A 52-year-old man presents with acute onset right-sided weakness of the arm and leg, but preserved pain and temperature sensation on the right side. Pain and temperature sensation is lost on the left side below the level of lesion. Which spinal cord tract dysfunction best explains the dissociated sensory loss?

Explanation

## Brown-Séquard Syndrome: Tract Dissociation **High-Yield:** This is a classic presentation of hemisection of the spinal cord (Brown-Séquard syndrome), which demonstrates the different decussation levels of motor and sensory pathways. ### Clinical Presentation Analysis **Right-sided findings:** - Motor weakness (arm and leg) → **ipsilateral corticospinal tract lesion** - Preserved pain and temperature sensation → **spinothalamic tract intact on right** **Left-sided findings:** - Loss of pain and temperature sensation below lesion → **contralateral spinothalamic tract lesion** (already decussated at spinal cord level) ### Why This Pattern Occurs ```mermaid flowchart TD A[Right-sided spinal cord hemisection]:::outcome --> B[Corticospinal tract damaged on right]:::action A --> C[Spinothalamic tract damaged on left side of cord]:::action B --> D[Ipsilateral motor loss below lesion]:::outcome C --> E[Contralateral pain/temp loss below lesion]:::outcome F[Spinothalamic tract decussates at spinal cord level]:::decision --> E G[Corticospinal tract decussates at medulla]:::decision --> D ``` ### Tract-Specific Decussation Timing | Tract | Decussation Level | Lesion Effect | |-------|-------------------|---------------| | **Corticospinal tract** | Medullary pyramids | Ipsilateral motor loss (lesion before crossing) | | **Spinothalamic tract** | Spinal cord (1–2 segments above entry) | Contralateral pain/temp loss (lesion after crossing) | | **Dorsal columns** | Medulla (fasciculus gracilis/cuneatus) | Ipsilateral proprioception/vibration loss | **Key Point:** A **right spinal cord hemisection** damages: 1. Right corticospinal tract → ipsilateral (right) motor loss 2. Left spinothalamic tract fibres (already crossed at lower spinal levels) → contralateral (left) pain/temperature loss **Mnemonic:** **LOSS** = **L**esion **O**n **S**ame Side (motor), **S**ensory on opposite (pain/temp) **Clinical Pearl:** The preserved pain and temperature sensation on the right side confirms that the right spinothalamic tract is intact—the lesion is not affecting it. The loss on the left confirms damage to left-sided STT fibres that had already decussated. [cite:Snell's Clinical Neuroanatomy Ch 4; Harrison 21e Ch 435]