Acute and chronic pain remain important health problems both in the United States and worldwide. With the rise in the prevalence of many chronic degenerative diseases across the globe, the distribution and absolute numbers of persons experiencing acute and chronic pain have continued to increase.[1] As a result, pain management has come to the forefront of the public health community.

The manifestation of pain itself typically involves the peripheral and central nervous systems.  Pain can classify as nociceptive, neuropathic or nocicplastic in origin. Nociceptive pain, also referred to as physiologic pain, results from activation of primary nociceptive afferents by actual or potential tissue-damaging stimuli. In nociceptive pain, large nerve integrity remains preserved as sensory receptors are stimulated within visceral and somatic structures. In contrast to nociceptive pain, neuropathic pain results for direct injury or disease affecting the somatosensory system and tends to be more disabling than nociceptive pain.[2]  Neuropathic pain subdivides into peripheral (e.g. diabetic neuropathy) and central (e.g., spinal cord injury or central poststroke pain), while nociceptive pain subcategorizes into somatic or visceral (e.g., inflammatory bowel disease). Recently, the International Association for the Study of Pain added a third category to pain classification of taxonomy for conditions that do not neatly fit into neuropathic or nocicplastic categories.  Nociplastic pain refers to pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage or evidence a disease or lesion directly affecting the somatosensory system.  Conditions considered to be nocicplastic pain include fibromyalgia, complex regional pain syndrome (CRPS) type I, and irritable bowel syndrome.  One should keep in mind that there are several pain conditions, such as failed back surgery syndrome, that contain elements of more than one pain category, and can be classified as “mixed” pain states.  In addition, conditions that are clearly classified as nociceptive and neuropathic often contain overlapping mechanisms with nocicplastic pain in that they involve abnormal nociceptive processing (e.g. amplified pain signals, expansion of receptive fields, decreased descending modulation). 

Pain can also categorize as acute, chronic, or a combination of these types (e.g. sickle cell crisis). Acute pain arises from a specific disease or injury and its duration is typically self-limited. Acute pain is considered to serve a protective biological purpose and is often associated with muscle spasm and sympathetic nervous system activation. In contrast, chronic pain may be considered a disease state, with its duration outlasting the normal time of healing when associated with disease or injury. Chronic pain may also stem from psychological states and does not serve an apparent biological purpose. Unlike the self-limited nature of acute pain, chronic pain often does not have a recognizable endpoint.

Acute pain often follows trauma or surgery and constitutes a signal to the brain regarding the presence of noxious stimuli or ongoing tissue damage.[3] This pain signal is adaptively useful, providing a warning of danger to the individual. Thus, acute pain results directly from the outcome of a noxious event and presents as a symptom of underlying tissue damage or disease. Acute injuries, including surgery, typically manifest as nociceptive pain.[4]

Whereas acute pain symptoms dissipate with the removal of the painful stimulus, chronic pain persists beyond the useful period of the pain signal and often continues after the initial tissue damage has resolved. Chronic pain may not be directly related to the initial tissue injury or disease condition, but instead may result secondarily to changes in the pain detection system, either as neuropathic pain (e.g., post-traumatic neuropathy, CRPS type II) or nocicplastic pain (e.g., CRPS type I).[2][3] Therefore, while acute pain and traumatic injury may precede the development of chronic pain, the mechanisms underlying chronic pain may differ from those implicated in acute pain.[3]

Experiencing acute or chronic pains are co-morbidities and complications, respectively, after nerve or tissue injury. As a result of this and a host of overlapping pain mechanisms at multiple sites (e.g., periphery, spinal cord, brain, descending modulatory systems), the etiology of pain remains complex. The four most common causes of pain are cancer, osteoarthritis, and rheumatoid arthritis, surgeries and non-iatrogenic trauma, and spinal problems.[4] Back pain and arthritis, in particular, are among the most commonly reported causes of chronic pain, accounting for up to one-third of all reported cases.[1][5] These conditions, along with depression, are also among the top three causes of years lost to disability.  Other disease states and conditions also associated with acute and chronic pain are diabetes, heart disease, depression, fibromyalgia, and asthma. There is significant overlap between chronic pain and depression in terms of co-morbidity and treatment, with many therapies effective for both indications.[6]

Pain, in both acute and chronic forms, remains a significant health problem both in the U.S. and worldwide. Aging of the world’s population has led to an increased number of individuals experiencing both acute injuries and chronic disease.[7] Although mortality for these conditions has decreased, their non-fatal dimensions and associated psychiatric and other comorbidities have resulted in an increase in years lost to disability.[8]

Among the leading causes of years lost to disability worldwide, four of the top ten low-back pain, neck pain, migraine, musculoskeletal disorders are pain-related.[8] Globally, estimates suggest that 20% of adults suffer from chronic pain with another 10% diagnosed each year.[1] Although some estimates suggest that 20 to 25% of the world’s population suffers from acute or chronic pain, others indicate its prevalence may be as high as 45%.[9][10] In the U.S., the Institute of Medicine estimates that chronic pain impacts 1 in 3 persons.[8] Among those with chronic pain, 15 to 25% of these patients experience pain of neuropathic etiology.[11] Chronic pain has also been observed to have an increased frequency in women and older individuals.[3][12][13] Women present more often with headaches, abdominal pain, and widespread chronic pain. Furthermore, non-Hispanic whites and blacks report the highest rates of chronic back pain, leg, feet, arm, and hand pain, and widespread pain.[14]

Chronic pain is among the most prevalent reasons persons seek medical care, with chronic pain patients utilizing health care services almost five times more frequently than the rest of the population.[15] This increased usage is often due to ill-defined conditions, lower priority chronic disease, acute disease, and concomitant psychopathology.[16] Acute pain often presents in the postoperative period, with estimates suggesting that as many as 80% of surgical patients experience significant postsurgical pain.[17]