A 58-year-old man from Delhi presents with acute onset vertigo, nausea, and vomiting. On examination, he has nystagmus (beating towards the lesion), ipsilateral facial numbness, ipsilateral loss of pain and temperature sensation over the face, contralateral loss of pain and temperature sensation over the body, and ipsilateral Horner syndrome (miosis, ptosis, anhidrosis). MRI brain shows an acute infarct in the lateral medulla. Which of the following structures is most likely damaged in this syndrome?

Explanation

## Clinical Syndrome Identification This patient presents with **Wallenberg syndrome (lateral medullary syndrome)**, the most common brainstem stroke syndrome. The constellation of findings is pathognomonic. ## Anatomical Basis of Wallenberg Syndrome **Key Point:** Wallenberg syndrome results from infarction of the lateral medulla, typically from vertebral artery occlusion. The lesion damages multiple crossed and uncrossed tracts in a characteristic distribution. ### Structures Damaged and Clinical Correlates | Structure Damaged | Ipsilateral vs Contralateral | Clinical Finding | |---|---|---| | **Spinal trigeminal nucleus** | Ipsilateral | Facial pain/temperature loss (onion-skin distribution) | | **Spinothalamic tract** | Contralateral | Body pain/temperature loss (crossed decussation in spinal cord) | | **Sympathetic fibers** | Ipsilateral | Horner syndrome (miosis, ptosis, anhidrosis) | | **Inferior cerebellar peduncle** | Ipsilateral | Ataxia, vertigo, nystagmus | | **Nucleus ambiguus** | Ipsilateral | Hoarseness, dysphagia (CN X involvement) | | **Dorsal motor nucleus of vagus** | Ipsilateral | Palatal weakness | **High-Yield:** The **spinothalamic tract and spinal trigeminal nucleus** are the two most consistently damaged structures in Wallenberg syndrome, producing the classic dissociated sensory loss (facial pain/temperature ipsilateral, body pain/temperature contralateral). ### Why This Anatomy Matters **Clinical Pearl:** The **onion-skin distribution** of facial sensory loss (concentric rings from perioral to peripheral) occurs because the spinal trigeminal nucleus is somatotopically organized — perioral fibers are rostral, peripheral fibers are caudal. A medullary lesion damages rostral fibers, sparing caudal fibers. ```mermaid flowchart TD A[Lateral Medullary Infarction]:::outcome --> B[Spinal Trigeminal Nucleus Damaged]:::action A --> C[Spinothalamic Tract Damaged]:::action A --> D[Sympathetic Fibers Damaged]:::action A --> E[Inferior Cerebellar Peduncle Damaged]:::action B --> F[Ipsilateral Facial Pain/Temp Loss]:::outcome C --> G[Contralateral Body Pain/Temp Loss]:::outcome D --> H[Ipsilateral Horner Syndrome]:::outcome E --> I[Vertigo, Nystagmus, Ataxia]:::outcome ``` **Mnemonic:** **WHAMS** — **W**allenberg, **H**orner, **A**taxia, **M**edulla, **S**pinothalamic/trigeminal. ## Differential Diagnosis of Brainstem Syndromes | Syndrome | Location | Key Features | |---|---|---| | **Wallenberg** | Lateral medulla | Ipsilateral facial loss + contralateral body loss + Horner | | **Weber** | Midbrain | Ipsilateral CN III palsy + contralateral hemiparesis | | **Benedikt** | Midbrain | CN III palsy + contralateral tremor/ataxia | | **Foville** | Mid-pons | Ipsilateral CN VI/VII palsy + contralateral hemiparesis | | **Locked-in** | Ventral pons | Quadriplegia + preserved consciousness | [cite:Robbins 10e Ch 28]