A 58-year-old man from Delhi presents with severe, burning pain in his left leg following a motor vehicle accident 6 months ago. The pain is described as constant, with a sensation of 'pins and needles' and occasional electric shock-like quality. Physical examination reveals allodynia (pain with light touch) and hyperalgesia (exaggerated pain response to pinprick) in the affected limb. Nerve conduction studies and imaging are normal. He has already tried NSAIDs and paracetamol without relief. Which of the following mechanisms best explains the persistent pain despite normal structural imaging?

Explanation

## Neuropathic Pain and Central Sensitization **Key Point:** This clinical presentation is classic for neuropathic pain with central sensitization — a state of heightened neuronal responsiveness in the CNS despite normal peripheral anatomy. ### Pathophysiology of Central Sensitization Central sensitization occurs when repeated nociceptive input causes: 1. **NMDA receptor upregulation** — increased Ca²⁺ influx amplifies pain signal transmission 2. **Increased glutamate and substance P** — enhanced excitatory neurotransmitter release in the dorsal horn 3. **Loss of inhibitory tone** — reduced GABA and glycine-mediated inhibition 4. **Glial activation** — microglia and astrocytes release pro-inflammatory cytokines (IL-1β, TNF-α) **Clinical Pearl:** Allodynia (pain from non-noxious stimuli) and hyperalgesia (amplified pain from noxious stimuli) are hallmark signs of central sensitization, not peripheral nociceptor sensitization. ### Why Normal Imaging Does Not Exclude Neuropathic Pain | Feature | Peripheral Nociception | Central Sensitization | |---------|------------------------|----------------------| | Structural damage | Present | Absent or minimal | | Imaging findings | Abnormal | Normal | | Pain quality | Sharp, localized | Burning, diffuse, radiating | | Response to NSAIDs | Good | Poor | | Allodynia/hyperalgesia | Mild | Prominent | | Treatment | NSAIDs, local anesthetics | Gabapentin, pregabalin, SNRIs | **High-Yield:** The 6-month duration and failure of NSAIDs strongly suggest neuropathic pain with central sensitization rather than ongoing peripheral inflammation. ### Descending Pain Modulation Pathways ```mermaid flowchart TD A[Nociceptive input to dorsal horn]:::outcome --> B[NMDA receptor activation]:::action B --> C[↑ Intracellular Ca²⁺]:::action C --> D[Central sensitization]:::outcome D --> E[↓ Descending inhibition<br/>from brainstem]:::urgent E --> F[Loss of serotonin/<br/>norepinephrine tone]:::urgent F --> G[Amplified pain perception]:::outcome H[Periaqueductal gray<br/>Rostral ventromedial medulla] --> I[Serotonin/NE release<br/>to dorsal horn]:::action I --> J[Inhibition of pain transmission]:::action ``` **Mnemonic:** **CANS** — Central sensitization involves Amplified Nociception with Sensitized dorsal horn neurons. ### Treatment Implications Drugs that work in central sensitization: - **Gabapentin/Pregabalin** — block Ca²⁺ channels, reduce glutamate release - **SNRIs (venlafaxine, duloxetine)** — enhance descending serotonergic/noradrenergic inhibition - **Topical lidocaine** — reduces peripheral nociceptor input **Clinical Pearl:** NSAIDs are ineffective because the pain is no longer driven by peripheral inflammation but by central amplification of pain signals.