Brain bleeds have several possible causes, but the two most common are head trauma and chronic high blood pressure. Together, these account for the majority of cases. Other causes include ruptured aneurysms, blood vessel abnormalities, blood-thinning medications, and age-related changes in blood vessel walls. The type and severity of the bleed depends on where in the brain it occurs and what triggered it.
Four Types of Brain Bleeds
Not all brain bleeds are the same. They’re classified by location, and each type tends to have different causes and different outcomes.
Epidural bleeds happen between the skull and the brain’s outer protective layer. These are almost always caused by a blow to the head, typically to the temple area, where a skull fracture tears an artery running along the inside of the skull.
Subdural bleeds occur just beneath that outer layer. They happen when small blood vessels stretching between the brain and skull get torn. Head trauma is the most common trigger, but they can also occur spontaneously in people on blood thinners or with clotting disorders.
Subarachnoid bleeds involve bleeding into the fluid-filled space surrounding the brain. These are the type most associated with ruptured aneurysms, though they also result from trauma.
Intracerebral bleeds occur within the brain tissue itself. High blood pressure is the leading cause here, though blood vessel abnormalities, medications, and age-related vessel disease also play a role. On a CT scan, this type typically shows up as a dense bright spot in the deep structures of the brain, the cerebellum, or the back of the brain.
High Blood Pressure: The Leading Non-Traumatic Cause
Chronic high blood pressure is the single biggest risk factor for a brain bleed that isn’t caused by an injury. Over years, elevated pressure forces the walls of small arteries in the brain to remodel at the cellular level. The vessel walls thicken, stiffen, and lose their elasticity as the structural proteins that keep them flexible break down. Inflammation accelerates the damage, with immune cells infiltrating the vessel walls and weakening them further.
Eventually, tiny balloon-like weak spots called microaneurysms form along these damaged arteries. When pressure spikes, or sometimes with no obvious trigger at all, one of these weak points gives way. Blood surges into the surrounding brain tissue under high pressure, destroying cells and creating swelling. These bleeds tend to occur deep in the brain, in areas supplied by the smallest, most vulnerable arteries.
Head Trauma
Trauma is the most common cause of brain bleeds overall. A blow to the head, a fall, a car accident, or any sudden impact or rapid acceleration can tear blood vessels inside the skull. The type of bleed that develops depends on exactly which vessels are damaged and where.
A direct hit to the temple can fracture the thin bone there and sever an artery, causing a fast-developing epidural bleed. A hard fall might stretch and snap the small bridging veins between the brain and skull, producing a subdural bleed that develops more slowly. Violent shaking or whiplash-type forces can cause bleeding on the brain’s surface or tiny scattered hemorrhages deep in the tissue that may not even show up on a standard CT scan but are visible on MRI.
Ruptured Brain Aneurysms
A brain aneurysm is a weak, balloon-like bulge in a blood vessel wall. Most people with aneurysms never know they have one, but if it ruptures, blood floods the space around the brain, causing what’s often described as “the worst headache of my life,” a sudden thunderclap that peaks within seconds.
Size matters significantly for rupture risk. Aneurysms smaller than 7 millimeters have a rupture risk under 1% per year, while larger ones carry substantially higher lifetime risk. A Finnish study tracking patients over their lifetimes found that the risk varied dramatically based on a combination of factors. Women with large aneurysms (7 mm or more) had a 73% lifetime rupture risk, compared to 18% for men with the same size. Smoking amplified the danger further: women who smoked and had large aneurysms reached a 100% lifetime rupture risk in that study. Even small aneurysms carried a 20% to 30% lifetime risk depending on sex.
Blood Vessel Abnormalities
Some people are born with structural defects in their brain’s blood vessels that can bleed without warning. The most significant of these is an arteriovenous malformation, or AVM, a tangle of abnormal arteries and veins that connect directly to each other without the normal network of tiny capillaries in between. Because capillaries normally slow blood flow and reduce pressure, AVMs create high-pressure channels that are prone to rupturing.
