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The epidemiology of chronic pain in the community
Alison M Elliott, Blair H Smith, Kay I Penny, W Cairns Smith, W Alastair Chambers
Summary
Background Chronic pain is recognised as an important problem in the community but our understanding of the epidemiology of chronic pain remains limited. We undertook a study designed to quantify and describe the prevalence and distribution of chronic pain in the community.
Methods A random sample of 5036 patients, aged 25 and over, was drawn from 29 general practices in the Grampian region of the UK and surveyed by a postal self-completion questionnaire. The questionnaire included case-screening questions, a question on the cause of the pain, the chronic pain grade questionnaire, the level of expressed needs questionnaire, and sociodemographic questions.
Findings 3605 questionnaires were returned completed. 1817 (50-4%) of patients self reported chronic pain, equivalent to 46-5% of the general population. 576 reported back pain and 570 reported arthritis; these were the most common complaints and accounted for a third of all complaints. Backward stepwise logistic-regression modelling identified age, sex, housing tenure, and employment status as significant predictors of the presence of chronic pain in the community. 703 (48-7%) individuals with chronic pain had the least severe grade of pain, and 228 (15-8%) the most severe grade. Of those who reported chronic pain, 312 (17-2%) reported no expressed need, and 509 (28-0%) reported the highest expressed need.
Interpretation Chronic pain is a major problem in the community and certain groups within the population are more likely to have chronic pain. A detailed understanding of the epidemiology of chronic pain is essential for efficient management of chronic pain in primary care.
Lancet 1999; 354: 1248-52
Department of General Practice (A M Elliott BSC, B H Smith MRCGP) and Department of Public Health (Prof W C Smith MFPHM). University of Aberdeen Medicai School, Aberdeen; Department of Anaesthetics, Aberdeen Royai infirmary, Aberdeen (W A Chambers FRCP); and iVIedical Statistics Unit, University of Edinburgh iVIedical School, Edinburgh (K I Penny phD), UK
Correspondence to: Mrs Alison M Elliott, Department of General Practice and Primary Care, University of Aberdeen, Foresterhill Health Centre, Aberdeen AB25 2AY, UK
Introduction Chronic pain is recognised as a common problem within the community, and is known to affect general health,' psychological health," and social and economic well- being.'' Chronic pain is one of the most common reasons . why people seek medical care; patients with chronic pain use health services up to five times more frequently than the rest of the population.'' Chronic pain also results in many days lost from work.' Nevertheless, our understanding of the epidemiology of chronic pain remains limited. Several previous studies of the prevalence of chronic pain have been based on patients attending pain clinics,'"" but these patients represent one extreme of the population with chronic pain and are not likely to be representative of the general population.'^'"
Good community-based epidemiological studies have been difficult to do. Many approaches used to estimate the prevalence of chronic pain have had limitations, such as the use of non-specific measurement instruments.'''* Some studies have concentrated on patients with specific pain-related diagnoses, such as back pain, fibromyalgia, and chronic musculoskeletal pain,^"" and others have been small.'"' Previous estimates of the prevalence of chronic pain in the general population have ranged from 2% to 45%.^'"''^'"-"'
We present results from an epidemiological study of chronic pain, designed to quantify and describe the prevalence of chronic pain in the community, according to age, sex, and sociodemographic status.
Methods Participants and procedures The International Association for the Study of Pain defines chronic pain as "pain that persists beyond normal tissue healing time, which is assumed to be 3 months"." This definition does not allow for intermittent pain, so for the purposes of this study we defined chronic pain, after pilot testing, as "pain or discomfort, that persisted continuously or intermittently for longer than 3 months".
The study was done in the Grampian region of northeast Scotland. All practices that used the General Practice Administrative System for Scotland (GPASS) were invited to collaborate. We had to limit the study to these practices because GPASS was required for a related study. Of the 67 practices eligible to participate in the study, 29 (43%) were keen to be included. These practices accounted for about 35% of the total Grampian population and were representative of the practices in Grampian in terms of size of practice, geographical location, and socioeconomic status of those attending. There was no follow-up of the remaining practices.
