Bleeding in the brain happens when a blood vessel inside the skull ruptures or leaks, allowing blood to pool in or around brain tissue. The causes range from chronic conditions like high blood pressure to sudden events like head injuries or ruptured aneurysms. High blood pressure is the single most common cause, but the full picture involves several distinct mechanisms, each affecting different types of blood vessels in different parts of the brain.
High Blood Pressure: The Leading Cause
Chronic high blood pressure is responsible for more brain bleeds than any other condition. Over years, elevated pressure forces the tiny arteries deep inside the brain to remodel at a cellular level. The walls of these small vessels thicken, stiffen, and gradually lose the proteins (elastin and collagen) that keep them flexible and strong. This process, called lipohyalinosis, leaves the vessels brittle and prone to rupture.
When one of these weakened vessels finally gives way, high-pressure blood pushes directly into the surrounding brain tissue. These bleeds tend to occur in the deeper structures of the brain, which is why they’re sometimes called “deep hemorrhages.” Chronic inflammation plays a role too: the vessel walls become infiltrated by immune cells that further degrade their structural integrity from the inside out.
Keeping blood pressure under control is the most effective way to prevent this type of brain bleed. Current guidelines from the American Heart Association recommend a target below 130/80 for people who have already had a stroke or transient ischemic attack, and below 140/90 for first-time stroke prevention.
Ruptured Brain Aneurysms
A brain aneurysm is a balloon-like bulge in a blood vessel wall. Many people live their entire lives without knowing they have one. But when an aneurysm ruptures, it releases blood into the space between the brain and the thin tissues covering it, a type of bleed called a subarachnoid hemorrhage. This is the most common cause of that specific type of brain bleed.
Ruptures happen most often in people between ages 55 and 60, with women in that age range at particularly elevated risk. Several factors increase the chance of both developing and rupturing an aneurysm: smoking, heavy alcohol use, high blood pressure, and stimulant drugs like cocaine or methamphetamine. Having a first-degree blood relative (parent, sibling, or child) with a brain aneurysm also raises your risk. Certain inherited connective tissue disorders, including Ehlers-Danlos syndrome, Marfan syndrome, and polycystic kidney disease, make aneurysms more likely as well.
Amyloid Protein Buildup in Older Adults
In people over 65, a condition called cerebral amyloid angiopathy (CAA) becomes an increasingly important cause of brain bleeds. In CAA, a sticky protein called beta-amyloid gradually deposits along the walls of small arteries near the brain’s surface. Over time, these deposits weaken the vessel walls and make them fragile.
Unlike the deep bleeds caused by high blood pressure, CAA-related bleeds tend to occur in the outer layers of the brain, closer to the skull. They can range from tiny microbleeds, sometimes detected only on MRI, to large hemorrhages. CAA is a significant contributor to stroke-related death and cognitive decline in older populations, and the severity is graded on a scale from no deposits to widespread buildup throughout the brain’s surface arteries. There is no cure for CAA itself, which makes blood pressure management and careful medication choices especially important for people who have it.
Head Injuries
Physical trauma to the head can cause bleeding in several distinct locations, each with its own characteristics.
- Epidural bleeding occurs between the skull and the outermost membrane covering the brain. It’s most common in children and often results from a skull fracture that tears an artery running along the inside of the skull bone. Because arterial blood is under high pressure, this type of bleed can expand rapidly.
- Subdural bleeding happens in the layer just beneath that outer membrane. It typically results from torn bridging veins, the small vessels that connect the brain’s surface to the surrounding membranes. Subdural bleeds can be acute (developing within hours) or chronic (accumulating slowly over weeks, which is more common in older adults or people on blood thinners).
- Contusions are small hemorrhages within the brain tissue itself, usually found near the skull at the point of impact or on the opposite side of the brain (the “countercoup” injury, where the brain rebounds against the skull). These sometimes occur alongside subdural or subarachnoid bleeding.
Blood-Thinning Medications
Medications that reduce blood clotting are essential for many people with conditions like atrial fibrillation, but they carry a real risk of brain hemorrhage. A long-term study tracking 546 patients found that 180 eventually developed a spontaneous brain bleed. Among warfarin users, about 41% developed a hemorrhage over the follow-up period. Newer anticoagulants showed a comparable overall rate, but the bleeds tended to occur earlier, with a median time to hemorrhage of 2.5 years compared to about 4 years for warfarin.
Aspirin carried the lowest risk in the same study, with roughly 11% of users developing a bleed. This doesn’t mean blood thinners should be avoided. For most people prescribed these medications, the risk of a clot-related stroke without them is greater than the risk of a bleed with them. But the numbers illustrate why doctors carefully weigh the balance for each patient.
Vascular Malformations
Some people are born with abnormal tangles of blood vessels in the brain called arteriovenous malformations, or AVMs. Normally, blood flows from arteries through a network of tiny capillaries before reaching veins. In an AVM, blood bypasses the capillaries entirely, rushing directly from high-pressure arteries into thin-walled veins that aren’t built to handle that force.
The annual risk of an AVM bleeding is around 2% to 3% per year. That may sound small, but it compounds over a lifetime. A 20-year-old with an undetected AVM faces a substantial cumulative risk over the following decades. Many AVMs are discovered only after they bleed, though some are found incidentally on brain imaging done for other reasons.
Cocaine and Other Stimulant Drugs
Cocaine and methamphetamine can trigger brain bleeds even in young, otherwise healthy people. Cocaine blocks the normal recycling of certain signaling chemicals in the nervous system, flooding the body with norepinephrine and dopamine. The result is a rapid spike in blood pressure and heart rate, sometimes within minutes of use. These sudden hypertensive surges can rupture both normal and already-weakened blood vessels in the brain.
There’s a second mechanism at work too. Cocaine and its breakdown products are potent vasoconstrictors, meaning they cause blood vessels to clamp down tightly. This constriction can cut off blood flow to parts of the brain, creating small areas of tissue damage. When blood flow returns, these damaged areas can convert into hemorrhages. Amphetamines work through similar pathways, which is why stimulant drug use of any kind is a recognized risk factor for brain bleeds across all age groups.
How These Causes Overlap
In practice, brain bleeds rarely have a single neat explanation. An older adult with both high blood pressure and amyloid deposits in their vessel walls faces compounding risks. Someone on anticoagulants who also has uncontrolled hypertension is at higher risk than someone with either factor alone. A person with an undiagnosed AVM who uses cocaine could trigger a rupture that might otherwise not have happened for years.
The location of a brain bleed often gives doctors a strong clue about its cause. Deep hemorrhages in the center of the brain point toward high blood pressure. Bleeds near the brain’s surface in someone over 65 suggest amyloid angiopathy. Bleeding in the spaces surrounding the brain raises suspicion for a ruptured aneurysm. And bleeding that conforms to the shape of the skull’s inner surface, especially after trauma, points toward epidural or subdural hemorrhage. Understanding the cause matters because it directly shapes both the immediate treatment and the long-term strategy for preventing another bleed.