A lesion at the T5 spinal cord level on the right side results in loss of pain and temperature sensation on the left side starting 2–3 segments below the lesion. Which tract is damaged?

Explanation

## Spinothalamic Tract — Anatomy and Crossed Sensory Loss **Key Point:** The spinothalamic tract carries pain and temperature sensation and decussates at the **spinal cord level** (1–2 segments above entry). A lesion on one side causes **contralateral** loss of pain/temperature **below** the lesion. ### Anatomical Organization ```mermaid flowchart TD A[Peripheral nociceptor/thermoreceptor]:::outcome --> B[First-order neuron enters dorsal root]:::outcome B --> C[Synapse in dorsal horn at same level]:::outcome C --> D[Second-order neuron decussates 1-2 segments above entry]:::action D --> E[Ascend contralaterally as spinothalamic tract]:::action E --> F[VPL thalamus]:::outcome F --> G[Primary sensory cortex]:::outcome ``` ### Clinical Correlation: Right T5 Lesion | Finding | Explanation | | --- | --- | | **Lesion side** | Right T5 | | **Sensory loss side** | Left (contralateral) | | **Sensory modality** | Pain and temperature | | **Level of loss** | T7–T8 and below (2–3 segments below lesion) | | **Reason for offset** | Decussation occurs 1–2 segments above the entry level | **High-Yield:** Right-sided spinal cord lesion → **left-sided pain/temperature loss** (spinothalamic tract crosses). This is the hallmark of **Brown-Séquard syndrome** (hemisection of spinal cord). **Mnemonic:** **STT = Spinothalamic Tract crosses early** (at spinal cord, not brainstem like DCML). **Contralateral pain/temp loss below the lesion.** **Clinical Pearl:** In Brown-Séquard syndrome: - **Ipsilateral** loss of motor function and proprioception (corticospinal tract and DCML cross at brainstem) - **Contralateral** loss of pain/temperature (spinothalamic tract crosses at spinal cord) ### Why Not the Other Tracts? - **Fasciculus gracilis:** Carries proprioception (not pain/temperature) and crosses at the medulla (not spinal cord). - **Dorsal spinocerebellar tract:** Carries proprioception to cerebellum, does not cross, and does not cause sensory loss to cortex. - **Dorsal root entry zone:** Damage here would cause **ipsilateral** segmental loss at the level of injury, not contralateral loss below.