The nature of the transition from acute to chronic pain still eludes explanation, but chronic pain resulting from surgery provides a natural experiment that invites clinical epidemiological ...investigation and basic scientific inquiry into the mechanisms of this transition. The primary purpose of this article is to review current knowledge and hypotheses on the transition from acute to persistent postsurgical pain, summarizing literature on clinical epidemiological studies of persistent postsurgical pain development, as well as basic neurophysiological studies targeting mechanisms in the periphery, spinal cord, and brain. The second purpose of this article is to integrate theory, information, and causal reasoning in these areas. Conceptual mapping reveals 5 classes of hypotheses pertaining to pain. These propose that chronic pain results from: 1) persistent noxious signaling in the periphery; 2) enduring maladaptive neuroplastic changes at the spinal dorsal horn and/or higher central nervous system structures reflecting a multiplicity of factors, including peripherally released neurotrophic factors and interactions between neurons and microglia; 3) compromised inhibitory modulation of noxious signaling in medullary-spinal pathways; 4) descending facilitatory modulation; and 5) maladaptive brain remodeling in function, structure, and connectivity. The third purpose of this article is to identify barriers to progress and review opportunities for advancing the field. This review reveals a need for a concerted, strategic effort toward integrating clinical epidemiology, basic science research, and current theory about pain mechanisms to hasten progress toward understanding, managing, and preventing persistent postsurgical pain.
The development of chronic pain after surgery is a major clinical problem that provides an opportunity to study the transition from acute to chronic pain at epidemiologic and basic science levels. Strategic, coordinated, multidisciplinary research efforts targeting mechanisms of pain chronification can to help minimize or eliminate persistent postsurgical pain.
Neuropathic pain can develop after nerve injury, when deleterious changes occur in injured neurons and along nociceptive and descending modulatory pathways in the central nervous system. The myriad ...neurotransmitters and other substances involved in the development and maintenance of neuropathic pain also play a part in other neurobiological disorders. This might partly explain the high comorbidity rates for chronic pain, sleep disorders, and psychological conditions such as depression, and why drugs that are effective for one condition may benefit others. Neuropathic pain can be distinguished from non-neuropathic pain by two factors. Firstly, in neuropathic pain there is no transduction (conversion of a nociceptive stimulus into an electrical impulse). Secondly, the prognosis is worse: injury to major nerves is more likely than injury to non-nervous tissue to result in chronic pain. In addition, neuropathic pain tends to be more refractory than non-neuropathic pain to conventional analgesics, such as non-steroidal anti-inflammatory drugs and opioids. However, because of the considerable overlap between neuropathic and nociceptive pain in terms of mechanisms and treatment modalities, it might be more constructive to view these entities as different points on the same continuum. This review focuses on the mechanisms of neuropathic pain, with special emphasis on clinical implications.
Chronic pain is a major source of suffering. It interferes with daily functioning and often is accompanied by distress. Yet, in the International Classification of Diseases, chronic pain diagnoses ...are not represented systematically. The lack of appropriate codes renders accurate epidemiological investigations difficult and impedes health policy decisions regarding chronic pain such as adequate financing of access to multimodal pain management. In cooperation with the WHO, an IASP Working Group has developed a classification system that is applicable in a wide range of contexts, including pain medicine, primary care, and low-resource environments. Chronic pain is defined as pain that persists or recurs for more than 3 months. In chronic pain syndromes, pain can be the sole or a leading complaint and requires special treatment and care. In conditions such as fibromyalgia or nonspecific low-back pain, chronic pain may be conceived as a disease in its own right; in our proposal, we call this subgroup "chronic primary pain." In 6 other subgroups, pain is secondary to an underlying disease: chronic cancer-related pain, chronic neuropathic pain, chronic secondary visceral pain, chronic posttraumatic and postsurgical pain, chronic secondary headache and orofacial pain, and chronic secondary musculoskeletal pain. These conditions are summarized as "chronic secondary pain" where pain may at least initially be conceived as a symptom. Implementation of these codes in the upcoming 11th edition of International Classification of Diseases will lead to improved classification and diagnostic coding, thereby advancing the recognition of chronic pain as a health condition in its own right.
Background
Topical analgesic drugs are used for a variety of painful conditions. Some are acute, typically strains or sprains, tendinopathy, or muscle aches. Others are chronic, typically ...osteoarthritis of hand or knee, or neuropathic pain.
Objectives
To provide an overview of the analgesic efficacy and associated adverse events of topical analgesics (primarily nonsteroidal anti‐inflammatory drugs (NSAIDs), salicylate rubefacients, capsaicin, and lidocaine) applied to intact skin for the treatment of acute and chronic pain in adults.
