3.1.3. Phenotyping methods
The identified studies could be classified into 3 broad categories, according to their reason for phenotyping neuropathic pain: (1) to determine whether any pain was neuropathic (identification of cases and controls)2,4,8,12,15,16,20,22,23,34–36,42,45,49; (2) to identify endophenotypes within cohorts with neuropathic pain (ie, phenomics)6,11,21,27,40,46,49; and (3) to identify pain (not specifically neuropathic) in a neurological condition, eg, multiple sclerosis.16
Table 3 and Supplementary Digital Content 2 (available online as Supplemental Digital Content at http://links.lww.com/PAIN/A148) provide summaries of the phenotyping methods described in these articles. Overall, “clinical examination” was the most frequently reported phenotyping method; described in 15/21 studies, with “brief examination” reported in 6 studies, including in total 1742 cases, and “detailed examination” reported in 9 studies (1346 cases). “Pain-rating scales” (either visual analogue or numerical rating scales, 2786 cases) and “history” (2552 cases) were the next most common phenotyping method described, with 13/21 studies each. With “Pain-rating scales,” only half of these articles described what the relevance of the pain score was: for example, to be used as a cutoff for “caseness” to categorize patients as “cases” or “controls.” Nine of the articles reporting a “history” provided a “brief history” (2226 cases), and 4 of the articles provided a “detailed history” (326 cases). All other phenotyping methods were described in fewer than half of the articles, with the least frequently reported phenotyping methods being “nerve conduction studies” (2/21), “intraepidermal nerve fibre density” (2/21), “inflammatory markers” (2/21), “body chart of perceived pain” (1/20), and related “psychiatric measures” (1/21). In 2 studies, there was phenotyping heterogeneity, manifesting in the use of more than 1 sample population cohort within the same study.11,15
The only specific neuropathic pain symptom questionnaire reported was the AIDS Clinical Trials Group Brief Peripheral Neuropathy Screen.9,21,49 No studies reported the use of validated case ascertainment tools such as the Douleur Neuropathique en 4 questions (DN4),5 Leeds Assessment of Neuropathic Symptoms and Signs (LANSS),3 or painDETECT.18
The control groups described in the studies could be divided into 3 broad categories: (1) healthy volunteers,2,4,16,35,36,42,45 (2) population-specific national reference cohorts,15,22,23 and (3) diseased controls.6,8,15,16,20,27,34,49 Three studies used more than 1 category of control group.15,16,45 In the first 2 categories (healthy controls and population-specific national reference cohorts), little or no information was provided on the phenotyping of controls. Typically, the information provided in these studies was restricted to whether the controls were healthy, genetically unrelated, and matched the cases for various demographic variables. Diseased controls were defined as having the same disease or aetiology as the cases, but were not considered to have neuropathic pain at the time of the study.
Four studies modelled genotype against intensity of neuropathic pain and thus did not include control groups.11,21,46,49 Similarly, 2 case reports did not include control groups at all.12,40
3.1.4. Grading framework: “possible,” “probable,” or “definite” neuropathic pain
Only 1 of the 20 studies cited the NeuPSIG neuropathic pain grading system, and that single study indicated that cases satisfied the criteria for “definite” neuropathic pain.40 Table 4 summarises the grading criteria met by each study based on the description of the phenotyping methodology in the article text. Only 1 study did not describe assessing a history.6 Although only 1 study reported using a body chart of perceived pain16 to assess whether pain occurred in a plausible anatomical distribution, the assessment of pain distribution could be inferred from the phenotyping descriptions provided in 13 studies, even if the method of assessment was not explicitly described.8,11,12,15,20–23,27,34,35,49 Fourteen studies described assessing clinical signs,6,8,12,15,16,20–23,27,35,40,49 and 8 studies described the use of methods that could identify a nerve lesion (either radiological imaging and/or QST).4,6,12,20,22,23,27,35 In summary, 4 studies included patients identified as “possible” neuropathic pain cases,11,34,45,46 5 included patients identified as “probable” neuropathic pain cases,8,15,16,21,49 and only 7 included patients identified as “definite” neuropathic pain cases.12,20,22,23,27,35,40 Six studies could not be graded according to the criteria proposed by Treede and et al.,47 and were classified as “undefined.”
