🧠 Brain Herniation Syndromes
When ICP takes the path of least resistance — the four herniation types, what gets crushed, and the clinical signs that tell you exactly what is happening.
Brain Herniation — The Four Syndromes
The skull is a rigid box. When ICP rises beyond the compensatory capacity of the brain, the only exit routes are through the fixed apertures of the skull base and the gaps in the rigid dural folds. Brain tissue — under pressure — is forced through whichever opening offers least resistance. What gets crushed determines the clinical signs.
The Anatomical Framework — The Dural Compartments
The dura mater forms two major rigid folds that subdivide the intracranial space:
🔵 Falx Cerebri — a vertical sickle-shaped fold descending in the interhemispheric fissure between the two cerebral hemispheres. Herniation under this = subfalcine herniation.
🟣 Tentorium Cerebelli — a horizontal shelf separating the supratentorial space (cerebral hemispheres) from the infratentorial space (cerebellum, brainstem). Has a central oval opening — the tentorial notch — through which the brainstem passes. Herniation through this opening = uncal / central herniation.
🔴 Foramen Magnum — the only exit from the posterior fossa into the spinal canal. The last resort for escaping pressure = tonsillar herniation.
Anatomical Diagram — The Four Herniation Pathways
A coronal (front-on) cross-section showing the dural compartments and where each herniation type occurs. Labels on the left describe what gets displaced; labels on the right show the consequence.
Quick Reference Master Table
| Type | Structure Displaced | What Gets Crushed | Classic Clinical Sign | Common Cause |
|---|---|---|---|---|
| Uncal (Transtentorial) | Uncus of temporal lobe | CN III (blown pupil) + cerebral peduncle (hemiparesis) | Ipsilateral fixed dilated pupil + contralateral hemiparesis | EDH, unilateral SDH, temporal tumour |
| Subfalcine (Cingulate) | Cingulate gyrus under falx | Anterior Cerebral Artery (ACA) compressed | Contralateral leg weakness (ACA supplies medial motor cortex) | Unilateral SDH, hemisphere tumour |
| Tonsillar (“Coning”) | Cerebellar tonsils through foramen magnum | Medulla oblongata — vital cardiorespiratory centres | Cushing’s Triad → Respiratory arrest → Death | Any severe raised ICP; iatrogenic LP in raised ICP |
| Central (Transtentorial) | Diencephalon / bilateral hemispheres | Sequential brainstem compression — midbrain → pons → medulla | Cheyne-Stokes → Decorticate → Decerebrate → Flaccid → Death | Diffuse cerebral oedema, large bilateral masses |