What Causes an Eye Stroke? The Underlying Risks

An eye stroke happens when blood flow to the retina or optic nerve is suddenly reduced or cut off, starving delicate tissue of oxygen. The blockage can involve an artery, a vein, or the tiny vessels feeding the optic nerve head, and the underlying causes overlap heavily with the same cardiovascular risk factors that drive brain strokes and heart attacks. While age and high blood pressure top the list, the full picture involves a surprisingly wide range of systemic conditions, from diabetes and atrial fibrillation to sleep apnea and inflammatory blood vessel diseases.

Not One Condition but Several

The term “eye stroke” is informal. Clinically, it covers at least three distinct problems. Retinal artery occlusions happen when a clot or piece of debris blocks the central retinal artery or one of its branches, cutting off the blood supply to part or all of the retina. Retinal vein occlusions occur when a vein draining blood out of the retina becomes blocked, causing blood to back up, leak, and damage the tissue. And anterior ischemic optic neuropathy (AION) involves reduced blood flow to the optic nerve head itself. Although all three share certain cardiovascular risk factors and the common theme of ischemia, their mechanisms, treatment options, and what they signal about a person’s overall health differ substantially.

Retinal artery occlusions tend to behave more like miniature brain strokes. They are often caused by emboli, fragments of plaque or clot that travel from the heart or carotid arteries and lodge in a retinal vessel. Retinal vein occlusions, by contrast, are driven more by local compression and blood-flow dynamics within the eye, though systemic vascular disease still sets the stage. Ischemic optic neuropathy sits somewhat apart: the non-arteritic form is linked to a “crowded” optic disc and overnight blood pressure drops, while the arteritic form is a medical emergency tied to giant cell arteritis, an inflammatory condition of the blood vessels.

How Arteries Get Blocked

The retinal artery is an end artery, meaning there is essentially no backup blood supply if it becomes obstructed. The most common cause of a retinal artery occlusion is an embolus that originates elsewhere in the cardiovascular system. Cholesterol plaques in the carotid artery are a frequent source: in one study of patients with various ocular vascular findings, about 70% were found to have greater than 50% narrowing of the carotid artery on the same side.¹ Blood clots that form in the heart, particularly in people with atrial fibrillation, can also embolize to retinal vessels. The connection is direct: atrial fibrillation facilitates clot formation that can travel to the retinal circulation.²

Less commonly, the artery itself can become inflamed or narrowed at its origin. Giant cell arteritis, the most common large-vessel inflammatory condition in people over 50, can cause devastating retinal artery occlusions and ischemic optic neuropathy by inflaming the short posterior ciliary arteries that supply the optic nerve and parts of the retina.³ This form of eye stroke is considered a medical emergency because untreated giant cell arteritis carries a real risk of permanent bilateral blindness.⁴

Why Veins Occlude

Retinal vein occlusions are the more common type of eye stroke, and the mechanism is different from the arterial version. Inside the eye, retinal arteries and veins share a common outer sheath at the points where they cross each other. When the artery stiffens or thickens from years of high blood pressure and atherosclerosis, it can physically compress the vein at these crossing points. Histological studies show that the venous wall develops damage in these areas: the basement membrane thickens, and the endothelial cells lining the vein become elongated and dysfunctional.⁵ The narrowing increases blood flow velocity at the crossing, which further damages the vein wall and can trigger a clot. Most of the resulting damage occurs downstream of the crossing point, consistent with the idea that hemodynamic turbulence beyond the narrowing is what actually initiates clot formation.⁶

This is why branch retinal vein occlusion, the most common subtype, nearly always occurs at an arteriovenous crossing. Central retinal vein occlusion, where the main vein draining the entire retina is blocked, tends to occur at a different bottleneck: the point where the vein exits the eye through the lamina cribrosa, a sieve-like structure in the optic nerve head. In either case, the underlying story is the same. Systemic vascular disease stiffens arteries, which then compress the adjacent veins, setting up the conditions for a clot.

High Blood Pressure Is the Dominant Risk Factor

If you had to pick a single modifiable cause of eye strokes, high blood pressure would be it. Hypertension damages both arteries and veins throughout the body, but the retinal vasculature is especially vulnerable because the vessels are tiny, the arteriovenous crossings are tightly packed, and the tissue they serve has enormous oxygen demands. A large cohort study found that people with stage 2 hypertension had about a 32% higher risk of retinal vascular occlusion compared to people with normal blood pressure, while those with only mildly elevated readings had a more modest increase.⁷ One key clinical recommendation for any retinal vein occlusion is to optimize blood pressure control, regardless of what other workup is done.⁸

The effect of blood pressure on the eye is not limited to daytime readings. Nocturnal blood pressure drops can also play a role, particularly in optic nerve ischemia. In patients already being treated for hypertension, an excessive overnight decline in blood pressure was associated with progressive visual field loss in ischemic optic neuropathy. Average systolic blood pressure dropped about 26% and diastolic about 33% at night, and in vulnerable individuals, this dip may reduce blood flow to the optic nerve head below a critical threshold.⁹ The implication is counterintuitive: aggressively lowering blood pressure at night can sometimes cause harm in people whose optic nerve blood supply is already compromised.

