J Pain. 2016 (Sep); 17 (9 Suppl): T1–9 ~ FULL TEXT
Robert H. Dworkin, Stephen Bruehl, Roger B. Fillingim,
John D. Loeser, Gregory W. Terman, Dennis C. Turk
Departments of Anesthesiology, Neurology, and Psychiatry,
University of Rochester,
Rochester, New York.
A variety of approaches have been used to develop diagnostic criteria for chronic pain. The published evidence of the reliability and validity of existing diagnostic criteria is limited, and these criteria have typically not been used in clinical practice. The availability of a widely accepted, consistently applied, and evidence-based taxonomy of diagnostic criteria would improve the quality of clinical research on chronic pain and would be of great value in clinical practice. To address the need for evidence-based diagnostic criteria for the major chronic pain conditions, the Analgesic, Anesthetic, and Addiction Clinical Trial Translations, Innovations, Opportunities, and Networks (ACTTION) public-private partnership with the US Food and Drug Administration and the American Pain Society (APS) have collaborated on the development of the ACTTION-APS Pain Taxonomy (AAPT). AAPT provides a multidimensional framework that is applied systematically in the development of diagnostic criteria. This article (1) describes the background and rationale for AAPT; (2) presents the AAPT taxonomy and the specific conditions for which diagnostic criteria have been developed (to be published separately); (3) briefly reviews the 5 dimensions that constitute the AAPT multidimensional framework and describes the 7 accompanying articles that discuss these dimensions and other important issues involving AAPT; and (4) provides an overview of next steps, specifically, the general processes by which the initial set of diagnostic criteria (for which the evidence base has been drawn from the literature, systematic reviews, and secondary analyses of existing databases) will undergo additional assessments of reliability and validity.
PERSPECTIVE: To address the need for evidence-based diagnostic criteria for the major chronic pain conditions, the AAPT provides a multidimensional framework that is applied systematically in the development of diagnostic criteria. The long-term objective of AAPT is to advance the scientific understanding of chronic pain and its treatment.
KEYWORDS: Chronic pain; classification; diagnostic criteria; taxonomy
From the FULL TEXT Article:
In many areas of medicine, the validity and utility of diagnostic criteria have increased appreciably in recent decades. Evidence-based diagnostic criteria have had widespread adoption in clinical research and clinical practice, as has occurred, for example, with the Diagnostic and Statistical Manual (DSM) criteria for psychiatric disorders. [1, 2] Unfortunately, the diagnosis of acute and chronic pain conditions has not been a focus of sustained efforts to improve validity and utility. The International Association for the Study of Pain (IASP) Classification of Chronic Pain was first published in 1979 and revised in 1994  (with an update in 2011–2012). Although it describes many chronic pain conditions, research has not confirmed the reliability of the diagnostic clasification  and it has seen limited application in clinical research and practice. In addition, despite widespread interest in the development of a mechanism-based approach to pain diagnosis, [16, 30, 32–34] current knowledge of the pathophysiologic mechanisms of acute and chronic pain and methods to identify these mechanisms in patients is not sufficiently advanced to provide the basis for classification and diagnosis. Thus, there is a critical unmet need for standardized, consistent, and evidence-based diagnostic criteria for acute and chronic pain conditions that incorporate the neurobiological and psychosocial mechanisms and consequences of pain.
