Brain bleeds happen when a blood vessel in or around the brain ruptures or leaks, allowing blood to pool where it doesn’t belong. Over 60% of spontaneous brain bleeds are caused by chronic high blood pressure, making it the single most common culprit. But trauma, blood vessel abnormalities, medications, and substance use can all trigger bleeding too. With roughly 25 cases per 100,000 people each year and a median 30-day mortality rate around 40%, understanding the causes matters.
Types of Brain Bleeds by Location
Not all brain bleeds are the same. The name depends on where the blood collects, and the cause often differs by type.
- Epidural hemorrhage: Blood pools between the skull and the brain’s tough outer covering. Almost always caused by head trauma, typically a skull fracture that tears an artery near the temple.
- Subdural hemorrhage: Blood collects just beneath that outer covering. Happens when veins bridging the brain and skull get stretched or torn, often from a blow to the head or a fall.
- Subarachnoid hemorrhage: Bleeding into the fluid-filled space surrounding the brain. Ruptured aneurysms are the classic cause.
- Intracerebral hemorrhage: Bleeding directly into the brain tissue itself. High blood pressure is the leading cause of this type.
High Blood Pressure: The Leading Cause
Chronic high blood pressure is responsible for more brain bleeds than any other single factor. Years of elevated pressure damages the tiny arteries that supply blood deep inside the brain. The vessel walls undergo degenerative changes, becoming stiff and weakened. Over time, small balloon-like bulges called microaneurysms form along these fragile vessels.
When blood pressure spikes or the weakened wall simply gives out, the vessel ruptures and blood pours into surrounding brain tissue. These bleeds tend to occur in deep brain structures, areas critical for movement, sensation, and consciousness. The damage comes not just from lost blood supply but from the expanding pool of blood compressing and destroying nearby brain cells.
Head Trauma
A hard blow to the head can cause bleeding in several ways, depending on the force and angle of impact. In epidural hematomas, a fracture of the thin temporal bone near the temple tears the artery running along its inner surface. Because this is arterial bleeding, blood accumulates rapidly and can become life-threatening within hours.
Subdural hematomas work differently. The brain shifts inside the skull during impact, stretching or tearing small veins that bridge between the brain’s surface and the skull’s inner lining. Because veins bleed more slowly than arteries, symptoms can develop gradually, sometimes over days or weeks. Older adults are especially vulnerable because the brain naturally shrinks with age, putting more tension on these bridging veins even from minor falls.
Aneurysms and Blood Vessel Malformations
A brain aneurysm is a weak, balloon-like bulge in an artery wall. Most people who have one never know it. But if the aneurysm ruptures, blood floods the space around the brain, causing a subarachnoid hemorrhage. The hallmark symptom is a sudden, explosive headache, often described as the worst headache of a person’s life.
Arteriovenous malformations (AVMs) are tangles of abnormal blood vessels where arteries connect directly to veins, bypassing the normal capillary network. Blood rushes through these connections at abnormally high pressure and speed, creating constant stress on vessel walls. AVMs are a particularly important cause of brain bleeds in younger people. The risk of rupture is highest when these malformations sit in certain locations, particularly near the base of the brain where blood pressure in the vessels tends to be higher. Some AVMs also develop aneurysms along their feeding arteries, further increasing the danger.
Amyloid Buildup in Older Adults
In people over 65, a condition called cerebral amyloid angiopathy becomes an increasingly common cause of brain bleeds. A sticky protein called beta-amyloid gradually deposits in the walls of small arteries near the brain’s surface. Over time, this buildup destroys the smooth muscle cells that give blood vessels their strength, causing the walls to thicken, narrow, split, and eventually form microaneurysms.
When these weakened vessels rupture, the bleeding typically occurs in the outer portions of the brain (called lobar hemorrhages), which distinguishes it from the deep bleeds caused by high blood pressure. Many people with this condition also have tiny “microbleeds” scattered across the brain that show up on MRI scans. Doctors use a set of validated criteria called the Boston criteria to diagnose this condition during a person’s lifetime, primarily by identifying multiple bleeds restricted to these outer brain regions with no other obvious cause.
Blood-Thinning Medications
Medications that prevent blood clots, while essential for conditions like atrial fibrillation and deep vein thrombosis, increase the risk of brain bleeds as a side effect. They don’t cause blood vessels to rupture, but they make it harder for the body to stop bleeding once it starts, even from a tiny leak that would normally seal itself.
Not all blood thinners carry equal risk. Older-generation blood thinners (vitamin K antagonists like warfarin) carry roughly four times the risk of brain hemorrhage compared to newer direct oral anticoagulants. The risk climbs further for people who already have tiny microbleeds in their brain tissue. Those with five or more existing microbleeds face a substantially elevated risk, which is why doctors sometimes weigh the benefits of anticoagulation against the bleeding danger in these patients.
Cocaine, Methamphetamine, and Other Stimulants
Stimulant drugs like cocaine and methamphetamine can trigger brain bleeds even in otherwise healthy young people. These substances flood the nervous system with stress hormones, causing sudden, dramatic spikes in blood pressure and intense constriction of blood vessels. A vessel that might have held up under normal pressure can rupture when blood pressure surges unexpectedly.
The danger isn’t limited to a single use. Chronic stimulant use causes ongoing damage to blood vessel walls through persistent high blood pressure and repeated vasoconstriction. Over time, this can lead to weakened arteries throughout the body, including the brain. Stimulant-related brain bleeds are one of the most common stroke-related causes of death in adults under 45 who use these drugs.
Other Contributing Causes
Several additional factors can cause or contribute to brain bleeds. Brain tumors, whether cancerous or benign, can erode into nearby blood vessels or develop their own fragile, leaky blood supply. Liver disease impairs the body’s ability to produce clotting factors, making spontaneous bleeding more likely. Blood disorders that reduce platelet counts or impair clotting function carry similar risks.
Hemorrhagic conversion of an ischemic stroke is another important cause. When a blood clot blocks an artery and brain tissue begins to die, the damaged blood vessels in that area can start leaking once blood flow is restored, turning a clot-based stroke into a bleeding stroke.
Warning Signs of a Brain Bleed
Symptoms depend on the location and size of the bleed, but certain red flags demand immediate emergency care: a sudden severe headache unlike anything you’ve experienced before, loss of consciousness after a head injury, vomiting, blurred vision, difficulty staying steady, confusion, slurred speech, or loss of movement on one side of the body. Pupils that appear different sizes are another warning sign.
As blood continues to accumulate and pressure builds inside the skull, symptoms tend to worsen progressively. A headache that keeps getting worse, increasing drowsiness, and seizures all suggest expanding bleeding. After any significant blow to the head, a headache that persists rather than fading is reason enough to seek emergency evaluation, even if you initially felt fine.