Introduction

Pain is prevalent worldwide and is among the most common symptoms leading patients to consult a physician in the United States (US) [1]. Recurrent and chronic non‐cancer pain (CNCP) are not a set of single, cohesive disorders. Instead, recurrent and CNCP are generic classifications that include a wide range of disorders.

Individuals with recurrent pain and CNCP comprise disparate groups, with varying underlying pathophysiology, and widely diverse impacts on quality of life, function and demands on the healthcare provider and society. Thus, CNCP and recurrent pain have not only significant health consequences, but also personal, economic and societal implications. These conditions have both direct costs of health care and indirect costs (e.g., lost paid employment, disability compensation). In this chapter we provide a summary of the prevalence of some of the most common CNCP and recurrent pain disorders and describe their economic impact.

Epidemiology is, “The study of the occurrence and distribution of health‐related states or events in specified populations, including the study of the determinants influencing such states, and the application of this knowledge to control the health problems” [2]. It is important to clarify the meaning of epidemiology and key concepts of incidence and prevalence.

Incidence is the number of new cases of a disease developing during a particular time period in a population at risk of developing the disease. Prevalence is the proportion of the at‐risk population affected by a condition (i.e. total number of cases of disease present in the population at a specified time divided by the total number of persons in the population at that specified time). In this chapter, we will focus on prevalence of chronic pain in general as well as in specific diagnostic groups.

It is important to acknowledge at the outset that a number of factors will influence the prevalence rates of any chronic pain diagnosis as population estimates for the prevalence of chronic pain vary widely. Some of the variability in prevalence estimates reported in the literature result from the case definition used. In addition, ascertainment methods (telephone interview, in person interviews), wording of questions (e.g., any pain, pain that prevent respondent from daily activities, pain severe enough to induce healthcare seeking), timeframe (e.g. recall bias, differential time intervals such as pain over the last month versus last week, retrospective vs. current), sample (e.g., population‐based) and time, place and population sampled (internet vs. in‐person) will all influence the survey results.

One particularly important problem in establishing the prevalence of different chronic pain conditions is the inherent subjectivity of pain presents a fundamental impediment to increased understanding of its mechanisms, control, and the epidemiology. The language used by any two individuals attempting to describe a similar injury and their pain experience often varies markedly. Similarly, clinicians and clinical investigators commonly use multiple terms that at times have idiosyncratic meanings. Needless to say, appropriate communication requires a common language and a classification system that is used in a consistent fashion.

In order to identify target groups, conduct research, prescribe treatment, evaluate treatment efficacy, to develop policy and for decision making, it is essential that some consensually validated criteria are used to distinguish groups of individuals who share a common set of relevant attributes. The primary purpose of such a classification is to describe the relationships of constituent members based on their equivalence along a set of basic dimensions that represent the structure of a particular domain. Infinite classification systems are possible, depending on the rationale about common factors and the variables believed to discriminate among individuals. The majority of the current taxonomies of pain are “expert‐based” classifications.

Expert‐Based Classification of Pain

Classifications of disease are usually based on a preconceived combination of characteristics (e.g., symptoms, signs, results of diagnostic tests), with no single characteristic being both necessary and sufficient for every member of the category, yet the group as a whole possesses a certain unity [3]. Most classification systems used in pain medicine (e.g., ICD [4], classification and diagnostic criteria for headache disorders, cranial neuralgias, and facial pain [5], IASP Classification of Chronic Pain [6], CRPS [7], whiplash‐associated disorders [8], Research Diagnostic Criteria [RDC] for Temporomandibular Disorders in dentistry [9,10] and the Analgesic, Anesthetic, and Addiction Clinical Trial Translations, Innovations, Opportunities, and Networks [ACTTION]‐American Pain Society Pain Taxonomy [AAPT] [11] and ACTTION‐American Pain Society‐American Academy of Pain Medicine [AAPM] Pain Taxonomy [AAAPT][12]) are based on the consensus reached by a group of “experts”. In this sense, they reflect the inclusion or elimination of certain diagnostic features depending on agreement.

