Which clinical finding best distinguishes a patient with cerebellar vermis lesion from one with cerebellar hemisphere lesion?

Explanation

## Cerebellar Anatomy and Functional Divisions **Key Point:** The cerebellum is functionally divided into three longitudinal zones: vermis (midline), intermediate zone, and lateral hemispheres. Each zone controls different aspects of motor coordination. ### Vermis Lesions **Key Point:** The vermis receives input from the vestibular system and controls axial (trunk) and proximal muscles. - Causes **truncal ataxia** — inability to maintain balance while sitting or standing - Gait disturbance with broad-based, unsteady walking pattern - **Limb coordination remains relatively preserved** (this is the discriminating feature) - Nystagmus may be present but is not the primary finding ### Hemisphere Lesions **Key Point:** Lateral cerebellar hemispheres receive input from the cerebral cortex via pontocerebellar pathways and control distal limb movements. - Causes **ipsilateral intention tremor, dysmetria, and dysdiadochokinesia** - Limb incoordination is prominent; gait may be relatively spared early - Nystagmus is common, often with horizontal/rotatory components ### Comparison Table | Feature | Vermis Lesion | Hemisphere Lesion | | --- | --- | --- | | **Primary deficit** | Truncal ataxia | Limb ataxia | | **Gait** | Severely affected | May be relatively preserved | | **Limb coordination** | Preserved | Severely affected (dysmetria, tremor) | | **Nystagmus** | Variable | Common | | **Sitting balance** | Lost | Preserved | **High-Yield:** Vermis lesion = **trunk problem** (can't sit or stand); hemisphere lesion = **limb problem** (can't coordinate fingers/hands). **Clinical Pearl:** A patient who cannot sit upright unsupported but has normal finger-nose-finger testing suggests vermis pathology (e.g., midline tumor, atrophy). A patient with normal sitting but clumsy finger-nose-finger testing suggests hemisphere pathology. **Mnemonic:** **VERMIS = TRUNK** (Vermis Equilibrium Regulation, Midline Instability, Sitting dysfunction). **HEMISPHERE = LIMBS** (Hemisphere Executes Limb Intentions, Skilled movements Disrupted).