Introduction

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Epidemiology provides a framework for determining the prevalence, incidence, and risk of pain in populations. Epidemiological studies use the paradigm of exposure-disease assessment. Classical epidemiological research focuses on the distribution of diseases and their determinants within populations and relies on field-tested measures of exposure (usually self-reported) in defining disease-exposure associations. More recently, advances in molecular technology have made possible the measurement of genetic markers of disease, prognosis, and therapeutic response. This newer molecular epidemiology paradigm represents the confluence of sophisticated advances in molecular biology and field-tested epide-miological methodologies. Currently, interest in the use of biological markers in epidemiology to enhance assessment of exposures to disease, provide insight into disease mechanisms, understanding susceptibility to diseases, and refining assessment of risk of disease is increasing. In this chapter we present the results of studies of the epidemiology of pain using classical epidemiological methods; describe the basic principles of epidemiology; and introduce the concepts and approaches for exploring the molecular epidemiology of pain.

associated with depressive disorders or psychological distress and anxiety.27,28,29

Pain has a significant economic impact. In the United States alone, lost productive time resulting from common pain conditions among active workers costs an estimated $61.2 billion annually.30 An estimated 2.9 million Americans (1.1 percent of the population) receive treatment annually from chronic pain specialists. In the United Kingdom, the mean cost per adolescent experiencing chronic pain was approximately £8000 per year, including direct and indirect costs.31

PREVALENCE OF PAIN

As many as 11 to 60 percent of general adult populations suffer from chronic pain1,2345678 * 10 11 12 13 14 15 16 17,18,19 (Table 5.1). It should be noted that estimates of the prevalence of pain have varied widely, mainly because of a lack of uniformity or standardization in the definitions of and assessment measures used for pain (i.e. no gold standard exists) and the heterogeneity of pain conditions (nociceptive versus neuropathic). Other factors contributing to the wide variation in results include, but are not limited to, the heterogeneity of disease conditions and the types of treatment settings (outpatient versus inpatient versus community) in which the studies were conducted.

Adverse impact of pain

Left untreated, pain adversely affects function and daily activity. One study showed that individuals with persis tent pain were more likely to experience severe activity limitations than those without persistent pain (odds ratio, 1.63; 95 percent confidence interval, 1.41-1.89).20

Another study found that among individuals with abdominal pain, more than 65 percent reported some activity limitations.21 Up to 20 percent of older adults in the general population in the United States have reported significant pain resulting in activity limitations.22

The adverse impact of pain is not limited to function.

For example, individuals with chronic pain have up to a four-fold increase in the incidence of psychological dis orders when compared with those without chronic pain.20,

23,24 pain is predictive of the development of depression.25

Several studies have addressed the relationship between depression and pain and found that depression has either a causal or mediating effect on pain. One study in parti cular found a strong correlation between pain severity and depression in older patients but a weak and insignificant correlation between the two in younger patients.26 Although the causal relationship between depression and pain remains debatable, at least in primary care settings, studies have shown that symptoms such as pain are in fact

Variance in pain by age, sex, and ethnicity/ culture/race

Although the pattern of pain prevalence in older individuals is unclear, several community-based studies of pain suggest that pain prevalence increases from the early adult years up to approximately 60 years of age32, 33 and thereafter reaches a plateau and may even decline in extreme old age. It is generally accepted that increased pathological load is an overriding factor contributing to increased pain complaints with advancing age,33 as older adults are at greater risk for diseases that cause pain, such as arthritis and cancer. Patients older than 60 years of age have a two-fold increase in the incidence of painful conditions relative to younger patients34 and older patients are less likely to receive adequate analgesic treatment.35 Furthermore, although they are more likely to experience pain than younger individuals, older adults tend to be less likely to complain of pain.36 Factors such as other medical problems, cognitive and sensory impairment, and depression are possible contributors to the underreporting of pain among older adults.