Methods
We identified systematic reviews in acute and chronic pain published to February 2017 in the Cochrane Database of Systematic Reviews (the Cochrane Library). The primary outcome was at least 50% pain relief (participant‐reported) at an appropriate duration. We extracted the number needed to treat for one additional beneficial outcome (NNT) for efficacy outcomes for each topical analgesic or formulation, and the number needed to treat for one additional harmful outcome (NNH) for adverse events. We also extracted information on withdrawals due to lack of efficacy or adverse events, systemic and local adverse events, and serious adverse events. We required information from at least 200 participants, in at least two studies. We judged that there was potential for publication bias if the addition of four studies of typical size (400 participants) with zero effect increased NNT compared with placebo to 10 (minimal clinical utility). We extracted GRADE assessment in the original papers, and made our own GRADE assessment.
Main results
Thirteen Cochrane Reviews (206 studies with around 30,700 participants) assessed the efficacy and harms from a range of topical analgesics applied to intact skin in a number of acute and chronic painful conditions. Reviews were overseen by several Review Groups, and concentrated on evidence comparing topical analgesic with topical placebo; comparisons of topical and oral analgesics were rare.
For at least 50% pain relief, we considered evidence was moderate or high quality for several therapies, based on the underlying quality of studies and susceptibility to publication bias.
In acute musculoskeletal pain (strains and sprains) with assessment at about seven days, therapies were diclofenac Emulgel (78% Emulgel, 20% placebo; 2 studies, 314 participants, NNT 1.8 (95% confidence interval 1.5 to 2.1)), ketoprofen gel (72% ketoprofen, 33% placebo, 5 studies, 348 participants, NNT 2.5 (2.0 to 3.4)), piroxicam gel (70% piroxicam, 47% placebo, 3 studies, 522 participants, NNT 4.4 (3.2 to 6.9)), diclofenac Flector plaster (63% Flector, 41% placebo, 4 studies, 1030 participants, NNT 4.7 (3.7 to 6.5)), and diclofenac other plaster (88% diclofenac plaster, 57% placebo, 3 studies, 474 participants, NNT 3.2 (2.6 to 4.2)).
In chronic musculoskeletal pain (mainly hand and knee osteoarthritis) therapies were topical diclofenac preparations for less than six weeks (43% diclofenac, 23% placebo, 5 studies, 732 participants, NNT 5.0 (3.7 to 7.4)), ketoprofen over 6 to 12 weeks (63% ketoprofen, 48% placebo, 4 studies, 2573 participants, NNT 6.9 (5.4 to 9.3)), and topical diclofenac preparations over 6 to 12 weeks (60% diclofenac, 50% placebo, 4 studies, 2343 participants, NNT 9.8 (7.1 to 16)). In postherpetic neuralgia, topical high‐concentration capsaicin had moderate‐quality evidence of limited efficacy (33% capsaicin, 24% placebo, 2 studies, 571 participants, NNT 11 (6.1 to 62)).
We judged evidence of efficacy for other therapies as low or very low quality. Limited evidence of efficacy, potentially subject to publication bias, existed for topical preparations of ibuprofen gels and creams, unspecified diclofenac formulations and diclofenac gel other than Emulgel, indomethacin, and ketoprofen plaster in acute pain conditions, and for salicylate rubefacients for chronic pain conditions. Evidence for other interventions (other topical NSAIDs, topical salicylate in acute pain conditions, low concentration capsaicin, lidocaine, clonidine for neuropathic pain, and herbal remedies for any condition) was very low quality and typically limited to single studies or comparisons with sparse data.
We assessed the evidence on withdrawals as moderate or very low quality, because of small numbers of events. In chronic pain conditions lack of efficacy withdrawals were lower with topical diclofenac (6%) than placebo (9%) (11 studies, 3455 participants, number needed to treat to prevent (NNTp) 26, moderate‐quality evidence), and topical salicylate (2% vs 7% for placebo) (5 studies, 501 participants, NNTp 21, very low‐quality evidence). Adverse event withdrawals were higher with topical capsaicin low‐concentration (15%) than placebo (3%) (4 studies, 477 participants, NNH 8, very low‐quality evidence), topical salicylate (5% vs 1% for placebo) (7 studies, 735 participants, NNH 26, very low‐quality evidence), and topical diclofenac (5% vs 4% for placebo) (12 studies, 3552 participants, NNH 51, very low‐quality evidence).
In acute pain, systemic or local adverse event rates with topical NSAIDs (4.3%) were no greater than with topical placebo (4.6%) (42 studies, 6740 participants, high quality evidence). In chronic pain local adverse events with topical capsaicin low concentration (63%) were higher than topical placebo (5 studies, 557 participants, number needed to treat for harm (NNH) 2.6), high quality evidence. Moderate‐quality evidence indicated more local adverse events than placebo in chronic pain conditions with topical diclofenac (NNH 16) and local pain with topical capsaicin high‐concentration (NNH 16). There was moderate‐quality evidence of no additional local adverse events with topical ketoprofen over topical placebo in chronic pain. Serious adverse events were rare (very low‐quality evidence).
GRADE assessments of moderate or low quality in some of the reviews were considered by us to be very low because of small numbers of participants and events.
