Traumatic Brain Injury
- Traumatic brain injury (TBI) is an umbrella term, not one disease — it spans skull fractures, bleeds, bruised brain, and torn axons.
- The non-contrast head CT is the workhorse: fast, everywhere, and great at finding fresh blood and fractures.
- Split injuries into extra-axial (blood outside the brain) and intra-axial (damage inside the brain itself) — it organizes everything.
- The scary part is rarely the first bruise; it's the swelling, the expanding bleed, and the brain getting squeezed afterward.
- A "normal" CT does not equal a normal brain — diffuse axonal injury can hide from CT entirely.
Someone's head met something it shouldn't have — a windshield, a sidewalk, a hockey puck with ambitions. Now they're in front of you and everyone wants to know one thing: is the brain okay? "Traumatic brain injury" sounds like a single diagnosis, but it's really a whole genre of bad days happening at once. Your job is to sort them into tidy buckets so nothing gets missed.
Why we reach for CT first
The brain lives in a sealed box, and when it's bleeding or swelling, that box has no room to spare. We need answers now, which is why the first study is almost always a non-contrast head CT. It's quick, it's in every emergency department on Earth, and fresh blood lights up bright white against gray brain. No contrast needed — acute blood is already dense enough to show off on its own.
Think of acute blood on CT as a fresh coffee stain on a gray carpet: obvious, bright, and unmistakable. Over days that stain fades — bleeds become less dense as they age — which is a handy clock for guessing how old an injury is.
The two big questions a head CT answers: Is there blood? and Is the midline where it belongs? Everything else is detail.
The great divide: outside vs inside the brain
The single most useful sorting move is asking where the blood sits relative to the brain itself.
| Bucket | What it means | Classic players |
|---|---|---|
| Extra-axial | Blood collecting around the brain, in the layers between brain and skull | Epidural and subdural hematoma, subarachnoid hemorrhage |
| Intra-axial | Damage inside the brain substance itself | Contusions, intraparenchymal hemorrhage, diffuse axonal injury |
Extra-axial bleeds are covered in depth on their own page, so here I'll keep my focus on what trauma does to the brain inside — plus the bone around it. For the full menu of blood-in-the-head, the intracranial hemorrhage page is your map.
Contusions: the brain's bruises
A contusion is exactly what it sounds like — a bruise of brain tissue, usually where the soft brain slammed against the hard inner ridges of the skull. They love the undersides of the frontal lobes and the front of the temporal lobes, because that's where the skull is bumpiest. On CT they look like patchy areas of mixed bright (blood) and dark (swelling), often described as "salt and pepper."
Contusions characteristically blossom — they look small on the first scan and bigger on the one a day later. So a worrisome mechanism plus a "mild" first CT still earns a careful watch. The early picture is a preview, not the final cut.
You'll also hear about coup and contrecoup injury: the brain gets bruised both where it was hit (coup) and on the opposite side (contrecoup), because it sloshes inside the skull and bangs into the far wall. One impact, two bruises — physics being rude.
Diffuse axonal injury: the one CT loves to hide
Here's the humbling one. Diffuse axonal injury (DAI) happens when rapid rotation and deceleration stretch and shear the brain's long wiring — the axons — at the junctions between gray and white matter. The patient can be deeply unconscious while the CT looks almost boringly normal, because the damage is microscopic tearing, not a big puddle of blood.
A near-normal CT in a patient whose consciousness doesn't match it is a red flag, not reassurance. DAI is best seen on MRI — especially gradient-based sequences that hunt for tiny specks of blood — so when the story and the CT disagree, the brain may be telling you to keep looking.
Don't forget the bone
The brain gets the headlines, but the skull is part of the story. Always flip to bone windows — the same CT data, displayed to make bone bright and crisp — to hunt for fractures. A fracture that crosses a groove where an artery runs is a setup for an arterial bleed, and a fracture through the skull base can let air in or spinal fluid leak out.
Read every head CT twice: once on brain windows for the soft stuff, once on bone windows for fractures. Two different displays of the same scan, two different jobs.
What actually kills: mass effect and herniation
Almost nothing in TBI hurts people because of the initial injury alone — it's the secondary injury afterward: swelling, expanding blood, and rising pressure inside that sealed box. As pressure climbs, the brain gets shoved out of its compartments, the dreaded herniation syndromes. So on every trauma scan, beyond spotting blood, you're tracking the consequences:
- Is the midline pushed across?
- Are the normal fluid spaces (ventricles, basal cisterns) being squashed or erased?
- Is one side of the brain effaced and swollen?
These are the findings that turn a CT read into a phone call.
Effaced basal cisterns — the fluid spaces at the base of the brain disappearing — are a sign of dangerously high pressure and impending herniation. That's a "pick up the phone before you finish dictating" finding.
The one-paragraph version
TBI isn't a diagnosis; it's a checklist. Get a non-contrast head CT, then ask in order: blood around the brain (extra-axial), blood or bruising inside it (intra-axial), fractures on bone windows, and — most importantly — is the brain being squeezed? A clean CT with a not-so-clean patient means look harder, often with MRI. In TBI, the first scan is the opening scene; the swelling writes the ending. (For how this fits into the whole battered-patient workup, see the polytrauma CT.)