5036 patients aged 25 years and older who attended participating practices were selected randomly from the community health index (a list of all patients registered with a general practitioner). The sample was stratified for sex and age (six age-groups ranging from 25-34 years to 75 years and over) and was weighted to ensure an equal representation of patients in each of the subgroups. Each list of patients was screened by the relevant general practitioner so that inappropriate or insensitive inquiries could be prevented (for example, in the
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388543 people in GPASS practices
185747 people in participating practices
5036 randomly selected
425 excluded by general practitioner
4611 sent questionnaires
232 questionnaires not delivered 176 incorrect or unknown
addresses 56 not contactable
4379 questionnaires delivered
774 did not reply
3605 respondents
Study profile
case of terminal illness). No specified criteria for the exclusion of patients were provided. General practitioners were simply asked to score through the name of anyone whose inclusion they felt would be unsuitable or inappropriate.
Questionnaire Participants were surveyed by a postal self-completion questionnaire, developed through piloting and validation. This questionnaire included case-screening questions, a question on the cause of the pain, the chronic pain grade questionnaire, the level of expressed needs questionnaire, and sociodemographic questions.
Chronic pain cases were identified through two questions: one question to assess whether pain or discomfort was present, and a second to establish whether this pain or discomfort had started longer than 3 months ago. Patients were asked to say whether they knew what caused their pain and were given a choice of responses (based on pilot studies) of angina, arthritis, back pain, injury, women's problems, don't know, and other.
The chronic pain grade questionnaire'™ is a seven-item questionnaire that measures severity of chronic pain in three dimensions: persistence, intensity, and disability. The results of the questionnaire enable patients to be classified into five hierarchical grades: grade 0 (pain free), grade I (low disability, low intensity), grade II (low disability, high intensity), grade III (high disability, moderately limiting), and grade IV (high disability, severely limiting). This questionnaire has been tested
Education No educational qualifications Secondary schooi qualifications Higher education quaiifications
Marital status Never married Currently married No longer married
Housing tenure Owned or mortgaged Council rented Privately rented and other
Number of occupants Living alone Living with others
Employment status Empioyed Retired Not able to work owing to iilness/disabiiity Others not empioyed
Social class (occupation based) Highly skilled Skilled Part or non-skilled
Number of respondents
1637 960 735
376 2528
659
2436 898 208
751 2639
1809 1086
161
428
708 719 351
Odds ratio* (95% CI)
1-OOt 0-93 {0-78-110) 0-99 (0-82-119)
l o o t 1-23 (0 98-1-54) 1-20 (0-91-1-57)
1-OOt 1-29 (1-10-1-51) 0-97 (0-72-1-29)
1-OOt 0-99 (0-83-1-17)
1-OOt 1-33 (1-03-1-72) 8-15 (5-03-13-21)
0-92 (0-73-1-16)
1-OOt 1-02 (0-82-1-26) 1-07 (0-82-1-40)
P
0-40 0-89
0-08 0-20
0-01 0-82
0-87
0-03 <0-001
0-50
0-87 0-61
•Adjusted for age and sex. tReference category.
Table 2 : Association of demograpiiic factors witii ciironic pain
for use in a UK population,^' and was found to be acceptable, valid, and reliable for use as a self-completion postal questionnaire. Only those patients who responded positively to the case-screening questions, which identified whether chronic pain was present, were asked to complete the chronic pain grade questionnaire.
The level of expressed needs questionnaire is based on previous work.̂ ^ It represents a measure of patients' response to chronic pain in a way that reflects demand for and uptake of health-service resources and has meant that a series of increasing amounts of help-seeking behaviour can be identified. The questionnaire enquires about the treatment sought by patients in an attempt to relieve their pain, as measured by how recently and frequently patients used painkillers or visited health professionals. For the purposes of this study, this questionnaire consisted of four questions: Have you sought treatment for your pain or discomfort recently? Have you sought treatment for your pain or discomfort ofren? Have you taken painkillers for your pain or discomfort recently? Have you taken painkillers for your pain or discomfort ofren?
From the number of positive responses we were able to identify five levels of expressed need for patients with chronic pain from level 0 (no expressed need; answered no to all four questions) to level 4 (high expressed need; answered yes to all four questions).
The sociodemographic section included questions on education, marital status, housing tenure, employment status, occupation, and the number of occupants living in the patient's home. Although occupational data can be used to determine social class for people in work, housing tenure is a valid substitute to assess socioeconomic status.^'
Age-group (years) 25-34 35-44 45-54 55-64 65-74 ©75
Total
Men
.n
76 102 139 173 189 173
852
Rate of chronic pain (range*)
31-8 (19-1-58-9) 38-3 (25-6-58-8) 47-6 (35-8-60-6) 53-9 (44-5-62-0) 56-6 (49-6-61-9) 59-9 (49-6-66-8)
48-9 (37-0-61-4)
Women
n
94 140 170 198 190 173
965
Rate of chronic pain (range*)
31-6 (23-3-49-6) 43-6 (35-0-54-8) 53-1 (42-9-62-1) 60-2 (50-4-66-7) 57-9 (50-0-63-7) 64-3 (51-3-71-5)
51-8 (41-8-61-1)
•Minimum rates assume no non-responders had chronic pain; maximum rates assume ail non-responders had chronic pain.
