A 52-year-old man with a 6-month history of progressive gait ataxia and dysarthria undergoes MRI showing a lesion in the cerebellar hemisphere. On examination, he has intention tremor and dysdiadochokinesia on the ipsilateral side, but no vertigo or nystagmus. Which feature best distinguishes a cerebellar hemisphere lesion from a cerebellar vermis lesion?

Explanation

## Cerebellar Hemisphere vs. Vermis Lesions: Key Discriminators ### Anatomical Basis The cerebellum has distinct functional zones that determine the clinical presentation of lesions: - **Cerebellar hemisphere**: Receives input from contralateral cerebral cortex via pontocerebellar tracts; outputs via dentatorubrothalamic pathway → ipsilateral motor control - **Cerebellar vermis**: Controls axial and proximal muscles; coordinates bilateral lower limbs and trunk ### Clinical Feature Comparison | Feature | Cerebellar Hemisphere Lesion | Cerebellar Vermis Lesion | |---------|------------------------------|-------------------------| | **Limb ataxia** | Ipsilateral (same side as lesion) | Bilateral or absent | | **Trunk/gait** | Relatively preserved | Severely affected (truncal ataxia) | | **Intention tremor** | Present (ipsilateral limb) | Absent | | **Dysdiadochokinesia** | Present (ipsilateral) | Absent | | **Vertigo/nystagmus** | Absent | Present (especially downbeating) | | **Stance** | Can stand with feet together | Wide-based, unsteady gait | ### Why Option 1 (Ipsilateral Limb Ataxia with Preserved Midline Stability) is Correct **Key Point:** Cerebellar hemisphere lesions produce **ipsilateral limb dyscoordination** (intention tremor, dysdiadochokinesia) while **sparing midline function** because the vermis remains intact. The patient can maintain trunk stability and stance. **Clinical Pearl:** The mnemonic **"DANISH"** helps recall cerebellar signs: - **D**ysarthria - **A**taxia (ipsilateral for hemisphere) - **N**ystagmus (absent in hemisphere, present in vermis/flocculonodular) - **I**ntention tremor - **S**can speech - **H**ypotonia In this case, the presence of intention tremor, dysdiadochokinesia, and absence of vertigo/nystagmus confirms hemisphere pathology. ### Vermis Lesion Pattern (for contrast) Vermis damage → **truncal ataxia, gait disturbance, vertigo** with relatively preserved limb coordination. Classic presentation: patient cannot sit or stand without support despite normal arm movements. **High-Yield:** Hemisphere = **limb signs**; Vermis = **midline/gait signs**. This distinction is tested frequently in NEET PG. [cite:Snell's Clinical Neuroanatomy 8e Ch 7]