Best Practice & Research Clinical Rheumatology 27 (2013) 591–600

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Low back pain across the life course Kate M. Dunn, Reader in Epidemiology a, *, Lise Hestbaek b,1, J. David Cassidy, Professor of Epidemiology and Senior Scientist c, 2 a

Arthritis Research UK Primary Care Centre, Research Institute for Primary Care & Health Sciences, Keele University, Staffordshire ST5 5BG, UK Nordic Institute of Chiropractic and Clinical Biomechanics and Institute of Sports Science and Biomechanics, University of Southern Denmark, Campusvej 55, Odense M DK-5230, Denmark c University Health Network, University of Toronto, Suite 200, Box 115, 700 Bay Street, Toronto, ON M5G1Z6, Canada b

s u m m a r y Keywords: Back pain Children Adolescence Adults Elderly Natural history Clinical course

Back pain episodes are traditionally regarded as individual events, but this model is currently being challenged in favour of treating back pain as a long-term or lifelong condition. Back pain can be present throughout life, from childhood to older age, and evidence is mounting that pain experience is maintained over long periods: for example, people with pain continue to have it on and off for years, and people without pain do not suddenly develop long-term pain. A number of factors predict back pain presence in epidemiological studies, and these are often present, and predictive, at different life stages. There are also factors present at particular life stages, such as childhood or adolescence, which predict back pain in adulthood. However, there are little published data on longterm pain patterns or predictors over the life course. Such studies could improve our understanding of the development and fluctuations in back pain, and therefore influence treatment approaches. Ó 2013 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: þ44 (0) 1782 734703; fax: þ44 (0) 1782 733911. E-mail addresses: [email protected] (K.M. Dunn), [email protected] (L. Hestbaek), [email protected] (J.D. Cassidy). 1 Tel.: þ45 6550 4530. 2 Tel.: þ1 416 400 2656. 1521-6942/$ – see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.berh.2013.09.007

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Background Historically, the epidemiology of back pain was most commonly studied in adults, predominantly working age adults. Risk factors for back pain were also principally those factors present in adults at the time of onset, or shortly preceding the apparent onset of the condition. This focus was due at least in part to the prevailing belief that back pain was usually due to an injury, often occurring in the workplace. Research studies were often orientated around this model and focussed on the onset or persistence of episodes of pain, or studying risk factors present at a time point when an individual did not have pain and predicting the presence of pain at a later point. This model has been challenged over the last couple of decades as a result of three factors: (1) developments in our understanding of the natural (or clinical) course of back pain, highlighting that individuals often experience repeated episodes, and that these episodes are not independent of each other; (2) increasing knowledge about the presence of back pain in children, young people and older age adults; and (3) the emergence of the biopsychosocial model of back pain [1], which indicated that psychological and social factors, as well as biomedical factors, might be related to the occurrence of back pain. One result of this is that studying single episodes of pain, or presence of pain at one-off time points, is limiting our understanding of the condition. Similar issues have been faced by researchers of other conditions and they have led to the development of life-course epidemiology. This can be defined as “the study of long-term biological, behavioural, and psychosocial processes that link adult health and disease risk to physical or social exposures acting during gestation, childhood, adolescence, earlier in adult life, or across generations” (Ref. [2], pg 3). Application of methods and principles from life-course epidemiology to problems such as cardiovascular disease and respiratory conditions has led to improved understanding of the widespread nature of their influence on health. Back pain researchers are beginning to take a longer-term or life-course perspective, [3] and this chapter aims to summarise some of the work that has been done and point to potential future directions. Taking a lifetime perspective The first challenge in moving on from our historical models of back pain is conceptual – thinking of back pain as a long-term or recurrent condition rather than a series of unrelated episodes. A simple ‘yes/no’ question on previous history of back pain has usually been the only information collected about prior pain experience. It is important to understand that people with no back pain at a point in time are not all the same: some may never have had back pain previously, whereas others may have had one or more significant episodes of pain in the past. Ignoring this fact, and simply studying the onset of a new episode, can introduce biases such as incidence-prevalence bias [4] – people who have had back pain before (from which they recovered) will be different from people who have never had back pain, as they might, for example, have certain illness perceptions or health behaviours as a result of their prior pain experience. In classical epidemiology, a risk factor is present in someone without the disease and is related to future onset of disease, but in someone with a prior history of back pain, is a risk factor really a prognostic factor? Although there is a general understanding that prior history of back pain is related to occurrence of future episodes (e.g., Ref. [5]), little is known about the specific influence of the timing, nature and duration of those episodes – principles from life-course epidemiology relating to chains of risk and cumulative exposure might help elucidate these issues. Our lack of understanding about back pain over the life course may be limiting treatment, as health care is usually focussed on discrete episodes of pain. There may be alternative approaches to the prevention and management of back pain that could be highlighted by taking a life-course approach. Epidemiology of back pain over the life course Prevalence of back pain over the life course A recent systematic review has estimated the point prevalence of low back pain to be 12%, with a 1month prevalence around 23% [6]. There is no strong evidence that these figures are different when

