Imaging pain

Tracey I : Imaging pain. British J Anaesthesia 2008;101(1):32-39

  • blood oxygen level dependent (BOLD)-functional magnetic resonance imaging(FMRI) detects is between 6 and 9s
  • Pain is a conscious experience, and interpretation of the nociceptive input influenced by memories, emotional, pathological, genetic, and cognitive factors.
  • Pain if highly subjective experience, as illustrated by the definition of the International Association for the Study of Pain: ‘An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.’
  • pain matrix
    • leteral (sensory-discriminatory involving areas such as primary somatosensory(S1), secondary somatosensory(S2), thalamus, and posterior parts of insula)
    • medial (affective-cognitive-evaluate areas like the anterior parts of insula, anterior cingulate cortex(ACC) and prefrontal cortex(PFC))
  • genetics
    • Individuals homozygous for the met158 allele of the catechol-O-methytransferase polimorphism (val158met) showed diminished regional mu-opioid system responses to pain
  • Cognition : attention and distraction
    • From these studies, regions that appear critical during the attentional modulation fo pain include the descending pain modulatory system and key elements of the pain matrix
  • The descending pain modulatory system
    • PAG periaqueductal grey
    • Valet and colleagues extended the work further and showed that the cingulofrontal cortex exerts top-down influenced on the PAG and posterior thalamus to gate pain modulation during distraction.
  • Context: the placebo effect
    • the author found that placebo analgesia was related to decreased brain activity in classic pain-processing brain regions (thalamus, insula, and ACC) but was additionally associated with increased activity during anticipation of pain in the PFC; an area involved in maintaining and updating internal representations of expectations.
  • Mood
    • For both chronic and acute pain, the sufferer’s mood and emotional state has a significant impact on resultant pain perception and ability to cope.
    • Critical regions involved in amplifying or exacerbating the pain experience include the antorhinal complex, amygdalae, anterior insula, and prefrontal cortices.
  • Another study on fibromyalgia patients found that pain catastrophizing, independent of the influence of depression, was significantly associated with increased activity in brain areas related to anticipation of pain (dorsal ACC, DLPFC), emotional aspects of pain (claustrum, closely connected to amygdala), and motor control.
  • The results by Gracely and colleagues pain perception through altering attention and anticipation, and also heightening the emotional responses to pain.
  • The PFC: controlling pain
    • We are beginning to unravel only now the roles of specific PFC regions in pain perception; it is thought that they reflect emotional, cognitive, and interoceptive components of pain conditions, and also perhaps processing of negative emotions, response conflict, and detection of unfavourable outcomes in relation to self for more medial PFC.
  • Specific role for the lateral PFC as a ‘pain control centre’ has been advanced in a study of experimentally induced allodynia in healthy subjects. In this study, increased lateral PFC activation was related to decreased pain affect, supposedly by inhibiting the functional connectivity between medial thalamus and mid-brain, thereby driving endogenous pain-inhibitory mechanisms.
  • These latest findings suggest that severe chronic pain could be considered a neurodegenerative disorder that especially affects this region.
  • Injury
    • There has been convincing evidence regarding the differential involvement of the periaqueductal grey(PAG), rostroventromedial medulla(RVM), parabrachial neclesu(PB), dorsal reticular nucleus, and nucleus cuneiformis (NCF) in the generation and maintenance of central sensitization states and hyperalgesia in both animal models, and ,for the first time, in human model of secondary hyperalgesia.
    • Changes within the descending pain-modulatory network in chronic pain in terms of patients having either a dysfunctional descending inhibitory system or an activated and enhanced descending facilitatory system, are clearly implicated in these and increasingly in other studies.
  • conclusions
    • Knowledge regarding how pain is perceived at a central level in humans is growing. An extensive network is recruited that is highly modifiable depending upon genetics the environment, mood, and the particular injury sustained. Combined, these produce a unique cerebral signature that produces an individualized pain experience.