Table 1 : Prevaience of self-reported ciironic pain
All respondents
n
170 242 309 371 379 346
1817
Rate of chronic pain (range*).
31-7 (21-3-54-3) 41-2 (30-3-56-8) 50-5 (39-4-61-4) 57-1 (47-4-64-3) 57-3 (49-8-62-8) 62-0 (50-4-69-1)
50-4 (39-4-61-2)
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Sex Men Women
Number of respondents
1511 1610
Odds ratio* (95% CI)
Total with Grade I chronic pain
Grade II Grade III Grade IV
1-00* 1-24 (1-07-1-43) 0-003
Sex Men 693 Women 752
348 (50-2%) 176 (25-4%) 355 (47-2%) 177 (23-5%)
70 (10-1%) 99 (14-3%) 9 1 (12-1%) 129 (17-2%)
Age-group (years) 25-34 35-44 45-54 55-«4 65-74 375
505 545 562 567 543 399
1-00* 1-53 (1-19-1-96) 2-02 (1-58-2-59) 2-38 (1-85-3-07) 2-25 (1-63-3-09) 2-90 (2-07-4-05)
<0-001 <0-001 <0-001 <0-001 <0-001
Age-group 25-34 35-44 45-54 55-64 65-74 375
159 211 276 299 280 220
90 (56-6%) 120 (56-9%) 140 (50-7%) 144 (48-2%) 138 (49-3%)
71 (32-3%)
40 (25-2%) 48 (22-7%) 74 (26-8%) 70 (23-4%) 69 (24-6%) 52 (23-6%)
11 (6-9%) 17 (8-1%) 22 (8-0%) 35 (11-7%) 38 (13-6%) 38 (17-3%)
18 (11-3%) 26 (12-3%) 40 (14-5%) 50 (16-7%) 35 (12-5%) 59 (26-8%)
Housing tenure Owned or mortgaged Council rented Privately rented and other
2224 732 165
1-00* 1-23 (1-03-1-47) 0-92 (0-66-1-29)
0-02 0-63
Employment type Empioyed Retired Unable to work owing to iiiness/disabiiity Others not empioyed
1672 917 136
396
1-00* 1-32 (1-00-1-74) 7-75 (4-59-13-09)
0-96 (0-76-1-22)
0-047 <0-001
0-73
* Reference category.
Table 3: Logistic-regression modei of factors significantiy associated witii ciironic pain
Data analysis Each patient was sent a copy of the questionnaire accompanied by a letter, signed by his or her own general practitioner and the general practitioner in the research group. Up to two reminders were sent after intervals of 2 weeks to maximise the response rate.
Questionnaire responses were entered into the Microsoft Access database (version 2) and the data were analysed with SPSS (version 6). x̂ tests were used to test for associations between sociodemographic variables and questionnaire responses, and Mantel-Haenszel tests were used to investigate trends across age-groups. To identify the sociodemographic factors that were associated with chronic pain, odds ratios (adjusted for age and sex), 95% CI, and p values for a two- sided test were calculated. We considered for inclusion in a model all factors for which the odds ratio had a p value below 0-2,̂ '' that were deemed clinically important, or that had had a significant association with chronic pain in previous studies. Backward stepwise logistic-regression modelling was then used to obtain a subset of sociodemographic factors that were associated with chronic pain. We used likelihood ratio tests to judge whether or not to remove factors.
