Brain herniation syndromes: what is squeezed, and where
Brain herniation syndromes are told apart by the structure compressed and the signs it produces: subfalcine, uncal, central, tonsillar and upward, plus the Kernohan notch and Cushing triad.
Chapters (12)
Distinguish subfalcine, uncal, central and tonsillar herniation by the structures compressed and the resulting signs.
The short version
- Subfalcine herniation is the cingulate gyrus pushed under the falx; it can squeeze the anterior cerebral artery and cause leg weakness.
- Uncal herniation is the uncus sliding over the tentorial edge: a dilated pupil on the side of the lesion and weakness on the opposite side.
- The Kernohan notch is a false localising sign: weakness on the same side as the lesion.
- Central and tonsillar herniation squeeze the brainstem, ending in coma and then failure of breathing.
- Cushing triad is the late warning that herniation may be imminent.
The rigid box: falx cerebri, tentorium and the tentorial notch
The skull is a rigid box, divided by two main folds of dura. The falx cerebri runs between the two hemispheres. The tentorium cerebelli separates the cerebrum from the cerebellum, leaving a gap for the midbrain called the tentorial notch.[1][3]
When pressure rises, brain is pushed past a fold or through an opening. That shift is a herniation, and brain herniation syndromes are named after the part of the brain that moves or the fold or opening it passes.[1]
Subfalcine herniation: the cingulate gyrus under the falx
In subfalcine herniation the cingulate gyrus is pushed under the falx. It is the commonest type.[1][6]
It can squeeze the anterior cerebral artery, so think of leg weakness, usually on the opposite side.[1][6] On its own it does not cause pupil or eye movement signs.
Uncal herniation: pupil, eye, visual field and limbs
Uncal herniation is the one the exam loves. The uncus, the medial edge of the temporal lobe, slides over the tentorial edge.[1][3] It compresses three structures that lie close together.
| Structure compressed | Sign | Side of the sign |
|---|---|---|
| Oculomotor nerve | Dilated, sluggish pupil first, then the eye turns down and out | Same side as the lesion |
| Posterior cerebral artery | Occipital cortex infarct: homonymous hemianopia | Opposite side |
| Cerebral peduncle | Weakness of the arm and leg (corticospinal fibres) | Opposite side |
The parasympathetic fibres run on the outside of the oculomotor nerve, so they are compressed first. The first sign is therefore a dilated, sluggish pupil on the same side, followed by the down-and-out eye.[1][4]
The peduncle carries the corticospinal fibres above their crossing, so the weakness is on the opposite side.[5][6]
The Kernohan notch: a false localising sign
There is one trap. The shifted midbrain can push the opposite peduncle against the tentorial edge. That is the Kernohan notch: weakness on the same side as the lesion, a false localising sign.[5]
Central and tonsillar herniation: the brainstem under pressure
Central, or downward transtentorial, herniation pushes the diencephalon and midbrain down through the notch. Stretched basilar branches tear and bleed, the Duret haemorrhages, in the midbrain and pons. The patient loses consciousness and shows abnormal posturing.[1][6]
Tonsillar herniation drops the cerebellar tonsils through the foramen magnum, where they press on the medulla, which holds the respiratory and cardiac centres. Breathing stops.[1][6]
This explains the video's opening case: a man with a large cerebellar tumour has a worsening headache, becomes drowsy, then stops breathing. The tonsils were squeezed through the foramen magnum onto the medulla.
A fifth, rarer type is upward herniation: a mass in the back of the skull pushes the cerebellum and midbrain up through the notch.[1][6]
Brain herniation syndromes compared
| Type | What is squeezed | Key signs |
|---|---|---|
| Subfalcine | Cingulate gyrus under the falx; anterior cerebral artery | Leg weakness, usually opposite side |
| Uncal | Uncus over the tentorial edge; oculomotor nerve, posterior cerebral artery, peduncle | Same-side dilated pupil, down-and-out eye; opposite hemianopia and weakness |
| Central | Diencephalon and midbrain pushed down the notch | Falling consciousness, abnormal posturing, Duret haemorrhages |
| Tonsillar | Cerebellar tonsils through the foramen magnum; medulla | Breathing stops |
| Upward | Cerebellum and midbrain pushed up the notch | Cause: a mass in the back of the skull (posterior fossa) |
Cushing triad: the late warning
Whatever the type, rising pressure on the brainstem can bring Cushing triad: high systolic blood pressure with a wide pulse pressure, a slow heart rate, and irregular breathing.[2][6]
It is the late warning that herniation may be imminent.[2]
Practice question
Original · USMLE styleA 74-year-old man is seen 2 days after a fall. Imaging shows a large right-sided subdural hematoma. Over the past hour he has become drowsy. His right pupil is 7 mm and does not react to light. The right eye is deviated down and out. He has weakness of the left arm and leg.