The original IASP Classification of Chronic Pain included 5 axes [i.e. (1) body region; (2) system whose abnormal functioning that might produce the pain; (3) temporal characteristics of pain and pattern of occurrence; (4) onset and intensity of pain; and (5) presumed etiology] and each diagnosis resulted in a unique code number. Thus, this approach moved beyond the location of symptoms and system involved.

Recently, IASP proposed a classification of chronic pain for inclusion in the ICD‐11 [4]. The classification includes seven categories (i.e. “primary”, cancer, postsurgical/posttraumatic, neuropathic, headache and orofacial, visceral and musculoskeletal). The primary category is somewhat of a mixed collection of pain disorders that cannot be explained by other chronic pain conditions and includes back pain that is neither identified as musculoskeletal nor neuropathic, chronic widespread pain, fibromyalgia (FM) or irritable bowel syndrome. The primary category is consistent with the lumping of this set of disorders in the category of The American Academy of Pain Medicine’s (AAPM’s) diagnosis of maldynia and central sensitivity disorders advocated by Clauw [13] and Yunus [14] among others. There may be some concern that this poorly defined category may imply the discredited psychogenic classification; that is, an artificial dichotomy where either the condition has a physical (i.e. somatogenic) basis or the absence is “primary” (i.e. psychogenic).

Recently, a consortium composed of Analgesic Clinical Trials Translations, Innovations, Opportunities, and Networks (ACTTION) (a public‐private partnership support by the US Food and Drug Administration) partnered with the American Pain Society to create a chronic pain taxonomy – ACTTION American Pain Society Pain Taxonomy (AAPT) [11] and with the American Academy of Pain Medicine [12] to create an acute pain taxonomy – ACTTION, American Pain Society, and American Academy of Pain Medicine Pain taxonomy (AAAPT). AAPT and AAAPT are evidence‐based pain taxonomies in which a multidimensional diagnostic framework has been applied to the most prevalent and important chronic and acute pain conditions. A major impetus for the AAPT/AAAPT initiative derived from observing the transformative impact of evidence‐based diagnostic classifications that have been published by different medical specialties.

AAPT categorizes chronic pain conditions by organ system and anatomic structure, distinguishing (1) peripheral and central neuropathic pain, (2) musculoskeletal pain, (3) spine pain, (4) orofacial and head pain and (6) abdominal/pelvic/urogenital pain. Because certain types of chronic pain cannot be included in one of these groups, an additional category for disease‐related pain not classified elsewhere includes pain associated with cancer and pain associated with sickle cell disease (pain associated with Lyme disease and with leprosy, among other conditions, would also be included in this group). It is important to emphasize that all types of headache were intentionally excluded from AAPT because the International Classification of Headache Disorders provides systematic, valid and widely used diagnostic criteria for these conditions [5].

The AAPT multidimensional framework comprises five dimensions that can be applied to all chronic pain conditions. This can be contrasted with the new IASP taxonomy in which psychosocial factors are “optional specifiers” for each diagnosis beyond the classification of “chronic primary pain”; psychosocial factors are given a prominent role as are interference with activities and participation in social roles (somewhat of a departure from the original IASP taxonomy where psychosocial factors are a significant consideration only for one diagnostic classification; namely, chronic primary pain) (see also [11]). Other than prioritizing core diagnostic criteria, which is the first AAPT dimension, the order of the dimensions does not reflect their importance. Indeed, as noted earlier, it is anticipated that AAPT diagnostic criteria will ultimately be based on the mechanisms of the specific chronic pain conditions, whereas in the current version of the taxonomy, these mechanisms constitute the final dimension.

Like the IASP classification, the AAPT also includes seven but somewhat different categories of chronic pain (i.e. peripheral nervous systems; central nervous system; spine; musculoskeletal; orofacial and head; visceral, pelvic, and urogenital; other [e.g., cancer, sickle cell]). The AAPT classification incorporates five dimensions for each condition within the seven categories (core diagnostic criteria [symptoms, signs, and diagnostic findings required for the diagnosis]; common features [including pain characteristics, non‐pain features, lifespan], common medical and psychiatric comorbidities; neurobiological, psychosocial and functional consequences; and putative neurobiological and psychosocial mechanism, risk factors, and protective factors. Although the AAPT integrates important components of the classification of chronic pain conditions, there are no epidemiological data, thus far, that have been reported using this classification.