Results Of the 5036 individuals randomly selected, 425 were excluded from the survey by general practitioners (figure). The reason for exclusion was not always given. 267 exclusions were explained: 72 patients had died; 99 were no longer registered with the practice; 18 had terminal illness; 43 were demented, frail, or very elderly; nine had mental illness; 12 were in hospital; and 14
Back pain
Arthritis Injury Angina Women's Don't problems know
Sex Men 259 (14-9%) 239 (13-7%) 133 (7-6%) 85 (4-9%) 0 90 (5-2%) Women 317 (17-0%) 331 (17-8%) 80 (4-3%) 76 (4-1%) 139 (7-5%) 64 (3-4%)
Age-group 25-34 64 (11-9%) 6 (1-1%) 35-44 97 (16-5%) 29 (4-9%)
108 (17-6%) 76 (12-4%) 120(18-5%) 131(20-2%) 101 (15-3%) 171 (25-8%) 86 (15-4%) 157 (28-1%)
36 (6-7%) 1 (0-2%) 31 (5-3%) 0 36 (5-9%) 7 (1-1%) 33 (5-1%) 4 1 (6-3%) 37 (5-6%) 50 (7-6%) 40 (7-2%) 62 (11-1%)
28(5-2%) 16(3-0%) 47(8-0%) 24(4-1%) 31(5-1%) 32(5-2%) 17(2-6%) 30(4-6%)
6(0-9%) 26(3-9%) 10(1-8%) 26(4-7%)
45-54 55-64 65-74 S 7 5
Total 576 (16-0%) 570 (15-8%) 213 (5-9%) 161 (4-5%) 139 (3-9%) 154 (4-3%)
Percentages are of totai in each subgroup.
Table 4: Seif-reported cause of pain
Total 1445 703(48-7%) 353(24-4%) 161(11-1%) 228(15-8%)
Percentages are of total with chronic pain in each subgroup who completed the chronic pain grade questionnaire.
Table 5: Seif-reported ciironic pain severity
were excluded for other reasons. Of 4611 questionnaires mailed, 176 questionnaires were not delivered because the address was unknown or incorrect, and were returned to us. 56 people did not receive their questionnaires; this group consisted of people who had died recently, who were in hospital, or who were working overseas. The patients' families notified us of these individuals.
3605 questionnaires were completed and returned—a response rate of 82-3%. The response rate varied with age and sex. 1864 (83-9%) of 2223 women responded, compared with 1741 (80-8%) of 2156 men (p<0-01). The response rate increased with age from 72-3% (536 of 741) in the age-group 25-34 years to 88-9% (558 of 628) in the age-group 65 years and older (p<0-001).
The proportion of respondents reporting chronic pain was 50-4% (table 1). There was no significant difference between men and women in the proportion reporting chronic pain (48-9 vs 51-8%) but the proportion significantly increased with age from 31-7% for the youngest age-group to 62-0% for the oldest age-group (p<0-001). By standardising the sample to the age and sex distribution of the total population of patients registered with participating practices, we estimated the prevalence of chronic pain in the general population as 46-5% (95% CI 44-8-48-2).
To assess the possible effect of differences in response in the various age-sex groups, we calculated the minimum possible rate of chronic pain (based on the assumption that none of the non-responders had chronic pain), and the maximum possible rate (based on the assumption that all of the non-responders had chronic pain).
Table 2 gives odds ratios for pain associated with various sociodemographic factors. Individuals living in rented council accommodation were more likely to have chronic pain than those who lived in owned or mortgaged property. Individuals who were retired or not able to work were also more likely to report chronic pain than those who were employed. To obtain a subset of sociodemographic variables associated with chronic pain, backward stepwise logistic-regression modelling was done (table 3). Age, sex, housing tenure, and employment status were identified as factors independently and significantly associated with chronic pain. Although sex was not a significant factor in the univariate analysis, it was included in the final model as a significant factor, with women more likely than men to report chronic pain. Chronic pain was also associated with older age, living in rented council accommodation, and being retired or unable to work.
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Sex Men Women
Age-group 25-34 35-44 45-54 55-64 65-74 a 7 5
Total
Total with chronic pain
852 965
170 242 309 371 379 346
1817
Level 0
174 (20-4%) 138 (14-3%)
32 (18-8%) 53 (21-9%) 55 (17-8%) 64 (17-3%) 60 (15-8%) 48 (13-9%)
312 (17-2%)
Level 1
158 (18-5%) 132 (13-7%)
28 (16-5%) 41 (16-9%) 63 (20-4%) 56 (15-1%) 62 (16-4%) 40 (11-6%)
290 (16-0%)
Level 2
195 (22-9%) 253 (26-2%)
57 (33-5%) 50 (20-7%) 80 (25-9%) 92 (24-8%) 89 (23-5%) 80 (23-1%)
448 (24-7%)
Level 3
110 (12-9%) 148 (15-3%)
19 (11-2%) 36 (14-9%) 37 (12-0%) 54 (14-6%) 62 (16-4%) 50 (14-5%)
258 (14-2%)
Level 4
215 (25-2%) 294 (30-5%)
34 (20-0%) 62 (25-6%) 74 (23-9%)
105 (28-3%) 106 (28-0%) 128 (37-0%)
509 (28-0%)
Percentages are of total with chronic pain in each subgroup.
