CHAPTER 2 Epidemiology
INTRODUCTION
Epidemiology is the branch of medicine that deals with the study of the causes, distribution, and control of disease in populations.1 Epidemiology of spine pain provides insight into the scope of the problem and allows us to evaluate the impact of various treatment methods and preventative strategies. Without reliable epidemiologic data it is impossible to evaluate treatment or prevention with any accuracy. In reviewing the literature on the epidemiology of spine pain, it quickly becomes evident that there are significant gaps in our knowledge which require sound evidence-based medicine for resolution. Until we have reproducible data with set criteria for spine pain, in general and specific populations, we will be unable to accurately define its natural history or the benefit of selected treatments. Historically, the medical profession has held a variety of opinions on the cause of spine pain with associated treatments. This led to the teaching of treatments without any clear scientific evidence and has propagated potentially ineffectual approaches to ill-defined causes of spine problems. The long history of opinion-based clinical medicine and medical education is coming to a close. In this era of evidence-based medicine it is essential to determine the epidemiology of spine problems so we can proceed to focusing on effective treatment and prevention.
The terms often used in studying the effect of spine pain on populations are incidence and prevalence. Prevalence is the percentage of a population that is affected with a particular disease or symptoms at a given time or during a specific set time interval. There are many factors contributing to prevalence including, but not limited to, the number of new cases, the duration of symptoms, and individuals with spine pain moving in or out of the study population.2 The determination of prevalence only requires sampling at one time point. A cross-section of the population of interest should be sampled to ensure that the data will be generalizable to the population as a whole. The study population must reflect the population to which the information will be applied or the information will have little or no utility. Point prevalence is thought to be fairly accurate when obtained in surveys, whereas prevalence over long periods of time or an individual’s lifetime is often less accurate. Memory fades with time, particularly if pain has resolved.
Prevalence and incidence of spine pain allow us to better define the scope of the problem. They also allow for the formulation of theories of etiology by analysis of associated factors. They do not determine causation.3 The estimates of costs to society further define the problem and include economic, medical, and disability-related costs.
CHALLENGES
Consistency among studies with clearly defined criteria for an episode of spine pain would allow for comparison and pooling of data. Fleeting, transient, mild neck pain should not be evaluated in the same category as severe, intense, disabling, chronic neck pain. The study should include the question used and how it was administered. The length of the particular questions used and the method of administration can alter the responses obtained. The prevalence period must be defined. Only the same prevalence periods should be compared. Point prevalence represents the most reliable information to obtain in survey studies since memory is not required. The longer the recall period the more this is apt to be affected by memory.4 This can cause errors in both directions. Memory may fade with time or events may be remembered as occurring more recently than they actually occurred.5,6 Point prevalence avoids this issue.
Generalizability of the information from a study population is frequently the goal. To allow for the extrapolation of findings from the sampled population to the larger population requires that the sample population be representative of the group as a whole. It is essential that the study population be a random sample of the target population. It is important to define the population prior to sampling so the outcome is relevant.
Scope of the problem
Spine pain is nearly ubiquitous in industrial societies. It is among the most common medical problems in developed countries. It is present in rural workers and in sedentary through heavy-duty occupations. In the majority of cases causation remains muddled. The often repeated causal factors including obesity, heavy work, leg length discrepancy, and others have not been proven. The data for low back pain vary but the lifetime prevalence in industrial nations is high, 50–85% or greater.7,8 The annual incidence is approximately 5% with some reports up to 15%.8 Back pain accounted for 15 million physician visits in 1990 in the US.9 It is a major factor in lost work days and the first or second most common cause of disability.10 In people under 45 year of age it is the most common cause of disability in the US.10
History
Back pain has been present since the earliest of recorded time. In the Edwin Smith papyrus circa 1500 BCE there is a description of back pain, including the examination and diagnosis. Neanderthal skeletons and Egyptian mummies revealed degenerative spine changes. Hypocrites (460–370 BC) noted that back pain with sciatic pain lasted about 40 days and affected men 40–60 years old.11 Historically, chronic back pain was not thought to be secondary to injury until the mid nineteenth century. This was the time of the industrial revolution and the building of the railways. It was called railway spine and thought to be related to work, or even travel on the railroad, even if there was no identifiable injury.12 This led to the acceptance of spine pain as an occupational injury.
