A brain hemorrhage happens when a blood vessel inside or around the brain breaks open, releasing blood into surrounding tissue. That escaped blood damages brain cells directly and creates pressure that can injure even more tissue nearby. The causes range from chronically high blood pressure slowly weakening tiny arteries over years to a sudden head injury that tears vessels in an instant.
How High Blood Pressure Damages Brain Vessels
Chronic high blood pressure is the single most common cause of spontaneous brain hemorrhage. It works slowly and silently. Years of elevated pressure force the walls of small arteries deep inside the brain to remodel at a cellular level. The smooth, flexible lining of these vessels gradually thickens and stiffens, a process called lipohyalinosis. Proteins that normally keep vessel walls strong and elastic, like collagen and elastin, break down. Inflammation accelerates the damage as immune cells infiltrate the vessel walls.
Over time, some of these weakened arteries develop tiny balloon-like bulges. Eventually, the wall of one of these arteries gives way entirely. Blood under high pressure floods directly into the deep brain tissue, forming a clot that compresses and destroys the surrounding cells. This type of bleeding typically strikes the deeper structures of the brain rather than the outer surface.
Aneurysms and Why They Burst
A brain aneurysm is a weak, bulging spot on an artery wall, often shaped like a small berry hanging off a branch. Most people with aneurysms never know they have one. The danger comes when the wall of that bulge thins enough to tear open.
What determines whether an aneurysm ruptures has a lot to do with how blood moves through it. Aneurysms that eventually burst tend to have complex, swirling blood flow with multiple rotating currents inside them, rather than a single smooth vortex. That turbulent flow creates areas along the wall where the force of blood against the vessel lining is unusually low and constantly changing direction. Those conditions trigger inflammation, damage the inner lining of the artery, and gradually degrade the wall until it fails. In one study of ruptured versus unruptured aneurysms, 61% of ruptured aneurysms had these complex flow patterns, while 75% of unruptured ones had simple, stable flow.
When an aneurysm bursts, blood typically spills into the fluid-filled space surrounding the brain rather than deep inside it. This is called a subarachnoid hemorrhage, and its hallmark symptom is a sudden, explosive headache, often described as the worst headache of a person’s life, reaching maximum intensity within seconds.
Traumatic Injuries That Tear Vessels
A blow to the head or a sudden acceleration-deceleration force (like a car crash or fall) can tear blood vessels mechanically. Even without the skull fracturing, the brain can slam against the inside of the skull, bruising tissue and ripping small vessels. This is sometimes called a coup-contrecoup injury: the brain is damaged both at the point of impact and on the opposite side as it bounces back.
The shearing forces are especially destructive to the brain’s internal lining and small blood vessels. Depending on which vessels tear, traumatic hemorrhages can occur between the skull and the brain’s outer covering, just beneath that covering, or deep within the brain tissue itself. MRI scans detect far more of these injuries than CT scans. In one study of closed head injuries, MRI identified bleeding or tissue damage in 61 out of 62 patients, while CT found abnormalities in only 19.
Amyloid Buildup in Aging Blood Vessels
In older adults, a condition called cerebral amyloid angiopathy becomes an increasingly common cause of brain bleeds. Sticky amyloid proteins, the same type involved in Alzheimer’s disease, gradually deposit along the walls of small arteries in the brain. These deposits make the vessels brittle and prone to leaking or breaking.
Unlike hypertension-related hemorrhages, which tend to occur deep in the brain, amyloid-related bleeds happen in the outer layers of the brain (the cortex). This pattern is one way doctors distinguish between the two. Cerebral amyloid angiopathy also raises the risk of dementia independently of any large bleeds, because even small, repeated leaks damage surrounding tissue over time.
Abnormal Blood Vessel Tangles
Arteriovenous malformations (AVMs) are tangles of abnormal blood vessels where arteries connect directly to veins without the usual network of tiny capillaries in between. People are born with them, though symptoms most commonly appear between ages 20 and 40. Without treatment, an AVM carries a 2 to 4% chance of bleeding each year. That risk compounds over a lifetime, which is why these are often discovered in younger adults. An AVM that has already bled once is far more dangerous: roughly 17% of previously ruptured AVMs bleed again within a year.
Blood-Thinning Medications
Anticoagulant and antiplatelet medications reduce the blood’s ability to clot, which is their purpose for preventing strokes and heart attacks. But that same effect means that if a small vessel in the brain does break, the bleeding is harder to stop and more likely to expand. The degree of risk varies by medication. Newer blood thinners like rivaroxaban have shown higher rates of brain bleeding compared to aspirin, while apixaban appears to carry a risk closer to aspirin’s. Your overall risk depends on which medication you take, your blood pressure control, and your age.
What a Brain Hemorrhage Feels Like
Symptoms depend on where and how fast the bleeding occurs, but they almost always come on suddenly. A subarachnoid hemorrhage from a ruptured aneurysm typically announces itself with that thunderclap headache, often accompanied by nausea, vomiting, a stiff neck, vision changes, or brief loss of consciousness.
Hemorrhages deeper inside the brain may cause different symptoms depending on the area affected: sudden weakness or numbness on one side of the body, difficulty speaking, loss of coordination, or confusion. Unlike a headache that builds gradually, these neurological changes appear within minutes. Some people lose consciousness quickly if the bleeding is large or creates significant pressure.
How Doctors Identify the Bleeding
A non-contrast CT scan is usually the first test performed because it’s fast and widely available in emergency rooms. CT is good at spotting fresh blood inside the skull. However, it misses a significant number of smaller bleeds and subtle injuries, particularly after trauma. MRI is far more sensitive. In head injury patients whose CT scans looked normal or near-normal, MRI consistently revealed injuries that CT could not detect, including small hemorrhages in the brainstem and deep brain structures.
Once bleeding is confirmed, doctors use additional imaging (sometimes including angiography, which maps blood vessels in detail) to identify the source: an aneurysm, AVM, or damaged vessel from another cause.
Survival and Recovery
Brain hemorrhages are serious. In a study of 368 adults with spontaneous bleeding inside the brain, 28.3% died within 30 days. Outcomes depend heavily on the size and location of the bleed, how quickly treatment begins, and the person’s overall health. Small bleeds in non-critical areas can be survivable with good recovery, while large hemorrhages that cause significant brain compression carry much higher mortality.
For survivors, recovery is often measured in months to years rather than weeks. Many people experience lasting effects like weakness, speech difficulties, or cognitive changes. Rehabilitation, including physical, occupational, and speech therapy, plays a central role in regaining function. The brain’s ability to rewire itself means improvement can continue well beyond the first few months, though the degree of recovery varies widely from person to person.