The association between headache and low back pain: a systematic review

Low back pain and headache are leading causes of disability worldwide [1, 2]. Each headache disorder has specific diagnostic criteria [3]. The commonest types of headache are migraine, tension-type (both primary headaches) and medication overuse headache (a secondary headache) [4]. Migraine and tension-type headache featured in two of the eight causes of chronic disease and injury, each affecting more than 10% of the world’s population [1]. Chronic headache is a severely disabling condition affecting around 3–4% of adults worldwide [5]. It is defined as a headache occurring on ≥15 days per month for more than 3 months [3].

Low back pain has a high healthcare burden, and in the most recent global burden of disease study, both low back pain and migraine were featured in the five leading causes of years lived with disability [1]. Around 4% of the UK population take time off work because of low back pain, resulting in around 90 million working days lost and between 8 and 12 million General Practitioner (GP) consultations per year [6]. Chronic low back pain is defined as pain felt in the area between the bottom of the rib cage and the buttock creases for more than 3 months [7].

There is a considerable focus in headache management in achieving a precise diagnosis in line with the International Classification of Headache Disorders [3]. Once a diagnosis is identified, management is focused accordingly [3, 8]. In contrast once serious causes of low back pain are excluded (malignancy, vertebral fractures, inflammatory disorders or infection) non-specific low back pain is diagnosed.^{2, 3,}

People with persistent low back pain and people with chronic headache disorders are typically managed by clinicians from specific clinical specialities rather than experts in the management of chronic pain syndromes [9]. Whilst this approach may be appropriate for those living with one of these chronic conditions, it may be different if people have both. A previous systematic review of twin studies has identified a possible independent association between headache and low back pain [10]. People with both headache disorders and low back pain might constitute a neglected group that could have both conditions managed in combination rather than as separate entities. Here, we describe a systematic review of observational studies reporting the association between headaches (primary headaches, and chronic headaches) and persistent low back pain.

We sought to identify all case control, cohort and cross-sectional studies reporting the relationship between primary headache disorders and persistent low back pain. We used a wide definition of headache disorders to reflect that exact headache type is often poorly defined and setting tight diagnostic criteria for study inclusion would exclude much of the available literature. Here, we use the term persistent to define low back pain duration rather than the term chronic [11]. This reflects that duration of low back pain is often poorly defined and using a strict definition for chronicity of back pain would exclude much of the available literature. For this review, we have reported the definitions of headache and low back pain used by the original authors of the included studies. This will allow the reader to interpret the findings cognisant of the definitions used.

The protocol for the review is registered on PROSPERO (PROSPERO 2018 CRD42018086557). This can be accessed via this link: http://​www.​crd.​york.​ac.​uk/​PROSPERO/​display_​record.​php?​ID=​CRD42018086557.

Studies were heterogeneous, but a consistent positive association between headache and low back pain was found. This is consistent across countries, populations and study design but variable in magnitude. The odds ratios range from OR 1.72 (95% CI 1.38–2.15) [15, 16] to 8 (95% CI 5.3–12.1) [10].

Four studies were of children/adolescents (one cohort [17], three cross-sectional [18‐20],) ten were adult studies (one cohort [21], four cross-sectional [10, 22‐24], one cross-sectional older adult study [25], one cohort twin study [26], three cross-sectional twin studies) [15, 16, 27, 28] (Table 1).

Studies came from; Denmark (four studies) [15, 16, 22, 26, 27], USA (2 studies) [18, 28], and one each from Germany [10], Iran [25], Norway [19], Switzerland [21], Tunisia [23], UK [17], and Qatar [24]. One large multinational study included participants from 29 countries from Europe, North America and Israel [20, 28].

Study size ranged from 88 participants [19] to a large international study with 404,206 participants [20]. Three studies had 8,000–10,000 participants. Overall, we include data on studies with 460,195 participants. Ages ranged from 9.8 years [20] to 102 years [15, 16] (Table 1).

In this review, we aimed to identify and describe an association between primary headache disorders and persistent low back pain. These are conditions that generate considerable morbidity globally,¹ [33], but that have not been widely managed as co-morbidities. We have identified 14 studies reporting an association between primary headache disorders and persistent low back pain. Overall, there was a positive association in all studies, with odds ratios estimating the relationship between primary headache and low back pain ranged from 1.55 (95% CI 1.13–2.11) and 8.00 (95% CI 5.3–12.1). This appears to be a consistent finding irrespective of population studied or the study design. Although methodological quality varied, the larger epidemiological studies of Swain and Yoon are likely to illustrate a true population association due to the large population covered [10, 20]. Although, because of the extreme heterogeneity of study designs, definitions and populations studied, we cannot provide a pooled estimate the results indicate that a relationship exists between headache and low back pain. Thus, indicating reasonable grounds for further study.

We have followed the PRISMA guidance for systematic reviews during this review process [12]. The studies we identified had a large range of number of participants, a variety of methodologies, a large variety of countries, represented and participants of all ages. The total sample size represented in this review was 460 435. The review is limited by the varying definitions of chronic headache and low back pain, and in many cases, there was little explanation as to what was taken as a case definition of either headache (chronic or otherwise) or low back pain (Table 1). Some studies reported migraine specifically, but this was self-reported diagnosis, therefore may include non-migrainous headache subtypes [15, 16]. That no studies used current diagnostic or classification criteria of headache constrains the inferences that can be drawn from the existing literature. We looked for the association between chronic headache disorders and persistent low back pain but chronicity was not always determined in the published literature, and where chronicity was clarified the definitions used were variable.

