A 58-year-old man presents with acute onset vertigo, nausea, and inability to walk in a straight line. Examination reveals positive Romberg sign, broad-based gait, and inability to perform tandem walking, but finger-nose-finger test and rapid alternating movements are normal. Which cerebellar region is most likely damaged?

Explanation

## Clinical Analysis of Cerebellar Syndrome ### Key Clinical Features in This Case **Key Point:** The patient has: 1. **Truncal instability** — cannot walk straight, positive Romberg, cannot do tandem walking 2. **Preserved limb coordination** — normal finger-nose-finger test and rapid alternating movements 3. **Vestibular symptoms** — vertigo and nausea This constellation is pathognomonic for **vermis lesion**. ### Why Vermis? **High-Yield:** The cerebellar vermis has three functional subdivisions: - **Anterior lobe vermis** — receives proprioceptive input; controls proximal muscles and posture - **Posterior lobe vermis** — receives vestibular input; controls balance and eye movements - **Flocculonodular lobe** — receives vestibular input; controls balance and gait **Key Point:** In this patient: - Inability to walk straight + broad-based gait = **anterior/posterior vermis involvement** - Vertigo + nausea = **flocculonodular or posterior vermis involvement** (vestibular connections) - Normal limb coordination = **rules out hemisphere** ### Pathophysiology The vermis coordinates: 1. **Trunk and proximal limb muscles** via spinocerebellar pathways 2. **Balance and equilibrium** via vestibular inputs 3. **Gait control** through connections with brainstem nuclei Damage disrupts these integrative functions, leaving distal limb control (mediated by hemispheres) intact. ### Comparison: Vermis vs. Hemisphere Lesions | Clinical Feature | Vermis Lesion | Hemisphere Lesion | | --- | --- | --- | | **Gait** | Broad-based, ataxic | May be relatively normal | | **Romberg sign** | Positive | Negative | | **Tandem walking** | Cannot perform | Can perform | | **Finger-nose test** | Normal | Dysmetric, intention tremor | | **Rapid alternating movements** | Normal | Dysdiadochokinesia | | **Vertigo** | Common (vestibular input) | Rare | | **Sitting balance** | Lost | Preserved | **Clinical Pearl:** A patient who **cannot sit or stand unsupported** but **can touch nose accurately** has vermis pathology until proven otherwise. This is one of the most reliable bedside discriminators in cerebellar examination. **Mnemonic:** **VERMIS ATAXIA = AXIAL ATAXIA** (Vermis Equilibrium Regulation, Midline Instability, Sitting dysfunction = Axial control loss, Xylem-like trunk instability, Intact distal coordination, Ataxia of gait). ### Why Not the Other Options? See distractor explanations below.