regain useful vision with anti-VEGF treatment, though recovery is often incomplete and depends on how much damage the macula sustained before treatment started. The first few months of treatment typically bring the most rapid improvement, with slower gains thereafter. Some patients eventually come off injections entirely when the edema resolves, while others need periodic retreatment for years.
Living with the aftermath of a retinal vascular occlusion means adapting to whatever vision deficit remains and staying vigilant about cardiovascular health. Your ophthalmologist will want to see you regularly to monitor for late complications like new abnormal blood vessel growth, which can cause fresh bleeding or even retinal detachment months after the initial event. And your primary care doctor should be optimizing your blood pressure, cholesterol, and blood sugar with the understanding that the clot in your eye was not an isolated event but a manifestation of a systemic problem.
Subconjunctival Hemorrhage and Other Red Herrings
Many people searching for information about blood clots in the eye are actually looking at a bright red patch on the white of their eye and wondering if something dangerous is happening. In most cases, that visible redness is a subconjunctival hemorrhage, a tiny broken blood vessel between the clear surface membrane and the white of the eye. It looks alarming but is almost always harmless. Sneezing, coughing, straining, rubbing your eyes, or even sleeping in an awkward position can cause one. They resolve on their own in a week or two without treatment.
The key difference is simple: a subconjunctival hemorrhage looks terrible but does not affect your vision at all. A retinal vascular occlusion does not look like anything from the outside, but it causes noticeable vision loss or distortion. If you have a red eye with perfectly normal vision, you almost certainly have a subconjunctival hemorrhage and can relax. If you have sudden vision loss or blurriness in one eye with or without any external redness, treat it as an emergency.