A 58-year-old man from Delhi presents to the emergency department with acute onset of right-sided weakness affecting his face, arm, and leg equally. On examination, he has right facial droop, right-sided hemiparesis (3/5 power), hyperreflexia on the right side, and an extensor plantar response on the right. Speech is slurred. CT head shows a hyperdense lesion in the left basal ganglia region. Which anatomical structure has been damaged to produce this constellation of motor findings?

Explanation

## Anatomical Basis of the Clinical Presentation **Key Point:** The internal capsule is a critical white matter structure containing both ascending and descending motor and sensory fibres. A lesion here produces a characteristic syndrome affecting multiple body regions. ### Internal Capsule: Functional Anatomy The internal capsule has three main parts: | Part | Location | Contents | Clinical Significance | |------|----------|----------|----------------------| | **Anterior limb** | Between caudate and lentiform nucleus | Thalamocortical fibres (frontal lobe connections) | Cognitive, personality changes; frontal eye fields | | **Genu** | Medial bend | Corticobulbar tract (CN motor fibres) | Facial weakness, dysarthria | | **Posterior limb** | Between thalamus and lentiform nucleus | Corticospinal tract + sensory fibres (thalamocortical) | Hemiparesis, hemisensory loss | ### Why Posterior Limb? The patient presents with: 1. **Right-sided hemiparesis** (face, arm, leg) — indicates damage to the **corticospinal tract** 2. **Facial droop** — corticobulbar fibres (which run through the genu, but posterior limb damage can affect overall motor output) 3. **Hyperreflexia and extensor plantar response** — upper motor neuron signs confirming pyramidal tract involvement 4. **Slurred speech** — bulbar involvement from corticobulbar tract The **posterior limb of the internal capsule** contains the bulk of descending motor fibres (corticospinal tract) and ascending sensory fibres. A lesion here produces contralateral hemiparesis affecting all three regions (face, arm, leg) in a pyramidal distribution — exactly what this patient has. **Clinical Pearl:** A lacunar stroke affecting the posterior limb produces "pure motor stroke" — the classic presentation of internal capsule involvement. The left-sided lesion on imaging (basal ganglia region) affecting the right body is explained by the decussation of the corticospinal tract in the medulla. **High-Yield:** The **"FAST" rule** for internal capsule stroke: - **F**ace droop → genu involvement (corticobulbar) - **A**rm weakness → posterior limb (corticospinal) - **S**peech slurred → genu + posterior limb - **T**ime to treat → acute stroke protocol In this case, all three motor regions are affected equally, pointing to **posterior limb involvement** as the primary lesion. ### Why Not the Other Options? **Anterior limb:** Damage here causes personality changes, cognitive dysfunction, and loss of frontal eye fields — NOT motor weakness in a pyramidal pattern. **Genu:** While the genu contains corticobulbar fibres (explaining facial weakness and dysarthria), it does NOT contain the corticospinal tract to the limbs. A pure genu lesion would spare limb power. **External capsule:** This is a thin white matter layer between the lentiform nucleus and insula. Lesions here produce minimal motor deficits; it carries associative fibres, not pyramidal tract fibres.