A 58-year-old man with a 30-year history of heavy alcohol use presents with progressive gait disturbance. On examination, he walks with a wide-based, unsteady gait and cannot stand upright without support, but his finger-to-nose test and rapid alternating movements are relatively preserved. Imaging reveals selective degeneration of the anterior superior aspect of the structure marked **B** in the diagram. Which of the following best explains the predominant clinical finding in this patient?

Explanation

## Why "Truncal ataxia due to disruption of axial posture and gait control by the spinocerebellum" is right The structure marked **B** is the cerebellar vermis, which comprises the spinocerebellum and is responsible for control of truncal and axial posture and gait. Chronic alcoholism causes selective degeneration of the anterior superior vermis through a thiamine-dependent mechanism. This results in the classic presentation of truncal ataxia—a wide-based, unsteady "drunken sailor" gait with inability to maintain upright posture without support—while appendicular coordination (finger-to-nose, rapid alternating movements) remains relatively preserved until late stages. This dissociation between truncal and limb coordination is pathognomonic for vermian lesions. (Gray's Anatomy 42e Ch 22; Harrison 21e Ch 437) ## Why each distractor is wrong - **Ipsilateral intention tremor and dysmetria due to hemispheric cerebellar dysfunction**: Cerebellar hemispheric lesions (marked **C**) cause appendicular ataxia with intention tremor, dysmetria, and past-pointing—the opposite of what this patient exhibits. His limb coordination is preserved; his gait is impaired. - **Vertigo and nystagmus due to vestibulocerebellar involvement**: The flocculonodular lobe (marked **A**) is the vestibulocerebellum and controls balance and eye movements. Lesions here cause vertigo and nystagmus, not truncal ataxia with preserved limb coordination. - **Obstructive hydrocephalus with headache due to mass effect on the fourth ventricle**: While medulloblastomas of the vermis can cause obstructive hydrocephalus, this patient has degenerative disease from chronic alcohol use, not a mass lesion. The clinical picture is pure truncal ataxia without signs of raised intracranial pressure. **High-Yield:** Vermis lesions → truncal ataxia with wide-based gait and preserved limb coordination; hemispheric lesions → appendicular ataxia with intention tremor and dysmetria. [cite: Gray's Anatomy 42e Ch 22; Harrison 21e Ch 437]