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children, adolescents or the elderly are considered separately. A recent meta-analysis also estimated the point prevalence of back pain among children/adolescents at 12% [7]. This paper and another systematic review indicate that there is a clear rise in prevalence with age among children and adolescents, and adult prevalence levels appear to be reached by around the age of 18 years [7,8]. At the other end of the scale, among the elderly (>60 years), back pain again appears to have a similar prevalence as it does among people of middle age [9], although there is some evidence that severe forms of back pain may increase in prevalence with increasing age [10] and reduction of activities due to back pain has been shown to increase with age [11]. Long-term persistence Although studies of prevalence give indications of the proportion of the population affected by back pain at any one time, it is not possible to tell from these studies whether it is the same people, or different people, who have the pain at different ages and time points. However, as the lifetime prevalence of back pain can be over 80% [12], it appears that it is not the same 12% of the population that has back pain all the time. This fits with studies showing that at least 40% of people seeking health care do recover within a year of an episode [13], many much quicker [14]. Population-based studies with longer-term follow-up give further insights. One Swiss study with annual follow-ups over 5 years indicated that 14% of adults had back pain at all the follow-up points and only 35% of the population were back-pain free at all times [15]. Similarly, a Dutch population study collecting data from adults at three time points over a 10-year period found that 30% of the population were back-pain free at all follow-up points but only 6% had back pain at all points [16]. A Danish study with 5-year follow-up (which had more stringent criteria for being pain free) found around 23% of people consistently reporting no pain days during the previous year at each follow-up point, while around 10% reported >30 days of back pain at all points [17]. These studies, and others, indicate that while long-term persistent back pain is not common in the adult general population, the majority of people do experience back pain off and on over long periods of time. There is evidence from general population samples that previous experience of back pain is a risk factor for a new episode among all adults [5,17,18] and the elderly [19]. However, among patient populations, systematic reviews show that a history of back pain is not a consistent predictor of pain persistence [20], there is limited evidence of its value in predicting recovery [21] and therefore it is not considered to be useful [22]. While it seems intuitive that people who have had pain before are more likely to have persistent problems, the fact that a history of back pain is so common among this group (over 80% [23]) is perhaps one reason for the lack of predictive value. It may also be difficult for patients to separate previous episodes from the current episode where they have long-term symptoms. It is therefore possible that using more information about the timing (e.g., whether episodes occur during critical periods), number and duration (e.g., to investigate risk accumulation) or nature and severity (e.g., fluctuating/mild or severe persistent trajectories – see the chapter in this issue on back pain trajectories) of previous episodes would add value to information about back pain history. Back pain episodes Episodes of back pain can be conceptualised as having sudden onset, sometimes due to trauma, or gradual (or insidious) onset. Most back pain episodes do appear to have sudden onset, with a population-based study in the UK reporting 59% of back pain to be of sudden onset [24] and an Australian study in primary care reporting 80% of episodes to be of sudden onset [25]. Sudden onset of back pain has been linked with good early outcomes, but only among males [24]. However, how this onset of pain episode relates to the initial or first onset of back pain, or to the long-term course, is unclear, as over 75% of people with new episodes of pain report having back pain previously [26]. Furthermore, it is becoming increasingly clear that back pain is common in children and especially in adolescents [7]. Thus, the acute onset of back pain episodes commonly reported in general practice might be the result of problems beginning earlier in life.

