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Chapter 73 - Chronic Pain

John C. Rowlingson

The symptom of pain is prevalent, as reflected in data noting that it "is the number one reason Americans visit their doctors."^[1] At least 8% of all primary care visits are related to musculoskeletal disease, and 12% of all medications prescribed after primary care office visits are for pain relief. ^[2] As many as 65% to 80% of the population of the United States will experience an episode of low back pain in their lifetime,^[3] and 25 million Americans have osteoarthritis or rheumatoid arthritis.^[4] Harstall and Ospina reviewed international data in 13 studies about the incidence of chronic pain and reported a range of prevalence of 10.1% to 55.2% of the population. ^[5] The mismanagement or under-treatment of "pain" results in undesired, anguishing personal struggles, disruption of vocational and recreational pursuits, psychosocial consequences, patient dissatisfaction, adverse clinical and functional outcomes, and untold costs for health care. Estimates vary, but figures tabulate costs as high as $100 billion/year in this country alone.^{[1] [6]}

Whatever the form in which pain presents, the abiding definition of pain as a "sensory and emotional experience associated with actual tissue damage or described in terms of such damage" endures.^[7] The current perception is that a lengthy duration of pain creates more havoc both within and external to the central nervous system (CNS), making long-lasting pain a fundamentally distinct disease along the continuum of pain from that which is new in onset.^[8] Apfelbaum and colleagues have documented that even acute pain (e.g., postoperative pain) is under-treated in spite of our having a remarkable array of treatments available.^[9]

Perkins and co-workers provided insight into the reality that poorly managed acute pain can lead to the occurrence of chronic pain.^[10] They selected patients who were to undergo limb amputation, breast surgery, gallbladder surgery, lung surgery, or inguinal hernia repair because of the high incidence of pain associated with these procedures. They hoped to detect preoperative, intraoperative, and postoperative factors that predicted the patient's development of chronic pain and documented that the presence of pain preoperatively is a predictor of chronic pain in patients undergoing amputations, breast surgery, and cholecystectomy. Intraoperative nerve damage was a significant factor, as in patients undergoing thoracotomy and mastectomy (with intercostal and intercostobrachial nerves at particular risk). The severity of postoperative pain following breast, thoracic, and hernia surgery was a potent predictor of subsequent chronic pain. In a review of pain mechanisms, Basbaum reflected the prevailing contemporary view, stating that "persistent pain should be considered a disease of the nervous system, not merely a symptom of some other disease condition."^[11]

Dworkin has shown that an individual's psychosocial circumstances influence the transition from acute to chronic pain.^[12] Gatchell and associates showed that personality disorders, but not psychopathology, increased the likelihood that a patient with acute low back pain will develop chronic low back pain, because the former reflected psychosocial susceptibility and/or deficient coping skills.^[13] The study by Perkins and colleagues of acute pain operations leading to chronic pain syndromes identified this same "psychosocial vulnerability" in their patients.^[10]

Pain historically has been categorized as acute or chronic for purposes of clearly distinguishing these disease processes. The contemporary terms inflammatory and neuropathic, respectively, delineate more accurately the primary pathology involved in the cause of pain.^{[11] [14]} The expectation in patients having inflammatory (acute) pain (e.g., postoperative pain) is that there will be a thorough response to treatment, or spontaneous resolution of the pain. In neuropathic (chronic) pain, this is much less likely, because in addition to the disruption of neural function associated with inflammatory pain, there is a significant time element which provokes enduring alterations in the CNS processing of pain as well as the embedding of pain-related behaviors.^{[11] [15] [16]} Put another

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