Low back pain (LBP) is usually defined as pain, muscle tension, or stiffness localized below the costal margin and above the inferior gluteal folds, with or without leg pain (sciatica). LBP occurs in about 60–80% of people at some points in their lives, and can begin in childhood (1-7). It is a disorder with many possible etiologies, with many definitions, and occurring in many groups of populations. The vast literature available on prevalence of LBP is not only heterogeneous, but also sometimes contradictory. This variability may be due to differences among study factors such as the age of the sample, the definition of LBP, and the strategy for extracting data. The prevalence can be described in terms such as point prevalence (the number of persons in a defined population who have LBP at a particular point in time, usually the time the survey was carried out), period prevalence (the number of persons who have LBP at any time during a specified time interval), and lifetime prevalence (the number of persons who have LBP at some points in their life). LBP has also been shown to be associated with certain psychosocial factors, including presence of psychological conditions, maladaptive coping strategies, poor job satisfaction, higher physical work demands, poor general health or functional level, tobacco use, obesity, receipt of workers’ compensation or disability/sick leave, and unresolved litigation or compensation issues related to the back pain (8-10). There may be also cultural differences in the pain perception or reporting, with some ethnic minorities having the attitude that pain is to be endured without complaint (8,11).
The overall prevalence of LBP is higher in women than in men (12,13). Women are also affected by many chronic pain conditions and painful conditions of the musculoskeletal system in greater numbers than men are (14). A biopsychosocial model of chronic pain attributes sex differences in pain to interactions between biological, psychological, and sociocultural factors (15,16). The heightened pain sensitivity among women can also partially explain greater reports of pain by women compared to men (17,18). Menstrual cycle fluctuations in pain sensitivity may help to explain sex differences in pain reporting in younger adults (19). Biologic response to pregnancy and childbearing, physical stress of child-rearing, perimenopausal abdominal weight gain are additional causes for LBP (12). Population-based studies have shown that the prevalence of widespread pain increases with age, peaking in the seventh and eighth decades (20,21). Recently, it has been show that genetics also played a role in the development of LBP (1,22,23).
Lumbar disc degeneration and its associated changes such as disc space narrowing are related to LBP (24). Lumbar disc degeneration is a common musculoskeletal condition, the prevalence of which increases with age (25-29). Disc degeneration can progress to disk herniation, spinal canal stenosis, and, in conjunction with facet joint arthrosis, degenerative spondylolisthesis. In an analysis of published data of 600 autopsy specimens of young and middle-aged subjects younger than 50 years, intervertebral disc degeneration was observed in men in the second decade of life, occurring at an earlier age than in women; the severity of age matched disk degeneration was also being generally greater in men (25). In a later independent histologic study, Łebkowski (26) investigated 308 lumbar intervertebral discs at autopsy from 57 women (mean age, 41.8 years) and 79 men (mean age, 42.1 years). Disc degeneration became first readily apparent during the 2nd decade of life, though it was observed to occur in men almost a decade earlier than in women. In a MR imaging-based survey of young adults 20–22 years, lumbar disc degeneration was significantly more frequent in men (30). These results confirm the general perception that young men are more susceptible to disc degeneration than young women are, most likely due to increased mechanical stress and physical injury. However, after menopause which is around the age of 49–50 years, lumbar discs in females degenerate at a notably quicker rate than male lumbar discs in males, after later 50s, disc space narrowing become more apparent and more severe than age matched males (31-33).
Based on these backgrounds, we propose a hypothesis, i.e., while overall females have higher prevalence of LBP across all age groups, this male vs. female difference in LBP prevalence further increases after female menopause age. This is partially related to the accelerated lumbar disc degeneration and spine degeneration after female menopause (31-33). In this study we performed a literature review to look at whether this male vs. female difference in LBP prevalence exaggerated after female menopause age can be demonstrated.
Materials and methods
The literature search was performed on PubMed on January 2, 2016. The search word combination was (low back pain) AND prevalence AND [(males OR men) AND (females OR women)]. This search generated 2,897 paper titles. The titles and abstracts of these papers, and sometimes full papers when required, were screened, and the following criteria were used to include the papers for further synthetic analysis:
- Only English literatures on LBP were further analyzed. Review and meta-analysis articles were excluded.
- We only included prospective studies on general population of both males and female subjects studied using the same LBP criterion, ages-specific information available, and males and female subjects were age-matched. Patient-only studies, which tended to be retrospective, were excluded, as results from clinical groups are likely to be biased (4). The decision to seek medical care would be dependent on: (i) predisposition of the individual to use services, which is based on demographic and social characteristics as well as attitudes about medical care and efficacy of treatment; (ii) ability to obtain medical services; and (iii) subjective perception of severity of illness.