Diabetes, Cholesterol, and the Interplay of Metabolic Risk

Diabetes increases the risk of retinal vein occlusion, though the relationship is more nuanced than a blanket statement suggests. A meta-analysis pooling data from 37 studies found that diabetes was associated with roughly a 68% higher odds of retinal vein occlusion overall.¹⁰ But the risk was not uniform across subtypes: it was strongest for central retinal vein occlusion and weaker, even non-significant, for branch retinal vein occlusion. The severity of diabetes also matters. People with uncomplicated diabetes did not show an increased risk of central retinal vein occlusion, while those with end-organ damage from diabetes had about a 53% higher hazard.¹¹ In other words, it is poorly controlled or advanced diabetes that poses the real threat, not a diabetes diagnosis on its own.

High LDL cholesterol has long been considered a risk factor for retinal vein occlusion because of its role in atherosclerosis. LDL drives plaque formation in arteries, which stiffens them and contributes to the compression of retinal veins at crossing points. However, a genetic study using Mendelian randomization found that once the effects of other lipid components were accounted for, LDL cholesterol may not be an independent risk factor for retinal vein occlusion.¹² The picture with cholesterol, then, is less clean than with blood pressure or diabetes. It likely contributes indirectly through its role in overall vascular disease rather than acting as a standalone trigger.

Sleep Apnea and the Overnight Vulnerability

Obstructive sleep apnea has emerged as one of the stronger and more underappreciated risk factors for a specific type of eye stroke: non-arteritic anterior ischemic optic neuropathy (NAION). NAION is the most common acute optic nerve problem in people over 50, and it typically presents as sudden, painless vision loss in one eye upon waking. The timing is a clue. During sleep apnea episodes, blood oxygen levels drop repeatedly, and blood pressure can swing unpredictably. These fluctuations may reduce perfusion to the optic nerve head at a time when blood pressure is already at its natural overnight low.

A meta-analysis found that people with obstructive sleep apnea had more than six times the odds of developing NAION compared to those without sleep apnea.¹³ A large insurance database study confirmed the association, finding that patients with untreated sleep apnea had a significantly higher rate of NAION than matched controls.¹⁴ Earlier research had also noted that patients with sleep apnea who did not use CPAP therapy had about a 16% increased hazard of developing NAION compared to those without the condition.¹⁵

The practical takeaway is straightforward: if you’ve had NAION and snore heavily, experience daytime sleepiness, or have been told you stop breathing during sleep, getting evaluated for sleep apnea is worth doing. Treating sleep apnea may not reverse the damage already done, but it could reduce the risk of the other eye being affected.

Erectile Dysfunction Drugs and a Specific Subgroup

Reports linking phosphodiesterase-5 inhibitors (drugs like sildenafil, sold as Viagra) to ischemic optic neuropathy generated alarm when they first appeared. These medications dilate blood vessels, and the concern was that the resulting blood pressure changes could compromise optic nerve perfusion. The evidence, however, is more limited than the headlines suggested. A case-control study found that men with NAION overall were not significantly more likely to have used these drugs compared to controls of similar age. But among men who already had a history of heart attack, the association was much stronger, with roughly a tenfold increase in risk.¹⁶ A similar though statistically weaker pattern appeared in men with hypertension. The drug on its own does not seem to be the problem. Rather, in people whose cardiovascular system is already compromised, the additional blood pressure drop may tip the balance.

Eye Strokes as Warning Signs for Brain Strokes

One of the most important things to understand about retinal artery occlusions is that they share the same mechanism as ischemic brain strokes. Both are caused by emboli or in-situ clots in arterial circulation. The retinal artery is, after all, a branch of the ophthalmic artery, which comes directly off the internal carotid. A systematic review and meta-analysis found that about 30% of patients with acute central retinal artery occlusion had evidence of a concurrent acute cerebral ischemic event on brain imaging, even when they had no neurological symptoms. For branch retinal artery occlusion, the figure was about 25%.¹⁷ And looking forward in time, retinal artery occlusion is significantly associated with future stroke risk even after accounting for other cardiovascular conditions.¹⁸

That said, a more recent study has introduced some nuance. When isolated retinal artery occlusions (those without concurrent brain involvement) were compared to minor brain strokes or transient ischemic attacks, the retinal events carried a lower risk of subsequent cerebral infarction.¹⁹ So while a retinal artery occlusion is a genuine red flag for vascular disease and warrants urgent evaluation, it is not perfectly equivalent to a brain stroke in terms of what happens next. The appropriate response is still an emergency workup, not complacency.