STATISTICS
Europe
In Britain, a study in the general population with 4515 respondents from three general practices determined the prevalence of neck and back pain.13 An episode of spine pain was defined as lasting at least 1 week in duration. The 1-month prevalence of all spinal pain was 29%. The prevalence for back pain was 24.5% for women and 21.3% for men. For neck pain the prevalence was 16.5% for women and 10.7% for men. Of the total spine pain, 40% was disabling.
A British study with 12 907 respondents to a survey found a 1-year prevalence of 34% and a weekly prevalence of 20% for neck pain.14 Of the total respondents, 11% reported neck pain within the past year that interfered with their normal activities. An episode was defined as pain lasting 1 day or longer.
In one of the few prospective studies the lifetime and annual prevalence of low back pain in the UK was 59% and 42%, respectively.15 This was a mailed survey with 1455 respondents. An incidence rate of 4% was found. Age was associated with increased prevalence. An episode of back pain was defined as lasting longer than 1 day and not associated with menstrual cycle, pregnancy, or febrile illness.
Guez, in a Swedish study of 4392 adults, found an 18% prevalence of chronic neck pain with continuous pain lasting longer than 6 months.16 Of the subjects with neck pain, 30% had a history of trauma. No data were reported on the interval between trauma and neck pain. The definition of neck injury was injury that was severe enough to lead to a physician visit. In another Swedish population study with 6000 respondents, 48% of men and 38% of women reported neck pain on a self-administered questionnaire.17 The prevalence as a whole was 43% with women having a significantly higher prevalence than men. Chronic neck pain defined as lasting greater than 6 months was reported in 22% of women and 16% of men. A history of head or neck trauma was present in 25% of the subjects who developed chronic neck pain. Linton, in a Swedish study, surveyed 3000 persons and found a 2-year prevalence of 73% for low back pain.18 Of these, 17% utilized sick time and another 14% had been off work but did not use sick time.
In the Mini-Finland Health Survey 8000 people were interviewed and examined.19 Lifetime prevalence of neck pain was 71%. Chronic neck pain was diagnosed in 9.5% of the men and 13.5% of the women. An association was found between neck pain and history of injury and mental and physical stress at work. In a survey study of 10 000 Norwegians, the 1-year prevalence rate of neck pain was 34.4%.20 Neck pain lasting for more than 6 months had a prevalence of 13.8%.
In a telephone survey of 1964 participants in Catalonia, Spain, the 6-month prevalence of low back pain was 50.9%.21 Back pain was more common in women, manual workers and less-educated respondents. Back pain limited the daily activities in 36.7% and was responsible for time off work in 17% and disability pension in 6.5%.
In a Belgian study of 618 blue collar workers in the steel industry, lifetime prevalence was 66%, 1-year prevalence was 53%, and 1-week prevalence was 25%.22 An episode was any ‘problem in the low back.’ Most of these episodes were mild and categorized as fatigue or common low back pain. Only 17% sought medical advice and only 11% were limited in their occupational or domestic activities.
In the Netherlands in a survey with 3664 respondents, low back pain had a prevalence of 26.9% and neck pain 20.6%. Low back pain was the most common musculoskeletal pain and neck pain was the third most common.23
North America
National Health and Nutrition Examination Survey-CDC (NHANES 1999–2000) found the prevalence of low back pain (LBP) within the past 3 months to be 37.44% with a sample size of 4880.24 Neck pain over the previous 3 months lasting at least 1 day revealed a prevalence of 18.46%. Deyo analyzed the NHANES II data (1976–1980) with a survey population of 27 801 and found a lifetime prevalence in the US of LBP of 13.8% and prevalence in the previous year of 10.3%.25 In the Deyo study an episode of LBP was defined as lasting at least 2 weeks.
