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Cerebellar Signs – Double-Crossing Ipsilateral — SCE Neurology MCQ

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EasyCerebrovascular DiseaseCerebellar Signs – Double-Crossing IpsilateralSCE Neurology

A 70-year-old man with a right cerebellar hemisphere lesion has right-sided limb ataxia (dysmetria, dysdiadochokinesis, intention tremor). The ataxia affects the SAME side as the lesion. Why are cerebellar signs ipsilateral?

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Correct answer: E — The cerebellum projects to the ipsilateral thalamus, then to the contralateral motor cortex, which controls the contralateral body — the double-crossing (cerebellum→thalamus then cortex→spinal cord) means the NET effect is ipsilateral

Cerebellar signs are ipsilateral to the lesion because of a double-crossing arrangement: (1) the cerebellum projects via the superior cerebellar peduncle to the contralateral (red nucleus and) thalamus (first crossing), (2) the motor cortex then projects via the corticospinal tract to the contralateral spinal cord (second crossing — pyramidal decussation). Two crossings = the net effect is ipsilateral. A: The cerebellum does not connect directly to muscles. C: They are ipsilateral. D: There are two crossings. E: The net effect is ipsilateral.

Reference: Clinical Neuroanatomy