As the initial step in addressing this overarching goal, the Analgesic, Anesthetic, and Addiction Clinical Trial Translations, Innovations, Opportunities, and Networks (ACTTION) public-private partnership with the US Food and Drug Administration (FDA) and the American Pain Society (APS) have collaborated on the development of the ACTTION-APS Pain Taxonomy (AAPT) for chronic pain. AAPT is an evidence-based pain taxonomy in which a multidimensional diagnostic framework has been applied to the most prevalent and important chronic pain conditions. A major impetus for the AAPT initiative derived from observing the transformative impact of evidence-based diagnostic classifications in related medical specialties. For example, DSM-III revolutionized clinical research and practice in psychiatry by implementing structured evidence-based diagnostic criteria to replace previous theory-based approaches, which were often applied in an idiosyncratic manner and lacked reliability. [1, 17] The DSM-III multiaxial framework provided structured diagnostic criteria as well as a standardized format for additional clinically relevant information, including comorbid medical conditions, psychosocial stressors, and overall functioning. DSM-III has been revised on the basis of accumulating evidence (the most recent edition is DSM-52), and the DSM approach  can serve as a valuable model for the development and revision of evidence-based diagnostic criteria for chronic pain. Another successful diagnostic system is the International Classification of Headache Disorders (ICHD), an evidence-based classification of headache. [21, 22] The ICHD, now in its third edition,  was first released in 1988 and has made an invaluable contribution to headache research and clinical practice. The ICHD is the gold standard for clinical research, including clinical trials, and has greatly facilitated the development of evidence-based treatments for headache.
A consistent evidence-based taxonomy for classifying and diagnosing chronic pain conditions would have an important impact on pain research and practice. First, the taxonomy would be aligned with current knowledge of the biopsychosocial mechanisms of pain, which has increased greatly in the past few decades. Existing approaches to the classification and diagnosis of chronic pain are based primarily on symptoms, signs, and body location and only rarely include information regarding putative mechanisms and risk factors (eg, magnetic resonance imaging evidence of spinal stenosis, catastrophizing). Because pain treatments produce their therapeutic effects by targeting neurobiological or psychosocial mechanisms, a taxonomy that includes knowledge of these mechanisms would play an important role in developing and validating personalized approaches to pain treatment (ie, precision pain medicine).
In addition, a taxonomy in which diagnostic criteria for all chronic pain conditions are consistently implemented will facilitate communication about chronic pain research by ensuring comparability across studies of the same condition. At present, definitions of the same chronic pain condition can vary widely across clinical research studies, including clinical trials, which limits the ability to compare findings across studies and conduct meta-analyses. A widely accepted, consistently applied, and evidence-based taxonomy would help overcome this obstacle and thereby potentially accelerate the development of more efficacious treatments. In particular, a standard taxonomy could enhance clinical trial methodology by promoting consistently applied inclusion and exclusion criteria.
Moreover, diagnostic criteria possess high utility to the extent that they provide important information about prognosis, treatment response, and biological and psychosocial factors.  From this perspective, a diagnosis has utility when it has implications that “it is clinically unsound to ignore.”  The AAPT framework and diagnostic criteria can therefore make a major contribution to pain clinical practice and pain education by providing an evidence-based foundation for understanding and evaluating the signs, symptoms, and mechanisms of the most prevalent chronic pain conditions. Finally, by identifying gaps in the evidence regarding the diagnosis of particular pain conditions and their biopsychosocial mechanisms and consequences, AAPT highlights important unmet needs for future research.
To develop the multidimensional framework and chronic pain taxonomy, AAPT held a consensus meeting in May 2013, which brought together clinical and basic scientists with expertise in pain mechanisms, the major adult and pediatric chronic pain conditions, and the development of evidence-based diagnostic criteria for several specific chronic pain conditions. In tandem with a series of presentations, the meeting was devoted to building consensus regarding a multidimensional framework and the taxonomy of chronic pain conditions to which the framework would be applied. Fillingim and colleagues  presented the resulting multidimensional framework and taxonomy in a previous article.
