Epidural & Subdural Hematoma
- Both are bleeds outside the brain itself — the trick is figuring out which layer of "shrink-wrap" the blood got stuck under.
- Epidural: blood between skull and dura. Lens-shaped (biconvex), doesn't cross sutures, often arterial and fast.
- Subdural: blood between dura and the brain's surface. Crescent-shaped, crosses sutures, usually venous and slower.
- Shape and how it respects (or ignores) the skull's sutures is your single best clue for telling them apart on CT.
- Either one can squeeze the brain. Watch for midline shift and effaced spaces — that's the part that hurts people.
Picture the brain as a delicate dessert wrapped in a few layers of plastic. The outermost, leathery layer is the dura (think English: "tough mother" — it really does mean that). When you bleed between the skull and the dura, that's an epidural hematoma (EDH). When you bleed under the dura, on top of the brain, that's a subdural hematoma (SDH). Same neighborhood, different apartment. The whole skill here is reading the lease.
These are both flavors of bleeding inside the skull, so if the broad category feels fuzzy it's worth a quick detour through intracranial hemorrhage first. And nearly everything below is read on a non-contrast head CT, the workhorse of head trauma.
The shape tells the story
Here's the elegant part. The dura is glued down tightly to the skull at the sutures — those wiggly seams where skull bones meet. That single anatomical fact does most of the diagnostic work for you.
An epidural bleed has to pry the dura off the skull. The sutures act like tent stakes, so the blood can't spread past them. It piles up into a tight, biconvex lens shape — like a lemon, or a partially squeezed water balloon pressed against a wall. It bulges.
A subdural bleed is already on the inside of the dura, where there are no sutures to stop it. So it spreads thin and wide, hugging the curve of the brain in a crescent shape — like a banana, or frosting smeared along the inside of a bowl. It happily crosses suture lines because nothing's holding it back.
Who's bleeding, and how fast
The two also tend to come from different plumbing.
| Feature | Epidural (EDH) | Subdural (SDH) |
|---|---|---|
| Layer | Skull ↔ dura | Dura ↔ brain surface |
| Shape on CT | Biconvex lens | Crescent |
| Crosses sutures? | No | Yes |
| Crosses midline? | Can cross midline (dural attachments) | Limited at midline (falx) |
| Usual source | Often arterial (a torn meningeal artery) | Usually venous (torn bridging veins) |
| Classic tempo | Fast | Often slower |
The classic EDH is a torn middle meningeal artery under a skull fracture near the temple — arterial pressure means it can fill quickly. The classic SDH is torn bridging veins, the little veins that span from the brain to the dura. Low-pressure venous oozing means an SDH can build up over a longer time, sometimes from a bump the patient barely remembers.
You may have heard of a "lucid interval" with epidurals — the patient knocks their head, seems fine, then deteriorates. It's real and worth knowing, but don't treat it as a rule. Plenty of epidurals never have one, and waiting around for it is exactly the wrong move.
Age changes the color
Blood doesn't stay the same brightness on CT as it ages, and this matters a lot for subdurals (which love to be chronic).
- Acute (fresh): bright white — denser than brain. Fresh clot is packed with protein-rich material that soaks up the X-ray beam.
- Subacute (days to weeks): fades toward the gray of brain. This is the sneaky stage — an SDH that's the same shade as the brain can nearly vanish.
- Chronic (weeks+): dark, closer to the gray, watery look of cerebrospinal fluid.
The isodense subacute subdural is a classic miss. When the blood is the same gray as the brain, it doesn't shout at you — you find it by noticing indirect signs: the brain's surface grooves (sulci) pushed away from the skull, the midline nudged over, or a ventricle squashed. Hunt for the mass effect, not just the bright spot.
What actually hurts the patient
The bleed itself is rarely the lethal part. The danger is mass effect — a fixed box (the skull) with a growing blob inside it has nowhere to expand, so it shoves the brain.
The things I scan for: a midline shift (the brain's central seam pushed off-center), effaced sulci or ventricles (the normal dark spaces squeezed flat), and any sign the brain is being forced toward an exit — the start of herniation, which is the genuine emergency.
A small bleed with no mass effect and a small bleed actively shoving the midline are radically different problems. Always describe the mass effect, not just the blood.
Don't mix it up with the others
A couple of quick distinctions so you file these correctly. Subarachnoid hemorrhage sits one layer deeper — down in the cerebrospinal fluid spaces, tracing the brain's grooves rather than forming a tidy collection against the skull; the non-traumatic kind raises the question of an aneurysm. And while both EDH and SDH are usually trauma-related and overlap heavily with traumatic brain injury, they're extra-axial (outside the brain) — distinct from bleeding or bruising within the brain tissue itself.
So if you remember one thing: read the shape and ask whether it respects the sutures. Lens that stops at the seams, think epidural. Crescent that smears past them, think subdural. Then immediately look at what the bleed is doing to everything around it — because that's what decides how worried to be.