A 52-year-old man presents with progressive weakness of the left lower limb and loss of pain sensation on the right lower limb starting 3 days ago. MRI shows a lesion in the left half of the spinal cord at T10. Which tract dysfunction best explains the contralateral pain loss?

## Brown-Séquard Syndrome: Tract-Level Localization ### Clinical Presentation **Key Point:** The patient has a **hemisection (Brown-Séquard syndrome)** of the spinal cord at T10 on the left side, producing: 1. **Ipsilateral motor weakness** (left lower limb) → corticospinal tract damage at T10 2. **Contralateral pain loss** (right lower limb) → spinothalamic tract damage that had already crossed before the lesion ### Why Spinothalamic Tract Crossing Explains Contralateral Pain Loss **High-Yield:** The spinothalamic tract crosses the midline **1–2 segments rostral to its entry point**. - Pain fibers from the right lower limb enter at **T12–L1** (sacral dermatomes) - These fibers **cross at T10–T11** (1–2 segments above entry) - The left hemisection at **T10** damages the **already-crossed fibers** from the right side - Result: **Right-sided pain loss** (contralateral to the lesion) ### Anatomical Diagram: Spinothalamic Tract Crossing ```mermaid flowchart TD A["Pain input from right lower limb<br/>(T12-L1 dermatome)"]:::outcome B["First-order neuron synapses<br/>in dorsal horn at T12-L1"]:::action C["Second-order neuron crosses<br/>1-2 segments rostral<br/>(at T10-T11)"]:::action D["Crossed fibers ascend as<br/>spinothalamic tract on left side"]:::action E["LEFT hemisection at T10<br/>damages crossed fibers"]:::urgent F["Contralateral pain loss<br/>(right side)"]:::outcome A --> B --> C --> D --> E --> F ``` ### Comparison Table: Spinal Cord Lesion Patterns | Lesion Type | Motor Loss | Fine Touch Loss | Pain Loss | Explanation | |-------------|-----------|-----------------|-----------|-------------| | **Hemisection (Brown-Séquard)** | Ipsilateral | Ipsilateral | Contralateral | Corticospinal & dorsal columns ipsilateral; STT crosses early | | **Central cord** | Bilateral upper > lower | Bilateral | Bilateral (cape-like) | Damages crossing STT fibers in center | | **Anterior cord** | Bilateral | Bilateral | Bilateral | Damages ventral STT & corticospinal tracts | | **Posterior cord** | None | Bilateral | None | Dorsal columns only | **Clinical Pearl:** The **1–2 segment offset** between pain entry and crossing is the key to understanding why contralateral pain loss occurs with an ipsilateral spinal lesion. **Mnemonic:** **STT = Early Crosser** — spinothalamic tract crosses within 1–2 segments of entry, making it vulnerable to hemisection lesions that damage already-crossed fibers.