Authors' conclusions
There is good evidence that some formulations of topical diclofenac and ketoprofen are useful in acute pain conditions such as sprains or strains, with low (good) NNT values. There is a strong message that the exact formulation used is critically important in acute conditions, and that might also apply to other pain conditions. In chronic musculoskeletal conditions with assessments over 6 to 12 weeks, topical diclofenac and ketoprofen had limited efficacy in hand and knee osteoarthritis, as did topical high‐concentration capsaicin in postherpetic neuralgia. Though NNTs were higher, this still indicates that a small proportion of people had good pain relief.
Use of GRADE in Cochrane Reviews with small numbers of participants and events requires attention.
Accurate classification of chronic pain conditions requires reliable and valid pain assessment. Moreover, pain assessment serves several additional functions, including documenting the severity of ...the pain condition, tracking the longitudinal course of pain, and providing mechanistic information. Thorough pain assessment must address multiple domains of pain, including the sensory and affective qualities of pain, temporal dimensions of pain, and the location and bodily distribution of pain. Where possible, pain assessment should also incorporate methods to identify pathophysiological mechanisms underlying the pain. This article discusses assessment of chronic pain, including approaches available for assessing multiple pain domains and for addressing pathophysiological mechanisms. We conclude with recommendations for optimal pain assessment.
Pain assessment is a critical prerequisite for accurate pain classification. This article describes important features of pain that should be assessed, and discusses methods that can be used to assess the features and identify pathophysiological mechanisms contributing to pain.
Prevalence of pain as a recurrent symptom in children is known to be high, but little is known about children with high impairment from chronic pain seeking specialized treatment. The purpose of this ...study was the precise description of children with high impairment from chronic pain referred to the German Paediatric Pain Centre over a 5-year period.
Demographic variables, pain characteristics and psychometric measures were assessed at the first evaluation. Subgroup analysis for sex, age and pain location was conducted and multivariate logistic regression applied to identify parameters associated with extremely high impairment.
The retrospective study consisted of 2249 children assessed at the first evaluation. Tension type headache (48%), migraine (43%) and functional abdominal pain (11%) were the most common diagnoses with a high rate of co-occurrence; 18% had some form of musculoskeletal pain disease. Irrespective of pain location, chronic pain disorder with somatic and psychological factors was diagnosed frequently (43%). 55% of the children suffered from more than one distinct pain diagnosis. Clinically significant depression and general anxiety scores were expressed by 24% and 19% of the patients, respectively. Girls over the age of 13 were more likely to seek tertiary treatment compared to boys. Nearly half of children suffered from daily or constant pain with a mean pain value of 6/10. Extremely high pain-related impairment, operationalized as a comprehensive measure of pain duration, frequency, intensity, pain-related school absence and disability, was associated with older age, multiple locations of pain, increased depression and prior hospital stays. 43% of the children taking analgesics had no indication for pharmacological treatment.
Children with chronic pain are a diagnostic and therapeutic challenge as they often have two or more different pain diagnoses, are prone to misuse of analgesics and are severely impaired. They are at increased risk for developmental stagnation. Adequate treatment and referral are essential to interrupt progression of the chronic pain process into adulthood.
Background & Aims Chronic stress alters the hypothalamic–pituitary–adrenal axis, increases gut motility, and increases the perception of visceral pain. We investigated whether epigenetic mechanisms ...regulate chronic stress-induced visceral pain in the peripheral nervous systems of rats. Methods Male rats were subjected to 1 hour of water avoidance stress each day, or given daily subcutaneous injections of corticosterone, for 10 consecutive days. L4–L5 and L6–S2 dorsal root ganglia (DRG) were collected and compared between stressed and control rats (placed for 1 hour each day in a tank without water). Levels of cannabinoid receptor 1 (CNR1), DNA (cytosine-5-)-methyltransferase 1 (DNMT1), transient receptor potential vanilloid type 1 (TRPV1), and EP300 were knocked down in DRG neurons in situ with small interfering RNAs. We measured DNA methylation and histone acetylation at genes encoding the glucocorticoid receptor (NR3C1), CNR1, and TRPV1. Visceral pain was measured in response to colorectal distention. Results Chronic stress was associated with increased methylation of the Nr3c1 promoter and reduced expression of this gene in L6–S2, but not L4–L5, DRGs. Stress also was associated with up-regulation in DNMT1-associated methylation of the Cnr1 promoter and down-regulation of glucocorticoid-receptor–mediated expression of CNR1 in L6–S2, but not L4–L5, DRGs. Concurrently, chronic stress increased expression of the histone acetyltransferase EP300 and increased histone acetylation at the Trpv1 promoter and expression of the TRPV1 receptor in L6–S2 DRG neurons. Knockdown of DNMT1 and EP300 in L6–S2 DRG neurons of rats reduced DNA methylation and histone acetylation, respectively, and prevented chronic stress-induced increases in visceral pain. Conclusions Chronic stress increases DNA methylation and histone acetylation of genes that regulate visceral pain sensation in the peripheral nervous system of rats. Blocking epigenetic regulatory pathways in specific regions of the spinal cord might be developed to treat patients with chronic abdominal pain.
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