A 58-year-old man with hypertension presents with acute right-sided facial weakness, dysarthria, and preserved right arm and leg strength. Which part of the internal capsule is most likely affected?

Explanation

## Clinical Localization of Internal Capsule Stroke ### Clinical Presentation Analysis The patient presents with a selective motor deficit affecting bulbar muscles (face, speech) while limb motor power remains intact. This dissociation is the key diagnostic clue. **Key Point:** Facial weakness + dysarthria + preserved limb strength = **Genu involvement** (corticobulbar tract lesion). ### Anatomical Basis The genu of the internal capsule contains: - Corticobulbar (corticonuclear) tract fibres - Motor fibres to facial nucleus (CN VII), hypoglossal nucleus (CN XII), and nucleus ambiguus (CN IX, X) - Thalamocortical fibres from mediodorsal nucleus Damage to the genu produces: 1. **Contralateral facial droop** (upper and lower face affected — central facial weakness) 2. **Dysarthria** (weakness of tongue, pharynx, larynx) 3. **Preserved limb strength** (corticospinal tract is in posterior limb, not genu) ### Differential Localization by Syndrome ```mermaid flowchart TD A[Internal Capsule Stroke]:::outcome --> B{Clinical Pattern?}:::decision B -->|Facial weakness + dysarthria<br/>+ preserved limb power| C[Genu Lesion]:::action B -->|Contralateral hemiplegia<br/>+ facial weakness| D[Posterior limb<br/>anterior 2/3]:::action B -->|Hemiplegia + hemianopia<br/>+ sensory loss| E[Posterior limb<br/>posterior 1/3]:::action B -->|Cognitive/behavioural<br/>changes only| F[Anterior limb]:::action C --> G[Facial-sparing pattern<br/>of weakness]:::outcome D --> H[Classic lacunar stroke<br/>pure motor]:::outcome E --> I[Sensorimotor stroke<br/>with visual field cut]:::outcome ``` ### Comparative Table: Internal Capsule Syndromes | Location | Fibre Content | Clinical Syndrome | Key Distinguishing Feature | | --- | --- | --- | --- | | **Genu** | Corticobulbar + MD thalamic | Facial weakness, dysarthria, preserved limbs | **Bulbar weakness WITHOUT hemiplegia** | | **Anterior 2/3 of Posterior Limb** | Corticospinal tract | Pure motor hemiplegia | Weakness of face, arm, leg equally | | **Posterior 1/3 of Posterior Limb** | Sensory + visual radiations | Sensorimotor stroke + hemianopia | Hemiplegia + hemianopia ± sensory loss | | **Anterior Limb** | Frontopontine fibres | Cognitive/personality changes | Minimal motor deficit | **Clinical Pearl:** In this case, the **facial-sparing pattern** (bulbar weakness without limb weakness) is pathognomonic for genu involvement. If the patient had weakness of the arm and leg as well, the posterior limb would be implicated. **High-Yield:** Hypertensive lacunar strokes commonly affect the genu (causing facial weakness with preserved limb power) or the posterior limb (causing pure motor hemiplegia). The genu syndrome is less common than posterior limb strokes but is highly specific when present. **Mnemonic:** **GENU = Face** (Genu affects facial and bulbar muscles). **Posterior Limb = Limbs** (Corticospinal tract to extremities). [cite:Harrison 21e Ch 451; Snell's Clinical Neuroanatomy 10e Ch 7]