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Factors associated with back pain across the life course In this section, we present information about factors associated with, predictive of or prognostic for back pain at different points in the life course. Genetic factors Although genetic factors are present throughout life, they do not necessarily contribute equally to back pain at different life stages, because the relative contribution of various environmental factors varies with age. In a classical twin study, the total contribution of genetic factors to the overall probability of developing back pain decreases from around 70% at age 16 to 36% at age 40 [27], reflecting an increase in the relative environmental contribution over this age span. Associations between childhood factors and adult back pain There have been few studies specifically investigating links between factors identified in childhood or adolescence and the occurrence of back pain as adults. Two studies of Danish twins found associations between birth weight and back pain in young adults and some evidence of an association between social factors in adolescence and back pain later in life. [28,29] Data from the 1958 British Birth Cohort Study demonstrated links between obesity and weight gain in early adulthood, and the later presence of back pain, but no links with body mass index (BMI) in childhood [30]. In the 1966 Northern Finland Birth Cohort, researchers found links between being overweight and smoking at the age of 14 years and hospitalisation for sciatica as adults [31]. Other studies looking more broadly at chronic pain and functional limitations, or focussed on widespread pain or other pain sites, can provide further relevant information. For example, prospective studies report that childhood behaviour or adverse circumstances (such as road traffic accidents and maternal death), and the presence of symptoms such as abdominal pain and headaches, are associated with chronic widespread pain in adulthood [32–34]. Two studies using recalled information about childhood education, and childhood health and socioeconomic circumstances, showed that these were linked with adult functional limitation or chronic disabling pain, a proportion of which is likely to be caused by musculoskeletal pain [35,36]. Another study found that components of prenatal, prepubertal and pubertal growth are predictive of grip strength among adults [37], which is potentially associated with back pain. Further studies of this type would help us to understand whether, and how, childhood health and circumstances can influence back pain experience later in life [38]. Parental factors Maternal health, and thus pre-term birth or low birth weight, has an influence on the offspring’s susceptibility to disease [39,40], although the effect size is modest for back pain [41]. Associations have been reported between parental (predominantly maternal) and child pain (including back pain) in population-based samples [42,43]; but other studies have not found significant associations [44–46], with reasons for the inconsistent evidence unclear. However, there are indications of a relationship between prevalence of back pain and parents’ socioeconomic status [29], and parental education [47]. Any associations might be partly mediated by lifestyle factors, such as overweight and smoking. Smoking is associated with back pain at all ages [48,49]; but the association is stronger in adolescence than in adulthood [50,51]. Another potential explanatory factor is learned health-care-seeking behaviour between parents and children, although a recent review again reported conflicting evidence for the association between primary care consultations for back pain between parents and children [52]. The conceptual model suggested by Evans et al. presents a range of other potentially important factors in the links between parental and child pain, including socialisation, life events and family structure [53]. Good evidence for many of these links is, as yet, unavailable.

