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USMLE Step 1Neuroanatomy and lesion localisationEP 12 of 16

Cerebellar lesion localisation: vermis vs hemisphere

Name the site of a cerebellar lesion from the pattern of ataxia. The midline steadies the trunk, the hemispheres steady the limbs, and hemisphere signs stay on the same side.

  • 4:42 video
  • 3 min read
  • 10 sources
  • Published 15 Nov 2026
Video lesson · 4:42 · Watch on YouTubeSeries: Neuroanatomy & Lesion Localisation · @medservatory
Chapters (10)
By the end of this lesson

Localise a cerebellar lesion to the midline or to a hemisphere from the pattern of ataxia: truncal versus limb, and on which side.

The short version

  1. Vermis means trunk and gait: truncal ataxia, with swaying and a wide-based, staggering walk, and the limbs can be spared.
  2. Hemisphere means limb ataxia: dysmetria, intention tremor and dysdiadochokinesia, with reduced tone and sometimes slurred speech.
  3. Hemisphere signs appear on the same side as the lesion, because two crossings cancel.
  4. The flocculonodular lobe adds vertigo and nystagmus.
  5. A medial posterior inferior cerebellar artery stroke can mimic an inner-ear problem.

A staggering man with a perfect finger-to-nose

A 51-year-old man staggers as if drunk and sways even when he sits, though he has had nothing to drink. Yet he touches his nose and then the examiner's finger perfectly, with either hand. A cerebellar problem, with normal limbs: how?

The answer is cerebellar lesion localisation. The cerebellum coordinates movement and balance, and the signature of a lesion is ataxia: movement that is clumsy and poorly aimed. What matters is where the ataxia shows up, in the trunk or in the limbs, and on which side.[1][7][9]

The cerebellum from behind: vermis, hemispheres, flocculonodular lobe

Seen from behind, the vermis is the narrow strip down the middle and the hemispheres are the large lobes on either side. The flocculonodular lobe sits underneath, and it is the part tied to the vestibular system.[7][9]

The three cerebellar zones and what a lesion causes
ZoneControlsLesion gives
Vermis (midline)Trunk, posture and gaitTruncal ataxia, wide-based staggering gait
Flocculonodular lobeBalance, ocular reflexesVertigo and nystagmus
HemispheresThe limbs, with finer movement further outLimb ataxia on the same side

The midline: truncal ataxia and a wide-based gait

The midline, the vermis, with the flocculonodular lobe tucked underneath, controls the trunk, posture and gait.[2][7]

A vermis lesion gives truncal ataxia: swaying when sitting or standing, and a wide-based, staggering gait. The limbs can be spared, which is why finger-to-nose can be normal in a patient who cannot sit steady.[2][3][4]

The flocculonodular lobe adds vertigo and nystagmus.[2][9]

The hemispheres: limb ataxia, dysmetria and intention tremor

The hemispheres control the limbs, and the further out, the more skilled and fine the movement. A hemisphere lesion gives limb ataxia.[2][7]

  • Dysmetria: the finger overshoots or undershoots its target.[7][4]
  • Intention tremor: a shaking that appears on purposeful movement, as the hand approaches its target.[7][9]
  • Dysdiadochokinesia: rapid alternating movements become slow and clumsy.[4][9]
  • Tone drops, and speech can slur.[7][9]
  • Gait can be unsteady too, so check the limbs before calling it a vermis lesion.[2]

Finger-to-nose and heel-to-shin testing bring these limb signs out.[4]

The key rule: hemisphere signs are on the same side

Hemisphere signs appear on the same side as the lesion. A right cerebellar hemisphere lesion gives right-sided limb signs.[1][2][7]

The reason is a double crossing. The cerebellar output crosses to the opposite side in the midbrain, and the corticospinal tract crosses back at the pyramids, so the two crossings cancel.[5][6][8][10]

One warning: a medial PICA stroke can mimic an inner-ear problem

A stroke in the territory of the medial posterior inferior cerebellar artery, which feeds the nodulus and the lower vermis, can mimic an inner-ear problem. The picture is severe vertigo and nystagmus, with few limb signs.[2][3]

Midline versus hemisphere: two worked localisations

The rule in one line: midline means trunk, gait and balance; hemisphere means limbs, on the same side. Apply it to two patients.[2][7]

The man from the opening
  1. He sways and his gait is wide-based
  2. His limbs are normal
  3. Trunk and gait affected, limbs spared: a vermis lesion
A woman whose right hand overshoots
  1. Her right hand overshoots a target and shakes as it approaches
  2. She sits and walks without swaying, so the trunk is spared
  3. Check the side: right hand, right hemisphere
  4. Limb signs on the right, trunk spared: a lesion of the right cerebellar hemisphere

Practice question

Original · USMLE style
Your turnQB-MS-S1-NEURO-007-01

A 49-year-old man has had 2 weeks of clumsiness in his left arm and leg. His left hand overshoots a cup when he reaches for it. On finger-to-nose testing there is a left-sided intention tremor, and rapid alternating movements are slow and irregular on the left. He sits and stands without swaying. Muscle strength is normal.

