A patient with a unilateral cerebellar hemisphere lesion presents with ipsilateral dysmetria and intention tremor. Which cerebellar output nucleus is most likely damaged?

Explanation

## Cerebellar Output Nuclei and Clinical Syndromes **Key Point:** The dentate nucleus is the primary output of the lateral cerebellar hemisphere and projects to the contralateral motor cortex and brainstem motor centres. Damage here produces ipsilateral dysmetria and intention tremor — the hallmark of cerebellar hemisphere syndrome. ### Cerebellar Nuclei: Anatomy and Function | Nucleus | Location | Input Zone | Output Target | Lesion Syndrome | | --- | --- | --- | --- | --- | | **Dentate** | Lateral | Lateral hemisphere (Purkinje cells) | Contralateral VL thalamus → motor cortex; red nucleus | Ipsilateral dysmetria, intention tremor, hypotonia | | **Interposed (Globose + Emboliform)** | Intermediate | Intermediate zone | Contralateral red nucleus → spinal cord | Ipsilateral limb ataxia (less severe) | | **Fastigial** | Medial | Vermis | Bilateral vestibular + reticular nuclei | Truncal ataxia, nystagmus, gait disturbance | | **Vestibular** | (Extracerebellar) | Flocculonodular lobe | Direct inhibition of vestibular neurons | Vertigo, nystagmus, balance loss | ### Why Dentate Lesion Causes Ipsilateral Dysmetria + Intention Tremor 1. **Lateral hemisphere → dentate nucleus pathway** — Purkinje cells from the lateral hemisphere (which receive climbing fibre input from contralateral inferior olive) inhibit dentate neurons. 2. **Dentate output crosses** — Dentate axons decussate in the superior cerebellar peduncle and synapse in the contralateral VL thalamus and red nucleus. 3. **Recrossing at thalamus** — VL thalamus projects back to the ipsilateral motor cortex (second crossing), so a dentate lesion affects ipsilateral motor control. 4. **Loss of coordination signals** — Disruption of dentate output removes the cerebellar contribution to motor planning, causing dysmetria (overshoot/undershoot) and intention tremor (oscillation during purposeful movement). **High-Yield:** Ipsilateral signs + lateral hemisphere = dentate nucleus. Medial/vermis signs (truncal ataxia) = fastigial nucleus. **Mnemonic:** **DINT** — **D**entate = lateral hemisphere, **I**nterposed = intermediate, **N**uclei output crosses, **T**halamus receives dentate output. **Clinical Pearl:** Intention tremor is pathognomonic for cerebellar dysfunction; it worsens as the limb approaches the target (unlike Parkinsonian resting tremor). This is because the cerebellum uses error feedback to refine movement — loss of this feedback causes oscillation during the final approach.