Canada has been the site for many epidemiologic studies for both lumbar and cervical ailments. Cassidy et al., with 1131 respondents to a mailed survey in Saskatchewan, found 28.4% point prevalence and 84.1% lifetime prevalence of back pain.26 The 6-month prevalence was graded into five intensities and disability categories. This was an attempt to stratify the prevalence so that transient nondisabling pain could be differentiated from disabling back pain. Low intensity/low disability back pain accounted for 48.9% of the population that had back pain in the previous 6 months. High intensity/high disability back pain was reported by 10.7% of this population. The remaining 12.3% of the subjects in the 6-month prevalence group reported high intensity/low disability back pain. Women were twice as likely as men to report severe disabling back pain; low intensity was equal between genders. The authors conclude that general prevalence is not terribly useful information since the majority of responders who had episodes of back pain had low intensity/nondisabling episodes of back pain.
Cote et al. looked at the prevalence of neck pain in the same random survey of the Saskatchewan population.27 The lifetime prevalence of neck pain was 66.7% and point prevalence was 22.2%. Neck pain was defined as any pain between the occiput and third thoracic vertebrae as detailed on a mannequin diagram. Subjects were stratified by intensity of pain and disability in a fashion similar to the study on back pain. Women experienced more neck pain than men in all severity groups. Women had a 58.8% 6-month prevalence and men had a 47.2% 6-month prevalence. The 6-month prevalence of low intensity/low disability neck pain was 39.75% and 10.1% for high intensity/low disability neck pain. A total of 4.6% of the surveyed population reported highly disabling neck pain for the previous 6-month period. Interestingly, low intensity/low disability neck pain was found to decrease with age. High disability neck pain was more prevalent in women than in men.
Kopec et al., in a longitudinal study of households in 10 provinces in Canada, were able to determine the incidence of back pain.10 The interval of the two surveys was 2 years and the sample size was 11 063 subjects age 18 years or older. An episode of back pain was defined as lasting longer than or equal to 6 months in duration or expected duration. The 2-year incidence in females was 9.0% and in males was 8.1%. Of note is that this was a self-administered survey but the question was ‘Have you been diagnosed by a health professional with back problems, excluding arthritis?’ One could envision several potential biases of this longitudinal prospective study. The question asked is not defined in terms of intensity but only duration. The duration maybe 6 months or longer or in the alternative be expected to last 6 months or longer. This opinion on expected duration is that of the subjects. The diagnosis of back problems by health professionals is being self-reported by the subject and not by health professionals or their records. The validity of this second-hand information is unclear. George, in another Canadian survey with 1131 respondents, showed an 8% 6-month incidence of clinically significant low back pain by the Chronic Pain Questionnaire.28
The prevalence of low back pain in North America, as elsewhere, varies by study. In an attempt to reconcile the variability and determine reliable prevalence rates a methodological review of the literature was performed to identify acceptable studies and compare prevalence rates.29 They found 13 studies from 1981–1998 methodologically acceptable, but with variable assessments and definitions of an episode of back pain. The range of point prevalence in the studies varied from 4.4% to 33.0%. One-year prevalence rates ranged from 3.9 to 63%. The explanation for the variability is partially blamed on the differing durations of back pain required to constitute a reportable episode.
Asia
In a cross-sectional study of garment workers, battery/kiln workers, and teachers in Shanghai, People’s Republic of China, the overall yearly prevalence of back pain was 50%.30 The number of subjects in this study was 383. This was self-reported back pain with symptoms lasting a minimum 24 hours. Garment workers had the highest yearly prevalence of 74% while teachers had a prevalence of 40%. The 7-day prevalence was 45% for garment workers and 22% for teachers. The different occupations were thought to account for the variation in prevalence.
In a study of 800 workers in Russia, the lifetime prevalence was 48.2%, point prevalence was 11.5%, and the 1-year prevalence was 31.5%. The vast majority (88.2%) had pain for less then 2 weeks. Only 1.8% had pain for longer than 12 weeks.31
Low-income countries
Studies to determine statistics for spine pain in low-income countries are much less common than in wealthy industrialized nations. The literature on back pain is primarily from high-income countries accounting for less then 15% of the world population. In an attempt to test the hypothesis that in low-income countries, since physical labor is more common, back pain should have a higher prevalence, a systematic review of the literature for low-income countries was performed.5 The point prevalence was the benchmark and used for comparison. Interestingly, high-income countries had 2–4 times the point prevalence found in rural, low-income countries. The variation within both the high-income and low-income groups was twofold. This large disparity within categories of countries puts the methodology and therefore strength of the study into question. Notwithstanding the methodologic issues, manual labor does not appear to correlate with back pain. Perhaps physical activity is protective or even serves as treatment. This study in a general way lends evidentiary support to exercise as a treatment modality.
