1. Introduction
Pain is a subjective multidimensional experience inducing multiple repercussions on quality of life, sleep, mood, physical activity, self-perceived health, social relations, work, and income. Indeed, pain imposes a significant emotional and physical burden on those affected, and leads to physical, psychological, social, and economic vulnerabilities [
1,
2,
3]. Pain intensity depends on sensitivity, but is also modulated by emotional states, and mental processes (such as attention, interpretation, memorization and anticipation). Pain is closely linked to psychological distress, with reciprocal influences [
4,
5,
6]. It also depends on demographic and socioeconomic factors: it is more frequent in elderly, women, and persons with a lower level of education or income [
7].
Pain is a very frequent reason for medical visit [
8], and constitutes an important workload for health systems [
9]. For example, spinal pain is experienced by 80% of individuals at some point in their life in the United States [
10], and the prevalence in the elderly is estimated at 49% [
11]. In France, 54% of adults had suffered from pain in the past two years [
12], and 25% had experienced physical pain that was difficult to bear in the past year [
13]. Patients who reported pain consulted healthcare professionals twice as often as the others [
14].
In several chronic diseases, pain is one of the main symptoms affecting the daily life of patients. The general adult population in the European Union reported an average chronic pain prevalence of around 20% [
15,
16]. The etiology of chronic pain is complex, and influenced by biochemical, psychosocial and behavioral factors [
17]. Chronic pain is a source of disability, and major alterations in quality of life. It represents significant costs for health systems [
18,
19], including direct costs (medical visits, analgesics), but also indirect costs (depression, addictions). The annual additional medical cost is estimated at more than one billion euros in France [
14]. In addition, chronic pain represents a very significant proportion of the causes of sick leave, early retirement, and permanent disability [
20]. According to data from the National Health and Wellbeing Survey in five European countries, the presence of severe daily pain is associated with a reduced likelihood of being employed full time (25% against 45% for people with no pain) [
20]. Chronic pain induces a decrease in productivity at work. Thus, in addition to its considerable impact on the quality of life of individuals, it reduces the competitiveness of the working population.
Vulnerable people are more likely to be affected by chronic conditions [
21]. The prevalence of chronic conditions reaches 60% among beneficiaries of the Medicaid program in the United States (low-income people aged 18 to 64 years) against 50% in the general population (yet including people over 65 years) [
22]. In low-income countries, death rates from chronic diseases in 2005 were 54% higher for men and 86% higher for women compared to both men and women in high-income countries [
23]. Vulnerability can drive the incidence of chronic disease, through its influence on modifiable risk factors [
24,
25,
26,
27], and reduced utilization of health care systems, due to barriers such as geographical isolation, scarcity of public transport, or lack of social ties [
28,
29,
30]. People with low incomes find it less easy to adhere to a healthy lifestyle, and have poorer general health [
31,
32]. They generally happen to suffer most from the fragmentation of care services [
33]. But vulnerability can also be the consequence of a chronic condition: illness aggravates social vulnerability and can push people into precariousness through job loss [
34].
Although there is enough evidence to make it a high priority, as well as to conduct more research, chronic pain appears to have received few specific public health policy responses, especially in vulnerable populations. The research project EFFICHRONIC is a prospective and multicentric study, where socio-economically vulnerable participants with a chronic condition participate to the "Chronic Disease Self-Management Programme" (CDSMP) [
35].
In this article, we explore, at baseline i.e. before the beginning of the programme, the relation between pain intensity and: (1) socio-demographic variables (country, sex, age, education, income, and social network); (2) sedentary behaviour and physical exercise; (3) quality of life (mental and physical), and its components (sleep problems, fatigue, psychological distress, and general perceived health); (3) work absenteeism.
4. Discussion
This multicentric study allowed a thorough characterization of the factors associated with pain and its intensity in a large sample of persons with a chronic condition among vulnerable populations in five European countries (N=1,364).
In raw data, average income was lower in participants with higher pain intensity, and they had fewer social relationships, as compared to participants without pain. Besides, participants with pain were on average older than those without pain. Finally, women had on average a higher level of pain than men. Multivariable analyses, adjusting for the whole set of variables that were significant in univariable models, showed that women had more intense pain as well as participants with a lower education level. Although the participants had been included with vulnerability criteria, these results suggest an association of pain with low education level and low income even within vulnerable populations, and confirm the impact of socio-economic status among the determinants of pain.