In this chapter, we will use a hybrid approach to classification, as the available epidemiological data tend to follow classification by body location (e.g., back pain, headache, pelvic pain, temporomandibular disorders (TMDs), irritable bowel syndrome (IBS), wide‐spread) and etiology (i.e. osteoarthritis (OA), neuropathic whiplash‐associated disorders). In the future, epidemiological research may advance our understanding of the prevalence of the diverse set of chronic pain disorders by using the more comprehensive IASP and AAPT classification.

Epidemiology of Chronic Noncancer Pain and Recurrent Pain

CNCP, typically assessed as pain that persists for longer than six months, remains a significant public health issue affecting millions of people worldwide [15]. Worldwide the prevalence is estimated to be over 20% of all adults, with 10% newly diagnosed each year [16, 17]. In 2015 the global point prevalence of activity‐limiting low back pain (LBP) alone was 7.3% (540 million people) affected at any one point in time [18].

Based on data from the National Health Interview Survey (NHIS) conducted in 2012, a representative national population‐based survey conducted annually by personal, home‐based interviews by the National Center for Health Statistics, 25.3 million American adults report daily pain and 23.4 million reported having “a lot of pain” [19]. In a subsequent NHIS survey [20], 20.4% (50 million) of the adult US population reported chronic pain (defined as having pain on every day or most days over the past 6 months) and 8% (19.6 million) had “high‐impact chronic pain” severe enough to interfere with their lives (i.e. limited life or work activities on most days or every day during the past 6 months).

The presence of high‐impact chronic pain was strongly associated with an increased risk of disability after controlling for other chronic health conditions, where disability was more likely in those with chronic pain than in those with stroke or kidney failure, among other conditions [20]. In the US, pain (i.e. LBP, neck pain, other musculoskeletal pain, OA, migraine) accounts for 9.7 million years living with disability in comparison with 8.8 million years living with disability for the 12 leading medical conditions [21]. The high‐impact chronic pain population reported more severe pain and more mental health and cognitive impairments than persons with CNCP or recurrent pain without disability and was also more likely to report worsening of health, more difficulty with self‐care and greater health care use [22].

CNCP is estimated to account for 16.2% of all adult outpatient visits in 2015, having increased from 11.3% in 2000 [23]. CNCP is a highly common condition and accounts or 57% of health care encounters [24]. Interestingly, for some of the most prevalent pain conditions (e.g. LBP, FM, headache, pelvic pain) there is no clear objective evidence of any underlying physical pathology associated with reported pain in the majority of cases (e.g. [25–27]). One survey conducted in Australia, found that 65% of people had no clear medical diagnosis for their chronic pain and 33% identified no clear precipitant [28].

A secondary analysis of data from the 2016‐2017 National Survey of Children’s Health [29] indicated that CNCP and recurrent pain are not only problems for adults, but also for children. An estimated 8% of national sample (95% confidence interval [CI]: 7.5%‐8.6%) of children (6‐17) had chronic pain as rated by parents [30]. Chronic pain was defined by response to the question: “During the past 12 months, has this child had frequent or chronic difficulty with repeated or chronic physical pain, including headaches or other back or body pain?” The NSCH is an annual cross‐sectional survey, conducted via in‐person interviews of randomly sampled households, selected via a multistage process to represent the entire civilian, noninstitutionalized population of the US. Pediatric prevalence rates of chronic pain subtypes range across studies from 8% to 83% for headaches, 4% to 53% for abdominal pain, 14% to 24% for back pain, 4% to 40% for musculoskeletal pain, 4% to 49% for multiple pains and 5% to 88% for other pains. Several studies report that 5% of youth report experiencing severe pain that interferes with daily function [31,32].

Similar to adults, physical causes for reported pain are often difficult to identify. In only one‐fifth of the patients are specific causes or medical diagnoses for their pain condition able to be identified, which further underscores chronic pain (and its related disability) as a medical syndrome unto itself [33].

Persistent pain in youth may continue to adulthood. A retrospective review of the onset of pain in adults seeking treatment in a pain clinic [34] found that 80% of adults with chronic pain reported that their current pain was a continuation of chronic pain they had experienced during childhood. It is important to note, however, that this estimate is based on patients from a tertiary pain clinic and therefore likely overestimates the proportion of adults with chronic pain who had also experienced chronic pain in childhood. Indeed, much of the burden of chronic pain occurs in later life [19, 35]. because of age‐associated musculoskeletal conditions, such as OA.