Which of the following is the most likely site of the lesion?
Pick one option. Your answers stay on this device.

Common questions

How do you tell a vermis lesion from a cerebellar hemisphere lesion?

By the pattern of ataxia. A vermis lesion gives truncal ataxia, with swaying and a wide-based gait, and the limbs can be spared. A hemisphere lesion gives limb ataxia, such as dysmetria and intention tremor.

Which side are cerebellar hemisphere signs on?

The same side as the lesion. The cerebellar output crosses in the midbrain and the corticospinal tract crosses back at the pyramids, so the two crossings cancel.

What does the flocculonodular lobe do?

It is tied to the vestibular system. A lesion adds vertigo and nystagmus.

What is the difference between intention tremor and a resting tremor?

Intention tremor appears on purposeful movement, as in a cerebellar hemisphere lesion. The resting tremor of Parkinson disease appears at rest.

Can a cerebellar stroke look like an inner-ear problem?

Yes. A stroke in the territory of the medial posterior inferior cerebellar artery, which feeds the nodulus and lower vermis, can cause severe vertigo and nystagmus with few limb signs.

Transcript

Read the full narration, by chapter606 words

0:00The case: a staggering man with a perfect finger-to-nose

A 51-year-old man staggers as if drunk, and sways even when he sits, though he has had nothing to drink. Yet he touches his nose and then the examiner's finger perfectly, with either hand. A cerebellar problem, with normal limbs. How? Today: localise a cerebellar lesion to the midline or to a hemisphere from the pattern of ataxia. The cerebellum coordinates movement and balance,

0:24The cerebellum from behind: vermis, hemispheres, flocculonodular lobe

and the signature of a lesion is ataxia: movement that is clumsy and poorly aimed. Picture the cerebellum from behind: the vermis is the narrow strip down the middle, the hemispheres are the large lobes on either side, and the flocculonodular lobe sits underneath, the part tied to the vestibular system.

0:46The midline: truncal ataxia, wide-based gait, vertigo and nystagmus

Two zones matter clinically. The midline, the vermis, with the flocculonodular lobe tucked underneath, controls the trunk, posture and gait. A vermis lesion gives truncal ataxia: swaying when sitting or standing, and a wide-based, staggering gait. The limbs can be spared. The flocculonodular lobe adds vertigo and nystagmus.

1:10The hemispheres: dysmetria, intention tremor, dysdiadochokinesia

The hemispheres control the limbs, and the further out, the more skilled and fine the movement. A hemisphere lesion gives limb ataxia. The finger overshoots or undershoots its target: dysmetria. A shaking appears on purposeful movement: intention tremor. Unlike the resting tremor of Parkinson disease, it appears on movement, not at rest. Rapid alternating movements become slow and clumsy: dysdiadochokinesia. Tone drops, and speech can slur. Finger-to-nose and heel-to-shin testing bring these limb signs out.

1:49The key rule: hemisphere signs are on the same side (double crossing)

The key rule: hemisphere signs appear on the same side as the lesion. That is because the cerebellar output crosses to the opposite side in the midbrain, and the corticospinal tract crosses back at the pyramids, so the two crossings cancel.

2:05Warning: the medial PICA stroke that mimics an inner-ear problem

One warning: a stroke in the territory of the medial posterior inferior cerebellar artery, which feeds the nodulus and the lower vermis, can mimic an inner-ear problem, with severe vertigo and nystagmus but few limb signs.

2:20Midline vs hemisphere: back to our two cases

Midline means trunk, gait and balance. Hemisphere means limbs, on the same side. Back to our man. He sways, his gait is wide-based, and his limbs are normal. That is a vermis lesion. Now a woman whose right hand overshoots a target and shakes as it approaches, yet she sits and walks without swaying. Check the side: right hand, right hemisphere. Limb signs on the right, trunk spared: a lesion of the right cerebellar hemisphere.