Table 6: Expressed need of patients with chronic pain
Back pain and arthritis were the two most commonly reported causes of chronic pain, accounting for about a third of all reported causes (table 4). Back pain was the most common problem in men and in the younger age- groups, and arthritis was the most frequent cause of pain in women and in the older age-groups. A large number of other causes of pain were given. Some people identified the actual complaint and others reported an anatomical site for their symptoms. The most frequently reported complaints in this category were leg pain (1-7%) and headaches (1-0%). All other causes given were reported by less than 1% of the sample.
Table 5 shows the pain severity as measured by the chronic pain grade questionnaire. Of 1445 individuals who reported chronic pain, 703 (48-7%) had pain graded as least severe (grade I), and 228 (15 8%) had pain graded as most severe (grade IV). There was no significant difference in the distribution of grades of pain between men and women but the frequency of the more severe pain grades (grades III and IV) increased with age (p<0-001).
Of those patients with chronic pain, 312 (17-2%) showed no expressed need and 509 (28 0%) showed the highest level of expressed need (table 6). The small percentage of individuals with chronic pain who had no apparent expressed need suggests that patients with chronic pain are likely to make extensive use of health services. Women were more likely to report high expressed need than were men (p<0-001). The level of expressed need also varied significantly with age (p<0-001).
Discussion This study shows that chronic pain is a major problem in the community, with about half of the sample reporting chronic pain; after standardisation we estimate that 46-5% of the general population have chronic pain. This figure is higher than previous estimates of chronic- pain prevalence in general population studies."*'''"*""' This discrepancy is likely to reflect differences in the research methods and case definitions used; differences that previously accounted for the majority of variation in general-population prevalences." Prevalence of chronic pain also varies widely across different geographical locations.^
These findings expand our understanding of chronic pain in two important ways. Unlike many previous studies, our study was done in a community setting and used a definition of chronic pain based on the most commonly used standard definition, that of the International Association for the Study of Pain. Also,
our study is the first general population study of all chronic pain to use validated measures of the severity and significance of chronic pain. This approach allows stratification of chronic pain, which means that in addition to providing prevalence rates for any chronic pain we have established the prevalence of severe and significant chronic pain. These prevalence rates are likely to be of greatest clinical significance, and of more use, to physicians and health-service planners.
The response rate for the study was high, with slightly higher response rates from women than men, and increasing response rates with age. The increase in response rate with age is likely to be related to the fact that prevalence of chronic pain increases with age. To explore the possible effects of non-response on the prevalence of chronic pain, we calculated minimum and maximum rates for each subgroup of age and sex. The range between the minimum and maximum rates is widest for the youngest age-group, reflecting the lower response rate in these patients.
Analysis of the sociodemographic factors identified age, sex, housing tenure, and employment status as a subset of factors significantly associated with chronic pain. These findings highlight subsets of the general population who may be more likely to have chronic pain and are important in consideration of the onset and management of this important and common problem. Further longitudinal studies are needed to establish the direction of these sociodemographic associations, and to differentiate causes from effects.
Although the proportion of our sample who had chronic pain was high, most individuals (48-7%) reported pain of the mildest chronic pain grade. Even so, 17-2% of those with chronic pain reported no expressed need. A fairly small proportion (15-8%) reported the most severe grade of pain, yet 28-0% of all those with chronic pain reported the highest expressed need. These results suggest a disparity between reported severity of chronic pain and the expressed need of patients, but the disparity arises because the two instruments measure different features of chronic pain. The chronic pain grade assesses severity of chronic pain in terms of intensity and disability, and the level of expressed need questionnaire measures patients' response to their chronic pain in terms of treatment and analgesics. Our results suggest that response to chronic pain, as measured by the level of expressed needs questionnaire, is not solely dependent on pain intensity and disability and may reflect the multifactorial and subjective experience of, and response to, chronic pain. The disparity between the two measures is important since it may highlight areas of unmet need and highlight
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those patients who display high expressed need yet have mild chronic pain. Further research into the complexities of this issue is in progress.
There is evidence that many patients with pain, in many cases treated inadequately, could be relieved with improved management by general practitioners." Management strategies need to be practicable and appropriate for use in primary care, where the majority of patients who seek treatment will present. These management strategies must be evaluated by randomised control trials, the design and interpretation of which will be informed by epidemiological studies of chronic pain in primary care.