Common questions
What are the main brain herniation syndromes?
Subfalcine, uncal, central and tonsillar, with a rarer upward type. Each is defined by the structure that is pushed past a fold or through an opening.
What are the signs of uncal herniation?
A dilated, sluggish pupil on the same side as the lesion, then the eye turns down and out. Weakness of the arm and leg appears on the opposite side, and an opposite homonymous hemianopia can occur.
What is the Kernohan notch?
A false localising sign in which the shifted midbrain pushes the opposite peduncle against the tentorial edge, causing weakness on the same side as the lesion.
What is Cushing triad?
High systolic blood pressure with a wide pulse pressure, a slow heart rate and irregular breathing. It is the late warning that herniation may be imminent.
Why does tonsillar herniation stop breathing?
The cerebellar tonsils drop through the foramen magnum and press on the medulla, which holds the respiratory and cardiac centres.
Transcript
Read the full narration, by chapter640 words
0:00The puzzle: a tumour, then no breathing
A man with a large cerebellar tumour has a worsening headache, then becomes drowsy, and then stops breathing. What was squeezed, and where? Today: tell the herniation syndromes apart by what is squeezed and what the patient shows. The skull is a rigid box,
0:16The rigid box: falx cerebri, tentorium, tentorial notch
divided by two folds of dura. The falx cerebri runs between the two hemispheres. The tentorium cerebelli separates the cerebrum from the cerebellum, with a gap for the midbrain called the tentorial notch. When pressure rises, brain is pushed past a fold or through an opening. That shift is a herniation.
0:40Subfalcine herniation and the anterior cerebral artery
First, subfalcine. The cingulate gyrus is pushed under the falx. It can squeeze the anterior cerebral artery, so think of leg weakness, usually on the opposite side.
0:52Uncal herniation: pupil, eye, field and limbs
Second, uncal, the one the exam loves. The uncus, the medial edge of the temporal lobe, slides over the tentorial edge. It squeezes the oculomotor nerve. The parasympathetic fibres run on its outside, so the first sign is a dilated, sluggish pupil on the same side, then the eye turns down and out. It squeezes the posterior cerebral artery, which can infarct the occipital cortex and cause an opposite homonymous hemianopia. And it presses the cerebral peduncle, which carries the corticospinal fibres, so the opposite arm and leg are weak. Dilated pupil on the side of the lesion, weakness on the other side.
1:38The Kernohan notch trap
There is one trap. The shifted midbrain can push the opposite peduncle against the tentorial edge. That is the Kernohan notch: weakness on the same side as the lesion, a false localising sign.
1:52Central herniation and Duret haemorrhages
Third, central, or downward transtentorial. The diencephalon and midbrain are pushed down through the notch. Stretched basilar branches tear and bleed, the Duret haemorrhages, in the midbrain and pons. The patient loses consciousness and shows abnormal posturing.
2:11Tonsillar herniation and the medulla
Fourth, tonsillar. The cerebellar tonsils drop through the foramen magnum and press on the medulla, which holds the respiratory and cardiac centres. Breathing stops. That is our man with the cerebellar mass.
2:25Upward herniation
A fifth, rarer type is upward: a mass in the back of the skull pushes the cerebellum and midbrain up through the notch. Whatever the type, rising pressure on the brainstem brings Cushing triad:
2:37Cushing triad: the late warning
high systolic blood pressure with a wide pulse pressure, a slow heart rate, and irregular breathing. That is the late warning that herniation may be imminent. Your turn.
