A 52-year-old man from rural Maharashtra presents with severe, burning pain in his right lower limb that began 3 weeks ago following a crush injury to the foot. On examination, the limb is swollen with glossy skin and increased temperature. Pain is disproportionate to the clinical findings. He reports that even light touch (allodynia) and pinprick (hyperalgesia) intensify the pain dramatically. Passive range of motion of the ankle is restricted due to pain. Which of the following mechanisms best explains the disproportionate pain response in this patient?

Explanation

## Clinical Diagnosis: Complex Regional Pain Syndrome (CRPS) This patient presents with classic features of CRPS Type 1 (reflex sympathetic dystrophy): disproportionate pain, allodynia, hyperalgesia, swelling, skin changes, and motor dysfunction following a crush injury. ## Pathophysiology of Pain Amplification in CRPS **Key Point:** Central sensitization is the primary mechanism driving the exaggerated pain response in CRPS. This involves: 1. **Wind-up phenomenon**: Repeated C-fibre nociceptor stimulation causes progressive increase in dorsal horn neuron firing (temporal summation of EPSC). 2. **NMDA receptor activation**: Excessive glutamate release triggers NMDA-mediated calcium influx, lowering the threshold for future stimuli. 3. **Loss of descending inhibition**: Reduced noradrenaline and serotonin from brainstem (locus coeruleus and raphe nuclei) diminishes tonic inhibition of pain. 4. **Microglial activation**: Spinal cord glia release pro-inflammatory cytokines (IL-6, TNF-α), amplifying pain signalling. **High-Yield:** Allodynia and hyperalgesia in CRPS are hallmarks of central sensitization, NOT peripheral nociceptor dysfunction. The pain is amplified at the spinal cord level, not the periphery. ## Why Central Sensitization Explains the Clinical Picture | Feature | Mechanism | |---------|----------| | Allodynia (pain to light touch) | Lowered dorsal horn neuron threshold; Aβ fibres now activate pain neurons | | Hyperalgesia (exaggerated pain to noxious stimulus) | Wind-up and NMDA-mediated amplification | | Disproportionate pain | Central amplification exceeds peripheral tissue damage | | Swelling and vasomotor changes | Neurogenic inflammation from substance P and CGRP release | **Clinical Pearl:** CRPS pain is maintained by a vicious cycle: initial injury → nociceptor sensitization → central sensitization → sympathetic dysfunction → peripheral vasoconstriction → tissue hypoxia → further nociceptor activation. ## Gate Control Theory Context The gate-control mechanism (Melzack & Wall) is INTACT but overwhelmed in CRPS. Normally, Aβ fibre input closes the "pain gate" in the dorsal horn (via enkephalin interneurons). However, in central sensitization, the gate is forced open by excessive C-fibre input and loss of descending inhibition.