Subdural hematoma

What it means

The brain sits inside several protective layers, one of which is a tough membrane called the dura. Small veins cross the space just beneath the dura to drain blood from the brain's surface. A blow to the head, even a relatively minor one, can stretch and tear these veins, allowing blood to pool in that space. This pooled blood is called a subdural hematoma, and because it sits between the dura and the brain rather than inside brain tissue itself, it appears on a scan as a crescent-shaped area along the inner surface of the skull.

Why it appears on a CT or MRI report

A head CT is the standard first scan after any significant head injury, fall, or sudden change in alertness, precisely because it can detect this kind of bleeding quickly. Reports describe the size (thickness in millimeters), the age of the blood (acute blood looks bright, older blood looks darker and more like fluid), whether it is on one side or both, and — critically — whether it is causing "mass effect," meaning it is pushing on or shifting the brain. Radiologists also watch for a mix of bright and dark areas, which can suggest active, ongoing bleeding rather than a single past event.

What it usually means

How serious a subdural hematoma is depends heavily on its size, whether it is growing, and how much it is pressing on the brain. A thin, stable collection found incidentally, especially in an older adult with mild brain shrinkage (which stretches those bridging veins and makes small subdural bleeds more common with age or even minor bumps), may simply be watched with repeat imaging. A larger or rapidly forming collection is a medical emergency, because pressure on the brain can impair consciousness, cause weakness, or become life-threatening if it is not relieved. People on blood thinners are at higher risk of both developing one and having it grow larger than expected.

When to follow up

Any subdural hematoma should be discussed with a doctor promptly, and larger or symptomatic ones are treated as emergencies requiring hospital admission and close observation, sometimes with surgery to drain the blood and relieve pressure. Seek emergency care immediately if you or someone you are with has a worsening headache, repeated vomiting, increasing confusion or drowsiness, difficulty waking up, weakness on one side of the body, slurred speech, or seizures after a head injury — these signs can appear hours to days after the original impact, even if the person seemed fine at first. Small, stable collections are typically followed with a repeat scan within days to weeks to confirm they are not enlarging.

A plain-language way to picture it

Imagine the brain as a water balloon resting inside a hard helmet, with a few thin threads (the bridging veins) anchoring the balloon to the inside of the helmet. A jolt can snap one of those threads, and it slowly seeps blood into the gap between the balloon and the helmet, the way a leak might slowly spread a thin puddle between two nested bowls. A small puddle causes little trouble and the body can reabsorb it over time. A larger, faster leak fills that gap and starts pressing the inner bowl inward — which is why doctors act quickly to drain it before it squeezes the brain itself.