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abnormal hair or nail growth, and bone changes. ANS: autonomic nervous system; CNS: central nervous system; CRPS: complex regional pain syndrome types I and II; CRPS II: complex regional pain syndrome type II; HIV: human immunodeficiency virus. based on pain duration (i.e., acute vs. chronic pain) and underlying pathophysiology (i.e., nociceptive vs. neuropathic pain) are used most often (see I.B.9-10). This section of the monograph explores the distinction between acute and chronic pain. It also reviews elements of a mixed pain classification system in which pain is categorized as acute pain, cancer pain, or chronic noncancer pain (CNCP). 1. Acute Pain Acute pain was once defined simply in terms of duration. It is now viewed as a “complex, unpleasant experience with emotional and cognitive, as well as sensory, features that occur in response to tissue trauma.”22 In contrast to chronic pain, relatively high levels of pathology usually accompany acute pain and the pain resolves with healing of the underlying injury. Acute pain is usually nociceptive, but may be neuropathic. Common sources of acute pain include trauma, surgery, labor, medical procedures, and acute disease states. Table 3 summarizes its key features. Acute pain serves an important biological function, as it warns of the potential for or extent of injury. A host of protective reflexes (e.g., withdrawal of a damaged limb, muscle spasm, autonomic responses) often accompany it. However, the “stress hormone response” prompted by acute injury also can have adverse physiologic and emotional effects (see I.D.3).13 Even brief intervals of painful stimulation can induce suffering, neuronal remodeling, and chronic pain;10 associated behaviors (e.g., bracing, abnormal postures, excessive reclining) may further contribute to the development of chronic pain. Therefore, increasing attention is being focused on the aggressive prevention and treatment of acute pain to reduce complications, including progression to chronic pain states.88 2. Chronic Pain Chronic pain was once defined as pain that extends 3 or 6 months beyond onset or beyond the expected period of healing.98 However, new definitions differentiate chronic pain from acute pain based on more than just time (Table 3). Chronic pain is now recognized as pain that extends beyond the period of healing, with levels of identified pathology that often are low and insufficient to explain the presence and/or extent of the pain.99 Chronic pain is also defined as a persistent pain that “disrupts sleep and normal living, ceases to serve a protective National Pharmaceutical Council 11 Section I: Background and Significance Table 3. Key Features of Pain Types and Syndromes Type of Pain Features Acute pain Pain usually concordant with degree of tissue damage, which remits with resolution of the injury Reflects activation of nociceptors and/or sensitized central neurons Often associated with ANS and other protective reflex responses (e.g., muscle spasm, “splinting”) Chronic pain Low levels of identified underlying pathology that do not explain the presence and/or extent of the pain Perpetuated by factors remote from the cause Continuous or intermittent with or without acute exacerbations Symptoms of ANS hyperactivity less common Irritability, social withdrawal, depressed mood and vegetative symptoms (e.g., changes in sleep, appetite, libido), disruption of work, and social relationships Cancer pain Strong relationship between tissue pathology and levels of pain Limited time frame that permits aggressive pain management Rarely involves medical-legal or disability issues CNCP Weak relationship between tissue pathology and pain levels Prolonged, potentially life-long, pain May involve medical, legal, disability issues/conflicts, work or relationship problems, physical deconditioning, psychological symptoms (see chronic pain above) May progress to CPS CPS Preoccupation with somatic functioning Lifestyle centered on seeking immediate pain relief, with excessive, nonproductive, and often harmful use of health care services Repeated attempts to obtain pain-related financial compensation (e.g., Social Security, Veterans’ benefits) Numerous symptoms and signs of psychosocial dysfunction that the patient attributes to the pain (e.g., anger, depression, anxiety, substance abuse, disrupted work or personal relationships) Sources: References 88 and 98-100. ANS: autonomic nervous system; CNCP: chronic noncancer pain; CPS: chronic pain syndrome; VA: Veterans Administration. function, and instead degrades health and functional capability.”101 Thus, unlike acute pain, chronic pain serves no adaptive purpose. Chronic pain may be nociceptive, neuropathic, or both and caused by injury (e.g., trauma, surgery), malignant conditions, or a variety of chronic non-life-threatening conditions (e.g., arthritis, fibromyalgia, neuropathy). In some cases, chronic pain exists de novo with no apparent cause. Although injury often initiates chronic pain, factors pathogenetically and physically remote from its cause may perpetuate it.98 Environmental and affective factors also can exacerbate and perpetuate chronic pain, leading to disability and maladaptive behavior. 3. Cancer Pain Pain associated with potentially life-threatening conditions such as cancer is often called “malignant pain” or “cancer pain.” However, there is movement
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