AVMs can bleed at any age, but younger patients and women with certain structural features in their malformations, such as thinner vessel walls and smaller draining veins, may face a higher risk of hemorrhage. These bleeds are often the first sign that someone has an AVM at all.
Blood-Thinning Medications
Anticoagulant medications, commonly called blood thinners, increase the risk of a brain bleed 7 to 10 times compared to people not taking them. For patients already at higher stroke risk (who are the ones typically prescribed these drugs), the absolute risk of a brain bleed reaches roughly 1% per year.
Blood thinners don’t cause the vessel to break in the first place. Instead, they make it harder for the body to stop even minor bleeding. A small leak that the body would normally seal off quickly can expand into a life-threatening hemorrhage when clotting is impaired. This is why people on blood thinners need careful monitoring, and why even a seemingly minor head bump warrants attention.
Age-Related Vessel Disease
In older adults, a condition called cerebral amyloid angiopathy becomes an increasingly important cause of brain bleeds. It involves deposits of a protein called beta-amyloid building up in the walls of small arteries and capillaries near the brain’s surface. Over time, these deposits cause the vessel walls to thicken, the inner channel to narrow, and smooth muscle cells to die off. The vessels become brittle and prone to forming microaneurysms.
The bleeds caused by this condition tend to occur in the outer layers of the brain (the lobes), which distinguishes them from the deep bleeds caused by high blood pressure. They often start as tiny “microbleeds” that may not cause obvious symptoms, but each one signals increasing vessel fragility. The risk of a major bleed climbs with the number of microbleeds present: patients with 1 to 2 prior microbleeds had about a 14% to 17% chance of a significant hemorrhage within three years, while those with 6 or more had a 51% chance. This condition is also linked to progressive cognitive decline.
Symptoms of an Active Brain Bleed
Symptoms vary depending on the bleed’s location and speed, but the hallmarks include a headache that keeps getting worse, vomiting, increasing drowsiness, confusion, and dizziness. Slurred speech, weakness or paralysis on one side of the body (opposite the side of the bleed), and unequal pupil sizes are particularly telling signs. As bleeding continues and pressure builds inside the skull, seizures and loss of consciousness can follow.
Some bleeds, especially slow subdural ones, can develop over days or even weeks, with symptoms that come and go or gradually worsen. Others, like a ruptured aneurysm, produce catastrophic symptoms within seconds. Any combination of sudden severe headache, confusion, and neurological changes like vision problems or trouble walking is a medical emergency.
How Brain Bleeds Are Diagnosed
A CT scan of the head is the standard first test when a brain bleed is suspected. It’s fast, widely available, and nearly 100% sensitive at detecting fresh bleeding within the first 6 to 24 hours. For traumatic injuries, CT is almost always the starting point in the emergency department.
MRI is more sensitive for detecting very small bleeds, tiny scattered hemorrhages from concussive injuries, and older bleeds that CT might miss. Specialized MRI sequences can pick up microbleeds invisible on CT, which is particularly useful for diagnosing conditions like cerebral amyloid angiopathy or evaluating the full extent of a traumatic injury. Follow-up imaging with either CT or MRI is often repeated over the following hours and days to check whether the bleeding is expanding or whether swelling around the bleed is worsening.
Outcomes and Recovery
Brain bleeds are serious. For intracerebral hemorrhage specifically, about 16% of patients die within 30 days. Among survivors at 90 days, roughly 29% achieve functional recovery, meaning they regain enough independence to manage daily life. The remaining 54% of all patients are left with moderate to severe disability. Outcomes depend heavily on the bleed’s size, location, how quickly it’s treated, and the patient’s overall health beforehand.
Epidural bleeds, when caught quickly and treated surgically, can have excellent outcomes because the brain tissue itself may not be damaged. Subdural bleeds have more variable outcomes depending on how much underlying brain injury occurred. Subarachnoid hemorrhage from a ruptured aneurysm carries high mortality, but survivors who make it through the critical first two weeks often recover meaningful function. In every case, the speed of diagnosis and treatment is one of the strongest predictors of outcome.