Introduction

Chronic pain is one of the most frequent complaints in medical practice. The International Association for the Study of Pain (IASP) estimates that chronic pain, including musculoskeletal and joint pain, neck and back pain, cancer pain, trauma and post-chirurgical pain, and chronic headache, afflicts about 20 % (10–55 %) of the adult population, worldwide [1]. It is considered as a major social and economic burden to individuals, to families, and to society, with important physical and psychological consequences to sufferers [2].

Fibromyalgia (FM) is one of the main causes of chronic widespread pain (CWP). It represents a situation in which central nervous sensitization is manifested by CWP, which is the cardinal symptom of FM, and generalized tender points (hyperalgesia) [3, 4]. Other associated symptoms may be present, including fatigue, sleep disturbances, difficulties with memory and concentration, irritable bowel syndrome, headache, depression. It is debatable if FM is a distinct clinical entity or part of spectrum of CWP [5]. There are still some physicians who deny the validity of this diagnosis, attributing the pain complaints as a manifestation of other clinical and/or psychiatric disorders.

In 1990, the American College of Rheumatology (ACR) published some criteria for the classification of CWP and FM [6]. The proposed criteria for FM were: CWP in combination with tenderness at 11 or more of 18 specific tender point sites. CWP was defined as pain for at least three months, in the axial skeleton plus pain in the left and the right side of the body, and pain above and below the waist. Ten years later (2010), the ACR introduced new preliminary diagnostic criteria [7•], which would also be suitable for use by primary care physicians, as it did not require tender points examination, referred by many as difficult to apply and to interpret. The examination of tender points was also an impediment for doing large, nationwide epidemiological studies on FM, as they required all subjects with CWP to be examined by specialists. In 2011, the same group published a modification of the 2010 ACR criteria [8••], developing a survey questionnaire for epidemiological and clinical studies, which allows easier future larger, nationwide surveys.

Epidemiological studies are important to better understand the extent of the problem in general populations or specific settings, in order to calculate the appropriated resources to provide adequate assistance to FM sufferers.

Prevalence of Fibromyalgia

The prevalence of FM has been estimated in many studies in different settings, areas and countries, and on four continents: Africa, the Americas, Asia, and Europe. There was only one study in Africa, in Tunisia [9], and none in Oceania. The epidemiological studies in adults of the general populations are depicted in Table 1. The global mean prevalence of FM was 2.7 %, ranging from 0.4 % in Greece [26] to 9.3 % in Tunisia [9]. The mean rate was 3.1 % in the Americas, 2.5 % in Europe, and 1.7 % in Asia. In women, the mean prevalence was 4.2 % and in men 1.4 %, with a female-to-male ratio of 3:1. Most of these studies were done in some specific areas/towns/cities of the country; nationwide prevalence rates were only estimated in Canada [12], France [21, 22•], Finland [23], Germany [24•, 25•], Israel [3], Italy[24•], Portugal [24•], and Spain [24•, 28].

Table 1 Prevalence of fibromyalgia in the general population
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There were three recent studies using the 2010 ACR criteria for diagnosing FM, in the USA [14•], in Germany [25•], and in Thailand (Abstract) [19]. Although this new criteria has an increased sensibility and decreased specificity in relation to the 1990 ACR criteria, it seems that it does not result in higher prevalence rates of FM in epidemiological surveys [25•].

The prevalence of FM was also estimated in specific populations or settings. In women, the rates were 10.5 % in Arendal, Norway (20 to 49 years) [31] and 3.6 % in Trabzon, Turkey (20 to 64 years) [32]. In Mexican school children, aged 9–15 years, the prevalence was 1.2 % [33]. In elderly subjects, 65 years or older, in São Paulo, Brazil, a rate of 5.5 % was found [34]. In hospitalized patients of a primary care unit of Seoul, South Korea, the rate was 1.7 % [35]. Gallinaro et al. [36] found a 10.4 % prevalence of FM in metalworkers without repetitive strain injuries, and 58.8 % in those with this condition. Among textile workers in Denizli, Turkey, the rate was 7.3 % (9.0 % in females and 0.8 % in males) [37]. In hospital workers in Japan, the rate was 2.0 % in women and 0.5 % in men [38]. In the Amish community of London, Ontario, Canada, the rate was 7.3 % (10.4 % in females and 3.7 % in males) [39]. In a low socioeconomic status population in Embu, São Paulo, Brazil, assisted by the public primary health care system, the rate was 4.4 % [40]. In patients of 16 general practices of Marche, Italy, the rate 2.2 % [27], and in patients of a health insurance company in Germany, the rate was 0.4 % of women and 0.05 % of men [41].