3.2. Delphi survey
Twenty experts, of the 28 approached to take part in the survey, completed round 1 (responder rate: 20/28, 71%), and 17/20 (85%) indicated that they were willing to participate in subsequent survey rounds. Round 2 invitations were sent to 17 experts, 16 of whom completed the questionnaire (responder rate: 16/17, 94%). Invitations to round 3 of the survey were sent to the same round 2 17 experts, 15 of whom completed the questionnaire (responder rate: 15/17, 88%).
3.2.2. Validity and feasibility of assessing symptoms, signs, and completing additional investigations
Figure 2 shows the level of agreement of respondents after 2 survey rounds with regards to whether symptoms, clinical signs, and additional investigations are sensitive and specific assessments of neuropathic pain, and whether they are feasible for nonexperts and study participants to complete by themselves. There was good consensus that testing clinical signs was a sensitive method of detecting neuropathic pain as means of identifying true “cases,” but poor consensus was achieved for either symptoms or additional investigations alone. There was good consensus that clinical signs and additional investigations (but not symptoms) were specific methods to enable the identification of true “controls.” There was good consensus that symptoms could be reliably assessed by nonexperts and by study participants, but that additional investigations were not feasible for nonexperts or study participants to assess, and that study participants could not reliably self-assess clinical signs. The levels of agreement across each round of the survey are shown in Supplementary Digital Content 3 (available online as Supplemental Digital Content at http://links.lww.com/PAIN/A149).
3.2.3. Informative symptoms, signs, and additional investigations
Table 5 shows the final consensus on respondents' choices and rankings of symptoms used in the diagnosis of neuropathic pain for a genetic study. Strong or good consensus was achieved for only 2 symptoms: “hot/burning” and “pain evoked by light touch.” Fewer than one-third of respondents included “spontaneous pain,” “paroxysmal pain,” “evoked pain,” “painful cold,” “itching,” or “stabbing pain” in their top 5 symptoms for diagnosing neuropathic pain.
Respondents' choices and rankings of clinical signs used in the diagnosis of neuropathic pain are shown in Table 6. Strong or good consensus was achieved for only 2 signs: “dynamic mechanical allodynia” and “altered sensation to punctate mechanical stimuli.” Fewer than one-third of respondents included “temporal summation,” “altered vibration sense,” “thermal hyperalgesia,” “altered reflexes,” “static mechanical hyperalgesia,” or “deep mechanical hyperalgesia” in their top 5 signs to assess.
Table 7 shows the final consensus of respondents' choices and rankings of their top 5 additional investigations for diagnosing neuropathic pain. The 2 top-ranked investigations were, “QST” and “intraepidermal nerve fibre density.” In round 3, when the feasibility of nonspecialists making accurate measurements was assessed for each of the 5 additional investigations, only QST was rated as being feasible by 11/14 (79%) of respondents. More than 70% of respondents agreed that all other methods of assessment were not feasible for nonspecialists to assess neuropathic pain.
3.2.4. Perceived diagnostic certainty
Figure 3 shows the level of perceived diagnostic certainty, according to the neuropathic pain grading system,47 indicated by respondents for various assessment modalities used alone or in combination with each other.2 There was unanimous consensus that a combination of “additional investigations, clinical signs, symptoms, body chart, and patient history” provided a diagnosis of “definite” neuropathic pain. All respondents also indicated that combinations of: “additional investigations, clinical signs and symptoms,” and “clinical signs, symptoms, body chart, and patient history” provided a diagnosis of at least “probable” neuropathic pain. The poorest perceived diagnostic certainty was associated with assessments that only included “additional investigations.” The levels of agreement across each round of the survey are shown in Supplementary Digital Content 4 (available online as Supplemental Digital Content at http://links.lww.com/PAIN/A150).
3.2.5. Additional phenotyping measures
The level of agreement on which measurements should be included in an assessment of neuropathic pain (in addition to symptoms, signs, and additional investigations) is shown in Table 5 (and Supplementary Digital Content 5, available online as Supplemental Digital Content at http://links.lww.com/PAIN/A151). After 2 rounds of the survey, there was strong consensus that assessment should include a history and a body chart to indicate pain distribution. Prompted by discussions at the consensus meeting (see below), in round 3 of the survey, we probed what the body chart and history should include. Having ≥70% of respondents in agreed or strong agreement, the strongest consensus was for the following items assessing patient history: “a previous diagnosis of neuropathic pain by another clinician” and “risk factors for having neuropathic pain” (see Supplementary Digital Content 6, available online as Supplemental Digital Content at http://links.lww.com/PAIN/A152). On the body chart of perceived pain, 75% (12/16) of respondents indicated that, in the event of more than 1 pain site, the “main pain and any other pains” should be identified. Almost two-thirds (64%, 9/14) believed that a checklist of body regions was a suitable substitute for a body chart cartoon.
