A 52-year-old man presents with chronic neuropathic pain in his lower limbs following a spinal cord injury 6 months ago. Clinical examination reveals allodynia and hyperalgesia below the level of injury. To assess the functional integrity of the spinothalamic tract and confirm the diagnosis of central neuropathic pain, which investigation would be most appropriate?

Explanation

## Investigation of Choice for Neuropathic Pain Assessment ### Why Quantitative Sensory Testing (QST) is Correct **Key Point:** QST is the gold-standard bedside investigation for assessing spinothalamic tract function and characterizing neuropathic pain phenotypes. It measures thermal (warm and cold) and mechanical (touch, vibration, pressure) thresholds objectively. **High-Yield:** In central neuropathic pain syndromes (spinal cord injury, stroke), QST can detect: - Loss of thermal sensation (spinothalamic tract damage) - Preserved vibration/proprioception (dorsal column intact) - Allodynia and hyperalgesia (central sensitization) **Clinical Pearl:** QST is non-invasive, reproducible, and directly tests the nociceptive pathways. It helps differentiate central vs. peripheral neuropathic pain and guides treatment decisions. ### Comparison of Investigation Options | Investigation | Measures | Utility in Neuropathic Pain | Limitation | |---|---|---|---| | **QST** | Thermal & mechanical thresholds | Direct assessment of spinothalamic tract function | Operator-dependent, requires patient cooperation | | fMRI | Brain activation during pain | Research tool for pain processing | Expensive, not diagnostic, no bedside utility | | PET | Metabolic activity | Research on central sensitization | Radiation exposure, not specific for neuropathic pain | | EMG/NCS | Peripheral nerve conduction | Peripheral neuropathy only | Cannot assess central pathways or spinothalamic tract | **Mnemonic:** **QSTNP** — QST for Neuropathic Pain assessment - **Q** = Quantitative (objective measurement) - **S** = Sensory (tests sensation directly) - **T** = Tract (assesses spinothalamic tract integrity) - **NP** = Neuropathic Pain (gold standard for phenotyping) ### Why QST Confirms Central Neuropathic Pain 1. Detects dissociated sensory loss (loss of pain/temperature with preserved touch/vibration) — hallmark of spinothalamic tract lesion 2. Quantifies allodynia and hyperalgesia objectively 3. Establishes baseline for monitoring treatment response 4. Non-invasive and repeatable for longitudinal assessment **Warning:** Do not confuse QST with simple bedside pin-prick testing — QST uses calibrated stimuli and threshold measurement, not subjective clinical examination.