Women have a greater risk of pain and report more severe pain, more frequent pain, and longer pain durations than men do.37 These differences are partially attributed to the action of sex hormones, which may influence central and peripheral mechanisms of noci-ceptive pain transmission, pain sensitivity, and pain perception.38 Studies have also shown sex differences in responses to treatment with analgesics, especially opioids.39

The epidemiology of pain in ethnic minority communities has been a matter of intense investigation in racially and culturally diverse countries such as the United States. Studies of chronic pain conditions showed that African Americans consistently reported greater pain severity and disability than did those in other racial and ethnic groups.40,41 A recent study of a nationally representative sample of adults in the United States showed that Latino (Hispanic people) and Anglos (non-Hispanic white people) reported lower rates of activity impairment resulting from pain than did African Americans (non-Hispanic black people).42 Also, more African Americans than Anglos and Latinos reported functional impairment

Table 5.1 Population-based studies of chronic pain. Author Country Study design

Setting

Age range (years)

Andersson Bergman et al.2 Blyth et al.3 Blyth et al4 Breivik et al.5

Cassidy et al.6 Catala et al.7 Chrubasik et al.8

Croft et al.9 Elliot et a/.10

Eriksen et al.v Hassan et al.u

Sweden

Sweden

Australia

Australia

Fifteen European countries3 and Israel Canada

Spain

Germany

Denmark

Saudi Arabia

Cross-sectional Cross-sectional Cross-sectional Cross-sectional Cross-sectional

Cross-sectional Cross-sectional Cross-sectional

England Cross-sectional

United Kingdom Cross-sectional

Cross-sectional

Cross-sectional

Two primary health care 25-74

districts, rural areas

West coast of Sweden, 20-74

community-based

Nationwide, community- >16 based

Northern Sydney, >18

community-based

Community-based >18

Nationwide, community- 20-69 based

Nationwide, general 18-95

population Regierungsbezirk 18-80

Karlsruhe County, general population Northern England, general >18

population Grampian region, sample >25 of patients from 29 general practice/ primary care facilities Nationwide, random >16

sample, patients without cancer Ten regional health care centers

Number of Data collection subjects methods: pain assessment tools

1625 Mailed questionnaire

2425 Mailed questionnaire

17,543 Computer-assisted telephone interview 2092 Telephone survey

46,394 Screening questionnaire

1131 SHBPS

5000 Telephone survey

1304 Mailed survey

Pain prevalence (type)

19.0% (chronic); half reported having received inadequate treatment 22.2% (depressive symptomatology) 54.0% (chronic); 23.4 % of population 47.0% (unduly prolonged pain)

2034 Mailed survey

3605 Chronic Pain Grade questionnaire

10,066 SF-36

50.4% (chronic), 16.0% (back), and 15.8% (arthritis)

41.0% (neuropathic), 59.0% (nociceptive)

(Continued over)

Table 5.1 Population-based studies of chronic pain (continued).

Author

Country

Study design

Setting

Age range (years)

Number of subjects

Data collection methods: pain assessment tools

Pain prevalence (type)

Moulin et al.13

Canada

Cross-sectional

Nationwide, random

>18

2012

Telephone survey

29.0% (chronic, not cancer-

samples and patients

related)

prescribed pain

medication

Ng et al.14

China

Cross-sectional

Hong Kong, random

>18

1051

Telephone interview

10.8% (chronic)

sample

Rustoen et al.15

Norway

Cross-sectional

Nationwide, general

18-91

1912

Mailed questionnaire

24.4% (chronic)

population

Saastamoinen

Finland

Cross-sectional

Helsinki, city employees

40, 45, 50,

D010

Chronic Pain Grade

24.0% male, 29.0% female

et al.16

55, and 60

questionnaire

(chronic)

Taylor17

New Zealand

Cross-sectional

North Island, general

DO >18

329

Mailed questionnaire

40.0-60% musculoskeletal

population

Torrance et al.18

United Kingdom

Cross-sectional

Aberdeen, Leeds, and

>18

3120

S-LANSS

48.0% (chronic)

London; random

samples generated by

six family practices

Yu et al.19

Taiwan

Cross-sectional

Taipei City, multiple-stage

> 65

219

Interview

42.0% (chronic)

random sampling technique random sampling technique aThe 15 European countries were Finland, Norway, Sweden, France, Belgium, Spain, Italy, Poland, Ireland, Denmark, The Netherlands, United Kingdom, Switzerland, Austria, and Germany.

CRP, chronic regional pain; CWP, chronic widespread pain; SF-36, Short-Form-36; SHBPS, Saskatchewan Health and Back Pain Survey; S-LANSS, Leeds Assessment of Neuropathic Symptoms and Signs score.

as a result of pain, a result that approached statistical significance at mild levels of pain severity.

Many have argued that the association between race/ethnicity and poor health is largely a result of poor socioeconomic conditions among racial and ethnic minorities.43,4445 A report from the Institute of Medi-cine45 in the United States suggested that factors such as stereotyping and bias on the part of healthcare providers, the clinical appropriateness of care, and persistent racial and ethnic discrimination are among the reasons for racial and ethnic disparities in health care.

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