The Workup Gap

Given the connection to systemic vascular disease, you might expect that every person who shows up with a retinal artery occlusion would receive thorough cardiovascular screening. The reality falls short. A U.S. national trends study found that more than half of patients presenting to emergency departments with central retinal artery occlusion did not receive any stroke workup imaging.²⁰ Even in a dedicated retina clinic setting, only about half of patients completed both carotid imaging and echocardiography, the two studies needed to identify the most actionable sources of emboli.²¹

The reasons for this gap are partly structural. Eye doctors and emergency physicians may not always coordinate effectively, and patients sometimes fail to follow through on referrals. But part of the problem is awareness. Many people, and some clinicians, think of sudden vision loss as an eye problem rather than a vascular emergency. If you or someone you know experiences sudden painless vision loss in one eye, treating it with the same urgency as symptoms of a brain stroke (because it may literally be one) is the most important thing you can do.

Who Gets Eye Strokes and at What Age

Age is the single strongest non-modifiable risk factor. The prevalence of retinal vein occlusion increases steadily with each decade of life, though it can occur in younger people, especially those with clotting disorders. A large pooled analysis of population studies estimated that the overall prevalence of retinal vein occlusion varied by ethnicity: roughly 3.7 per 1,000 in white populations, 5.7 per 1,000 in Asian populations, and 6.9 per 1,000 in Hispanic populations.²² Branch retinal vein occlusion drove most of this ethnic variation; central retinal vein occlusion showed less difference across groups. Gender did not appear to affect risk.

The mechanisms may also differ across populations. A study comparing African American and Caucasian patients with retinal artery occlusions found that Caucasian patients had a much higher rate of severe carotid artery narrowing on the same side (about 41% compared to roughly 3% in African American patients), despite similar rates of visible emboli in the eye and similar overall cardiovascular risk factor profiles after adjusting for the fact that African American patients in the study were younger on average.²³ This suggests that the source of the embolus, and therefore the optimal prevention strategy, may differ depending on the patient. Carotid disease seems to be a bigger driver in some groups, while cardiac or small-vessel disease may be more important in others.

Clotting Disorders in Younger Patients

When an eye stroke occurs in someone under 50 without obvious cardiovascular risk factors, the workup shifts. In these patients, inherited or acquired clotting disorders become a more prominent consideration. Conditions like antiphospholipid syndrome, Factor V Leiden mutation, and elevated homocysteine levels can predispose the blood to clot in situations where it otherwise wouldn’t. For retinal vein occlusions specifically, clinical guidelines recommend investigating for thrombophilia in patients under 50.⁸ The yield of these tests is higher in younger patients and lower in older patients, where conventional vascular risk factors explain most cases.

Oral contraceptives and hormone replacement therapy, both of which increase clotting risk, are also considered when evaluating younger women who develop retinal vein occlusions. The combination of a mild inherited clotting tendency with an external clotting promoter like hormonal contraception can sometimes push the balance just far enough to cause an event that neither factor would trigger alone.

Retinal Ischemia as a Window Into Cardiovascular Health

The retina is the only place in the body where blood vessels can be directly observed without surgery, which makes it uniquely useful as a window into systemic vascular health. Researchers have been exploring whether subtle signs of retinal ischemia, even below the threshold that causes noticeable vision loss, could predict broader cardiovascular disease. One study found that retinal ischemic perivascular lesions (small patches of ischemic damage visible on retinal imaging) were associated with roughly a 60% higher odds of having cardiovascular disease, and people with higher cardiovascular risk scores had significantly more of these lesions than low-risk individuals.²⁴

The retina’s extreme sensitivity to blood supply disruption is partly a matter of anatomy. Computational models of the optic nerve head have shown that the single most influential factor in its oxygenation is vessel radius: even small narrowing of the tiny arterioles that supply the lamina cribrosa can dramatically reduce oxygen delivery.²⁵ This helps explain why the retina is often one of the first tissues to show damage from systemic vascular disease and why an eye stroke, even a small one, should be taken seriously as a marker of what is happening throughout the body’s vasculature.