A brain bleed happens when a blood vessel inside or around the brain ruptures or leaks, allowing blood to pool in places it doesn’t belong. The most common cause is long-term high blood pressure, which gradually weakens artery walls until they give way. But head injuries, blood vessel abnormalities, certain medications, drug use, and age-related changes can all trigger bleeding too.
High Blood Pressure Is the Leading Cause
Chronic high blood pressure is behind the majority of spontaneous brain bleeds. The mechanism is straightforward: years of elevated pressure damages the cells lining the inside of your arteries, making those walls stiffer and less elastic. Over time, the constant force of blood pushing through weakened vessels can cause a section of artery wall to bulge outward, forming a small balloon-like pouch called an aneurysm. That pouch can eventually burst.
Even without forming an aneurysm, high blood pressure can cause small arteries deep inside the brain to simply break open or leak. The tiny penetrating arteries that supply areas like the thalamus and the deep brain structures are especially vulnerable because they branch directly off larger, high-pressure vessels with little cushioning. About one third of these deep brain bleeds expand into the fluid-filled chambers at the center of the brain, which makes them more dangerous. This is why managing blood pressure matters so much: the damage is slow and invisible until a vessel finally fails.
Structural Abnormalities in Blood Vessels
Some people are born with or develop defects in their brain’s blood vessels that make rupture more likely. The two most important are aneurysms and arteriovenous malformations (AVMs).
A brain aneurysm is a weak, ballooned-out spot on an artery wall. Most are small and cause no symptoms for years. When one ruptures, blood spills into the space surrounding the brain, causing an intense, sudden headache often described as the worst of your life. This type of bleeding, called a subarachnoid hemorrhage, is a medical emergency.
An AVM is a tangle of arteries connected directly to veins without the normal network of tiny capillaries in between. Capillaries normally slow blood flow and reduce pressure before blood reaches the veins. Without that buffer, the veins in an AVM absorb the full force of arterial pressure, and the abnormal connections can eventually rupture. Some people with AVMs also have aneurysms, compounding the risk. AVMs tend to cause bleeding in younger adults and are often present from birth, though they may not cause problems until they rupture.
Head Injuries
Trauma is one of the most common causes of brain bleeds overall, and the type of bleeding depends on exactly which vessels get damaged. Falls and car accidents are the most frequent triggers.
A hard blow to the side of the head can fracture the temporal bone and tear an artery running just beneath the skull. Blood collects rapidly between the skull and the brain’s outer covering, forming what’s called an epidural hematoma. Because it involves arterial bleeding, pressure builds fast and symptoms can worsen within minutes to hours.
A different pattern occurs when the impact tears the small veins that bridge the gap between the brain’s surface and the skull’s inner lining. This produces a subdural hematoma, where blood collects more slowly beneath the brain’s outer covering. In older adults, these bridging veins are more fragile and stretched, which is why even a minor fall can cause a subdural bleed that develops over days or weeks. The person may not connect their worsening headaches or confusion to the original injury.
Age-Related Protein Buildup
In people over 55, a condition called cerebral amyloid angiopathy becomes an increasingly important cause of brain bleeds. Amyloid proteins, the same type implicated in Alzheimer’s disease, gradually deposit along the walls of small arteries in the brain. These deposits make the vessels stiff and brittle.
The bleeding pattern is distinctive: it tends to occur in the outer layers of the brain (the cortex) rather than in the deep structures where hypertension-related bleeds typically happen. This matters because lobar bleeds in an older person without a history of high blood pressure often point to amyloid angiopathy as the underlying cause. The condition also raises the risk of recurrent bleeds over time and is linked to cognitive decline.
Blood-Thinning Medications
Anticoagulant medications, prescribed to prevent blood clots in conditions like atrial fibrillation or after certain surgeries, carry a real but relatively small risk of brain bleeding. For people taking older blood thinners like warfarin, the annual risk of a brain bleed runs between 0.3% and 0.6%. Newer anticoagulants cut that risk roughly in half, to about 0.1% to 0.2% per year.
Those numbers sound small, but they add up over years of use, and the bleeds that do occur on blood thinners tend to be larger and harder to stop. The medication’s very purpose, preventing clotting, works against the brain’s ability to seal off a ruptured vessel. This is why doctors weigh the risk of a brain bleed against the risk of a stroke from blood clots when deciding whether to prescribe these drugs.
Stimulant Drugs and Substance Use
Cocaine and methamphetamine are significant causes of brain bleeds in younger adults who would otherwise be at low risk. These drugs cause sharp, sudden spikes in blood pressure that can overwhelm even healthy arteries. Methamphetamine in particular triggers a dose-dependent surge in blood pressure that can directly rupture the small penetrating arteries deep in the brain.
The damage goes beyond just pressure spikes. Methamphetamine also causes blood vessels in the brain to constrict, and it breaks down the blood-brain barrier, the protective lining that normally keeps blood components separated from brain tissue. Animal studies show this barrier breakdown can begin within hours of a single dose. The combination of extreme pressure, vessel spasm, and barrier damage creates a perfect setup for bleeding. Heavy alcohol use is another contributor: it both raises blood pressure over time and impairs the blood’s ability to clot.
Warning Signs of an Active Bleed
Most people experiencing a brain bleed notice a sudden, severe headache as the first symptom. Unlike a normal headache that builds gradually, this one typically hits at full intensity within seconds. Beyond the headache, bleeding inside the brain can produce:
- One-sided weakness or numbness in the face, arm, or leg
- Slurred speech or difficulty finding words
- Confusion or sudden sleepiness
- Nausea and vomiting
- Vision changes or loss of coordination
If the bleeding involves the space around the brain rather than inside it, a stiff neck and sensitivity to light are common additional symptoms. Seizures and loss of consciousness can occur with any type. The specific symptoms depend on which part of the brain is affected, since bleeding in the brainstem, for example, can disrupt breathing and heart rate and carries a mortality rate of roughly 75% within the first 24 hours.
How Brain Bleeds Are Diagnosed
When someone arrives at an emergency room with stroke-like symptoms, the first priority is determining whether the brain is bleeding or whether a clot is blocking blood flow. The treatments are opposite: clot-busting drugs that save lives in a clotting stroke could be fatal during a bleed.
A CT scan without contrast dye is the standard first step because it can detect fresh blood in the brain within minutes. Blood shows up as a bright white area on the scan, making it easy to spot. If the CT is unclear or doctors need more detail about the source of bleeding, MRI can detect blood products that CT misses by picking up the magnetic properties of deoxygenated hemoglobin. Once a bleed is confirmed, additional imaging like CT angiography can map the blood vessels to find the exact source, whether it’s an aneurysm, an AVM, or a ruptured small artery.