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Psychological factors There are few prospective population-based studies of the risk of back pain among children and adolescents [54]. In the studies that have been published, conduct (behavioural) problems and high levels of hyperactivity were significant predictors of future back pain in a British sample [55] and psychological distress was a risk factor in a Canadian sample [47]. Other psychological factors such as depression [56] have also been associated with back pain in cross-sectional studies among adolescents. Among adults, factors such as stress, anxiety and depression are commonly reported risk factors for the development of back pain [57]. In addition, recent systematic reviews have found that depression, psychological distress, passive coping strategies and fear-avoidance beliefs are independently associated with the transition from acute to chronic back pain [58] and low levels of fear avoidance are associated with recovery from back pain [22]. In prospective studies among elderly people, depressive symptoms scores have been linked with the onset of back pain [19,59]. Further information about psychological factors in back pain is presented in another chapter within this issue (Pincus & McCracken). Social determinants Social determinants for back pain can be identified throughout life, although the nature probably changes during the life course. Problems with peer relationships in adolescents predict persistent back pain [60]. In working age, factors relating to the workplace have been more intensively studied and associations with factors such as low job satisfaction and perceived workload have been demonstrated [61–63]; and in old age, social support seems to have an impact on back pain [64], perhaps more so than among younger people [65]. Socioeconomic factors contribute to the course of back pain in different ways: parents’ socioeconomic status might play a role in back pain in adolescents [29], educational background is important throughout life [66] and possible litigation processes and compensation systems play a major role, mainly in the working population [67–69]. Low back pain after motor vehicle collisions The World Health Organization estimates that between 20 million and 50 million people are injured in road traffic collisions worldwide each year (http://www.who.int/mediacentre/factsheets/fs358/en/ index.html; cited 28 August 2013). In the developed nations, the most common injuries occur to the soft tissues of the neck and lower back, and these account for about 80% or more of all motor vehicle injuries [69,70]. Most patients with whiplash injury to the neck also complain of low back pain [71], and this is one of the reasons why the Quebec Government Task Force coined the term ‘whiplashassociated disorders’, to encompass the spectrum of soft-tissue injuries and symptoms after traffic injuries [72]. Overall, about 50% of those injured in traffic collisions complain of collision-related low back pain [73]. However, for the majority of those injured, it is unlikely that this would be the first time they had experienced low back pain, and it is likely, but yet unproven, that previous low back pain is a risk factor for low back pain after traffic collisions. There is evidence that whiplash injuries to the neck also increase the risk for future pain in the neck, thoracic spine and lower back [74]. Further, there is a poor correlation between the severity of the collision in terms of force generated and subsequent low back symptoms [75]. Results like these argue that we need to be acutely aware that neck and back complaints are closely related and share risk factors, even in the context of an obvious trauma. From a life-course perspective, back problems might be more prevalent after injury in those who are less resilient to traumatic events because of cumulative exposure. Further research is needed in this area. Physical activity Leisure time physical activity is usually considered to have an impact on back pain at all ages, but the evidence is conflicting. There are more consistent associations found between less back pain and more physical activity in the elderly than in younger populations [76–79]. Contrary to prior beliefs, and in support of the concept of ‘lifetime perspective’, occupational lifting does not seem to be an independent causative factor for the development of back pain [80]. Workload predicts back pain in