Harlow found a 29.8% prevalence of low back pain, a 38.3% prevalence of upper back pain, and a 26.4% prevalence of neck pain in women in Tijuana, Mexico.32 In a study in urban Zimbabwe of 10 839 respondents, back pain was the second most disabling condition after headaches.33 Omokhodion, in 840 Southwest Nigerian office workers, found a 12-month prevalence of low back pain of 38% and point prevalence of 20%.34 The overall rate of disability was 5.6%. In a cross-sectional study in rural Tibet with n=499, the point prevalence of low back pain was 34.1%, the 12-month prevalence was 41.9%.35 Subjects also reported functional disability related to their pain. In rural China36 the prevalence was found to be 12.1% and in Nepal37 18.4% for low back pain. Sharma reported that 23% of patients seen for medical care in outdoor rural India were seen for back pain.38 The information from rural nations may be helpful in our understanding of the factors important in developing spine pain and its prevention.
COST
The cost of back pain to various societies is hard to quantify. This is due to the lack of central data collection and variation in methodology. Extrapolating data from worker’s compensation claims in the US and then projecting to the population as a whole reveals staggering costs.39 In 1988 the estimate was 22.4 million cases of back pain with 149.1 million lost work days. This loss of workdays alone is estimated to cost more than US$13.3 billion. This does not take into account health care, personal expenses, and insurance costs. Estimates of total cost in the US range from US$50 to US$100 billion per year. A Swedish study found that 6% of sufferers accounted for over 50% of the costs.18 In Australia the cost is estimated at US$10 billion per year with a lifetime prevalence of 80%.40 In the Netherlands back pain is the most common cause of lost days at work and disability. In 1991 the direct costs of medical care for back pain in the Netherlands was US$367.6 million and the indirect costs were US$4.6 billion.41
In a 2003 study in the US, back pain was the second most common pain condition resulting in lost time from work after headache.42 Out of the total work force, 3.2% lost time from work as a result of back pain. Pain-related loss of productive work time cost an estimated US$61.2 billion. The majority was because of decreased productivity while at work and not due to absence from work.
FACTORS
The cause of most episodes of spine pain is uncertain. The purported risk factors are numerous. Heavy lifting, particularly on a repetitive basis, has often been suggested as an inciting event. Age, gender, and psychological distress have all been implicated, but not consistently. Cigarette smoking and obesity have been related to back pain in some studies.43–46 Socioeconomic status has been identified as a risk factor in some studies43,47 but not all studies.10 Before leaping from associated factors to causation and postulated mechanisms, we must have better data for prevalence and incidence so the significance of these potentially inconsequential associations can be adequately evaluated.
Cote et al. analyzed the Saskatchewan Health and Back Pain Survey data to determine the factors associated with neck pain and disability.48 In the sample population, 15.9% reported prior neck injury in a motor vehicle accident. This history and headaches were strongly associated with all grades of neck pain. Subjects with cardiovascular or digestive problems had a higher 6-month prevalence of disabling neck pain but not milder neck pain. There was an association between low back pain and neck pain.
The Mini-Finland Health Survey found chronic neck pain strongly associated with back pain and shoulder disorders, but only weakly associated with osteoarthritis, cardiovascular, and mental disorders.19 Trauma to the neck or low back was associated with chronic neck pain.
Kopec et al., in a prospective study, tried to identify factors in the development of back pain in the general population.10 General health and psychosocial factors were important in both sexes. Other factors in men were age, usual activity pattern, lack of gardening, and height. For women the other factors were self-reported arthritis or rheumatism and a history of psychological trauma. If a woman has none of these identified factors her risk of developing back pain in a 2-year period is 6%. For a woman who has activity restriction, has been diagnosed with arthritis or rheumatism, has two or more traumatic events in childhood, and reports a high level of personal stress, her risk of developing back pain is 32% in a 2-year period.