2:53Your turn: a practice question

Your turn. A 49-year-old man has two weeks of clumsiness in his left arm and leg. His left hand overshoots a cup, finger-to-nose testing shows a left-sided intention tremor, and rapid alternating movements are slow and irregular on the left. He sits and stands without swaying, and his strength is normal. Which of the following is the most likely site of the lesion? A, left cerebellar hemisphere. B, right cerebellar hemisphere. C, cerebellar vermis. D, flocculonodular lobe. E, right primary motor cortex. Pause here and decide.

3:37The answer, one option at a time

B is out: wrong side. Cerebellar hemisphere signs are on the same side as the lesion. C is out: a vermis lesion causes truncal ataxia, and his trunk is steady. D is out: the flocculonodular lobe causes vertigo, nystagmus and imbalance, not isolated limb ataxia. E is out: a motor cortex lesion causes weakness on the opposite side, and his strength is normal. So the answer is A. Dysmetria, intention tremor and slow alternating movements on one side, with a steady trunk, localise to the cerebellar hemisphere on that same side.

4:18Recap pocket card and the next lesson

Three things to keep. One: vermis means trunk and gait, with a wide-based staggering walk. Two: hemisphere means limb ataxia, on the same side as the lesion. Three: the flocculonodular lobe adds vertigo and nystagmus. Next, brain herniation syndromes.

Sources

Numbers in the text, e.g. [1], point here
  1. Location of lesion determines motor vs cognitive consequences in cerebellar stroke. Europe PMC PMC5079414. europepmc.org/article/PMC/PMC5079414
  2. Neuro-otological aspects of cerebellar stroke syndrome. Europe PMC PMC2706413. europepmc.org/article/PMC/PMC2706413
  3. Ischemic posterior circulation stroke: a review. Europe PMC PMC3985033. europepmc.org/article/PMC/PMC3985033
  4. Ataxia in children: early recognition and clinical evaluation. Ital J Pediatr 2017, PMC5347818. europepmc.org/article/PMC/PMC5347818
  5. Cerebellum Lecture: the Cerebellar Nuclei, Core of the Cerebellum. Cerebellum 2024, PMC10951048. europepmc.org/article/PMC/PMC10951048
  6. Corticospinal vs Rubrospinal Revisited. Front Neurosci 2021, PMC8226017. europepmc.org/article/PMC/PMC8226017
  7. Wikipedia. Cerebellum. en.wikipedia.org/wiki/Cerebellum
  8. Wikipedia. Superior cerebellar peduncle. en.wikipedia.org/wiki/Superior_cerebellar_peduncle
  9. TeachMeAnatomy. The cerebellum. teachmeanatomy.info/neuro/structures/cerebellum/
  10. Gray's Anatomy, plate 764 (public domain). commons.wikimedia.org/wiki/File:Gray764.png

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: icons by DBCLS via bioicons.com, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined, resized, zones tinted): "Cerebellum dorsal view" and "Division of the cerebellum". The stick figures, bedside-test sketches, traces, crossing diagram, cards and all handwriting are drawn in code (original). No AI-generated teaching images.

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-NEURO-007.

For exam study only, not medical advice. Read the medical disclaimer.

In this series

Playlist on YouTube
← Previous · EP 11Basal ganglia direct and indirect pathways: D1, D2 and movement
  1. EP 01Upper vs lower motor neuron lesions: signs and localisation5:08
  2. EP 02Dorsal column vs spinothalamic tract: where each pathway crosses5:01
  3. EP 03Spinal cord syndromes compared: rebuild each from three tracts5:06
  4. EP 04Cranial nerve nuclei: a brainstem map by level and position5:10
  5. EP 05Brainstem crossed findings: the localisation rule4:50
  6. EP 06Medial vs lateral medullary syndrome: artery, structures and signs5:30
  7. EP 07Facial nerve palsy: why the forehead tells you the lesion4:59
  8. EP 08Stroke syndromes by artery: ACA, MCA and PCA4:41
  9. EP 09Lacunar stroke syndromes: pure motor, pure sensory and more4:54
  10. EP 10Visual pathway lesions and field defects4:42
  11. EP 11Basal ganglia direct and indirect pathways: D1, D2 and movement5:19
  12. EP 12Cerebellar lesion localisation: vermis vs hemisphere4:42
  13. EP 13Brain herniation syndromes: what is squeezed, and where4:56
  14. EP 14CSF flow and hydrocephalus: where the flow fails5:12
  15. EP 15Corticospinal tract: from cortex to anterior horn4:59
  16. EP 16Hypothalamic nuclei and their functions5:24