3.3. Consensus meeting
The primary goal of the meeting was the development of a consensus statement on “entry level” phenotyping requirements for genetic studies of neuropathic pain in human adults.
3.3.1. Consensus statement development
Participants agreed that there is an inverse relationship between accurate case ascertainment of neuropathic pain and large-scale implementation thereof in population-based studies. We agreed that the entry-level phenotyping requirements included in the statement should form the basis of establishing neuropathic pain “caseness” for genetic studies. This basic phenotyping must allow the addition of more in-depth measures for higher level phenotyping to allow the identification of genetic loci for specific aspects of neuropathic pain other than just presence or absence of neuropathic pain. The entry-level requirements should provide a framework to guide researchers on study design and provide a platform to appraise their findings to allow feasible and valid assessment for the presence or absence of neuropathic pain in large population-based genetic studies.
3.3.2. “Entry level” phenotyping for genetic studies, a consensus
Box 1 shows the agreed NeuroPPIC entry-level requirements that would allow the classification of participants as having “possible” neuropathic pain. Consistent with the findings of the Delphi survey, a neuropathic pain “case” must have:
- (1) pain with neuropathic characteristics (ie, positive on a validated screening questionnaire; or pain, ie, “hot/burning” and/or “pain evoked by light touch”);
- (2) a distribution or location of this pain that is anatomically consistent with an underlying somatosensory lesion or disease (as indicated by a body chart or body parts checklist); and
- (3) further characterisation in the form of a clinical history (duration and intensity of pain and presence of other pains) and demographic information relevant to the population/disease being studied.
Box 1 NeuroPPIC consensus statement on entry-level phenotyping for genetic studies of neuropathic pain in humans. Cited Here...
A neuropathic pain “control” would be negative for (1) and (2), but will be characterised as (3) as clinically relevant.
The panel agreed that if there are discrepancies between the findings from the screening tool, anatomical distribution, and patient history, then the participant should be excluded from the study. Overall, these 3 components only satisfy criteria 1 and 2 of the NeuPSIG grading system, and thus the level of diagnostic certainty achieved is “possible” neuropathic pain.47
While acknowledging the lack of a “gold standard,” we agreed that achieving greater diagnostic certainty (ie, “probable” or “definite” neuropathic pain) is desirable. Such greater diagnostic certainty requires additional assessments that confirm the presence of positive and/or negative sensory signs (for “probable” neuropathic pain) or provide direct evidence of a lesion to the somatosensory nervous system (for “definite” neuropathic pain) in the affected area. More extensive phenotyping could allow case stratification into additional pain phenotypes based on sensory and psychological parameters (eg, pain catastrophizers vs noncatastrophizers; pinprick hypoaesthesia vs pinprick hyperaesthesia).
The panel additionally agreed that it could not currently be prescriptive in the selection of additional phenotyping measures to use beyond determination of “caseness,” as the choice was dependent on factors such as the feasibility of making measurements based on cost and practicality, the population and disease being studied (eg, central vs peripheral neuropathic pain), and the research question being addressed. However, choice of additional measures to use when assessing “caseness” should be consistent with the NeuPSIG grading system for neuropathic pain. See Supplementary Digital Content 7 for examples of additional phenotypes voiced at the meeting (available online as Supplemental Digital Content at http://links.lww.com/PAIN/A153).
We agreed that misclassification of cases and controls significantly affects the power of studies to detect true genetic associations and the reproducibility of findings. We identified several key factors that may contribute to misclassification of cases and controls (Box 2). These factors can be divided into 2 broad categories: (1) misclassification due to on incomplete pain history; and (2) misclassification due to an imperfect sensitivity and specificity of available assessments for neuropathic pain. Ideally, where reasonable doubt arises after applying the phenotyping criteria proposed here, an individual should be considered as neither a case nor a control, and removed from analysis.
Box 2 Possible factors that may contribute to misclassification of neuropathic pain. Cited Here...