2:50Your turn: a subdural haematoma with a blown pupil
A 74-year-old man is seen 2 days after a fall. Imaging shows a large right-sided subdural haematoma. Over the last hour he has become drowsy. His right pupil is seven millimetres wide and does not react to light, and the right eye is deviated down and out. He has weakness of the left arm and leg. Which of the following best explains these findings? A, central transtentorial herniation. B, subfalcine herniation. C, tonsillar herniation. D, uncal herniation. E, upward cerebellar herniation. Pause here and decide.
3:36The options, one at a time, and the answer
A is out: central herniation brings falling consciousness with bilateral, midline brainstem signs, not an early one-sided dilated pupil with opposite weakness. B is out: subfalcine herniation squeezes the anterior cerebral artery, so it gives leg weakness without pupil or eye signs. C is out: tonsillar herniation compresses the medulla and stops breathing and circulation, without a third nerve palsy. E is out: upward herniation comes from a mass in the back of the skull, and his bleed is above the tentorium. So the answer is D. The right uncus is pressing the right oculomotor nerve and the right cerebral peduncle, giving a blown right pupil and left weakness.
4:25Three things to keep and the next lesson
Three things to keep. One: subfalcine is the cingulate gyrus under the falx, and uncal is the uncus over the tentorium, with a same-side blown pupil and opposite weakness. Two: central and tonsillar herniation squeeze the brainstem, ending in coma and then failure of breathing. Three: Cushing triad is the late warning. Next, CSF flow and hydrocephalus.
Sources
Numbers in the text, e.g. [1], point here- Brain herniation. Wikipedia. en.wikipedia.org/wiki/Brain_herniation
- Cushing reflex. Wikipedia. en.wikipedia.org/wiki/Cushing_reflex
- Law et al. Malays J Med Sci 2026 (tentorial incisura anatomy and uncal herniation). europepmc.org/article/PMC/PMC13739295
- Hutchinson pupil, Int J Emerg Med 2026. europepmc.org/article/PMC/PMC13015004
- Kernohan notch phenomenon; Cureus 2025. europepmc.org/article/PMC/PMC6957800 · europepmc.org/article/PMC/PMC11795214
- Herniation syndromes and clinical staging. europepmc.org/article/PMC/PMC12138751 · europepmc.org/article/PMC/PMC11610721 · europepmc.org/article/PMC/PMC13518026 · europepmc.org/article/PMC/PMC12548535 · europepmc.org/article/PMC/PMC12162276
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined, labelled): "Cerebrospinal fluid" (sagittal brain, flow arrows removed), "Brain section" (coronal), "Circle of Willis". Dura mater plate (falx cerebri and tentorium cerebelli, "Falxcerebri") and midbrain section ("Gray711", Henry Vandyke Carter, Gray's Anatomy, 1918): public domain, via Wikimedia Commons. Original diagrams: herniation arrows, glows and the tentorial edge, eye and pupil sketches, visual-field sketch, decorticate figure, schematic respiration trace and Cushing monitor. No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-NEURO-013.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Upper vs lower motor neuron lesions: signs and localisation5:08
- EP 02Dorsal column vs spinothalamic tract: where each pathway crosses5:01
- EP 03Spinal cord syndromes compared: rebuild each from three tracts5:06
- EP 04Cranial nerve nuclei: a brainstem map by level and position5:10
- EP 05Brainstem crossed findings: the localisation rule4:50
- EP 06Medial vs lateral medullary syndrome: artery, structures and signs5:30
- EP 07Facial nerve palsy: why the forehead tells you the lesion4:59
- EP 08Stroke syndromes by artery: ACA, MCA and PCA4:41
- EP 09Lacunar stroke syndromes: pure motor, pure sensory and more4:54
- EP 10Visual pathway lesions and field defects4:42
- EP 11Basal ganglia direct and indirect pathways: D1, D2 and movement5:19
- EP 12Cerebellar lesion localisation: vermis vs hemisphere4:42
- EP 13Brain herniation syndromes: what is squeezed, and where4:56
- EP 14CSF flow and hydrocephalus: where the flow fails5:12
- EP 15Corticospinal tract: from cortex to anterior horn4:59
- EP 16Hypothalamic nuclei and their functions5:24