A subdural hematoma happens when blood collects between the brain and its outer protective covering, usually because small veins bridging the gap between the brain and skull tear and bleed. The most common cause is head trauma, but the severity of that trauma varies enormously. A young person typically needs a significant blow to the head, while an older adult can develop one from something as minor as a stumble.
How Bleeding Starts in the Subdural Space
Your brain floats inside the skull, tethered to the surrounding membranes by small “bridging veins” that stretch across the space between the brain’s surface and the tough outer lining called the dura. When the head experiences a sudden acceleration or deceleration, the brain moves at a different speed than the skull. That mismatch puts shearing force on these bridging veins, and if the force is strong enough, they tear. Blood then pools in the subdural space, the narrow gap between the brain and the dura, and begins pressing on brain tissue.
The bleeding source is almost always venous or capillary rather than arterial, which means blood accumulates more slowly than in some other types of brain bleeds. This is why symptoms can develop gradually over days or even weeks, depending on the type of subdural hematoma.
Acute Subdural Hematoma: High-Impact Trauma
Acute subdural hematomas form rapidly after a forceful injury, typically within hours. The blood forms a solid clot that presses on the brain and can become life-threatening quickly. Roughly 11% to 20% of people with significant head injuries develop an acute subdural hematoma.
The most common scenarios include falls, motor vehicle collisions, and physical assaults. Any blow that causes the head to accelerate or stop abruptly can generate enough force to rupture bridging veins. In teenagers and young adults, these injuries skew toward males, largely because of higher rates of accidental trauma.
Chronic Subdural Hematoma: Slow Bleeds
Chronic subdural hematomas develop over weeks to months, often from injuries so minor the person doesn’t remember them. Instead of a solid clot, the blood becomes liquefied and encapsulated by thin membranes that the body forms around it. These membranes contain fragile new blood vessels that are prone to re-bleeding on their own, which is why chronic subdural hematomas can gradually enlarge without any additional injury.
The outer membrane surrounding the hematoma has many tiny, fragile vessels that frequently bleed in multiple spots. This creates a cycle: the body tries to wall off the blood, the new membrane leaks, and the collection slowly grows. Both the body’s clotting and clot-dissolving systems become overactive inside the hematoma cavity, which prevents the blood from forming a stable clot and resolving on its own. Current estimates place chronic subdural hematoma incidence between 1.7 and 20.6 per 100,000 people, with rates climbing sharply in older populations.
Why Older Adults Are at Higher Risk
Age is the single biggest risk factor for chronic subdural hematoma, and the reason comes down to brain shrinkage. As the brain naturally loses volume with age, it pulls away from the skull, creating more space between the two. This extra distance stretches the bridging veins tighter, like rubber bands pulled closer to their breaking point. A trivial bump or jolt that a younger person’s veins would easily absorb can snap these overstretched vessels.
The physics are straightforward: as the brain shrinks, negative pressure develops inside the skull. The greater the atrophy, the longer the distance bridging veins must span, and the more vulnerable they become. On whichever side of the brain the shrinkage is more pronounced, the veins stretch further, which is why subdural hematomas sometimes form on one side more than the other. Once the distance exceeds a certain threshold, even the supportive tissue strands between the brain and its membranes start to separate, essentially creating a ready-made pocket where blood can collect.
Blood Thinners and Medication Risk
Blood-thinning medications significantly raise the risk of subdural hematoma because they impair the body’s ability to stop bleeding once a vein tears. Among these medications, older-generation oral anticoagulants like warfarin carry the highest risk. A meta-analysis published in the American Heart Association’s journal Stroke found that people taking warfarin-type anticoagulants had three times the odds of developing a subdural hematoma compared to those on antiplatelet drugs like aspirin or clopidogrel.
Newer anticoagulants that directly target specific clotting factors appear to carry lower risk. Compared to warfarin, direct clot-blocking medications reduced the odds of subdural hematoma by roughly 45%, and factor Xa inhibitors reduced it by about 65%. This doesn’t mean newer blood thinners are risk-free, but the difference is meaningful. If you take any type of blood thinner, even a minor head bump deserves attention, because the bleeding that would normally stop on its own may continue slowly enough to cause problems days or weeks later.
Chronic Alcohol Use
Heavy, long-term alcohol use contributes to subdural hematoma through two separate pathways that compound each other. First, chronic alcohol consumption causes the brain to shrink, creating the same stretched-bridging-vein vulnerability seen in older adults. Second, alcohol damages the liver, which is responsible for producing most of the proteins your blood needs to clot properly. The combination of easier tearing and impaired clotting makes chronic drinkers especially prone to subdural hematomas, even from minor injuries they may not recall.
Causes in Infants and Children
In newborns, subdural hematomas most often result from birth itself, particularly deliveries involving vacuum extraction devices or other mechanical assistance. The newborn skull is soft and the bridging veins are delicate, making them vulnerable to the compressive and stretching forces of delivery.
After the newborn period through age two, the picture shifts. Research published in the journal Pediatric Neurosurgery found that nonaccidental head injury was the most common cause of subdural bleeding in infants under one year old, present in over 82% of abuse-related deaths. In what is commonly called shaken baby syndrome, violent back-and-forth shaking causes the infant’s brain to move in the opposite direction of the skull. Because the baby’s bridging veins are fragile and the brain moves with significant inertia relative to its size, the veins tear repeatedly with each motion. Accidental falls and blunt injuries account for the remaining cases in this age group.
Less Common Causes
Not every subdural hematoma comes from an obvious head injury. Several less common conditions can create the right circumstances for bleeding.
- Cerebrospinal fluid leaks. When spinal fluid leaks out through a tear or defect in the membranes surrounding the spinal cord, the brain sags downward inside the skull. This pulls on bridging veins much like brain atrophy does. A study of 165 patients with spontaneous low spinal fluid pressure found that about one in five developed a subdural hematoma. These leaks can be caused by connective tissue disorders like Marfan syndrome, disc herniations that puncture the dura, or previous spinal procedures.
- Connective tissue disorders. Conditions that affect the structural proteins in your body, particularly those involving abnormal fibrillin or elastin, can weaken the membranes around the brain and spinal cord. This weakness makes the dura more prone to tearing and leaking fluid, indirectly setting the stage for subdural bleeding.
- Acute-on-chronic bleeding. People who already have a small, stable chronic subdural hematoma are vulnerable to sudden worsening. Even a minor slip can tear either the bridging veins or the fragile new vessels in the membrane surrounding the old blood collection, causing fresh acute bleeding on top of the chronic hematoma. Because the clotting system inside the hematoma cavity is already disrupted, this new blood often fails to clot normally.
Overdrainage of a brain shunt, placed to manage conditions like hydrocephalus, can also lower pressure inside the skull enough to stretch bridging veins and trigger bleeding. Rarely, a direct connection can form between the spinal fluid space and nearby veins, draining fluid continuously and producing the same low-pressure effect.