A 58-year-old male with a 20-year history of alcohol use disorder presents with progressive gait instability and difficulty walking in a straight line. MRI brain shows selective atrophy of the structure marked **B** in the sagittal view. On examination, he has a wide-based gait and incoordination of the legs, but his upper limb coordination and speech are relatively preserved. Which of the following best describes the functional zone affected in this patient?

Explanation

## Why Spinocerebellum is right The structure marked **B** (cerebellar vermis) is the midline component of the spinocerebellum, which also includes the intermediate hemispheres. The spinocerebellum receives proprioceptive input via the spinocerebellar tracts and is responsible for gait control and proximal posture. Lesions affecting the vermis (as in alcoholic cerebellar degeneration, which classically affects the anterior superior vermis) produce gait ataxia and leg incoordination while relatively sparing fine distal limb coordination and speech — exactly the clinical picture described. This is the hallmark presentation of vermial/paravermial cerebellar pathology (Sutton Radiology, cerebellar functional anatomy). ## Why each distractor is wrong - **Vestibulocerebellum**: While the posterior vermis is part of the vestibulocerebellum, lesions here produce truncal ataxia, nystagmus, and vestibular signs — not the selective gait and leg ataxia with preserved arm coordination seen here. The anterior superior vermis (affected in alcoholic degeneration) is spinocerebellar, not vestibular. - **Cerebrocerebellum**: This zone comprises the lateral hemispheres and is responsible for distal limb fine coordination. Lesions produce appendicular ataxia, intention tremor, and dysdiadochokinesia — not gait ataxia. The patient's preserved upper limb coordination excludes this. - **Dentato-rubro-olivary pathway**: This is a brainstem output pathway associated with palatal myoclonus and ocular oscillations (Guillain-Mollaret triangle syndrome), not gait or proximal ataxia. **High-Yield:** Alcoholic cerebellar degeneration affects the anterior superior vermis (spinocerebellar zone) → gait + leg ataxia with relatively preserved arms — a classic paravermial syndrome. [cite: Sutton Radiology — Cerebellar Anatomy and Functional Zones]