Tissue injury tends to fuel neuroplastic changes within the nervous system, which results in both peripheral and central sensitization. Clinically, this can manifest as hyperalgesia (exaggerated pain response to a normally painful stimulus) or allodynia (painful response to a typically nonpainful stimulus) (Fig. 56-3: Pain sensitization).
- The four elements of pain processing are transduction, transmission, modulation, and perception (Fig. 56-4: The four elements of pain processing are transduction, transmission, modulation and perception).
- Modulation of pain transmission involves altering afferent neural transmission along the pain pathway. The dorsal horn of the spinal cord is the most common site for modulation of the pain pathway, and modulation can involve either inhibition or augmentation of the pain signals. Examples of inhibitory spinal modulation include release of inhibitory neurotransmitters (γ-aminobutyric acid, glycine) and activation of descending efferent neuronal pathways (release of norepinephrine, serotonin, and endorphins in the dorsal horn).
- Spinal modulation that results in augmentation of pain pathways is a consequence of neuronal plasticity. The phenomenon of “wind-up” is an example of central plasticity that results from repetitive C-fiber stimulation of wide-dynamic range (WDR) neurons in the dorsal horn.
- A multimodal approach to pain therapy should target all four elements of the pain processing pathway.