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adolescents [33,81], whereas ergonomics does not play a role in relation to body comfort [82] and school-bag weight does not predict back pain [55]. Co-morbidity A high correlation between back pain and pain in other parts of the musculoskeletal system, most commonly other areas of the spine, has been reported, with about one-third of back pain patients also complaining of neck pain [83–85]. This correlation seems to start at an early stage of the pain course, and pain in the three spinal regions behaves quite similarly with respect to occurrence and consequences, which may reflect the same, or similar, underlying aetiology [86,87]. This may in part be explained by genetic factors as heritability estimates for pain in the three regions are quite similar [86] and genetic factors are twice as important for the occurrence of concurrent back and neck/shoulder pain as for each of them individually [88], indicating a common genetic basis for a high proportion of spinal pain. The high degree of co-occurrence is especially important because multisite pain may have more serious consequences than single-site pain [89]. The risk of poor prognosis increases when spinal pain is accompanied by pain elsewhere [23,90], and functional problems have been shown to increase with increasing number of pain sites in both adolescence [91] and old age [92]. Finally, a higher number of pain sites are also strongly associated with work disability [93]. There are also strong links between back pain and other diseases and general health in adolescents and adults [84,94,95]. Similar associations are found for other types of musculoskeletal disorders [96,97]. The earlier section on psychological factors also describes co-morbid mental health issues. The mechanisms behind the relationships between back pain and general physical and mental health are poorly understood but they appear to be present throughout the life course. The associations described above are mainly based on cross-sectional studies. However, there are also longitudinal studies where somatisation has been associated with increased risk of back pain among adolescents [55] and risk of poor prognosis among adult populations [20,98], indicating that somatisation is a risk factor for back pain. However, the reverse relationship has also been demonstrated where back pain leads to increased somatisation [99]. Thus, rather than one being a risk factor for the other, there might be some degree of common origin, which could be genetic, social, psychological, physical or a combination, and range across all ages. Discussion It is evident that back pain is present from childhood to old age. Three key points draw us to the conclusion that factors present or developing in early life have a lifelong influence on the experiences of back pain: (i) Many of the factors associated with back pain among adults are also present (and associated with pain) among children; (ii) episodes of back pain are related to each other, even when preceded by a traumatic event; and (iii) evidence on the persistence of pain indicates that it may be the same groups of people who have pain either consistently, or on and off, throughout their life. What exactly these early life factors are, and how they influence the pain experience is unclear. One possibility is that certain groups of individuals have a susceptibility or vulnerability, which might affect their likelihood of experiencing pain in the first place, but which, probably more importantly, might influence their ability to recover from an episode of pain (whatever the immediate ‘cause’ of the episode), thereby placing them at increased risk of developing a long-term back pain problem. It seems likely, from the evidence, that this vulnerability results from a combination of genetic and environmental factors, such as socioeconomic situation, parental influences, psychological factors and presence of co-morbidities (pains and other health problems). Evidence from long-term studies indicates that people with long-term problems can have pain episodes separated by periods that are pain free, continuous mild pain with low impact or severe pain with a large impact on their lives – whether these differences are due to different levels of vulnerability or a cumulative impact of the pain experience remains to be seen. The epidemiology of back pain in children clearly shows an increase of pain prevalence and healthcare use with age, indicating that this is the age group in which any vulnerability develops or becomes apparent. It would therefore appear appropriate that studies of the incidence and development of back pain in childhood are important.

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So where does this leave us in terms of the management of back pain? Management of individual episodes will continue to be important, and treating factors beyond the pain itself (i.e., comorbidity, psychological or work-related factors) is justified based on evidence throughout the life course. However, as highlighted in the chapter in this issue by Foster et al., current treatments have limited or inconsistent benefits for people with back pain. Taking a life-course perspective, and considering the strong correlation between back pain and general health, it seems appropriate to develop a more public health-centred approach for prevention of long-term back pain in younger people. One example of such an approach is the Svendborg Project in Denmark, which is a public health initiative to treat firstonset back pain in children and to prevent it though activities and education [100]. Such an approach is important because limitations caused by musculoskeletal pain have a potential to influence all aspects of health and quality of life. Considering the potential consequences, it is apparent that we need to increase our knowledge about the epidemiology of musculoskeletal complaints in children and adolescents, including course, patterns of co-existence with other disorders, prognosis and risk factors. Summary Back pain research has had a traditional focus on studying episodes of pain, but evidence is mounting that the pain experience should be viewed as a long-term problem, with pain episodes closely linked with each other. Back pain is common throughout the life course, and similar factors are associated with the pain at different times. There is also strong evidence for the links between back pain, pain at other sites and other health problems. This evidence leads to the potential conclusion that vulnerability for long-term back pain develops at an early age, likely in childhood, and influences the occurrence of, and recovery from, episodes of back pain. However, strong evidence directly supporting this is currently lacking but could have important implications for the prevention and management of back pain. Practice points  Treating back pain episodes as entirely unrelated events should be abandoned, and back pain episodes should be viewed in the context of a lifelong pain experience, including other aspects of health.  Prevention and management of back pain among children might have long-term influences on back pain and wider health issues in later life.

Research agenda  Researchers should consider the lifetime experience of back pain, as well as the duration of the current episode, when studying the condition.  Studies should investigate whether factors associated with back pain have different influences at different points across the life course.  More research is needed to look at childhood and parental influences on the development of long-term back pain problems.  Researchers should investigate whether there is ‘vulnerability’ for developing long-term back pain, how this may develop and what can influence it in the long term.  More public health or population-based interventions that could address or mitigate cumulative risk for back pain should be studied.

Acknowledgements KMD is supported by the Wellcome Trust (083572).

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