[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Central nervous system](https://durgical.com/en/general-pathology/areas/cns/) › Cerebral edema, hydrocephalus and herniation

# Cerebral edema, hydrocephalus and herniation

- Mechanism

The skull is a closed box: swelling, extra fluid or a mass raises pressure until brain is pushed through openings, which can kill.

Updated Nov 12, 2026

## Key points

- Vasogenic edema: leaky blood-brain barrier, extracellular fluid, mostly white matter (tumours, abscess, contusion).
- Cytotoxic edema: cells swell from energy failure, intracellular fluid (ischemia, hypoxia, toxins); interstitial edema follows obstructive hydrocephalus.
- Hydrocephalus: noncommunicating (blocked ventricles) vs communicating (poor absorption by arachnoid villi); hydrocephalus ex vacuo is volume loss, not raised pressure.
- Subfalcine herniation compresses the anterior cerebral artery; uncal (transtentorial) herniation compresses the third nerve and posterior cerebral artery.
- Duret hemorrhages in the midbrain and pons follow downward herniation; tonsillar herniation compresses the medulla and stops breathing.

## What it is

Brain, blood and cerebrospinal fluid share a fixed space. When one grows (edema, hydrocephalus, hemorrhage, tumour), the others compensate only briefly, then pressure rises steeply and blood flow falls. Brain then shifts across the rigid dural folds and the foramen magnum. These herniations compress vessels and nerves and are a common final pathway of death in many brain diseases.

## Structure

- Edematous brain: soft, heavy, flattened gyri with narrow sulci and slit-like ventricles.
- Hydrocephalus: enlarged ventricles with thin overlying cortex; in infants before suture fusion the whole head enlarges.
- Herniation leaves grooves and necrosis on the uncus or tonsils, and streaky hemorrhages in the midline brainstem.

## Step by step

- A mass, edema or extra CSF takes up space inside the skull.
- CSF and venous blood are pushed out first; pressure stays nearly normal while reserve lasts.
- Reserve runs out: small extra volume now causes a steep rise in intracranial pressure.
- Perfusion pressure falls, adding ischemia and more cytotoxic edema (a vicious circle).
- Brain shifts under the falx, through the tentorial notch or into the foramen magnum.
- Compression of nerves, arteries and the brainstem gives coma, pupil changes and respiratory arrest.

## Herniation syndromes

| Type | What moves | What is compressed | Typical sign or lesion |
| --- | --- | --- | --- |
| Subfalcine (cingulate) | Cingulate gyrus under the falx | Anterior cerebral artery branches | Leg weakness from frontal infarct |
| [Transtentorial (uncal)](https://durgical.com/en/conditions/raised-intracranial-pressure-and-brain-herniation/) | Medial temporal lobe over the tentorium | Third cranial nerve, posterior cerebral artery, opposite cerebral peduncle | Fixed dilated pupil on the side of the mass; occipital infarct; Kernohan notch |
| [Central (downward)](https://durgical.com/en/conditions/coma-and-brain-death/) | Diencephalon and brainstem | Perforating branches of the basilar artery | Duret hemorrhages in midbrain and pons |
| Tonsillar | Cerebellar tonsils into the foramen magnum | Medulla | Respiratory and cardiac arrest |

## Clinical notes

- Raised pressure gives headache, vomiting, papilledema and falling consciousness; the Cushing response is hypertension with bradycardia.
- Normal-pressure hydrocephalus of older adults gives gait disturbance, urinary incontinence and dementia.

## Memory aid

The skull is a full suitcase: push in one more shirt and something squeezes out at the zipper, the falx, the notch or the hole at the bottom.

## Test yourself

Think of the answer, then tap the question to see it.

## Which type of cerebral edema surrounds a metastasis?

Vasogenic edema: the tumour's leaky vessels break the blood-brain barrier and fluid spreads through the white matter.

## Why does uncal herniation dilate the pupil on the side of the mass?

The herniating uncus compresses the third cranial nerve, whose outer parasympathetic fibres constrict the pupil.

## What is a Duret hemorrhage?

A linear bleed in the midbrain or pons from tearing of perforating arteries when the brainstem is pushed downward.

## What is hydrocephalus ex vacuo?

Ventricles that look large because brain tissue has been lost (atrophy, infarcts), with normal pressure.

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## Elsewhere in DurgiPedia

- [Cerebral blood flow, brain metabolism and intracranial pressure](https://durgical.com/en/physiology/cerebral-blood-flow-brain-metabolism-and-intracranial-pressure/)
- [CSF and the blood–brain barrier](https://durgical.com/en/physiology/csf-and-the-blood-brain-barrier/)
- [Blood–brain barrier, choroid plexus and cerebrospinal fluid](https://durgical.com/en/histology/blood-brain-barrier-choroid-plexus-and-cerebrospinal-fluid/)
- [Ventricles and cerebrospinal fluid](https://durgical.com/en/embryology/ventricles-and-cerebrospinal-fluid/)
- [Intracranial hypertension syndrome](https://durgical.com/en/clinical-exam/intracranial-hypertension-syndrome/)

## Connections

## Related 6

- [Cerebral infarction](https://durgical.com/en/general-pathology/cerebral-infarction/)
- [Brain and spinal cord trauma](https://durgical.com/en/general-pathology/brain-and-spinal-cord-trauma/)
- [Epidural vs subdural hematoma](https://durgical.com/en/general-pathology/epidural-vs-subdural-hematoma/)
- [Brain malformations: forebrain and posterior fossa](https://durgical.com/en/general-pathology/brain-malformations-forebrain-and-posterior-fossa/)
- [CNS tumours: an overview](https://durgical.com/en/general-pathology/cns-tumours-an-overview/)
- [Edema and effusions: the four mechanisms](https://durgical.com/en/general-pathology/edema-and-effusions-the-four-mechanisms/)

## Related conditions 9

- [Raised intracranial pressure and brain herniation](https://durgical.com/en/conditions/raised-intracranial-pressure-and-brain-herniation/)
- [Hydrocephalus](https://durgical.com/en/conditions/hydrocephalus/)
- [Idiopathic intracranial hypertension](https://durgical.com/en/conditions/idiopathic-intracranial-hypertension/)
- [Coma and brain death](https://durgical.com/en/conditions/coma-and-brain-death/)
- [Traumatic brain injury](https://durgical.com/en/conditions/traumatic-brain-injury/)
- [Intracerebral haemorrhage](https://durgical.com/en/conditions/intracerebral-haemorrhage/)
- [Brain abscess](https://durgical.com/en/conditions/brain-abscess/)
- [Glioblastoma](https://durgical.com/en/conditions/glioblastoma/)
- [Oculomotor (third) nerve palsy](https://durgical.com/en/conditions/oculomotor-nerve-palsy/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Hydrocephalus](https://www.ncbi.nlm.nih.gov/books/NBK560875/) (StatPearls (NCBI Bookshelf), 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/cerebral-edema-hydrocephalus-and-herniation/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/cerebral-edema-hydrocephalus-and-herniation/