Recallab › Question bank › Anatomy › Brain › #50015
Cerebellar localisation
A 61-year-old man has a three-week history of clumsiness. On examination the finger-to-nose test shows a coarse tremor that worsens as the finger approaches the target, and rapid alternating movements are irregular, both confined to the right arm. He walks steadily and can stand with his feet together. Where is the lesion?
- correctA. Right cerebellar hemisphere
- B. Left cerebellar hemisphere
- C. Cerebellar vermis
- D. Right frontal lobe
Explanation
Key fact
The cerebellum acts on its own side. Limb signs are ipsilateral; midline structures give truncal, not limb, ataxia.
Right cerebellar hemisphere. Intention tremor and dysdiadochokinesia in the right arm, with a normal gait, localise to the right hemisphere.
Why
The hemispheres coordinate the limbs; the vermis coordinates trunk and gait.
Normal walking and normal standing therefore put the vermis in the clear.
Signs are confined to one arm, so the lesion is one hemisphere.
Cerebellar output crosses twice - once in the superior peduncle, once in the corticospinal decussation.
Two crossings cancel, so the effect appears on the same side as the lesion.
Crosses twice, so it does not cross at all
Why not the others
- B. Left cerebellar hemisphere — would produce these signs in the left arm. This is the double-crossing trap.
- C. Cerebellar vermis — causes a wide-based truncal ataxia with normal limb coordination when the patient is lying down - the reverse of this picture.
- D. Right frontal lobe — would give left-sided pyramidal weakness, not an intention tremor, and the arm here is strong.
Remember
Cerebellum crosses twice, so it does not cross at all. Right hemisphere, right arm.
Go deeper
Chronic alcohol damages the anterior superior vermis, giving gait and lower limb ataxia while the arms are relatively spared.
A midline tumour in a child, typically medulloblastoma, presents as truncal ataxia with falls, plus headache from fourth ventricle obstruction.
Snell's Clinical Anatomy by Regions, 11th ed. (Wolters Kluwer, 2024); Gray's Anatomy for Students, 5th ed. (Elsevier, 2023).