Brain Herniation Syndromes
- The skull is a sealed box. When something inside swells, bleeds, or grows, the brain has nowhere to go but out — squeezing through the few openings it can find.
- Herniation is a shift, and the shift is the diagnosis. Look for things crossing the midline or sliding through gaps they shouldn't.
- It is the consequence of a problem (bleed, tumor, swelling), not a problem on its own — so always hunt for the cause that's doing the pushing.
- The deadly ones squash the brainstem, which runs your breathing and heartbeat. This is a "call the surgeon now" finding, not a "mention it at the end" finding.
Imagine packing a suitcase that's already full, then trying to stuff in a winter coat. Something has to give — a zipper bulges, socks squirt out a gap, the whole thing strains. Your skull is that overstuffed suitcase, except it can't even bulge. It's rigid bone with a fixed volume, and inside it the brain, blood, and cerebrospinal fluid (CSF) share every cubic centimeter. Add anything new — a clot, a tumor, swollen tissue — and the only way to make room is to push the brain somewhere else. That somewhere is a herniation syndrome.
The one idea behind all of them
Here's the rule that ties the whole topic together: the brain shifts toward the path of least resistance. There are only a handful of openings and stiff edges inside the skull, so there are only a handful of directions the brain can be shoved. Learn the doorways, and the syndromes name themselves.
Two stiff folds do most of the dividing. The falx cerebri is a vertical curtain hanging down the midline between the two hemispheres. The tentorium cerebelli is a horizontal shelf separating the big upper brain from the cerebellum and brainstem below. The herniations are basically "which fold did the brain get pushed past?"
Don't memorize the syndromes as a list to regurgitate. Memorize the anatomy of the doorways. Once you can picture the falx and the tentorium, every herniation is just "the brain went under, around, or through this thing."
The lineup, doorway by doorway
| Type | What gets pushed where | The tell on imaging |
|---|---|---|
| Subfalcine | Brain slides under the falx, across the midline | Midline structures shoved to the opposite side; the ventricle on one side gets squashed |
| Uncal (a type of transtentorial) | The inner temporal lobe (uncus) slips over the tentorial edge | That basal cistern space next to the brainstem gets effaced; brainstem looks shoved |
| Central transtentorial | The whole upper brain presses straight down through the tentorial notch | Brainstem pushed downward, basal cisterns crowded out |
| Tonsillar | Cerebellar tonsils squeeze down through the foramen magnum | Tonsils dangling below the bottom of the skull, crowding the brainstem |
| External / transcalvarial | Brain pushes out through a skull defect (e.g., after surgery or fracture) | Brain bulging beyond the bone edge |
Subfalcine is usually the first domino. Something on one side pushes the brain under that midline curtain, and the midline drifts. The casualty here can be an artery that runs along the falx — pinch it, and you can cause a stroke on top of the original problem. Cruel.
Uncal is the famous one for a reason. As the temporal lobe creeps over the tentorial edge, it leans on the brainstem and on the nerve that controls the pupil — which is why a classic clinical sign is a blown (dilated) pupil on the same side. Imaging shows the breathing room around the brainstem disappearing.
Why this is a true emergency
Notice where most of these doorways lead: toward the brainstem. That little stalk runs your breathing, your heart rate, your level of consciousness — the non-negotiable stuff. Squeeze it and the patient deteriorates fast.
Tonsillar herniation is the basement exit. The only hole at the very bottom of the skull is the foramen magnum, where the brainstem becomes the spinal cord. When pressure builds, the cerebellar tonsils get extruded down through it like toothpaste, clamping the brainstem against bone.
This is the reason we get nervous about doing a lumbar puncture in someone with high intracranial pressure and a mass. Drop the pressure at the bottom and you can suck the brainstem down through the foramen magnum. The herniation findings on a head CT are part of how we decide it's not safe.
How not to miss it
The good news: herniation is a shift, and shift is visible. On a non-contrast head CT — your first stop in any acute brain emergency (see the approach to the head CT) — train your eye on two things:
- The midline. Is it actually in the middle? Follow the septum pellucidum between the ventricles. If it's slid sideways, something is pushing.
- The cisterns — the fluid-filled moats of CSF that normally cushion the brainstem. They should look like crisp dark spaces. When they get crowded out and effaced, the brainstem is in trouble.
Don't stop at "I found the bleed" and forget to grade the consequences. The clinically urgent line in your report is often the mass effect — the shift, the effaced cisterns, the trapped ventricle — not just the lesion itself. A small bleed with brainstem compression is scarier than a big one without.
And always remember herniation is a symptom, not the disease. Behind every shift is a culprit doing the pushing: an intracranial hemorrhage, a swelling ischemic stroke, a brain tumor, an abscess, or backed-up CSF in hydrocephalus. Find the shift, then find what's causing it — and say both, loudly, because somebody needs to make room in that suitcase before the brainstem pays the bill.