The unequivocal identification of genetic sequence variants correlating with complex diseases requires sample sizes ranging from large cohorts (including hundreds to thousands of participants) when interrogating a few candidate genes, to much larger sample sizes that include tens of thousands of participants when studying the whole genome31,51 and extensive phenotyping.50 Unlike Mendelian disorders, the genetic contribution to complex diseases such as chronic pain is generally attributed to contributions of a high number of risk variants, including single-nucleotide polymorphisms, most with a small effect. In neuropathic pain, a number of relevant candidate genes have been identified in animal studies, and an even smaller number show some association in humans, as shown above. However, none of these single-nucleotide polymorphisms has been consistently replicated in large human cohorts.32 This is an essential step in confirming the validity of these genetic findings.7 Failure to replicate, in humans, a finding originally made in humans may be because the original finding was spurious, study cohorts were underpowered, there was heterogeneity in the studied pain condition or study populations, or there were differences in the phenotyping of cases and/or controls. The ideal control in a genetic study would be an age- and sex-matched individual who was exposed to the same lesion or disease of the somatosensory system as the case, but who did not develop neuropathic pain.30 Rigorous, standardised phenotyping is therefore crucial. Failure to translate a finding from rodent models to humans may likewise be because the original finding was spurious, the human cohort was underpowered, the phenotypes were different, or the gene under study does not play a role in the tested human condition. Pain geneticists who seek candidate neuropathic pain genes in animal models should try to approximate their phenotypes to those used by human pain geneticists, although animal phenotypes also require considerable further refinement in terms of both internal and external validity.37
The need for a consensus-based, rather than purely evidence-based approach to phenotyping neuropathic pain for genetic studies reflects the absence of “gold standards” for both case definition (what is neuropathic pain?) and case attribution (how do we know who has neuropathic pain?). Through a process of systematic literature review, Delphi survey, and expert consensus meeting, we have identified an agreed “entry level” approach to phenotyping cases and controls for studies of genetic associations with neuropathic pain in adults. This phenotype is based on establishing the presence or absence of “possible” neuropathic pain, according to the most internationally agreed system for classifying neuropathic pain clinically.47 This uses validated symptom-based screening tools and a body chart or checklist to determine neuroanatomical relevance. Further details, also collected by questionnaire, include pain history (duration, intensity, and formal diagnoses) and demographic information (Box 1). We present this “entry level” approach as the minimum phenotyping standard for genetic association studies on neuropathic pain in adults, with the expectation that more extensive phenotyping should build on this “entry level” approach, as required and as resources allow. Adoption of these NeuroPPIC phenotyping guidelines will facilitate comparison and collaboration between researchers, including replication studies and meta-analyses. The guidelines are applicable to different types of genetic studies of neuropathic pain, from targeted studies interrogating a few genetic loci that use specific neuropathic pain traits to hypothesis-free genome-wide studies with large sample sizes. They will also apply to studies with a larger array of phenotypes, including more detailed pain traits, psychosocial traits, and pharmacological screens. Our systematic review identified the lack of a consistent approach to phenotyping neuropathic pain, poor reporting of phenotyping methodologies, and failure to adequately describe control groups (especially when “healthy controls” were used). These limitations are likely to have contributed to inconsistent and nonreplicated findings, and highlight the need for the NeuroPPIC process we have completed.
Although we have recommended the use of validated assessment instruments where possible, our guidelines are not intended to guide case definition, attribution, or classification for clinical research or neurological practice, where more individuals certainty may be required, based on empirical evidence of the presence or absence of neuropathic pain and the known likelihood of identifying this through any assessment. This will require separate work, and is the subject of an ongoing NeuPSIG project. Instead, our consensus-based development provides a standardised approach for use by researchers in different settings internationally, in the expectation that it will allow a common approach to identifying the presence or absence of “possible” neuropathic pain for genetic research. However, future research should validate the “entry level” phenotype against best available clinical assessments, with a view to calculating sensitivity, specificity, and positive and negative predictive values.
Delphi surveys are an accepted method of achieving consensus on complex issues in science and medicine,13,39 including pain medicine, in which a “gold standard” does not exist (as in neuropathic pain26). For example, previous studies have used Delphi methods to develop a standard definition for back pain14 and neuropathic pain44 for use in prevalence studies, and to develop guidelines for the management of hip and knee osteoarthritis.52 Identified limitations of the method include its basis in the opinions of a selected group of survey participants, and the potential for the outcome to be a diluted version of “best opinion,”39 criticisms that could also be levelled at consensus meetings. There is no agreed optimum number or composition of panellists or rounds for Delphi surveys or consensus meetings. Our intention was to include a range of disciplines rather than to be comprehensive in our inclusion, and to include a similar number of participants and rounds as in previous successful studies.32,44,52 This would allow the processes to remain manageable, while ensuring a contribution of expertise from each relevant discipline and consistency with existing research. Contributors were identified on the basis of their published track record in neuropathic pain phenotyping and/or genetics. It is possible that a different group of contributors would have produced different outcomes, and unanimity was not obtained for any item in the survey. It is, though, notable that good or very good consensus was achieved in many useful aspects of this Delphi survey, and agreement on the NeuroPPIC “entry level” phenotyping approach was achieved after intense deliberations. We are, therefore, confident that our findings reflect the views of the relevant research community, resulting in adoption of NeuroPPIC's approach in future genetic studies.