In developing the AAPT framework, attention was devoted to 3 previous efforts that developed evidence-based diagnostic criteria for different chronic pain conditions:
(1) the research diagnostic criteria for temporomandibular disorders (RDC/TMD),  which on the basis of multiple studies of reliability and validity have become the diagnostic criteria for temporomandibular disorders (DC/TMD), an evidence-based diagnostic system suitable not only for research but also for clinical practice 
(2) the revised IASP criteria for complex regional pain syndrome (CRPS), [12, 13] which have been shown to have greater specificity than earlier criteria, and
(3) the International Spinal Cord Injury Pain Classification, an effort to reconcile existing spinal cord injury (SCI) pain taxonomies and develop valid diagnostic criteria for pain associated with SCI. [4, 5]
These 3 initiatives show the feasibility of implementing an evidence-based approach to the diagnosis of diverse pain conditions. However, these diagnostic criteria differ substantially from each other, reflecting the absence of a consistently applied framework to guide their development. AAPT established such a framework to produce a consistent taxonomy that would include the most common chronic pain conditions.
An essential characteristic of the AAPT multidimensional framework, taxonomy, and diagnostic criteria is that they are based on the best available evidence regarding symptoms, signs, mechanisms, and consequences rather than on expert opinion alone. This coordinated effort can be applied across all chronic pain conditions, and as has been true of other diagnostic criteria, AAPT will be revised periodically on the basis of accumulating evidence. Another critical aspect of the AAPT chronic pain taxonomy is that it reflects the multidimensional biopsychosocial nature of chronic pain, in which psychological and social risk factors and consequences are integrated with neurobiological mechanisms and outcomes. In addition, an essential characteristic is that the taxonomy is intended to be applicable for both research and clinical settings; it is recognized, however, that widespread clinical use is likely to develop gradually as the clinical utility of the criteria become apparent and as the evidence base increases. Finally, the initial version of AAPT is based on currently available evidence and the goal is to systematically update the criteria on the basis of new evidence, especially the results of studies of reliability, validity, and neurobiological mechanisms.
An important consideration in the development of the AAPT framework and taxonomy involved the extent to which it should depart from current approaches to classification and diagnosis that are based on disease states or body location and instead emphasize mechanisms of chronic pain. Such a mechanism-based approach to the diagnosis of chronic pain would be consistent with recent attention to precision (ie, personalized) medicine and could involve replacing current diagnoses with constellations of pathophysiologic and psychosocial mechanisms. Although the importance of including mechanisms in the development of a pain taxonomy is indisputable, existing knowledge of the mechanisms of chronic pain does not provide a sufficient basis for classifying and diagnosing chronic pain; furthermore, such an approach would be unlikely to have clinical utility at the present time.  However, it is probable that revisions of AAPT will incorporate increasing knowledge of pathophysiologic and psychosocial mechanisms and that the diagnosis of chronic pain will ultimately be based on its mechanisms.
An additional consideration in developing diagnostic criteria involves whether thresholds for clinical significance should be included. For example, should chronic osteoarthritis pain that is mild in intensity and does not interfere with physical functioning be considered a chronic pain condition? In psychiatry, associations between symptoms (or syndromes) and distress or disability have been considered evidence of clinical significance, which has been evaluated by assessing whether symptoms have been discussed with a clinician, whether medication has been used, and whether symptoms have interfered with activities.  Although clinical significance is generally not required to make medical diagnoses, it would be possible to incorporate thresholds for determining whether chronic pain is of sufficient severity to be considered clinically significant in the AAPT diagnostic criteria. However, conjoining such disability and distress thresholds with symptoms and signs in core diagnostic criteria may impede research on biopsychosocial mechanisms and consequences ; for example, risk and protective factors for the development of distress or disability are not necessarily the same factors that lead to the development of a chronic pain condition. For this reason, the adverse impacts and consequences of chronic pain are considered a separate AAPT dimension and are not included in the core diagnostic criteria.
Most approaches to the diagnosis of medical conditions are based on existing understanding, however limited, of etiology and pathophysiology. When knowledge of underlying biology is limited, diagnostic classifications typically adopt a syndromal approach, that is, clusters of symptoms and signs are emphasized. In important respects, AAPT can be considered to represent a hybrid of these 2 approaches. Although the AAPT core diagnostic criteria dimension specifies the signs and symptoms upon which diagnoses should be based, the taxonomy also includes a dimension on which mechanisms contributing to the condition should be indicated.