Musculoskeletal pain

Musculoskeletal pain is perhaps the most commonly reported set of CNCP conditions. Based on the 2012 NHIS (n = 34,525) more than 50% of US adults (approximately 125 million) experience one or more musculoskeletal pain disorders [36]. In an earlier epidemiological study that differentiated among musculoskeletal conditions, the NHIS of 2007, reported 29,019,000 (12.8%) had neck pain, 57,070,000 (25.4%) had pain in the lower back and 9,062,000 (4%) had pain in the face or jaw in the 3 months preceding the interview [37].

Among musculoskeletal locations, the most commonly afflicted region is the lower back. Low LBP accounts for 34 million office visits annually by family physicians and primary care interests [38]. LBP is amongst the top six costliest health conditions, and one of the top three most disabling health conditions [39]. In fact, LBP is the highest ranked condition contributing to years lived with disability worldwide according to the most recent Global Burden of Disease Study and is associated with significant societal and individual cost [21,40, 41]. LBP is also the most common of chronic pain conditions reported by adolescents, however, the range of prevalence rates across studies is quite large, namely, 8–44% [42]. Some of the features of epidemiological surveys listed above many account for the variability observed.

Epidemiologic surveys in the US report a prevalence rate of 25% for LBP any time during a 3‐month period [43]. Other industrialized nations, with prevalence rates for chronic LBP ranging 13–28% [42] with19% prevalence rate for CLBP during a 12‐month period and a lifetime prevalence rate of 29.5% [44].

Rheumatological diagnoses

Osteoarthritis (OA) is a chronic debilitating condition typically observed within three specific areas in order of decreasing frequency; the hand, knee and hip. It is the most common rheumatological diagnosis [45]. As of 2020, there are an estimated 32.5 million adults in the US who have OA [46]. Using the 2013‐15 data, Barbour et al. [47] concluded that 23.7 million (43.5%) had arthritis‐attributable activity limitation (an age‐adjusted increase of approximately 20% in the proportion of adults with arthritis activity limitations since 2002 [p‐trend <0.001]) [47]. The prevalence of OA is projected to increase by about 40% in the next 25 years [48] with the number of affected individuals expected to rise to 78 million by 2040 [47].

The prevalence increases of OA steadily with age, affecting 29.9% in men aged 18–64 years, 31.2% in women aged 18–64 years, 55.8% in men aged 65 years and older and 68.7% in women aged 65 years and older [49]. Symptomatic knee OA alone affects approximately 12% of those aged 60 years and older. OA is more prevalent in women than in men, with a prevalence ratio varying between 1.5:1 and 4:1 [50]. In addition to age and sex, the prevalence of OA can vary due to several risk factors such as ethnicity, level of obesity, physical activity levels, bone density and trauma, as well as global factors such as geographical location [50, 52]. Barbour and his colleagues [47] tracked NHIS data from 2002‐2014 and that by 2014 prevalence of OA was especially high among Blacks (42.3%) and Hispanics (35.8%).

Although OA is one of the most common diagnoses in general practice in the US [52], there are a number of other pain‐related rheumatological disorders. Prevalence estimates for rheumatoid arthritis suggest that 1.3 million adults have this diagnosis. In addition, juvenile arthritis is estimated to affect 294,000 children; spondylarthritides affects 0.6‐ 2.4 million adults; systemic lupus erythematosus affects from 161‐322,000 adults, systemic sclerosis affects 49,000 adults; and primary Sjogren’s syndrome affects from 0.4 million to 3.1 million adults [48].

Headache

Headache, an almost universal human experience, is one of the most common complaints encountered in medicine and, perhaps for this reason, the preponderance of data on the epidemiology of CNCP and recurrent pain disorders are found for headaches. According to the World Health Organization [53], half to three quarters of adults aged 18–65 years in the world have had headache in the last year and, among those individuals, 30% or more have reported migraine. When considering more chronic headache, the WHO estimated that the prevalence of headaches on 15 or more days every month affects 1.7–4% of the world’s adult population. The life‐long prevalence of headache is estimated to be 96% [54]. The 2011 NHIS [55] results reveal that 16.6% of US adults 18 or older reported having migraine or other severe headaches in the last 3 months.

