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Vachon-Presseau E, Centeno MV et al. J Dent Res 2016; 95: 605–612

The authors set a scholarly tone for this Critical Review by drawing the distinction between qualia ('raw feels' that is a perception in isolation from any effect it may have on behaviour), and chronic pain that 'persists in the absence of stark inputs' and is 'no longer coupled with an appropriate behavioral repertoire.' Looked at in another way, nociceptive mechanisms protect the body from harm with the pain subsiding as the injury resolves, in contrast to chronic pain that manifests itself as 'persistence of pain past the normal healing period'.

Human neuroimaging studies in those with chronic pain have shown 'that different chronic pain conditions are associated with distinct brain properties' challenging the proposition that chronic pain is the result of 'an increased and/or sustained nociceptive barrage impinging on the cortex'; factors such as 'demographics, affective states, lifestyle, comorbidities, and others' account for only a relatively small amount of variance in those suffering from chronic pain.

The emotional brain (corticolimbic system), which is pivotal to reward and motivated behaviour, is also a modulator of acute pain but a mediator for chronic pain. When considering chronic pain, connections between the nucleus accumbens and medial prefrontal cortex (mPFC) predict those who will develop persisting pain following acute back pain. Then chemical and structural changes in the brain have been shown in those with temporomandibular disorder in that there is 'a decrease in striatal dopaminergic uptake...and with changes in gray and white matter properties of corticolimbic regions'. In addition, patients with burning mouth syndrome show 'structural differences in the hippocampus and mPFC, as well as increased functional connectivity between frontal and limbic regions...'. And those who suffer chronic pain following dental surgery have 'higher cerebral blood flow in the thalamus, frontal regions and sensorimotor cortex'. This does not, however, refute the observation that psychological and personality traits have a role in chronic pain. The amygdala and hippocampal volumes have been associated with several psychiatric disorders and chronic pain conditions.

As recurring pain would appear to reorganise the brain circuitry, a research priority is to identify optimal timings for therapeutic interventions to minimise this transition from acute to chronic pain. D-cycloserine, used also as a second-line drug for the treatment of tuberculosis, and sarcosine, found in several foods and used to manufacture SLS in toothpaste and dopamine, may each have a role in preventing the transition to and the management of chronic pain.