General health was a strong predictor of back pain in a study in the UK by Croft et al.49 They studied 2715 individuals from two general practices in Manchester. The relative risk was 1.5 for men and 2.2 for women who had poor general health.
Weight
Weight has often been cited as a risk factor in spine pain, most often in low back pain. Webb found an association between obesity and back pain with disability but not with neck pain or low-intensity back pain.13 Kopec et al. found no significance but weight was close to significant as a factor in women.10 Croft found weight to be a significant factor for women but not men.49 Gyntelberg, in a study of Danish men, found an association between height and low back pain but not weight.50
Occupation
In a British survey with 12 907 respondents, no association for neck pain was found for lifting, vibratory tool use, or professional driving.14 There was an association found with above-the-shoulder activity for >1 hr/day. Stronger associations were found with tiredness or frequent stress. Occupations with the highest prevalence were, in descending order; construction workers, nurses, armed services members, and the unemployed. No association was found between physical workload, postures, or exposure to vibration and low back pain in steel workers.22
In a study in the Netherlands, scaffolders had a 60% 12-month back pain prevalence.51 Supervisors had similar rates for back pain and perceived disability but less severe back pain and lower absence rates than scaffolders.
Ehrlich opines that most spine pain is not related to work activities but may be related to psychosocial factors.52 Job dissatisfaction, stress, the system of compensation, and hiring a lawyer are all reported to decrease return-to-work rates. Hadler states how back pain is dealt with determines if it is disabling or not; secondary gain such as workers’ compensation increases the morbidity.53
Secondary gain
There is a long-standing controversy regarding the role of secondary gain in spine pain and disability. Disability from spine pain was not a significant problem until the industrial revolution. Cassidy et al. analyzed the effect of compensation on whiplash injury in Saskatchewan.54 On January 1, 1995, the tort compensation system for traffic injuries was changed to a no-fault system eliminating recovery for pain and suffering. This provided natural data collection points. It is important to note that Saskatchewan was the only insurer for motor vehicle injuries in the Province and all residents benefit from state health insurance. For the last 6 months of the tort claim system the 6-month cumulative incidence was 417 per 100 000 persons compared to 302 and 296 per 100 000 in the first and second 6-month periods of the no-fault system. This equates to a 28% decrease in claims for whiplash injury. The time from the date of injury to claim closure decreased from 409 days to 194 days in the same time interval. During this same period there was an increase in the number of vehicle-damage claims and distance driven.
Psychological
The development of back pain has often been associated with psychological factors.55–57 In the Manchester study, psychological factors were found to be predictive of low back pain.57 This was a prospective study of 4501 surveyed subjects. Subjects with no back pain but high scores for psychological distress were more likely to develop back pain than individuals with low scores.
Perez found an association prospectively between psychological factors and back pain in healthy workers.56 The only factors related to back pain in that study were age, depression, and general stress. Kopec et al. found general stress to be a factor in men and personal stress (a subset of general stress) a factor in women, as well as a history of psychological trauma in women.10 In a Finnish study, an association between depression and neck and back pain was found in both men and women.58 Power, in a British cohort study looking at early life variables, found psychological distress at age 23 to be the strongest predictor of low back pain.59 This doubled the risk of back pain later in life. In rural India 67%, of patients seen for low back pain had psychosocial issues, and 38% were dissatisfied with their current job.38
Smoking
Smoking has be implicated as a factor in developing spine pain.45,59 The Manchester study,49 Kopec et al.10 and Guez et al.16 among others did not find an association between spine pain and smoking. In a systematic review of the literature for 1976–1997 an association between smoking and non-specific back pain was found.45 The results revealed an association between smoking and back pain in men in 18 of 26 studies and in 18 of 20 studies in women. It is not clear if smoking preceded back pain or if there is a close relationship. The finding of a positive association does not imply causation.
Age
The highest prevalence of low back pain occurs between 40 and 60 years of age. Kopec et al., in one of the few studies looking at incidence and age, showed a peak incidence at 45–64 years of age.10 Rates of back pain seem to increase during adult life until age 65 and then they decrease.