NeuroPPIC has only provided a consensual statement on the nature of “entry level” phenotyping and instruments that can be used to accomplish successful classification of participants as having vs not having neuropathic pain. These NeuroPPIC guidelines are for the basic phenotyping requirements for any genetic study of neuropathic pain in adults. It is notable that this “entry level” phenotyping is based only on study participants' responses to questions and does not include clinical examination or further investigations. This “entry level” phenotyping approach will allow inclusion in large-scale questionnaire-based surveys. Our Delphi survey confirms that augmenting a simple symptom checklist by questions on pain bodily distribution and history will allow the identification of “possible” neuropathic pain. Although symptoms could be confined to a smaller list of verbal descriptors (eg, “hot/burning pain” and “pain evoked by light touch”), the use of a slightly longer but well-validated screening instrument will prevent the need for validation of a new questionnaire instrument. Available screening instruments include DN4, S-LANSS, and PainDETECT, but we are making no statement on which of these, if any, is to be preferred. This approach conforms to the “very general” approach outlined in the introduction and will be suitable for large-scale population-based studies, including those in which neuropathic pain is just one of a number of conditions to be phenotyped. More extensive phenotyping of additional aspects of neuropathic pain will require a more detailed set of assessments, to allow for inclusion of “very specific” phenotypes. Although NeuroPPIC was able to provide some pointers towards these levels, we have not yet furnished agreed guidelines on the nature of these phenotypes and the instruments to collect them, and this will require further work, building on the “entry level” approach. Any additional, more extensive phenotyping should, however, be consistent with the “entry level” approach, to allow specificity to be built in to the basic model. This will allow incremental approaches to phenotyping within a large cohort, and the testing for replication of associations identified in extensively phenotyped cohorts.
4.1. Reporting recommendations
The STREGA29 and GRIPS24,25 guidelines provide a framework for reporting genetic epidemiology studies and indicate that participant eligibility criteria and methods of participant selection and screening, including subsets of participants, should be reported. Moreover, in case–control studies, this information should be reported for both cases and controls. To facilitate clear and open reporting of phenotyping in genetic studies of neuropathic pain, we propose the following NeuroPPIC guidelines for describing study participants: (1) a description of sampling methodology and participant eligibility criteria; (2) an unambiguous description of the case definition used to define the presence or absence of neuropathic pain in cases and controls; (3) a description of the methods used to assess each component of the case definition and the criteria used to define abnormal function for each method used; and (4) a summative statement of whether the case definition satisfies the criteria for “possible,” “probable,” or “definite” neuropathic pain based on the latest grading guidelines (currently Treede et al.47). These recommendations are consistent with the recommendations made by STREGA and GRIPS and should be used in addition to those 2 guidelines.
By providing an “entry level” phenotype and making recommendations on the reporting of the criteria and methods used for caseness and its ascertainment, we believe that greater consistency and transparency can be achieved in studies on the genetics of neuropathic pain in adult humans. These improvements will facilitate advancements in the field by enabling collaboration between research groups, replication of discoveries of contributing genetic variants, meta-analyses, and translation from the laboratory to the general population, and back again.
