A 48-year-old woman from Delhi is admitted with acute onset vertigo, nausea, and vomiting following a sudden severe headache. Neurological examination reveals nystagmus, truncal ataxia, and inability to perform rapid alternating movements. CT brain shows acute hydrocephalus with compression of the fourth ventricle by a midline cerebellar mass. Which of the following structures is most directly responsible for the observed ataxia and loss of coordination?

Explanation

## Midline Cerebellar Lesion and Truncal Ataxia ### Anatomical Organization of the Cerebellum The cerebellum is functionally organized into three zones along the mediolateral axis: | Zone | Structure | Function | Lesion Effect | |------|-----------|----------|---------------| | **Midline** | Vermis | Truncal/axial control, balance, eye movements | Truncal ataxia, gait disturbance, nystagmus | | **Intermediate** | Paravermis | Proximal limb coordination | Limb dysmetria, dysarthria | | **Lateral** | Hemisphere | Distal fine motor, motor planning | Dysmetria, intention tremor, dysdiadochokinesia | ### Why the Vermis Causes Truncal Ataxia **Key Point:** The cerebellar vermis, particularly the anterior and posterior lobes, receives input from the spinocerebellar tracts (proprioception from trunk and proximal limbs) and projects to the fastigial nucleus. The fastigial nucleus output goes to the vestibular nuclei and reticular formation, which control axial muscles and balance. ### Clinical Features of Vermal Lesions 1. **Wide-based gait** — loss of truncal stability 2. **Positive Romberg test** — inability to maintain balance with eyes closed 3. **Nystagmus** — disruption of vestibulo-cerebellar circuits 4. **Inability to perform rapid alternating movements** — loss of coordination of trunk stabilization during limb movement 5. **Vomiting and hydrocephalus** — midline mass compressing fourth ventricle ### Functional Subdivisions **High-Yield:** The **flocculonodular lobe** (oldest part evolutionarily) is the **vestibular cerebellum** — it controls eye movements and balance via vestibular nuclei. The **anterior and posterior lobes** of the vermis control truncal and axial muscles via the fastigial nucleus and reticular formation. ### Mnemonic: Cerebellar Zones **MEPHD:** - **M**idline (Vermis) = balance, trunk, gait - **E**xternal (Hemisphere) = fine motor, distal limbs - **P**aravermis = proximal limbs - **H**emisphere = motor planning - **D**entate = output nucleus for hemispheres ```mermaid flowchart TD A[Midline Cerebellar Mass]:::outcome --> B{Which vermal region?}:::decision B -->|Flocculonodular lobe| C[Vestibular cerebellum]:::outcome C --> D[Nystagmus, vertigo]:::urgent B -->|Anterior & Posterior lobes| E[Truncal/axial cerebellum]:::outcome E --> F[Fastigial nucleus output]:::action F --> G[Vestibular & reticular nuclei]:::action G --> H[Truncal ataxia, gait disturbance]:::urgent A --> I[Fourth ventricle compression]:::urgent I --> J[Acute hydrocephalus]:::urgent ``` **Clinical Pearl:** Midline cerebellar lesions (especially vermis) classically present with **gait ataxia out of proportion to limb dysmetria** — the patient walks poorly but can perform finger-to-nose testing relatively well. This contrasts with hemispheric lesions, where dysmetria is prominent. [cite:Snell's Neuroanatomy 8e Ch 7; Robbins & Cotran 10e Ch 28]