What Causes Blood Clots in the Eye?

By Your Health Magazine Health Information Team

Blood clots in the eye are usually caused by a blockage in a retinal vein or artery—the blood vessels serving the light-sensitive tissue at the back of the eye. These blockages are associated with conditions that damage blood vessels or affect circulation, including high blood pressure, diabetes, high cholesterol, atherosclerosis, glaucoma, smoking, heart disease, and certain blood-clotting disorders. Sudden vision loss in one eye may be an eye stroke and requires immediate emergency care.

What Does a “Blood Clot in the Eye” Mean?

The phrase can describe several different problems. The most serious involve the retina, which converts light into signals the brain interprets as vision.

Condition	What Happens	Typical Symptoms
Retinal vein occlusion	A clot or reduced blood flow blocks a vein that carries blood away from the retina.	Sudden or developing blurry vision, partial vision loss, or occasionally no early symptoms
Retinal artery occlusion	A clot, fatty deposit, or other material blocks an artery supplying the retina.	Sudden, usually painless loss or darkening of vision in one eye
Subconjunctival hemorrhage	A small surface blood vessel breaks and leaks beneath the clear covering of the eye.	A bright red patch on the white of the eye without vision loss

A retinal vein occlusion may involve the central retinal vein or one of its smaller branches. The National Eye Institute’s information on central retinal vein occlusion explains that the blockage can cause blood vessel leakage, retinal swelling, and vision loss.

A retinal artery occlusion is more urgent because it interrupts oxygen delivery to retinal tissue. Central retinal artery occlusion is classified as a form of acute ischemic stroke. The American Heart Association recommends rapid emergency evaluation for suspected cases.

What Causes Retinal Blood Vessel Blockages?

A blockage may develop inside an eye vessel or may travel to the eye from elsewhere in the body. The specific cause is not always identified, particularly with retinal vein occlusion. Important causes and risk factors include:

High blood pressure: Long-term pressure can damage and narrow blood vessels, increasing the likelihood of an obstruction.
Atherosclerosis: Hardened, thickened arteries can press against nearby retinal veins or release material that travels into a retinal artery.
Diabetes: Elevated blood sugar can damage small vessels throughout the body, including those in the retina.
High cholesterol: Fatty deposits can contribute to narrowing of the carotid arteries and other blood vessels.
Heart conditions: Atrial fibrillation, heart valve disease, and certain other heart problems can allow clots to form and travel through the bloodstream.
Carotid artery disease: Material from an artery in the neck can break loose and block a retinal artery.
Glaucoma: Glaucoma and elevated pressure inside the eye are associated with retinal vein occlusion.
Smoking: Tobacco exposure damages blood vessels and increases cardiovascular and clotting risks.
Blood or clotting disorders: Less commonly, abnormal clotting or unusually thick blood may contribute, especially in a younger person without typical vascular risk factors.
Inflammation of an artery: Giant cell arteritis, also called temporal arteritis, can block blood flow to the eye, particularly in adults over age 50. New headache, scalp tenderness, jaw pain while chewing, and vision changes require emergency assessment.

Risk generally rises with age, but retinal vessel blockages can occur in younger people. In that situation, clinicians may investigate clotting conditions, inflammatory disease, medication use, and other less common causes. Blood clots can also occur alongside complex medical problems, as illustrated in this discussion of Deion Sanders’ health update involving blood clots and other conditions.

A Red Spot Is Usually Bleeding, Not a Clot

A sharply defined red patch on the white of the eye is commonly a subconjunctival hemorrhage rather than a blocked retinal vessel. It occurs when a tiny surface vessel breaks after coughing, sneezing, vomiting, straining, rubbing the eye, an injury, or an eye procedure. High blood pressure and blood-thinning medications may also make these hemorrhages more likely.

A typical surface hemorrhage does not cause pain, discharge, or a change in vision and usually clears gradually over two to three weeks. The MedlinePlus guide to subconjunctival hemorrhage provides additional information about this common condition. Recurrent episodes, easy bruising, other bleeding, pain, trauma, or altered vision should be evaluated.

How Are Retinal Clots Diagnosed and Treated?

An ophthalmologist or retina specialist may perform a dilated eye examination and check visual acuity, eye pressure, pupil responses, and the retina. Optical coherence tomography can detect retinal swelling, while retinal photography or fluorescein angiography may show blocked or leaking vessels.

Testing may also look for an underlying cardiovascular cause. Depending on the situation, this can include blood pressure and laboratory testing, an electrocardiogram, heart monitoring, an echocardiogram, or imaging of the carotid arteries.

Treatment depends on the type of blockage:

Retinal vein occlusion: Eye injections called anti-VEGF medicines may reduce macular edema—the swelling that often causes vision loss. Steroid treatment or laser procedures may be used in selected cases. These treatments address swelling and other complications rather than directly removing the original blockage.
Retinal artery occlusion: This requires immediate stroke-style evaluation. Emergency clinicians determine whether time-sensitive treatment is appropriate and look for a source in the heart, carotid arteries, or blood vessels.

Do not start aspirin or another blood thinner on your own for an eye problem. These medicines may be appropriate for some underlying conditions but can increase bleeding risk and should be used only with guidance from a qualified healthcare professional.

Can the Risk Be Reduced?

Not every retinal vessel blockage is preventable, but protecting overall vascular health may lower risk. Helpful measures include keeping blood pressure, blood sugar, and cholesterol under medical control; avoiding tobacco; staying physically active as appropriate; and attending recommended eye examinations. People with glaucoma, diabetes, cardiovascular disease, or a previous retinal occlusion may need closer monitoring.

Because many disorders can produce redness or vision changes, an overview of eye diseases and their symptoms can provide useful context, but it cannot replace an examination when symptoms appear suddenly.

When to Seek Care

Call 911 or go to an emergency department immediately for sudden, painless loss, dimming, or blurring of vision in one eye—even if the vision returns after a few minutes. Also seek emergency help if vision changes occur with facial drooping, weakness, numbness, severe dizziness, difficulty speaking, or a sudden severe headache.

Promptly contact an ophthalmologist or other healthcare professional for a new red eye accompanied by pain, light sensitivity, injury, discharge, or any change in vision. A painless red patch without vision changes is often less urgent, but it should be discussed with a clinician if it repeatedly returns, follows trauma, or occurs with unexplained bruising or bleeding.

This information is for general education and is not a diagnosis or a substitute for medical care.