This dimension specifies the general and specific neurobiological and psychosocial factors that contribute to the development and maintenance of the chronic pain and that are a source of the substantial individual differences in its clinical presentation. Thus, AAPT is a syndromal taxonomy that incorporates existing evidence of biopsychosocial mechanisms. These mechanisms not only contribute to the development of specific pain conditions but also provide the foundation for developing person-centered approaches to clinical assessment and treatment.
Taxonomy of Specific Chronic Pain Conditions
Table 1 A
Table 1 B
Chronic pain conditions can be classified by anatomic location (eg, upper extremity, spine, head) or by organ system (eg, nervous system, musculoskeletal system). AAPT is not based on anatomic site alone because this would group together chronic pain conditions that have different pathophysiologic mechanisms. For example, foot pain can be caused by painful diabetic peripheral neuropathy (DPN), Morton's neuroma, CRPS, and multiple other disorders. Conversely, the same chronic pain condition can affect different anatomic sites; for example, DPN can cause pain in the feet and hands, and CRPS can cause pain in the upper and lower extremities. AAPT therefore categorizes chronic pain conditions by organ system and anatomic structure, distinguishing peripheral and central neuropathic pain, musculoskeletal pain, spine pain, orofacial and head pain, and abdominal/pelvic/urogenital pain (see Table 1). Because certain types of chronic pain cannot be included in 1 of these groups, an additional category for disease-related pain not classified elsewhere includes pain associated with cancer and pain associated with sickle cell disease (pain associated with Lyme disease and with leprosy, among other conditions, would also be included in this group). It is important to emphasize that all types of headache were intentionally excluded from AAPT because the ICHD provides systematic, valid, and widely used diagnostic criteria for these conditions.
There are shortcomings of the AAPT classification of chronic pain conditions. For example, spine pain can result from either neuropathic or musculoskeletal mechanisms or both; temporomandibular disorders could be classified as a type of musculoskeletal pain; and certain urogenital conditions are believed to reflect inflammatory as well as neuropathic mechanisms. Such overlap is inevitable given the multiple mechanisms that are involved in the pathogenesis and maintenance of chronic pain. Moreover, AAPT treats chronic pain conditions that may have overlapping mechanisms as independent, for example, fibromyalgia, TMD, and irritable bowel syndrome. [6, 15] For these reasons, the AAPT diagnostic criteria for each of the chronic pain conditions will address relevant overlap within and between the different categories.
For each of the major categories of chronic pain, diagnostic criteria will be provided for several specific chronic pain conditions. These specific conditions, which are listed in Table 1, were most often selected on the basis of prevalence. However, some conditions were included for other reasons. For example, even although the FDA has considered it a rare disease (ie, affects fewer than 200,000 persons in the United States), we included postherpetic neuralgia because it has played an important role in the development of pharmacologic treatments for neuropathic pain.  Although CRPS is also considered a rare disease by the FDA, it is included not only because the development of diagnostic criteria for CRPS has served as a model for AAPT but also because it has been one of the most clinically challenging of all chronic pain conditions.
The AAPT multidimensional framework comprises 5 dimensions that can be applied to all chronic pain conditions. An overview of these dimensions is presented in Table 2, and each is briefly summarized in this section (see also Ref.10). Other than prioritizing core diagnostic criteria, which is the first AAPT dimension, the order of the dimensions does not reflect their importance. Indeed, as noted earlier, it is anticipated that AAPT diagnostic criteria will ultimately be based on the mechanisms of the specific chronic pain conditions, whereas in the current version of the taxonomy these mechanisms constitute the final dimension.