Conflict of interest statement
P. R. Kamerman, N. Attal, M. Haanpää, S. N. Raja, A. S.C. Rice, and B. H. Smith were members of the NeuPSIG Management Committee at the time of the research. O. van Hecke, D. L.H. Bennett, L. Diatchenko, R. Freeman, M. Haanpää, Z. Seltzer, and D. Yarnitsky declared no other conflicts of interest. N. Attal declared consultancy or lecture fees from Pfizer, AstraZeneca, Lilly, Grunenthal, Johnson and Johnson Sanofi Pasteur Mérieux outside the scope of this work. P. R. Kamerman declared consultancy fees from Reckitt Benckiser, lecture fees from Pfizer and Novartis, and travel support from Janssen. B. H. Smith declared occasional consultancy and lecture fees, on behalf of his institution, from Pfizer, Napp, and Grunenthal. He is the Scottish Government Lead Clinician for Chronic Pain. R. Baron has received grants/research support from Pfizer, Genzyme, Grünenthal, and Mundipharma. He is a member of the IMI “Europain” collaboration and industry members of this are: AstraZeneca, Pfizer, Esteve, UCB-Pharma, Sanofi-Aventis, Grünenthal, Eli Lilly, and Boehringer Ingelheim. German Federal Ministry of Education and Research (BMBF): German Research Network on Neuropathic Pain, NoPain system biology. German Research Foundation (DFG). He has received speaking fees from Pfizer, Genzyme, Grünenthal, Mundipharma, Sanofi Pasteur, Medtronic, Eisai, Lilly, Boehringer Ingelheim, Astellas, Desitin, Teva Pharma, Bayer-Schering, and MSD. He has been a consultant for Pfizer, Genzyme, Grünenthal, Mundipharma, Allergan, Sanofi Pasteur, Medtronic, Eisai, Lilly, Boehringer Ingelheim, Astellas, Novartis, Bristol-Myers-Squibb, Biogenidec, AstraZeneca, Merck, AbbVie, Daiichi Sankyo, and Glenmark Pharmaceuticals. G. Bjornsdottir and T. E. Thorgeirsson are employed by deCODE Genetics/Amgen Inc and have received funding from the European Commission (HEALTH-FP7-2013-Innovation-602891 to the NeuroPain Consortium) and the NIH (R01DE022905). D. L.H. Bennett declared occasional consultancy and lecture fees, on behalf of his institution, from Pfizer. M. I. Bennett declared consultancy and lecture fees from Pfizer, Bayer, and Grunenthal. R. Freynhagen declared research support, consulting, or lecture fees in the past 2 years from Astellas, Grünenthal, Janssen, Lilly, and Pfizer. T. S. Jensen declared consultancy fees for Pfizer, Grünenthal, Orion, and Astellas and lecture fees from Pfizer. S. N. Raja declared a research grant from Medtronic and that he has served as an Advisor for Mitsubishi Tanabe Pharma Inc. A. S.C. Rice declared Imperial College Consultants Consultancy work and Scientific Board Memberships contracted by Imperial College Consultants. In the last 3 years, these have included remunerated work for the following companies: Spinifex, Merck, Astellas, Medivir, Asahi Kasei Pharma, Servier, Relmada, Abide, Neusentis (Pfizer), Aquilas (Neuromax), and Mitsubishi; and ownership of share options in Spinifex. He also declared receipt of research grant support in the last 3 years, on behalf of his institution, from Pfizer (as part of London Pain Consortium and Neuropain) and Astellas (as part of EU Innovative Medicines Initiative grant-EUROPAIN). He is a member of the UK Department of Health (nonfinancial) Joint Committee on Vaccination and Immunisation, Varicella/Herpes Zoster subgroup. M. Haanpää declared consultancy fees for AbbVie, Allergan, Astellas, Eli Lilly, Janssen Cilag, Pfizer, and Sanofi-Aventis, and lecture fees from Astellas, Eli Lilly, Janssen Cilag, MSD, Mundipharma, Orion, and Pfizer. D. L.H. Bennett is a Wellcome Senior Clinical Scientist (Ref 095698z/11/z). N. Attal, D. Bouhassira, D.L.H. Bennett, R. Baron, T. S. Jensen, A. S.C. Rice, B. H. Smith, and D. Yarnitsky are members of the DOLORisk consortium funded by European Commission Horizon 2020 (ID633491).
This work was entirely funded by the Neuropathic Pain Special Interest Group (NeuPSIG) of the International Association for the Study of Pain.
O. van Hecke and P. R. Kamerman contributed equally to this work.
The authors thank the 20 responders to the Delphi survey for their input and Dr Harriet Wordsworth for taking comprehensive minutes of the expert panel meeting.
Supplemental Digital Content
Supplemental Digital Content associated with this article can be found online at http://links.lww.com/PAIN/A147; http://links.lww.com/PAIN/A148; http://links.lww.com/PAIN/A149; http://links.lww.com/PAIN/A150; http://links.lww.com/PAIN/A151; http://links.lww.com/PAIN/A152; http://links.lww.com/PAIN/A153.
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Keywords:© 2015 International Association for the Study of Pain
Neuropathic pain; Genetics; Phenotype; Systematic review; Delphi survey