Blood clots in the eye most often form when a vein or artery in the retina becomes blocked, a condition called retinal vascular occlusion. The most common causes are high blood pressure, diabetes, and high cholesterol, all of which damage blood vessel walls over time and set the stage for a clot. But “blood clot in the eye” can also refer to something much less serious: a bright red patch on the white of the eye, which is a completely different condition with different causes.
Surface Bleeding vs. Internal Clots
A red spot on the white of your eye is almost always a subconjunctival hemorrhage, a tiny broken blood vessel just beneath the surface. It looks alarming but is harmless. Sneezing, coughing, rubbing your eyes, or straining can cause one. It clears on its own in one to two weeks and doesn’t affect your vision at all.
A blood clot inside the eye is a different matter entirely. This happens deeper, in the blood vessels that supply the retina, the light-sensitive tissue at the back of your eye. When one of these vessels gets blocked, the retina loses blood flow, and vision can deteriorate quickly. This is the type that requires urgent attention, and it’s what the rest of this article covers.
How Clots Form in Retinal Blood Vessels
The retina is fed by a single central artery and drained by a single central vein. These vessels branch into smaller tributaries that spread across the retina’s surface. At certain points, arteries and veins cross over each other, sharing a thin outer sheath. When an artery stiffens from years of high blood pressure or cholesterol buildup, it can press on the vein beneath it. That compression narrows the vein, slows blood flow, and creates conditions for a clot to form.
This is why branch retinal vein occlusion, the most common type, typically occurs right at a point where an artery crosses over a vein. The clot doesn’t travel to the eye from somewhere else in the body. It forms locally because the vessel has been gradually compromised.
In a central retinal vein occlusion, the blockage happens at the main vein itself, near where it exits the eye through a tight anatomical opening called the lamina cribrosa. Because this vein drains the entire retina, a clot here affects a much larger area of vision than a branch occlusion does.
The Major Risk Factors
High blood pressure is the single biggest risk factor. It damages artery walls throughout the body, including the tiny arteries in the retina, making them thicker and less flexible. Over time, those stiffened arteries compress neighboring veins.
Diabetes causes its own form of blood vessel damage. Chronically elevated blood sugar weakens vessel walls and promotes inflammation, making clots more likely. High cholesterol contributes by building up fatty deposits inside artery walls, narrowing them further.
These three conditions, hypertension, diabetes, and high cholesterol, account for the majority of retinal vein occlusions. They often overlap in the same person, compounding the risk.
Eye-Specific Factors
Glaucoma and elevated eye pressure also play a role. High pressure inside the eye physically squeezes the veins that drain blood from the retina, slowing flow and encouraging clots. People with glaucoma face a meaningfully higher risk of retinal vein occlusion even if their overall cardiovascular health is decent.
Blood Clotting Disorders
In younger people (under 50), the usual cardiovascular culprits are less common, so inherited clotting disorders become more relevant. A study published in Ophthalmology Retina found that roughly 11.5% of patients under 50 with a central retinal vein occlusion had an underlying hypercoagulable state, meaning their blood was inherently more prone to clotting. Factor V Leiden, a genetic mutation that makes blood clot more easily, shows up at higher rates in retinal vein occlusion patients under 60. Notably, this rate was similar whether or not those patients also had diabetes, high blood pressure, or high cholesterol, suggesting these clotting disorders act as an independent risk factor.
Types of Retinal Vein Occlusion
There are three main types, defined by where the blockage occurs:
- Branch retinal vein occlusion (BRVO): A clot in one of the smaller tributary veins. It affects one quadrant of the retina or just the central macula. This is the most common type and generally carries a better visual outcome.
- Central retinal vein occlusion (CRVO): A clot in the main vein draining the entire retina. It affects more of the visual field and tends to cause more severe vision loss.
- Hemiretinal vein occlusion (HRVO): A blockage at the optic disc that affects either the top or bottom half of the retina. It falls somewhere between the other two in severity.
