IN LIGHT OF THE OPIOID EPIDEMIC, THE MEDICAL community is paying increased attention to pain. The annual volume of publications found under a PubMed search for “chronic pain” doubled in the decade between 2006 and 2016 and increased nearly fivefold in the 20-year period prior to 2016. In 2011 the Institute of Medicine published a landmark assessment entitled “Relieving Pain in America: A Blueprint for Transforming Prevention, Care, Education, and Research,” that outlined current shortcomings in pain.1 Most recently the consequences of opioid use and misuse have garnered substantial attention.2
These developments likely reflect, among other factors, the burden of disability due to pain and an evolving understanding of pain as a disorder. Evidence of the former exists in the 2013 Global Burden of Disease study, in which seven of the most common chronic conditions worldwide were primary pain conditions.3 Evidence of the latter comes from a large body of theoretical and experimental work, dating back to the mid-twentieth century, indicating that chronic pain (generally longer than 3 months’ duration) can be associated with changes throughout the central nervous system that are remote from external pain generators and may serve to sustain pain, distress, and disability independent of such generators. From the perspective of a specialist in physical medicine and rehabilitation (PM&R), chronic pain is a common cause of disability, loss of independence, and loss of life roles.
As research and clinical practice in pain have grown, the classification of painful conditions has been re-evaluated. The taxonomy of pain is still an evolving process that reflects these trends. In this chapter structures for the taxonomy of chronic pain and their rationales will be reviewed.
CHRONIC PAIN TAXONOMY CONSTRUCTS
Historically, pain conditions have been most indelibly associated with anatomic structures, thus hewing closely to chief complaints. An anatomically based pain taxonomy, with entities such as headache, low back pain, and pelvic pain, is readily understood by nonspecialist providers and people experiencing pain, and is in some sense canonized by virtue of inclusion in the International Classification of Disease coding system. Nonetheless each anatomic region includes several tissue types that can act as pain generators and several individual conditions with different postulated pathophysiological mechanisms.
A recent review illustrates this point with the example of chronic low back pain.4 The low back is a complex region composed of several tissue types, including bone, ligament, joints, disc, muscle, and neural elements including nerve terminals in multiple innervated structures, nerve roots, and sensory ganglia. The authors summarize the large body of experimental evidence of numerous pain mechanisms within and beyond these structures, including structural pathology that activates nociceptors innervating musculoskeletal elements, release of inflammatory cytokines in response to structural pathology, compression and subsequent dysfunction of neural elements in contact with structural pathology, and upstream modulation and modification of somatosensory structures. Furthermore, insofar as pain is ...