Predicting back pain by knowing the causation and associated factors would allow for an intelligent, scientific approach to prevention. The data currently available are often contradictory and difficult to explain. In one study, gardening is associated with a lower risk of low back pain.10 In other studies sports and nonoccupational home improvement increased the risk.49 Is gardening really associated with lower incidence of back pain or do people prone to back pain not garden? We must ask the right questions to determine the association between risk factors and the development of spine pain. Perhaps these are just chance associations. Without precise methodology, reproducibility, and multiple studies in agreement, the true associated factors remain uncertain and true causation beyond our reach.
RECURRENCE
The traditional notion that the great majority of episodes of non-specific back pain resolve has come under scrutiny. There is good evidence that a substantial fraction of back problems have recurrent symptoms. Miedema found that 28% of patients with an episode of back pain, for which they consulted their physician, went on to develop chronic back pain.41 Only 1 in 5 people with back pain consult their physician.
In a review of the literature the 1-year recurrence rate for low back pain was 20–44%.8 Lifetime recurrence rates were up to 72%. These studies were prospective studies for occupational back pain. Nurses and drivers had the highest recurrence rates while white collar workers had the lowest recurrence rates. In general, men had higher rates of recurrence than women. In a longitudinal cross-sectional study in the UK there was a 59% lifetime prevalence, with 42% of those respondents reporting persistent annual low back pain.15 Acute episodes recurred and in some patients turned into chronic, constant pain.
DISABILITY
Spine pain is a major cause of disability. It has been estimated that 1% of the population is disabled by back pain. It is the leading cause of disability in the US for the population under 45 years old and the second cause for those 45–65 years old.46 Back pain accounts for approximately one-fourth of worker’s compensation claims in the US. In a survey of 30 074 respondents 5256 subjects, or about 17.5%, self-reported back pain lasting at least 1 week.39 Construction workers in males and nurses aides in females had the highest prevalence rates of 22.6% and 18.8%, respectively. Subjects reported missing work or changing jobs in 12.1% of those with reported pain. Extrapolating this to national estimates yields staggering numbers of cases of back pain and lost work days.
There is little known about the extent of the disability spine pain produces in less-industrialized nations. In Nigerian office workers, only 5% of those surveyed reported lost days due to back pain, with a mean of 4.7 days per year.34 The incidence of back pain was similar to that in industrialized nations but the absence from work was not as significant.
The history of spine pain and disability helps illuminate some of the factors that transform spine pain, as an accepted part of life, into a disabling condition. Allan and Waddell review the history of back pain and disability.11 There was very little written about spine pain causing disability until the industrial revolution. Early reports of spine disability in railway workers led to much more frequent spine disability in the early twentieth century. This was coupled with the concept of compensation for work-related injury. During WWI the US draft board rejected recruits that had static problems of the spine to avoid backache. Recruits still developed backache, but could be made fit for service by special training battalions. This suggested that back pain might be a fitness problem and not a medical problem. In the British armed forces there was a fivefold increase in withdrawal from duty for back pain between WWI and WWII. In Britain in 1911 and the US in 1949, workers were covered for injury by Workman’s Compensation Insurance. As the breadth of compensation increased so did the extent of disability for back pain. The author’s concluded that disability is not a natural sequelae of back pain, but is secondary to how we compensate, manage, and treat patients with these aliments.
CONCLUSION
The rate of surgery varies by regions and by country with up to a 15-fold variation within the US. The use of various treatments including COX-2 antiinflamatory drugs, spinal injections, IDET and percutaneous discectomies, just to name a few treatments, vary greatly by geographic area. Treatment trends have changed throughout the history of medicine. The factors driving these shifts are, unfortunately, not always scientific in basis or in the patient’s best interest. This is perhaps driven more in the US by reimbursement trends and patients desire for specific treatments. The lay press and insurance industry fuel this, and not necessarily scientific evidence. Once these treatment modalities become common and patients ask for them, it is very difficult to study their effectiveness. The first step to evidence-based medicine in the treatment of spine pain is the collection of valid, consistent, epidemiologic data. This will serve as the foundation on which to build rational treatment in the future.
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