Contributors Blair Smith, Cairns Smith, and Alastair Chambers designed the study. Alison Elliott collected the data. Alison Elliott and Kay Penny analysed the data. The paper was drafted by Alison Elliott and edited by all other investigators.
Acknowledgments We acknowledge the support of the Scottish Home and Health Department, grant reference K/RED/4/C286, and the patients and general practices for their support.
References
1 Becker N, Thomsen AB, Olsen AK, Sjorgren O, Bech P, Eriksen J. Pain epidemiology and health related quality of life in chronic non- malignant pain patients referred to a Danish multi-disciplinary pain center. Pain 1997; 73: 393-400.
2 Magni G, Marchetti M, Moreschi C, Merskey H, Luchini SR. Chronic musculoskeletal pain and depressive symptoms in the National Health and Nutrition Examination—I: epidemiological follow-up stidy. Pain 1993; 53: 163-68.
3 Gureje O, Von Korff M, Simon GE, Gater R. Persistent pain and well-being: a WHO study in primary care. JAMA 1998; 280: 147-51.
4 Turk DC, Rudy TE. Toward an empirically derived taxonomy of chronic pain patients: integration of psychological assessment data. J Consult Clin Psychol 1988; 56: 233-38.
5 Latham J, Davis BD. The socioeconomic impact of chronic pain. DisabilRehabil 1994; 16: 39-44.
6 Locker D. Disability and disadvantage. London: Tavistock Publications, 1983.
7 Von Korff M, Dworkin SE, Le Resche L. Graded chronic pain status: an epidemiologic evaluation. Pain 1990; 279: 91.
8 Von Korff M, Wagner EH, Dworkin SF, Saunders KW. Chronic pain and use of ambulatory health care. Psychosom Med 1991; 53: 61-79.
9 Bowsher D, Rigge M, Sopp L. Prevalence of chronic pain in the British population: a telephone survey of 1037 households. Pain Clinic 1991; 4: 223-30.
10 Davies HTO, Crombie IK, Macrae WA, Rogers KM. Pain clinic patients in North Britain. Pain Clinic 1992; 5: 129-35.
11 Crook J, Weir R, Tunks E. An epidemiological follow-up survey of persistent pain sufferers in group family practice and speciality pain clinic. Pain 1989; 36: 49-61.
12 Crombie IK, Davies HTO. Selection bias in pain research. Pain 1998; 74: 1-3.
13 Smith BH, Smith WC, Chambers WA.Chronic pain-time for epidemiology, j'5oi; AfeJ 1996; 89: 181-83.
14 Crook J, Rideout E, Browne G. The prevalence of pain complaints in a general populaton. Pain 1984; 18: 299-314.
15 Croft PR, Macfarlane GJ, Papageorgiou AC, Thomas E, Silman AJ. Outcome of low back pain in general practice: a prospective study. BAfJ 1998; 316: 1356-59.
16 Croft P, Rigby AS, Boswell R, Scholium J, Silman A. The prevalence of chronic widespread pain in the general population. J Rheumatol 1993; 20: 710-13.
17 Brattberg G, Thorslund M, Wikman A. The prevalence of pain in a gneneral population: the results of a postal survey in a county of Sweden. Pain 1989; 37: 215-22.
18 Verhaak PF, Kerssens JJ, Dekker J, Sorbi MJ, Bensing JM. Prevalence of chronic benign pain disorder among adults: a review of the literature. Pain 1998; 77: 231-39.
19 International Association for the Study of Pain. Classification of chronic pain. Pain 1986; suppl 3: S1-S226.
20 Von Korff M, Ormel J, Keefe FJ, Dworkin SF. Grading the severity of chronic pain. Pain 1992; 50: 133-49.
21 Smith BH, Penny KI, Purves AM, et al. The Chronic Pain Grade Questionnaire: validation and reliability in postal research. Pain 1997; 71: 141-47.
22 Purves AM, Penny KI, Munro C, et al. Defining chronic pain for epidemiological research—assessing a subjective defmition. Pain Clinic 1998; 10: 139-47.
23 Blaxter M. Health and lifestyles. London: Tavistock/Routledge, 1990.
24 Rothman KJ, Greenland S. Modern epidemiology, 2nd edn. Philadelphia: Lippincott-Raven, 1998.
25 Diamond A. The future development of chronic pain relief. Anaesthesia 1991; 46: 83-84.
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