There is a rather abundant literature on the link between pain and vulnerability. Significant inverse associations have been found between a more severe pain and a lower educational achievement [
58,
59,
60]. Similarly, pain prevalence, intensity, and functional interference, were repeatedly found to be higher in people with a lower income [
1,
59,
61,
62]. This was also observed when comparing people living in the most deprived areas with those living in the most affluent ones [
63]. Low income and manual work seem to have cumulative effects on the odds of experiencing severe pain [
61]. Sickness absence at work due to pain is also inversely related to socio-economic status [
60].
In the Austrian Health Interview Survey [
7] (with more than 15,000 respondents), socio-economic status (based on education, income and profession) was inversely and gradually associated with the prevalence of severe pain, with the number of indicated painful body sites, with the intensity of pain, and with the subjective level of feeling disabled through pain. Moreover, even at the same intensity of pain and the same number of painful body sites, people in the lowest socio-economic class were twice to three times more likely to feel disabled through pain than people in the highest socio-economic class. Similarly, other studies have shown that people in deprived socio-economic situation not only run a higher pain and chronic pain risk, but also experience their pain as more severe/disabling than their more privileged counterparts [
64,
65]. The mechanisms underlying the association between socio-economic status and pain are not yet elucidated. Firstly, low income dramatically increases risk factors of chronic diseases that can induce chronic pain. Secondly, depressive symptoms could be one of the mediators of this relationship, in terms of limiting the individual’s strategies to manage pain [
66]. Indeed, higher levels of subjective socio-economic status significantly predicted lower odds of participants having been prescribed at least one analgesic drug in the previous six months, and this was true even after controlling for objective socio-economic status variables [
67].
Vulnerable people with chronic conditions should be a priority of health policy and interventions. Over the last decades, literature on both chronic care [
68] and integrated care [
69] have gradually recognised the importance of the social determinants in shaping people´s health. Both fields acknowledge that healthcare strategies and interventions should not only consider individual factors, but also people’s environment (including socio-economic determinants). It is precisely the most vulnerable people with chronic conditions who most need social and community support as well as effective and integrated care [
70]. Consequently, integrated healthcare strategies should prioritize them. However, individuals from vulnerable groups are usually hard-to-reach [
24,
28,
71,
72]. They are often reluctant to attend the care system and do not seek support through the usual channels [
73]. Areas with higher vulnerability are often left out from policy innovations and experiences due to their socio-economic complexity.
Another interesting finding of this study is that participants had less pain if they performed more physical exercise, and tended to have less pain if they had a less sedentary behaviour (i.e. a smaller number of hours sitting per day). This distinction between sedentary behaviour and physical exercise has already been highlighted by studies of metabolic diseases: epidemiological data showed that high volumes of sedentary behaviour are detrimental to metabolic health, even in the presence of regular exercise [
74], suggesting that the health effects of sedentary behaviour are independent from those of exercise, and that daily living physical activity is beneficial.
Finally, it is remarkable that pain was the main predictor of sleep problems, fatigue, depression, self-perceived health, physical and mental quality of life. This is in line with other studies showing the major impact of pain on an individual’s health and life [
1,
3]. Pain is frequent in persons with a chronic condition [
75], but the causal relation is not necessarily unidirectional. Chronic pain can increase the risk for metabolic diseases and cardiovascular diseases [
3], through an impact on physical exercise and sedentary behavior, and through the influence of pain on blood pressure [
76]. Chronic pain can also induce depression and anxiety [
4,
6], either directly or through negative consequences on social and professional interactions [
20], and on sleep [
3]. Anxiety can in turn be involved in immune diseases and cancers through an impact on inflammation [
77].
A strength of this study is the rather large sample size of 1,364 participants from five countries. However, the use of vulnerability maps and targeted recruitment strategies may introduce bias into the sample, impacting the representativeness of the sample. In addition, the use of individual recruitment strategies may introduce confounding variables. Finally, our results may not be generalizable outside the European context.
The three questionnaire items on pain were significantly correlated between them, we can therefore consider that our measure of pain intensity is reliable. Persons with any chronic condition have been included indiscriminately, as well as persons with diverse vulnerability criteria, and this is an originality of this research: the vast majority of other pain studies focus on a specific disease or a specific population, and are therefore difficult to generalize. At the same time, the absence of information on the type of chronic condition (articular, digestive, metabolic, etc.) is a weakness of this protocol, because these entities encompass a great diversity. Furthermore, other useful information is missing, such as the height and body weight of participants, their ethnic/cultural background, or the use of painkillers.