Migraine and tension‐type headaches are the most common primary headache disorders [56]. The main subtypes are migraine (vascular headache) with and without aura. An aura is a fully reversible set of nervous system symptoms, most often visual or sensory symptoms, that typically develops gradually, recedes, and is then followed by headache accompanied by nausea, vomiting, photophobia and phonophobia. Tension‐type headache is a dull, bilateral, mild‐ to moderate‐intensity pressure–pain without striking associated features that may be categorized as infrequent, frequent or chronic and is easily distinguished from migraine.

Migraine is more prevalent in females between the ages 18‐44, with the overall age‐adjusted 3‐month prevalence of migraine in females was 19.1% and in males 9.0%, but varied substantially depending on age [56, 57, 58]. Data suggest that 70% to 80% of migraineurs have a family history. In the US, the impact of migraine appears to be greater in those who work part time or are unemployed, those with low socioeconomic status, and the uninsured [58]. The 2010 Global Burden of Disease Survey reported that migraine was the third most prevalent disorder and the seventh‐highest cause of disability worldwide [56].

The National Headache Foundation estimated that more than 37 million American experience recurrent migraines [59]. However less than half of those who experience migraines have received a formal diagnosis from a health care provider [61]. Migraine alone affects 18% of women and 6% of men in the US and has an estimated worldwide prevalence of approximately 10% [60]. Pediatric prevalence rates range from 8% to 83%, depending on the sample, with the excessively high estimates based on clinical samples compared to the lower estimates from population samples [33].

After reviewing data from three national surveys (NAMCS, National Ambulatory Medical Care Survey (2010), the National Hospital Ambulatory Medical Care Survey (NHAMC [2010], and the NHIS (2005‐2010), Burch et al. [58] concluded that migraine is a highly prevalent medical condition, affecting approximately 1 out of every 7 Americans annually and these estimates have been relatively stable over a period of eight years. The American Migraine Prevalence and Prevention study subdivided the migraine prevalence data into definite migraine and probably migraine. The authors concluded that the overall prevalence of migraine of 11.7% and probable migraine of 4.5%, for a total of 16.2% [61].

In the US, migraine accounted for 0.5% of all visits and other headache presentations for 0.4% of all ambulatory care visits. Overall, 52.8% of all visits for migraine occurred in primary care settings, 23.2% in specialty outpatient settings and 16.7% in emergency department (EDs) [55].

Although typically not as severe as migraine, tension‐type headache is far more common, with a lifetime prevalence in the general population of up to 80%. The global active prevalence of tension‐type headache is approximately 40% and migraine 10% [56].

Headache or pain in the head was the fourth leading cause of visits to the ED in 2009‐2010, accounting for 3.1% of all ED visits. The 3‐month prevalence of migraine or severe headache was 26.1% [58].

Neuropathic Pain

Neuropathic pain arises as a direct consequence of a lesion or disease affecting the somatosensory system. It can be peripheral in origin as a result of nerve injury or disease (e.g. lumbar radiculopathy, postherpetic neuralgia, diabetic or HIV‐related neuropathy, or postsurgical pain), or central (e.g. poststroke or spinal cord injury). Other diseases known to cause neuropathic pain that are diagnosed during childhood include erythromelalgia, toxic and metabolic neuropathies, mitochondrial disorders, paroxysmal extreme pain disorder and Fabry disease. Moreover, there has been increasing recognition that some classically “nonneuropathic” painful conditions (e.g. OA, FM) can give rise to symptoms more commonly associated with neuropathic pain.

Neuropathic pain is characterized by unpleasant symptoms, such as shooting or burning pain, numbness, allodynia and other sensations that are very difficult to describe. “Definite” neuropathic pain can relatively rarely be confirmed, particularly in nonspecialist settings [63, 64]. Neuropathic pain conditions have proven to be particularly recalcitrant to treatment [63].