Core Diagnostic Criteria
The core diagnostic criteria for chronic pain consist of the symptoms, signs, and test results that provide the basis for the diagnosis. Core diagnostic criteria are intended to be applied in a consistent manner and they provide standardized decision rules for determining whether an individual fulfills criteria for a specific chronic pain condition. These criteria include not only the key pain-related aspects of the condition (eg, pain duration, location, and temporal qualities) but also any physical examination findings (eg, sensory abnormalities, edema) and disease-related laboratory or imaging findings (eg, radiographic evidence of pathology) that are considered pathognomonic of the condition. For example, for TMD, diagnostic symptoms can include periauricular pain and diagnostic signs could include palpation sensitivity, reduced pain-free range of motion, and popping or clicking joint sounds. [8, 25]
Differential diagnosis is a critically important aspect of the diagnostic process, and unless relevant alternative diagnoses are carefully evaluated, the reliability and validity of diagnostic criteria is severely limited. The major differential diagnoses that must be considered for diagnosing the specific chronic pain conditions are discussed in this section of the AAPT framework.
Core diagnostic criteria are unlikely to have reliability, validity, or utility in clinical practice unless relatively standardized approaches for their assessment are available. Fillingim and colleagues  review various methods for assessing the sensory and affective qualities, temporal characteristics, and anatomic location of chronic pain and also discuss provocative and behavioral pain measures. As can be seen from their article, a variety of different measures are available for the assessment of chronic pain, and careful consideration of their psychometric properties is necessary before deciding which measures should be used in diagnostic assessments.
This AAPT dimension provides additional information regarding symptoms, signs, and other features that are commonly associated with the chronic pain condition but that are not required for its diagnosis. For example, patient-reported numbness is a symptom of many neuropathic pain conditions, ranging widely in prevalence depending on the specific condition, but it would not be required for the diagnosis of any of these conditions. Additional examples are provided by postherpetic neuralgia, which frequently occurs in thoracic dermatomes and is often described as burning pain, but these characteristics are not a requirement for its diagnosis. In addition to pain-related features, this dimension will also include information regarding more objective findings (eg, from physical examinations, laboratory tests, and imaging) that are not considered components of the core diagnostic criteria.
Life span issues relevant to the chronic pain condition are also considered within this dimension, including those specific to pediatric and geriatric populations. Walco and colleagues  review the importance of developmental considerations in the biopsychosocial model of chronic pain. They emphasize the relevance to the AAPT diagnostic criteria of both normal and pathologic age-related factors and developmental processes and highlight the necessity of a life span perspective in chronic pain classification and diagnosis.
Information about the epidemiology of the chronic pain condition will also be included in this AAPT dimension. Depending on the specific condition, this information will provide an overview of incidence and prevalence as well as any gender differences and racial, ethnic, and cultural considerations that are relevant to the diagnostic evaluation of the condition.
Common Medical and Psychiatric Comorbidities
There are many medical and psychiatric disorders that are comorbid with chronic pain but that are not required for its diagnosis. This AAPT dimension discusses such comorbid conditions and the important role that they can play in the clinical presentation of specific chronic pain conditions. For example, if major depression or generalized anxiety disorder are considerably more common in patients with a given chronic pain condition than in the general population, these psychiatric disorders would be included as psychiatric comorbidities in this AAPT dimension.
Maixner and colleagues  discuss a different type of comorbidity that is common in certain chronic pain conditions. This is the comorbidity represented by “chronic overlapping pain conditions,” that is, specific chronic pain conditions that are comorbid with each other. These include fibromyalgia, irritable bowel syndrome, temporomandibular disorder, musculoskeletal low back pain, interstitial cystitis, pelvic pain, vulvodynia, and various types of headache. Although these conditions have different features and etiologies, they also often share signs and symptoms, epidemiology, and putative biopsychosocial mechanisms. This overlap presents challenges to the validity of distinct diagnostic criteria, and Maixner and colleagues  discuss how these issues can be addressed in future research.
Neurobiological, Psychosocial, and Functional Consequences
Chronic pain conditions have important neurobiological, psychosocial, and functional consequences and these are included as 1 of the AAPT dimensions. There are important differences among people with respect to these consequences of chronic pain; for example, some individuals fulfill diagnostic criteria for a given condition but have minimal or no impairment in their physical functioning, whereas others with the same condition present with considerable disability. The psychosocial and functional consequences of chronic pain can be used to subgroup individuals with a given condition, which can have important implications with respect to identifying efficacious treatments.