Each type is further classified as ischemic (where blood flow is severely cut off) or non-ischemic (where some flow continues). Ischemic occlusions carry a higher risk of complications.
Retinal Artery Occlusions
Less commonly, the blockage occurs in a retinal artery rather than a vein. Retinal artery occlusions tend to cause sudden, painless, and more dramatic vision loss because the retina is being starved of incoming oxygen rather than having trouble draining blood. These are often caused by a small embolus, a fragment of plaque or a tiny clot, that breaks off from a larger artery (typically the carotid artery in the neck or the heart) and lodges in the narrow retinal artery. A retinal artery occlusion is treated as a medical emergency similar to a stroke, because the underlying causes overlap.
Symptoms to Recognize
Retinal vein occlusions typically affect only one eye. The hallmark symptom is blurry vision or partial vision loss that may come on suddenly or develop over hours to days. You might also notice floaters, dark spots, or wavy lines in your field of vision. Pain or pressure in the eye can occur in more severe cases, but many people experience no pain at all. If you wake up one morning and notice that vision in one eye is noticeably blurrier than the day before, that pattern is characteristic of a retinal vein occlusion.
Retinal artery occlusions present differently. Vision loss is typically sudden and more complete, often described as a curtain dropping over part or all of the visual field in one eye.
How It’s Diagnosed
An ophthalmologist can often identify a retinal vein occlusion during a dilated eye exam by looking at the pattern of swelling and hemorrhages on the retina. To determine the severity, they may use optical coherence tomography (a scan that creates cross-sectional images of the retina) to check for fluid buildup in the macula, which is the area responsible for sharp central vision. A dye-based imaging test called fluorescein angiography maps blood flow through the retinal vessels and reveals exactly where the blockage is and how much of the retina has lost circulation.
Complications of Untreated Clots
When blood can’t drain properly from the retina, pressure builds in the smaller vessels, causing them to leak fluid. This fluid collects in the macula and causes swelling called macular edema, the most common reason people with retinal vein occlusions lose vision. Without treatment, this swelling can permanently damage the photoreceptors responsible for detailed, central vision.
In ischemic cases, where a large area of retina is starved of oxygen, the eye may respond by growing new, abnormal blood vessels. These fragile vessels can bleed into the eye or grow into the drainage angle where fluid exits, blocking it and causing a dangerous spike in eye pressure known as neovascular glaucoma. This is one of the most serious complications and can lead to significant, irreversible vision loss.
How Retinal Clots Are Treated
The clot itself is not typically removed. Treatment focuses on managing the damage it causes, especially macular edema. The standard approach involves injections of medication directly into the eye that block a protein responsible for abnormal blood vessel growth and leakage. These injections stabilize vision in about 9 out of 10 people and improve it in roughly 1 out of 3. Most people need multiple injections over months, with the frequency tapering as the eye responds.
Steroid implants placed inside the eye are an alternative for some patients, particularly those who don’t respond well to the standard injections. Laser treatment is sometimes used to seal leaking blood vessels or to treat areas of the retina that have lost circulation, reducing the risk of abnormal new vessel growth.
Beyond the eye itself, treatment always involves managing the underlying cause. If high blood pressure drove the occlusion, getting it under control is essential to protect the other eye and reduce the risk of recurrence. The same applies to diabetes and cholesterol management. For younger patients, blood testing for clotting disorders may reveal a treatable condition that reduces risk going forward.
Who’s Most at Risk
Retinal vein occlusions become more common with age, particularly after 50. Men and women are affected at similar rates. People with uncontrolled high blood pressure face the highest risk. Smokers, people who are significantly overweight, and those with a family history of clotting disorders also have elevated risk. Having a retinal vein occlusion in one eye increases the chance of eventually developing one in the other eye, which is why managing cardiovascular risk factors after a first episode matters so much.