A subdural is a type of bleeding that occurs between the brain and its protective outer covering. The full medical term is subdural hematoma, and it happens when blood collects in the thin space between two layers of tissue that surround the brain. Subdurals range from small, slow bleeds that develop over weeks to large, life-threatening emergencies that require immediate surgery.
Where the Bleeding Happens
Your brain is wrapped in three layers of protective tissue called meninges. The outermost layer, the dura mater, is a tough membrane that sits just inside the skull. Beneath it lies a thinner layer called the arachnoid mater. A subdural hematoma forms in the potential space between these two layers.
The bleeding typically starts when small veins that bridge the gap between the brain’s surface and the dura get torn. These “bridging veins” are vulnerable to stretching and tearing, especially during sudden head movements or impacts. Once a vein tears, blood leaks into the space between the dura and arachnoid, gradually pooling and pressing against the brain. As the blood accumulates and pressure inside the skull rises, it can actually force more blood out of the torn veins, creating a feedback loop that makes the bleeding worse.
Acute, Subacute, and Chronic Types
Subdural hematomas are classified by how quickly symptoms appear after the initial injury:
- Acute: Symptoms develop within 4 days of injury. These are the most dangerous, with historically reported mortality rates between 22% and 66%, though more recent data from a large trauma center found a rate closer to 14%.
- Subacute: Symptoms appear between 4 and 21 days after injury.
- Chronic: Symptoms develop more than 21 days after injury, sometimes weeks or months later. These involve slower, smaller bleeds that accumulate gradually.
The chronic type is particularly common in older adults. About 80% of chronic subdural hematoma patients are over 65. In many cases, the original head injury was so minor the person doesn’t even remember it happening.
Who Is Most at Risk
Age is the single biggest risk factor. As people get older, the brain naturally shrinks slightly within the skull. This creates more space for the brain to move around during an impact and stretches the bridging veins, making them easier to tear. Even a minor bump or fall can be enough to start a slow bleed in an older person.
Blood-thinning medications significantly increase the risk as well. In one study, 44% of patients with chronic subdural hematomas were taking some form of blood thinner before admission. Warfarin was the most commonly involved drug, used by about 28% of those patients, followed by aspirin at around 13%. Interestingly, research has found that aspirin-related bleeding tends to have better recovery outcomes than warfarin-related bleeding, likely because the two drugs affect clotting through different mechanisms.
Other risk factors include heavy alcohol use (which both increases fall risk and affects clotting) and any condition that makes blood less able to clot normally.
Symptoms to Recognize
Acute subdural hematomas typically cause severe, immediate symptoms: an intense headache, slurred speech, vision changes, and weakness on one side of the body. Nausea, vomiting, dizziness, and difficulty walking are also common. In serious cases, the person may lose consciousness, have seizures, develop breathing problems, or fall into a coma.
Chronic subdural hematomas are trickier to spot. Because the bleeding is slow, symptoms creep in gradually over weeks or months. Memory loss, confusion, personality changes, and disorientation may be the only signs, which is why chronic subdurals are sometimes mistaken for dementia in older adults.
One pattern worth knowing about is the “lucid interval.” A person may seem completely fine immediately after a head injury, with no symptoms at all, then develop problems days later as the blood slowly accumulates and starts compressing the brain.
How It’s Diagnosed
A CT scan of the head is the standard diagnostic tool. On the scan, a subdural hematoma appears as a distinctive crescent-shaped mass hugging the inner surface of the skull. This shape helps distinguish it from an epidural hematoma (bleeding above the dura), which typically looks more like a lens or egg shape. Unlike epidural bleeds, subdurals can spread freely across the brain’s surface because they aren’t restricted by the natural attachment points of the dura to the skull.
Doctors look at specific features on the CT scan to guide treatment decisions, including the thickness of the blood collection and how far it has pushed the brain’s midline structures to one side. A midline shift of 5 millimeters or more is one threshold that factors into whether surgery is needed.
Treatment Options
Small subdural hematomas that aren’t causing significant symptoms can sometimes be managed without surgery. The body can gradually reabsorb the blood on its own, and doctors monitor with repeat CT scans to make sure the collection isn’t growing.
Larger or symptomatic subdurals require surgical drainage. The two most common procedures are burr hole drainage and craniotomy. In a burr hole procedure, the surgeon drills one or two small holes in the skull and flushes the blood out with irrigation. In a craniotomy, a larger section of skull is temporarily removed to give direct access to the blood collection and its surrounding membranes.
For chronic subdural hematomas, research comparing the two approaches found that burr hole drainage had clear advantages. Patients who had burr hole surgery spent about 79 minutes in the operating room compared to 129 minutes for craniotomy. Their average hospital stay after surgery was about 7 days versus 10 days. Post-operative complications occurred in 21% of burr hole patients compared to 55% of craniotomy patients. And 66% of burr hole patients went home directly after discharge, compared to 52% of craniotomy patients.
Perhaps most notably, only about 7% of burr hole patients needed a second operation, while 24% of craniotomy patients did. Despite the theoretical advantage of a wider opening to clear out all the blood, the less invasive approach produced better outcomes across nearly every measure.
Recovery and Recurrence
Recovery depends heavily on the type and severity of the subdural. Acute subdurals from major trauma carry the highest risk. Even with modern care at specialized trauma centers, about 14% of patients with acute traumatic subdural hematomas do not survive their hospitalization, and this rate is consistent whether or not surgical evacuation is performed.
Chronic subdural hematomas have a much better prognosis overall, but recurrence is a real concern. About 12% of patients experience a return of the blood collection after surgical drainage. This is one reason why follow-up imaging and monitoring are standard after treatment. Patients on blood thinners face additional complexity, as doctors must balance the risk of the subdural recurring against the original reason the blood thinner was prescribed.
For those who recover, the timeline varies widely. Some people bounce back within weeks, while others, particularly older adults or those who experienced significant brain compression, may deal with lingering cognitive or physical effects for months.