Neurobiological and psychosocial processes also play an important role in the pathogenesis of chronic pain, and some processes can serve as both causes and consequences (eg, depression, generalized hyperalgesia). This can make it difficult to distinguish whether, for example, depression is an antecedent or a consequence of chronic pain in a given individual patient. These are not mutually exclusive possibilities, and the development of a chronic pain condition can also cause some risk factors to worsen (depression and generalized hyperalgesia again being good examples). Moreover, it is also possible that a third factor may account for a chronic pain antecedent that can also be a consequence of chronic pain; for example, catastrophizing could explain why depression not only increases the risk of chronic pain but also can worsen after its onset.
Information about the neurobiological, psychosocial, and functional consequences of chronic pain may be derived from the clinical examination or via administration of various patient-reported and laboratory measures designed to assess these domains. Turk and colleagues  review the evidence that the psychosocial and functional consequences of chronic pain conditions can have substantial impacts not only on the experience of pain but also on presentation to health care providers, responsiveness to and participation in treatment, and overall quality of life. These investigators describe a set of key psychosocial and behavioral factors (eg, mood, coping, expectations, sleep, physical function, and pain-related interference with daily activities) that are important consequences of chronic pain and provide an overview of measures and procedures that can be used to assess these factors within the AAPT framework. Fillingim and colleagues  review various approaches to assessing neurobiological mechanisms of chronic pain (eg, skin biopsy, imaging), but most of the approaches they discuss can also be used to assess neurobiological consequences of chronic pain.
Putative Mechanisms, Risk Factors, and Protective Factors
Although there have been great advances in understanding the neurobiological and psychosocial mechanisms of chronic pain, this knowledge does not yet provide an adequate foundation for classification and diagnosis. However, biopsychosocial mechanisms of chronic pain represent a critically important dimension within the AAPT framework because it is expected that these mechanisms will ultimately be incorporated within the core diagnostic criteria. This dimension includes neurobiological mechanisms, such as specific molecular or pathophysiologic processes that appear to contribute to the chronic pain condition. [29, 30] Because of the limitations of existing evidence, information regarding specific mechanisms and risk factors is not compelling for many chronic pain conditions, and this uncertainty has been emphasized in the description of this dimension by use of the word “putative.” However, for most chronic pain conditions, sufficient evidence of neurobiological mechanisms does exist to provide a basis for specific hypotheses. For example, it is widely accepted that generalized hypersensitivity to painful stimuli occurs in many patients with fibromyalgia, although specific peripheral and central nervous system mechanisms accounting for this phenomenon are unknown.
Vardeh and colleagues  review current knowledge of pathophysiological pain mechanisms and how these mechanisms can be identified to guide treatment selection. They emphasize that existing assessment methods fall far short of a mechanism-based approach (what has recently come to be called “precision medicine”) and propose a systematic approach to identifying the specific mechanisms of chronic pain phenotypes. Their approach consists of the successive identification of pain state, pain mechanism, and molecular target, and it is illustrated with a discussion of chronic low back pain, the most common chronic pain condition. Fillingim and colleagues  review several approaches to assessing neurobiological mechanisms of chronic pain, including quantitative sensory testing, skin biopsy, imaging, pharmacologic challenge, and genotyping. These are all rapidly evolving areas of research involving pain mechanisms, and there can be little doubt that such methods will play increasingly greater roles in the diagnosis of chronic pain.
