A 52-year-old woman with a 10-year history of rheumatoid arthritis presents with chronic joint pain inadequately controlled on NSAIDs and conventional DMARDs. She reports pain severity of 7/10 affecting her quality of life and functional capacity. Before initiating biologic therapy, the physician suspects neuropathic pain component. Which investigation is most appropriate to confirm the presence of neuropathic pain?

Explanation

## Diagnostic Approach to Neuropathic Pain **Key Point:** Quantitative Sensory Testing (QST) is the gold-standard non-invasive investigation for confirming small-fiber neuropathy and neuropathic pain phenotyping in chronic pain conditions. ### Why QST is the Investigation of Choice QST measures thermal thresholds (warm and cold detection), vibration perception, and pressure pain thresholds — directly assessing the sensory dysfunction characteristic of neuropathic pain. It provides objective, reproducible evidence of small-fiber dysfunction without invasiveness. **High-Yield:** QST is particularly valuable in: - Confirming small-fiber neuropathy when clinical suspicion is high - Phenotyping neuropathic pain (loss of function vs. gain of function) - Monitoring disease progression and treatment response - Distinguishing neuropathic from nociceptive pain in mixed pain syndromes ### Comparison with Other Investigations | Investigation | Role in Neuropathic Pain | Limitation | |---|---|---| | **QST** | Gold standard for phenotyping; confirms small-fiber dysfunction | Operator-dependent; requires patient cooperation | | **EMG/NCS** | Detects large-fiber neuropathy; useful for demyelinating/axonal patterns | Insensitive for small-fiber neuropathy; may be normal in early disease | | **MRI spine** | Evaluates structural causes (stenosis, disc herniation) | Does not confirm neuropathic pain mechanism; non-specific | | **Inflammatory markers** | Assess disease activity in RA | Do not diagnose neuropathic pain; measure inflammation, not neuropathy | **Clinical Pearl:** In rheumatoid arthritis patients, neuropathic pain may arise from: - Small-fiber neuropathy (RA-related or treatment-related) - Cervical myelopathy (structural) - Entrapment neuropathies (carpal tunnel) QST helps isolate the small-fiber component, guiding targeted treatment (e.g., gabapentinoids, duloxetine). **Warning:** EMG/NCS are often normal in small-fiber neuropathy because they assess large myelinated fibers; absence of EMG abnormality does NOT exclude neuropathic pain. [cite:Harrison 21e Ch 389]