A 72-year-old woman with a 15-year history of hypertension is brought to hospital by her family after they found her unable to speak clearly and drooling from the right corner of her mouth. On examination, she has right-sided facial droop, dysarthria, and mild weakness of the right arm, but normal strength in the right leg. Sensation is intact. CT head reveals a small hypodensity in the left internal capsule. Which component of the internal capsule is primarily affected in this clinical presentation?

Explanation

## Clinical Syndrome: Facial Weakness with Dysarthria The patient presents with **selective cranial nerve motor deficits** (facial droop, dysarthria) with **minimal limb weakness**, indicating selective involvement of the **genu of the internal capsule**. ### Genu Anatomy and Function **Key Point:** The genu of the internal capsule contains **corticobulbar fibres** that project from the motor cortex to the motor nuclei of cranial nerves (CN V, VII, IX, X, XII). These fibres control muscles of facial expression, mastication, tongue, pharynx, and larynx. ### Differential Presentation by Internal Capsule Region | Region | Primary Contents | Clinical Presentation | |--------|------------------|----------------------| | **Genu** | Corticobulbar fibres (CN motor) | Facial weakness, dysarthria, dysphagia; **minimal or no limb weakness** | | **Posterior limb** | Corticospinal tract | **Contiguous hemiparesis** (face, arm, leg); may have hemisensory loss | | **Anterior limb** | Thalamocortical radiations | Cognitive/personality changes, frontal lobe signs | | **Retrolenticular** | Optic radiations | Contralateral homonymous hemianopia | ### Why Genu Involvement Here? 1. **Facial droop** = CN VII motor nucleus involvement (corticobulbar pathway) 2. **Dysarthria** = CN XII (hypoglossal) and CN X (vagal) involvement 3. **Minimal arm weakness** = corticospinal tract largely spared (located in posterior limb, not genu) 4. **Intact sensation** = sensory radiations (posterior limb) unaffected **Clinical Pearl:** This pattern is called a **"pure dysarthria-clumsy hand" syndrome** or **facial weakness with dysarthria** — a classic lacunar stroke syndrome caused by small vessel disease in hypertensive patients. The genu is supplied by penetrating branches of the anterior choroidal artery and lenticulostriate arteries. **High-Yield:** Isolated cranial nerve motor deficits (especially CN VII + XII) with preserved limb strength = genu involvement. Contiguous limb weakness (face + arm + leg) = posterior limb involvement. **Mnemonic:** **GENU = Cranial Nerve motor (Facial, tongue, pharynx)** — think "genu" → "genuine" facial/speech problems. [cite:Snell's Neuroanatomy 8e Ch 10; Robbins 10e Ch 28]