When does acute pain become chronic?

Abstract

The transition from acute to chronic pain appears to occur in discrete pathophysiological and histopathological steps. Stimuli initiating a nociceptive response vary, but receptors and endogenous defence mechanisms in the periphery interact in a similar manner regardless of the insult. Chemical, mechanical, and thermal receptors, along with leucocytes and macrophages, determine the intensity, location, and duration of noxious events. Noxious stimuli are transduced to the dorsal horn of the spinal cord, where amino acid and peptide transmitters activate second-order neurones. Spinal neurones then transmit signals to the brain. The resultant actions by the individual involve sensory-discriminative, motivational-affective, and modulatory processes in an attempt to limit or stop the painful process. Under normal conditions, noxious stimuli diminish as healing progresses and pain sensation lessens until minimal or no pain is detected. Persistent, intense pain, however, activates secondary mechanisms both at the periphery and within the central nervous system that cause allodynia, hyperalgesia, and hyperpathia that can diminish normal functioning. These changes begin in the periphery with upregulation of cyclo-oxygenase-2 and interleukin-1β-sensitizing first-order neurones, which eventually sensitize second-order spinal neurones by activating N-methyl-d-aspartic acid channels and signalling microglia to alter neuronal cytoarchitecture. Throughout these processes, prostaglandins, endocannabinoids, ion-specific channels, and scavenger cells all play a key role in the transformation of acute to chronic pain. A better understanding of the interplay among these substances will assist in the development of agents designed to ameliorate or reverse chronic pain.

Effects of intra‐operative maintenance of general anaesthesia with propofol on postoperative pain outcomes – a systematic review and meta‐analysis

“…Accumulating evidence has shown that propofol may modulate NMDA receptors in the central nervous system. Activation of NMDA receptors is a vital step in the initiation and maintenance of central sensitisation induced by peripheral nociceptor input [32]. Ionotropic NMDA receptors can be phosphorylated on several crucial locations on the C-terminus, which may activate the receptors as well as trafficking to or from the membrane.…”

Acetazolamide attenuates chemical-stimulated but not thermal-stimulated acute pain in mice

“…Thermal and chemical stimuli are the two main pathways that induce acute pain and have different neurobiological mechanisms. First, thermal-stimulated pain is transmitted by Aδ and C fibers, whereas chemical-stimulated pain is only transmitted by C fibers [9] . Second, the role of ion channels and receptors in thermal-stimulated acute pain is not completely the same as in chemical-stimulated acute pain [10] .…”

ATP receptors gate microglia signaling in neuropathic pain

“…pathophysiological steps that alter the cellular, molecular, and anatomical organization of nociceptive neural networks in the spinal dorsal horn (Latremoliere and Woolf, 2009;Scholz and Woolf, 2002;Voscopoulos and Lema, 2010;Woolf and Salter, 2000). In this pathologically altered system, the balance of inhibitory and excitatory control is shifted such that inhibitory mechanisms are weakened while excitatory mechanisms are strengthened.…”