Reply to Hoegh et al.

Abstract

We thank the authors 3 for recognizing that "the suggested grading system (of nociplastic pain, italics added) may facilitate research"; however, we disagree with their view that "application in clinical settings seems premature."Hoegh et al. 3 argue that it is contradictory to talk about altered nociception in conditions lacking an identified source of nociceptive stimuli and that only hyperalgesia, not allodynia, can result from altered nociception. We thank the authors for raising this issue, giving us an opportunity to explain. The International Association for the Study of Pain (IASP) definition of "nociception" is "a process of encoding stimuli that is damaging or threatens damage to normal tissues." If the process of encoding these stimuli is altered resulting in increased responsiveness of nociceptive neurons to their normal input, or recruitment of a response to normally subthreshold inputs (ie, the IASP definition of sensitization), we end up with the activity in nociceptive pathways even in the absence of noxious stimuli. Sensitization results in the perception of normally nonpainful stimuli as painful (allodynia) and normally painful stimuli as even more painful (hyperalgesia); these are in fact the clinical signs of sensitization according to the IASP definition. Although we fully agree with Hoegh et al. 3 that sensitization is not specific for any pain type, be it nociceptive, neuropathic, or nociplastic, peripheral, or central, sensitization still remains a hallmark of nociplastic pain conditions. 1,5,6 Next, Hoegh et al. 3 question the inclusion of evoked pain (pain hypersensitivity) in the criteria and recommends only focusing on ongoing pain. However, we do not see how nociceptive, neuropathic, or nociplastic pain could be distinguished based solely on ongoing pain. In fact, sensory signs are required to classify probable neuropathic pain (and rightly so), although these sensory signs fail to distinguish between nonpainful and painful neuropathies. 2 On the contrary, the evoked pain hypersensitivity phenomena have been associated with the severity of nociplastic pain. 1,6 In addition, the proposed testing procedures for pain hypersensitivity are readily available in the clinic without the requirement of sophisticated technical equipment. 5 Furthermore, the authors 3 "find it problematic that the proposed algorithm relies on a new pain class, which has not been endorsed by the IASP (ie, pain of unknown origin)". Yes, we agree that this would be problematic; however, the nociplastic pain algorithm is not dependent on pain of unknown origin. Patients whose pain cannot be classified as nociceptive, neuropathic, or nociplastic have "pain not classifiable according to the current IASP mechanistic pain terminology", 4,5 in clinical practice referred to as "pain of unknown origin" or "idiopathic pain". In addition, Hoegh et al. 1 fear that patients not classified as having nociceptive, neuropathic, or nociplastic pain "may feel invalidated," yet they suggest implementing the term "pain of unknown origin...