- Literatures with focus on LBP of occupational causes, such as industrial workers, drivers, athletes, were excluded. Occupational populations represent a selective group of individuals. In general, workers tend to be healthier, and state of health itself may determine entry into a specific occupation. Rates of back pain among occupational groups may differ because of not only the work itself but also because of differential selection into various occupations. However, studies on teachers of general subjects and farmers were included. Studies on teachers were usually of convenience samples, and they are likely not to have specific occupational causes for LBP. Studies on farmers were usually of large sample size, and in many societies farmers constitute a very large portion of the society members. It is known in some societies females can be more involved in household work, in this study house wife were not excluded.
- The included studies were judged to have no major flaws. The characteristics of the population adequate, the sample size and response rate adequate.
- When a study applied a longitudinal design, only the prevalence rate at end of the study was recorded, or the prevalence rate which was judged to be most relevant was used for analysis.
- Regional and ethics factors were not considered in this study.
This study extracted 98 studies from PubMed for synthetic analysis, involved 772,927 participant subjects (Tables S1-S4). According to the information in the literature, participant subjects were divided into four age groups: (I) school age children group: 6–19 years; (II) young and middle age group: 20–50 years; (III) mixed age group: data from studies did not differentiate age groups; (IV) elderly group: ≥50 years old. When individual studies were not weighted by participant number and each individual study is represented as one entry regardless of their sample size, the median LBP prevalence ratio of female vs. males was 1.310, 1.140, 1.220, and 1.270 respectively for the four age groups (Figure 1). When individual studies were weighted by participant number, the LBP prevalence ratio of female vs. males was 1.360, 1.127, 1.185 and 1.280 respectively for the four groups (Figure 2).
This study demonstrated females had higher prevalence of LBP across all age groups. The female vs. males LBP prevalence ratio was highest for school age girls and boys. Compared with middle aged subjects, a further increased low back pain prevalence in females than in males was noted after menopause age.
Among all chronic pain problems and spinal pain conditions, LBP is the most common and important clinical, social, economic, and public health problem affecting the population indiscriminately across the world (8). LBP is known to be of multi-factorial causes (3,34,35). Employment and workplace factors, both physical and psychological, such as heavy lifting, pushing, pulling, vehicle driving, and prolonged walking or standing were found to be predictors of LBP and there are similar associations with stressful and monotonous work and dissatisfaction with work. Body mass index has been found to be linked to LBP in obese people (36). Associations between LBP and social class, low levels of educational and low income have been reported. Persons with greater education are more likely to be in professional, managerial, or other skilled occupations where there is more flexibility to eliminate pain-provoking job situations and physical demand (4,37). Compared with a lower or a higher frequency of exercise, a moderate frequency of exercise from one to five times a week was associated with a lower LBP risk level (4). LBP has been reported consistently in a higher proportion of females than males (3,4,12,13,38,39). Our synthetic analysis further confirmed this phenomenon. Gender prevalence ratios also revealed a higher prevalence of pain in females for headache, migraine, temporomandibular pain, burning mouth pain, neck pain, shoulder pain, back pain, knee pain, abdominal pain, and fibromyalgia (8). Women have shown to have a lower threshold of perception of pain and in reaction to it (40,41). Several authors have observed that although females are more likely to report symptoms, physician verified abnormalities are approximately equal to those of males (42,43). However, occupational LBP is seen in higher proportions in men (44,45).
Some data suggested LBP prevalence increased in the recent years (8,46). It could be that the actual prevalence has not changed but the reporting has; or it could be that the questions used to assess the prevalence have changed; or LBP prevalence really increased due to life style changes of the population. Harkness et al. showed there were significant differences in the prevalence of pain increasing from 2- to 4-fold between two surveys (47). Chronic pain is a common symptom and significant problem for older adults. Epidemiologic data in the elderly suggests that up to 50% of community-dwelling older adults and as many as 80% of residents of long-term care facilities experience persistent pain. Bressler et al. (48) undertook a systematic review of the literature from 1966 through 1998 and reported that persons over 65 years of age experience low back pain with greater frequency and have been under-represented in research, as well as in management. Further, age related prevalence of persistent pain appears to be much more common in the elderly associated with functional limitations and difficulty in performing daily life activities. In an evaluation of pain characteristics of adults 65 years of age and older referred to a tertiary pain care clinic, the older patients had relatively more physical problems concordant with their complaints, but fewer psychological factors contributing to disability than the younger pain patients (49). On the other hand, we expect the higher LBP prevalence in school age girls than in school aged boys is more likely due to psychological factors, female hormone fluctuation, and menstruation.
