Mass effect, oedema and herniation on CT
- Finding
The skull is a closed box: when a bleed, tumour or swelling takes up room, the brain is pushed aside and can be squeezed through its openings.
What it is
Mass effect is what a space-occupying lesion does to its neighbours: sulci flatten, the nearby ventricle is squashed, the midline shifts and the CSF spaces around the brainstem (the basal cisterns) disappear. Because the skull cannot expand, once CSF and venous blood have been pushed out, pressure rises fast and brain tissue moves from one compartment to another past rigid edges: under the falx, over the tentorium or down through the foramen magnum. This is herniation, and it can kill within hours. A plain head CT shows all of this in minutes, so the reader's job is to measure the shift, look at the cisterns and name the herniation before the patient's pupils do it for them.
What it looks like
- Early mass effect: effaced sulci on one side and a compressed ipsilateral lateral ventricle.
- Midline shift: measure how far the septum pellucidum lies from a line joining the front and back attachments of the falx; several millimetres is significant.
- The opposite lateral ventricle may enlarge (trapped) when the shift blocks its foramen of Monro.
- Basal cisterns (suprasellar, ambient, quadrigeminal) that are compressed or absent mean high pressure and danger of herniation.
- Subfalcine herniation: the cingulate gyrus slides under the falx; it can compress the anterior cerebral artery.
- Uncal herniation: the medial temporal lobe bulges over the tentorial edge and fills the ipsilateral ambient and suprasellar cistern.
- Tonsillar herniation: the cerebellar tonsils descend into the foramen magnum and the cisterna magna is lost.
- Diffuse oedema: loss of grey-white differentiation, effaced sulci and cisterns, small slit-like ventricles.
Why it looks that way
- Vasogenic oedema (tumour, abscess) leaks through a damaged blood-brain barrier into the white matter: finger-like low density that spares the cortex.
- Cytotoxic oedema (infarct, hypoxia) is cell swelling in grey and white matter together, so the grey-white boundary disappears; MRI diffusion shows it early.
- Interstitial oedema in acute hydrocephalus is CSF pushed through the ventricle lining: a smooth low-density rim around the frontal horns.
- In severe diffuse hypoxic injury the hemispheres turn dark while the cerebellum and brainstem stay relatively bright (the reversal or white cerebellum sign).
- Uncal herniation stretches the third cranial nerve (dilated ipsilateral pupil) and can kink the posterior cerebral artery, causing an occipital infarct.
- Downward brainstem displacement tears small perforating arteries: Duret haemorrhages, small midline bleeds in the midbrain and pons, carry a poor prognosis.
| Herniation | What moves | CT finding | Typical clinical sign |
|---|---|---|---|
| Subfalcine | Cingulate gyrus under the falx | Midline shift, compressed ipsilateral ventricle | Leg weakness if the anterior cerebral artery is compressed |
| Uncal (lateral transtentorial) | Medial temporal lobe over the tentorium | Ipsilateral ambient and suprasellar cistern filled | Dilated ipsilateral pupil, falling consciousness |
| Central (descending transtentorial) | Diencephalon and midbrain downward | All basal cisterns effaced; possible Duret haemorrhage | Small then fixed pupils, abnormal posturing |
| Upward transtentorial | Cerebellum up through the tentorial notch | Quadrigeminal cistern effaced, posterior fossa mass | Rapid decline, vomiting, gaze problems |
| Tonsillar | Cerebellar tonsils into the foramen magnum | Cisterna magna lost, crowded foramen magnum | Neck stiffness, irregular breathing, apnoea |
Differential and pitfalls
- A normal CT does not exclude raised pressure (for example idiopathic intracranial hypertension or early diffuse injury); the exam still matters.
- Kernohan notch: the opposite cerebral peduncle is pressed against the tentorium, so the weakness appears on the same side as the lesion (a false localising sign).
- Small ventricles in a young patient can be normal; judge them together with the sulci and the cisterns.
- A bilateral, symmetrical mass (for example bilateral subdurals) can raise pressure with little or no midline shift: look at the cisterns.
- Upward herniation from a posterior fossa mass effaces the quadrigeminal cistern; downward shunting or lumbar puncture can worsen it.
- Bone flap removed (craniectomy): brain bulging through the defect is external herniation, expected to a degree; compare with the previous scan.
Next step
- Midline shift, effaced basal cisterns or any herniation: call neurosurgery and the senior at once while the airway and position are secured.
- Do not do a lumbar puncture when the CT shows mass effect, obstructive hydrocephalus or a posterior fossa lesion.
- Repeat the CT promptly if the Glasgow score falls, a pupil dilates or new weakness appears.
- Tumour or abscess suspected: MRI with contrast (or contrast CT) to find the cause once the patient is stable.
- Diffuse oedema after cardiac arrest or trauma: discuss with intensive care; the radiologist's report helps with prognosis.
Test yourself
Think of the answer, then tap the question to see it.
Which CT finding most directly warns of imminent herniation even when the midline shift is small?
Compressed or absent basal cisterns (suprasellar, ambient, quadrigeminal): the CSF spaces around the brainstem have been squeezed out.
How do vasogenic and cytotoxic oedema differ on CT?
Vasogenic oedema is finger-like white matter low density that spares the cortex (tumour, abscess); cytotoxic oedema involves grey and white matter, erasing their boundary (infarct, hypoxia).
Why does an uncal herniation cause a dilated pupil on the same side?
The medial temporal lobe slides over the tentorial edge and compresses the ipsilateral third cranial nerve, whose outer parasympathetic fibres constrict the pupil.
What are Duret haemorrhages?
Small midline bleeds in the midbrain and pons caused by downward displacement of the brainstem tearing perforating arteries; they signal severe herniation and a poor prognosis.
Elsewhere in DurgiPedia
- Intracranial hypertension syndrome
- Pupils: light reflex and abnormal pupils (Marcus Gunn, Horner, Argyll Robertson)
- Level of consciousness and the Glasgow Coma Scale
- Brain death: the ethical and legal criteria in Colombia
Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Brain Herniation (StatPearls (NCBI Bookshelf), 2026); Transtentorial Herniation (StatPearls (NCBI Bookshelf), 2023); Cerebral Edema (StatPearls (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.