Abstract Oliver Wilder-Smith

Chronic pain is increasingly recognized as an undesirable outcome after surgery. It is an ideal model for investigating the development of chronic pain as it permits longitudinal study in easily recruited cohorts. Understanding the processes underlying the development of chronic pain is essential for reliable risk prediction for postoperative pain persistence, as well as for the achievement of effective prevention or treatment of chronic pain.

Research over the last decade using advanced quantitative sensory and other testing methods to probe pain sensitivity and modulation has made it possible to start understanding the alterations in central pain processing associated with chronic pain and its development. In the course of this research, it has become evident that chronic pain syndromes are typically characterized by a pronociceptive state of pain processing. This state comprises, firstly, supraspinal central sensitization manifest as generalized heterotopic hyperalgesia; secondly, a shift towards facilitatory descending pain modulation as demonstrated by sensitization after conditioned pain modulation paradigm testing, and thirdly, alterations in spontaneous and non-evoked pain processing as evidenced by alterations in resting state EEG or fMRI.

In agreement with this, the development and progression of chronic pain in the perioperative context have been shown to be accompanied by signs congruent with a shift towards a pronociceptive state. Thus the preoperative presence of hyperalgesia and facilitatory descending modulation appear to increase the risk of subsequent chronic pain. Furthermore, the abnormal persistence and spread of hyperalgesia postoperatively, best interpreted as an unchecked rostral neuraxial spread of central sensitization, are increasingly linked to the development and progression of chronic pain. Finally, we now have available first EEG evidence of altered resting states in patients with persistent pain after surgery.

These findings, which need further confirmation, not only provide valuable insight into the mechanisms underlying chronic pain and its development, but also suggest that perioperative quantitative testing of pain processing has the potential to become a useful clinical tool for assessing risk of chronic pain development and for managing its prevention and treatment.