What is a subdural hematoma?
A subdural hematoma is blood collected beneath the dura, the tough outer covering of the brain, and outside the brain tissue itself. “Subdural hemorrhage” and “subdural bleed” are related terms you may hear.
The collection can put pressure on the brain. Its effect depends on its size, how quickly it forms, where it is located, and the person’s overall health. It may occur on one side or both sides.
Where it forms
Three membranes, the meninges, cover the brain. From outside in they are the dura, the arachnoid, and the pia. Bridging veins cross the potential space under the dura. When those veins tear, blood can spread in a crescent along the inner skull.
Acute, subacute, and chronic
These labels describe the age and appearance of the blood.
| Type | What it usually means | Why it matters |
|---|---|---|
| Acute | A recent bleed, often after head trauma. | Can worsen rapidly and require emergency surgery. |
| Subacute | A collection evolving between the acute and chronic stages. | Symptoms and scan findings determine urgency. |
| Chronic | A collection that persists or develops over weeks. | May recur; treatment can include observation, drainage, MMAE, or a combination. |
These labels do not by themselves establish how safe it is to wait. A chronic collection can still cause serious pressure or develop new bleeding.
Why it happens
A fall or other head injury can damage vessels bridging the brain and its covering. In older adults, even a minor injury can matter, and the person may not remember it.
Risk can be higher with older age, repeated falls, blood-thinning medication, heavy alcohol use, or conditions that impair clotting. Occasionally there is no clear injury.
Do not stop a blood thinner on your own. The risk of bleeding must be balanced against the reason it was prescribed. Your treating team should make that decision.
How it is diagnosed
A clinician assesses symptoms, examines neurologic function, and reviews brain imaging. A head CT is commonly the first scan. MRI can provide additional information in some situations.
The team looks at the blood collection, its effect on nearby brain structures, changes from earlier scans, and any new bleeding. Symptoms and imaging need to be considered together. Learn more on the diagnosis guide.
What happens next
Worsening symptoms or substantial pressure may require urgent drainage. Stable chronic collections may be managed with observation, drainage, embolization, or combined treatment depending on the situation.
Sources & further reading
- MedlinePlus / National Library of Medicine — Subdural hematoma
- 2026 SNIS recommendations for MMA embolization in chronic subdural hematoma
Educational content from Neurovascular Centers. Individual treatment decisions require a clinical examination and review of your imaging.