From preemptive to preventive analgesia: time to reconsider the role of perioperative peripheral nerve blocks?

W hen I have read the article from Carvalho et al 1 published in this issue of Regional Anesthesia and Pain Medicine, two words caught my eyes, Bpreemptive[ and Bnerve block.[ Why? First, because in the light of our current knowledge about incisional pain mechanisms, the term preemptive is definitely obsolete. Second, because when we consider the terms preemptive analgesia and preventive analgesia in the literature, perineural techniques that have become increasingly popular, for example, plexus and peripheral nerve blocks, have received very little attention in comparison to neuraxial blocks and systemic treatments. Preemptive analgesia has been defined as an antinociceptive intervention that starts before surgical incision and is more effective in relieving acute postoperative pain than the same treatment starting after surgery. The rationale for preemptive analgesia was based on the block of central sensitization before it occurs. Effectively, nociceptive signals initiated by tissue injury induce a state of central nervous system hyperactivity, also called central sensitization, which facilitates pain (ie, amplification and memorization). However, studies in animal model of incisional pain have clearly shown that single analgesic treatment (either peripheral or neuraxial) before the incision does not reduce postoperative pain behaviors beyond the expected duration of the analgesic effect. When the block of nociceptive afferents abates, the wound is able to reinitiate central sensitization. Clinical trials have found similar results. Therefore, not the timing but both the duration and the efficacy of a perioperative analgesic intervention are important in treating postoperative pain and in preventing central sensitization. It is worth noting that when central sensitization has fully developed, it can become less dependent and even independent of peripheral nociceptive inputs. Treatments that attempt to reduce central neuronal hyperactivity should not be called preemptive analgesia, irrespective of the time the treatment is administered. Preventive analgesia is aimed to block the development of sustained pain. This broader definition includes any regimen given at any time during the perioperative period that will be able to control pain-induced sensitization. Central neuronal sensitization participates to the postoperative pain experience and might be one of the mechanisms underlying the development of persistent pain after tissue trauma.5 However, to date, the exact role of central sensitization in the magnitude and the duration of acute postoperative pain remains undetermined. Interestingly, drugs like systemic ketamine or spinal clonidine, which are able to modulate central sensitization and to affect the incidence of persistent postsurgical pain do not reduce acute postoperative pain. Continuous peripheral nerve blocks allow an effective control of postoperative pain, specifically pain associated to mobilization, which is the most difficult to alleviate, as well as reduce opioid-related adverse effects. Their success relies not only on the quite recent use of ultrasounds to perform the blocks but also on the safety of these techniques. As a consequence, perineural analgesia is currently favored over epidural techniques for limbs surgery and is now evaluated as an alternative to epidural blocks for thoracic procedures. Nevertheless, despite their clinical success, the real benefit of peripheral nerve blocks in preventive analgesia remains poorly determined. Because peripheral mechanisms strongly contribute to hypersensitivity and central sensitization after incision, an intense peripheral nerve block should prevent nociceptive inputs to reach the central nervous system and therefore should reduce the development of central sensitization and decrease the risk for persistent postsurgical pain. Unfortunately, there seems to be no further benefit of extending an overnight continuous peripheral nerve block to 48 hrs and even to 4 days after knee or hip arthroplasty on the improvement of health-related quality of life including pain between 6 weeks and 12 months after surgery. EDITORIAL