Our review may be susceptible to publication bias. Attempts at eliminating bias in observational studies have been made, including the STROBE guidelines, which provides a checklist for authors of observational studies to use when disseminating data and result [34]. This includes advice on reporting all outcome events or measures, which should therefore include negative results. However, despite a well-designed and a comprehensive search strategy in a systematic review, positive outcomes are more likely to be published than negative outcomes [35]. Since our initial scoping review indicated that data would be sparse we have included all published studies and not set a size or quality criterion to define study inclusion. Data published in languages other than English may also have been missed, however, given the consistency of our findings it is unlikely that any such studies will materially affect our overall conclusions.

We are constrained by the variable approaches to define headache disorders by the original authors. Nevertheless, we identified an important association between chronic headache and persistent low back pain in the two studies that used ICHD diagnostic criteria [3, 30] to define chronic headache and formally differentiate between primary headache disorders. Yoon et al. used ICHD to diagnose the primary headache disorders and chronic headache disorders; they also used our preferred criteria of diagnosing low back pain [10]. Ashina et al. also used ICHD criteria for headache diagnosis.¹⁰ [22], A combined approach to treating both conditions could reduce pill-burden, medication overuse, and provide a holistic treatment approach for two chronic pain syndromes.

A large-scale European study found moderate to severe chronic pain reported in 19% of adults, notably half of these felt their pain was inadequately managed [33] A number of countries have recognised the therapeutic benefits of less location-specific pain relief in recent evidence-base back pain management guidelines (including cognitive behavioural therapies and mindfulness training) [9, 36, 37]. Treating chronic pain remains a challenge for clinicians and unfortunately unrealistic expectations of short-term pain control can lead to pharmacological over prescription (opioids in particular), and even excess interventions (including surgery or nerve blockade) [38]. Although 90% of patients who consult their general practitioner for low back pain in the UK stop consulting within 3 months, most are still experiencing low back pain and related disability 1 year after consultation [39]. This suggests that either those affected feel that there is limited help available, or that for another reason consultation is not worthwhile [40].

Understanding chronic pain is complex. There is some evidence suggesting that chronic pain conditions can be exacerbated by changes in psychological processing [41]. A review article surmised that chronic pain can lead to a vicious cycle of progressive changes in psychological well-being which are aggregated by reduced dopaminergic effects, pain sensitisation leading to worsening pain states through anti-reward and stress related neuroadapations [41]. Another low back pain study found that people exhibiting deficits in emotional awareness or difficulties in processing feelings had higher odds of experiencing low back pain [42]. There is also a genetic overlap between migraine and depressive illness [43].

Recognition of the impact that psychological processing can have on chronic pain has been recognised by the use of non-pharmacological treatments in chronic pain management. Examples include exercise therapy, and psychological therapies such as cognitive behavioural therapy [44]. Behavioural therapies have been endorsed by American Headache society, who describe chronic headache as a biopsychosocial disorder, involving not just physical disease but recognising the potential contribution of psychological processing and stressors to chronic headache symptomology [45].

In addition to considering psychosocial determinants of chronic pain processing in persistent low back pain and chronic headache, there are possible biological explanations. A previous systematic review on low back pain and other comorbidities has suggested that differences in gene expression could be implicated in the process of pain perception and signalling [46]. Similarly in migraine research, certain genetic mutations have been linked with differences in pain sensation [47]. For example, mutations in NGF (nerve growth factor) causing loss of pain perception, and TRPM8 (transient receptor potential cation channel subfamily M member 8) a receptor-activated non selective cation channel involved in sensing cold and possibly modulating pain sensation [47]. Monoclonal antibodies targeting calcitonin gene- related peptide (CGRP) have also been shown to have some effect in migraine prophylaxis [48] CGRP has been related to other pain mechanisms, including facet joint pain, which is a recognised cause and contributor to low back pain. A 2017 systematic review on CGRP reported that numerous studies have found an association between measured CGRP levels and pain, concluding that CGRP could be a neuromodulator in pain syndromes other than migraine [49]. Indeed, CGRP fibres have been found in degenerated human intervertebral discs sensory nerve fibres, further suggesting it may a role in nociceptive back-related pain [50]. Therefore, it is biologically plausible that the neuromodulator CGRP is involved in an association between low back pain and migraine. In this review one study in particular, Yoon et al., identified a particularly strong association between migraine and chronic low back pain [10]. This lends some support to the notion that there might be a particular association between migraine and persistent back pain mediated through a specific biological mechanism.

Headache disorders, including migraine, may be associated with chronic painful disorders other than back pain. For example, the prevalence of co-morbid migraine in people with fibromyalgia has ranged from 18% to 36% in different studies [51]. There is a plausible link for this association through the role of central sensitisation in both disorders [51]. We are not aware of any systematic reviews quantifying the strength of the association between fibromyalgia and headache disorders. Further epidemiological work is needed to more fully explore the interrelationship between headache disorders and other painful disorders.

Further study should focus on the prospective and observational study of chronic headache patients and experience of persistent low back pain. In addition, study designs that examine apparent mechanisms that account for an association between persistent low back pain and primary headache disorders, in particular chronic headache would be beneficial. Such studies could lead us closer to finding concomitant management of both pain syndromes.