A subdural hematoma is a collection of blood that forms between the brain and its protective outer covering, called the dura. It happens when blood vessels tear, usually after a head injury, and blood pools in that narrow space. As the blood accumulates, it presses against the brain, and depending on how fast it builds up, a subdural hematoma can range from a slow-developing condition that causes gradual confusion over weeks to a life-threatening emergency that requires surgery within hours.
How the Bleeding Starts
Your brain sits inside the skull surrounded by layers of protective tissue. The outermost layer, the dura, is thick and tough. Between the dura and the brain’s surface, small veins called bridging veins stretch across the gap, draining blood from the brain into channels within the dura. These veins are delicate, and when the head takes a hard impact, the brain shifts inside the skull while the dura stays fixed in place. That sudden movement can tear one or more bridging veins.
Once a vein tears, low-pressure venous blood slowly seeps out and spreads along the surface of the brain. In some cases, a small artery on the brain’s surface gets damaged instead, which produces faster, more dangerous bleeding. Either way, the blood has nowhere to drain, so it collects in an expanding layer that pushes down on brain tissue.
Acute, Subacute, and Chronic Types
Subdural hematomas are classified by how quickly symptoms appear after the injury. Acute subdural hematomas develop within hours. They’re caused by severe trauma, like a car crash or a major fall, and produce rapid neurological decline. This is the most dangerous form. The mortality rate for acute subdural hematomas ranges from 50 to 90 percent, and only about 20 to 30 percent of patients recover full or partial brain function.
Subacute subdural hematomas show symptoms days to a few weeks after the initial injury. Chronic subdural hematomas develop over weeks to months, sometimes after a head bump so minor the person doesn’t remember it. In chronic cases, the blood slowly accumulates as tiny, repeated bleeds occur and the body’s attempts to reabsorb the blood fall behind. Symptoms creep in gradually and can mimic dementia or stroke, making diagnosis tricky.
Who Is Most at Risk
Older adults face the highest risk by a wide margin. The brain naturally shrinks with age, which stretches the bridging veins across a wider gap and makes them more vulnerable to tearing, even from minor jolts. The incidence of chronic subdural hematomas jumps dramatically with age: roughly 3 to 4 cases per 100,000 people per year under age 65, compared to as high as 286 per 100,000 in people over 80.
Blood-thinning medications add another layer of risk. Anticoagulants (like warfarin or direct oral anticoagulants) are associated with a roughly 2 to 3 times higher chance of rebleeding after a chronic subdural hematoma. Interestingly, antiplatelet medications like aspirin don’t appear to significantly increase rebleeding risk on their own, though combining blood thinners may compound the danger. People who drink heavily are also at elevated risk because chronic alcohol use can cause both brain shrinkage and clotting problems.
Falls are the leading trigger in older adults. For younger people, the typical causes are high-speed accidents, sports injuries, or assaults.
Symptoms to Recognize
The symptoms depend heavily on whether the hematoma is acute or chronic. An acute subdural hematoma typically causes a severe, unrelenting headache, nausea and vomiting, slurred speech, and rapid deterioration in consciousness. Because the bleeding is fast, the person may go from alert to unresponsive within hours.
Chronic subdural hematomas present very differently. Symptoms build slowly and can include:
- Persistent headache that may be mild at first but worsens over time
- Confusion or memory problems that family members may attribute to aging
- Balance issues or difficulty walking
- Weakness on one side of the body
- Vision changes
- Personality or behavioral shifts
In both types, increasing pressure inside the skull is the central danger. As the blood pool grows, it compresses brain tissue, and if that pressure isn’t relieved, it can lead to permanent brain damage, loss of consciousness, or death.
How It’s Diagnosed
A CT scan of the head is the primary diagnostic tool. It can show the size of the blood collection, where it’s located, and how much it’s pushing the brain out of its normal position (called midline shift). On a CT scan, an acute hematoma appears bright white because fresh blood is dense, while a chronic hematoma appears darker as the blood breaks down over time. In some cases, an MRI provides additional detail, particularly for smaller or subacute collections that may be harder to see on CT.
Treatment: Observation vs. Surgery
Not every subdural hematoma requires surgery. Small hematomas, those thinner than about 5 millimeters on a CT scan with no significant brain shift and no neurological symptoms, can sometimes be monitored with repeat imaging and close observation. The body may gradually reabsorb the blood on its own.
Surgery becomes necessary when the hematoma is large enough to compress the brain. Guidelines call for surgical removal when the blood collection is thicker than 10 millimeters or causes more than 5 millimeters of midline shift, regardless of how alert the patient is. Even smaller hematomas need surgery if the patient’s level of consciousness is declining, their pupils are unequal or non-reactive, or pressure inside the skull is climbing.
The type of surgery depends on the situation. For acute hematomas, surgeons typically remove a section of skull to access and drain the blood directly, then repair or cauterize any bleeding vessels they find. For chronic hematomas, less invasive approaches are common: a small hole drilled through the skull (called a burr hole) allows the older, liquefied blood to drain out. Some patients need a temporary drain left in place for a day or two to prevent re-accumulation.
Recovery and Long-Term Outlook
Recovery varies enormously depending on the type and severity. Chronic subdural hematomas that are caught early and drained have a generally favorable prognosis. Many patients, especially those who were neurologically intact before surgery, return to their previous level of function within weeks to a few months. The main concern after treatment is recurrence: the hematoma comes back in roughly 10 to 20 percent of cases, particularly in older adults or those on anticoagulants, sometimes requiring a second procedure.
Acute subdural hematomas carry a far grimmer outlook. The high mortality rate reflects the severity of the underlying brain injury, not just the blood collection itself. Among survivors, many face long-term challenges including persistent cognitive difficulties, physical disability, personality changes, and an elevated risk of seizures. Rehabilitation involving physical therapy, occupational therapy, and neuropsychological support plays a central role in recovery for these patients, and the process can stretch over months to years.
Age, overall health before the injury, how quickly treatment begins, and the degree of brain compression all influence outcomes. For acute cases, the speed from injury to surgery is one of the strongest predictors of survival.