That post menopausal women has higher incidence of LBP than men has been reported distinctively in a number of studies. In 1969 Lawrence (50) surveyed 713 males and 809 females aged ≥35 years with lumbar radiograph in Manchester, UK. Back-hip-sciatic pain was present at the time of the survey in 79 (11%) of the males and in 153 (19%) of the 809 females. In those with pain at the time of the survey the incidence had raised till age 40 in males and then remained constant, but in females it continued to rise sharply up to and over the age of 65 (Figure S1A). Nagi et al. (3) in 1973 showed a greater proportion of females (21%) reported back pain as compared to males (14%). It was suggested a number of women might have experienced back pain in connection with the biological processes associated with the menses. However, when age controls were introduced, Nagi et al. noted that women over 50 years of age were more likely than men to report back pain (26% and 17%, respectively). In 1995, Papageorgiou et al. (51) reported the South Manchester Back Pain Survey with study population of 4,501 (age: 18–75 years old). The 1-month period prevalence of LBP was 31.2%, 33.1%, 38.5%, 34.9% for the age range of 18–29, 30–44, 45–59, and ≥60 years respectively for males; 32.2%, 41.5%, 49.2%, 43.7% for the age range of 18–29, 30–44, 45–59, and ≥60 years respectively for females. In our own Osteoporotic Fractures in Men (Hong Kong) and Osteoporotic Fractures in Women (Hong Kong) Studies data published in 2013 (52). A total of 2,000 Chinese men and 2,000 Chinese women, aged ≥65 years, were prospectively recruited from local communities for a prospective cohort study from August 2001 to March 2003. The LBP prevalence was 30.6% for men and 53.3% for women (P<0.001). In 2010, Cho et al. (53) published LBP data collected for 4,181 subjects from a rural farming community in Korea, with a mean age of 56.6 years. 6-month prevalence of LBP was 38.5% for men and 55.6% for women. The prevalence of LBP increased significantly with age in women (Figure S1B).
Estrogen participates in a variety of biological processes through different molecular mechanisms. The collagen wasting is commonly observed in bone and skin in the postmenopausal period due to decreased estrogen levels (54,55). Estrogen has favorable effects on the lipid profile, antioxidant activity, and enhanced fibrinolysis (56). Estrogen may reduce the risk for arteriosclerosis, which has been considered a risk factor for LBP (57). Estrogen plays an important role in the etiology and pathophysiology of a variety of musculoskeletal degenerative diseases. The prevalence of osteoarthritis (OA) is higher among women than among men, and this prevalence increases considerably after menopause (58,59). Moreover, with the same degree of radiographic damage, OA is also more symptomatic in women (58,59). After menopause women have more severe disc space narrowing than age matched men (28,29,60). This may be associated with the physiological changes caused by relatively lower level of sex hormones after menopause in women, and the accelerated lumbar disc degeneration (33).
The expert views of hormone replacement treatment (HRT) evolved during the last 10 years since the publication of WHI trials (61,62). Dose regimen, combination of estrogen with progestins versus estrogen alone, the administration route and duration of treatment such as the choice of repetitive or periodic administration simulating the menstrual cycle are some of the factors that may be involved in the benefit discrepancies. The Estrogen and Thromboembolism Risk (ESTHER) study confirmed that oral estrogens increased venous thromboembolism risk, whereas transdermal estrogens had little or no impact on the development of thrombosis (63). The presence of gene polymorphisms may also be implicated. HRT may benefit a large number of postmenopausal women, but a subset of women may have higher risk of cardiovascular and thrombotic complications (64). Estrogen receptor modulators and phytoestrogens may retain the desired effects but avoid undesirable effects (65). HRT has been shown to be protective against menopause-associated OA (66,67). However, in one study Musgrave et al. (68) reported women taking HRT reported more back pain and back pain-related disability than did those not taking HRT. An in-depth understanding of the role of the gonadal hormones in LBP modulation remains unclear; whether HRT is useful for patients with severe LBP warrants further studies.
There are a number of limitations for the current study. Only English literatures were included in this review, and only database of PubMed was used. There was a lack of a universal definition and delimitation of LBP and the absence of important specifications of LBP such as the frequency of episodes, its intensity and duration in some studies; therefore we reported the ratio of female vs. male LBP prevalence instead of absolute LBP prevalence. The LBP in survey subjects are assumed to be nonspecific. Specific LBP is due to organic diseases that include spinal fractures, cancers, infections, and cauda equina syndrome can be identified. The probability that a particular case of back pain has a specific cause identified on back radiographs is less than 1% (69). Additionally, for the a few studies the age-specific grouping could only be approximated (Tables S1-S4). This study also did not establish an exact causal role of accelerated spine degeneration in post-menopausal women for the increased LBP prevalence. Our literature review is likely not being exhaustive; however, we believe this limitation is unlikely to have impact on the conclusion of our study.
In conclusion, our synthetic literature review demonstrated females had higher prevalence of LBP across all age groups. This female vs. male difference was highest for school age children. Compared with middle aged subjects, a further increased low back pain prevalence in females than in males was noted after menopause age. Clarification of the hormonal influences on pain modulation will advance our understanding of sex differences in clinical pain conditions such as LBP. The evidence showed in this study may open a new line of further clinical researches.
Conflicts of Interest: The authors have no conflicts of interest to declare.
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