In addition to neurobiological mechanisms, psychosocial variables have been shown to make important contributions to the development of many chronic pain conditions. Therefore, psychosocial risk and protective factors are also included in this AAPT dimension. These psychosocial variables undoubtedly reflect neurobiological processes, although the underlying biology is not well understood. For each chronic pain condition, available and recommended methods for assessing psychosocial and neurobiological mechanisms will be described. Edwards and colleagues  review psychosocial variables that are relevant to many medical and psychiatric conditions (eg, negative affect, childhood trauma, and social support) as well as psychosocial processes that are specific to pain, such as pain-related catastrophizing, self-efficacy, and coping. These investigators emphasize that this multidimensional array of interacting psychosocial risk factors, protective factors, and vulnerabilities makes a major contribution to accounting for individual differences in the development and maintenance of chronic pain and that such factors should be considered in the classification and diagnosis of all chronic pain conditions, regardless of their presumed etiology.
AAPT diagnostic criteria are being developed for the specific chronic pain conditions listed in Table 1, which as noted earlier, have been selected on the basis of either prevalence or clinical and research considerations. The diagnostic criteria are based on systematic reviews of previous criteria and disorder epidemiology, secondary analyses of epidemiologic data, and existing studies of reliability and validity, as evaluated by panels of individuals with research and clinical expertise relevant to the specific conditions, although not all of these different types of information are available for each of the conditions. Diagnostic criteria for specific pain conditions will be published in a series of individual articles, and it is anticipated that they will stimulate the development of criteria for other chronic pain conditions using the AAPT framework.
The initial versions of the AAPT diagnostic criteria will be as evidence based as possible given existing evidence. These criteria will then undergo formal studies to examine interrater reliability as well as diagnostic sensitivity and specificity, using research methods similar to those used in the development of diagnostic criteria for CRPS, TMD, and pain associated with SCI. [4, 5, 8, 12, 13] Bruehl and colleagues  discuss approaches to evaluating the different types of reliability and validity that are relevant to diagnostic criteria, emphasizing the challenges involved in validating criteria when pathophysiologic mechanisms cannot be identified and there is no gold standard. Their review serves as a roadmap for future empirical research on the AAPT criteria, the results of which will provide the basis for revision and improvements in reliability, validity, and utility.
AAPT provides a multidimensional framework and operational diagnostic criteria that are evidence based and systematically applied to the major chronic pain conditions. The AAPT framework explicitly includes a dimension in which information regarding the neurobiological and psychosocial mechanisms of chronic pain is provided, as well as a separate dimension for the neurobiological and psychosocial consequences of chronic pain. It is important to emphasize that these 2 dimensions, as well as the common comorbidities dimension, are not entirely discrete. Risk factors/antecedents, consequences, and comorbidities overlap in some cases and may be reciprocal or bidirectional. As noted earlier, this is often the case for psychosocial factors, which may be causes, consequences, or comorbid conditions, but can also be true of certain neurobiological processes.
The AAPT diagnostic criteria are intended for both research and clinical use, and the taxonomy and criteria will be updated and revised as new evidence emerges. As can be seen from Table 1, the current AAPT taxonomy comprises approximately 20 distinct chronic pain conditions. Although the way in which these conditions have been categorized is consistent with current research on the mechanisms and treatment of chronic pain, the AAPT taxonomy was not designed to provide a basis for regulatory decisions, especially the challenging issue of extrapolating evidence of treatment efficacy from 1 pain condition to others. As additional evidence becomes available regarding the validity of the AAPT diagnostic criteria and the mechanisms and treatment response of these distinct conditions, it can be expected that future AAPT taxonomies will provide a framework for extrapolating efficacy within groups of similar conditions and lead to evidence-based revisions of existing proposals for extrapolating acute and chronic pain treatment efficacy. 
As is true of diagnostic criteria for other disorders, it is likely that AAPT will initially be used most widely in clinical research as inclusion and exclusion criteria for identifying patient samples, but more widespread clinical use is likely to follow as the criteria evolve and as their clinical utility becomes apparent. In much the same way that evidence-based diagnostic criteria have had a transformative impact on research and treatment in psychiatry, the long-term objective of AAPT is to advance the scientific understanding of chronic pain and its treatment.
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