A 52-year-old woman from Mumbai is admitted with sudden onset of right facial weakness, dysarthria, and mild right arm weakness. MRI brain shows a small acute infarct in the left basal ganglia region. On detailed examination, her right eye cannot close completely, and she has difficulty pronouncing labial consonants. Her right arm power is 4/5, but right leg power is normal (5/5). Which part of the internal capsule is most likely affected?

Explanation

## Selective Vulnerability of the Genu in Lacunar Stroke **Key Point:** The genu of the internal capsule contains the **corticobulbar tract**, which carries motor fibres to the cranial nerve nuclei. A selective lesion here produces facial weakness and dysarthria WITHOUT significant limb weakness. ### Internal Capsule Anatomy: Functional Segregation The internal capsule has a somatotopic organization: | Anatomical Region | Fibre Type | Target | Clinical Signs with Damage | |-------------------|-----------|--------|---------------------------| | **Genu** | Corticobulbar tract | CN V, VII, IX, X, XII nuclei | Facial droop, dysarthria, tongue weakness | | **Posterior limb (upper fibres)** | Corticospinal (upper extremity) | Arm/hand motor cortex | Arm weakness | | **Posterior limb (lower fibres)** | Corticospinal (lower extremity) | Leg motor cortex | Leg weakness | ### Why Genu? This patient has: 1. **Right facial weakness** (eye closure failure, labial consonant difficulty) — **corticobulbar tract** 2. **Dysarthria** — bulbar motor involvement 3. **Mild right arm weakness (4/5)** — minimal pyramidal involvement 4. **Normal right leg power (5/5)** — **no lower extremity involvement** The **selective sparing of leg power** is the key diagnostic clue. If the posterior limb were involved, we would expect either: - Arm AND leg weakness together (if upper and lower fibres both affected), OR - Isolated leg weakness (if only lower fibres affected) Instead, the patient has facial + arm involvement with leg sparing. This pattern is best explained by **genu involvement with minor extension into the upper posterior limb**, affecting corticobulbar fibres primarily and only minimally affecting the proximal corticospinal fibres. **Clinical Pearl:** **"Facial-arm dysarthria stroke" (FADS)** is a recognized lacunar syndrome caused by genu lesions. It presents with: - Facial weakness - Arm weakness (often mild) - Dysarthria - **Leg sparing** (normal power) This is distinct from "pure motor stroke" (posterior limb), which affects all three regions equally. **Mnemonic: "GENU = Facial + Bulbar"** — Remember that the genu carries corticobulbar fibres to the face and bulbar muscles. **High-Yield:** Lacunar syndromes of the internal capsule: 1. **Pure motor stroke** → Posterior limb (all three regions affected) 2. **Facial-arm dysarthria (FADS)** → Genu ± upper posterior limb (face + arm, leg spared) 3. **Ataxic hemiparesis** → Posterior limb with cerebellar signs ### Why Not the Other Options? **Posterior limb (upper fibres only):** While this could explain arm weakness, it would NOT explain the prominent facial weakness and dysarthria. The corticospinal tract for the arm does not carry facial motor fibres. **Anterior limb with extension to genu:** The anterior limb carries thalamocortical fibres for cognition and frontal function. Extension to genu might explain facial weakness, but anterior limb involvement would add cognitive/personality changes, which this patient does not have. **Posterior limb (lower fibres only):** This would cause leg weakness, which the patient does NOT have. The patient's leg is completely normal, ruling out lower fibre involvement.