Much less is known about the prevalence of neuropathic pain disorders compared with other chronic and recurrent pain disorders (e.g. headache, back pain, arthritis). General population studies have reported prevalences of 8% and 6.9% in the United Kingdom and France, respectively [65, 66]. It is important to note that cases identified in these studies were described as having “pain of predominantly neuropathic origin” or “pain with neuropathic characteristics,” rather than “neuropathic pain” [64]. A systematic review of population‐based prevalence studies considered the true prevalence of pain with neuropathic characteristics to be 7% to 10% [67]. Furthermore, neuropathic pain is estimated to be present in 17% of adult patients with other CNCP disorders, with as many as 30% of adults seen in pain clinics are estimated to experience neuropathic pain [66, 68].

Pelvic Pain

Pelvic pain is characterized by intermittent or constant pain in the lower abdomen or pelvis for at least 6 months that may or may not be associated with menstruation. However, the pain should not be exclusively associate with menstruation, sexual intercourse or pregnancy. The most common diagnoses for pelvic pain are endometriosis, pelvic inflammatory disease and interstitial cystitis [bladder pain syndrome]. Pelvic pain is estimated to account for 20% of general practitioners’ referrals to gynecologists [69].

CPP has a considerable impact on the well‐being of women and is a cause of significant distress and disability. It has been reported to be associated with poor quality of life, fatigue, depression, anxiety and marital and sexual dysfunction [70]. Patients with CPP tend to spend days in bed due to illness and report poorer physical and mental health compared with the general population [71]. A study showed that 58.4% of women with CPP reported that they use analgesics and/or nonsteroidal anti‐inflammatory drugs on a weekly or daily basis without medical prescription [72].

The heterogeneity of the definitions used for chronic pelvic pain (CPP) introduced challenges for comparing results across different studies. The American Congress of Obstetricians and Gynecologists defines CPP as noncyclical pain in the pelvis, severe enough to require medical attention, located below the umbilicus in the region of the anterior abdominal wall, lumbosacral back or buttocks lasting for at least 6 months [73].

A number of studies have reported the prevalence of CPP in women, but most of them have used sampling frames such as hospital patients which are unable to provide accurate estimates of the prevalence of CPP in the general population. The relatively few population‐based studies have reported prevalence ranging from 6.4% [74] to 25.4% [75]. The studies were conducted using randomly selected women from representative sampling frames.

In a US population‐based study conducted by the Gallup Organization, 14.7% of eligible women (773/5263, 1 in 7) reported pelvic pain in the last 3 months, 61% of women with pelvic pain symptoms did not have a clear diagnosis, 15% of employed women with chronic pelvic pain reported that they lost time from pain work and 45% overall reported reduced work productivity due to their pain [70]. Worldwide estimates suggest that 24% of women experience CPP [76]. All the studies on the epidemiology of pelvic pain have been conducted among women of reproductive age. Older women who are believed to be less susceptible to CPP have been traditionally excluded from prevalence studies. More recent population studies have also confirmed significant reporting of CPP among older women. For example, the highest rate reported in one of study was in women aged 18–25 years (17%), whereas women older than 75 years had a rate of 13% [77].

Temporomandibular Disorders

Temporomandibular disorders (TMD) are disorders of the jaw muscles (i.e. muscle of mastication), temporomandibular joints and the nerves associated with chronic facial pain. Any problem that prevents the complex system of muscles, bones and joints from working together in harmony may result in a TMD. The exact cause of a person’s TMD is often difficult to determine. Pain may be due to a combination of factors, such as genetics, muscle hyperfunction, arthritis, jaw injury or hormonal influences. Some people who have jaw pain also tend to clench or grind their teeth (bruxism), although many people habitually clench or grind their teeth and never develop TMD.

TMDs are common, in some studies affecting approximately 25% of adults [78]. Further, TMD is associated with substantial morbidity, affecting quality of life and work productivity. As an example, it is estimated that for every 100 million working adults in the United States, TMD contributes to 17.8 million lost work days annually [79].

Over 2 decades of NHIS surveys (1989 to 2009), the prevalence of self‐reported TMD symptoms remained stable, affecting 5% of US adults [80]. Based on a national population‐based survey, it is estimated that 11.2‐12.4 million Americans have symptoms related to TMDs [81].

IASP Primary classification (also referred to as chronic widespread pain and Central Sensitivity Syndromes (CSS))

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