Incidence of Fibromyalgia

The incidence of FM has been estimated in two studies. Forseth et al. [42] found an incidence in females, aged 20–49 years, living in Arendal, Norway, of 5.83 new cases per 1,000 person–years. Weir et al. [43] reported an incidence rate of 6.88 new cases per 1,000 person–years for males and 11.28 new cases per 1,000 person–years for females, from a health insurance claims database.

Association of Fibromyalgia with Some Sociodemographic Variables

Many studies have shown that the prevalence of FM is higher either at the middle age (30 to 50 years) [10, 28, 37] or after 50 years of age [3, 12, 13, 23, 24•, 25•, 29, 30, 32]. White et al. [11] reported a peak prevalence in men in middle age, and in women the prevalence increasing steadily with age. The study of Vincent et al. [14•] was the only one that contrasted to the trend of increasing prevalence of FM with older ages; they described a higher rate in young ages (21 to 39 years).

All papers that studied the association of FM with the education level of subjects reported higher prevalence rates of this entity in low educated patients [11, 21, 23, 28, 32]. The same pattern was seen with socioeconomic status: the lower the household income, the higher the FM prevalence rate [11, 12, 28, 32, 37].

Regarding marital status, there was no consensus in the literature. Topbas et al. [32] found that FM was more frequent in widowed patients, Cobankara et al. [37] in married people, and White et al. [11] in divorced ones.

There was also a discrepancy about living in rural or urban areas. McNally et al. [12] in Canada, Mas et al. [8••] in Spain, and Hag et al. [15] in Bangladesh, all reported higher rates of FM in rural areas, whereas Turhanoglu et al. [30], in Turkey, found a higher prevalence in the urban population.

The association of FM with body weight was only mentioned by McNally et al. [12], with higher FM prevalence in obese women.

Fibromyalgia Comorbidity

The EPIFFAC Study [44], in Spain, reported that 84 % of patients with FM have one or more comorbid diseases: 67 % have other musculoskeletal conditions, 35 % psychological disorders, 27 % gastrointestinal disorders, 23.5 % cardiovascular disorders, and 19 % endocrinological disorders.

In hospitalized patients in the USA, the most common comorbidities when FM was the primary diagnosis were: non-specific chest pain, mood disorders, and spondylosis/intervertebral disc disorders/other back problems; with FM as a secondary diagnosis, the most common primary diagnoses were: essential hypertension, disorders of lipid metabolism, coronary atherosclerosis/other heart diseases, and mental disorders [45].

Wolfe et al. [46], in the USA, reported a significant association of FM with: hypertension, other cardiovascular conditions, depression, diabetes, lung diseases, asthma, liver diseases, neurological diseases, thyroid diseases, gastrointestinal disorders, mental illnesses, renal diseases, severe allergies, genitourinary disorders. FM patients have stronger comorbidity with these disorders than patients with rheumatoid arthritis.

Weir et al. [43], in a large health insurance database, in the USA, described that patients with FM were two to seven times more likely to have one or more of the following comorbid conditions: depression, anxiety, headache, irritable bowel syndrome, chronic fatigue syndrome, systemic lupus erythematosus, and rheumatoid arthritis. In Germany, in a statutory health insurance company, 51.9 % of patients with FM were diagnosed with a comorbid depression as well [41].

There is an overlap in the symptomalogy and also disease comorbidity among some "functional" conditions, including FM, chronic headache, chronic fatigue syndrome, low back pain, irritable bowel syndrome, temporomandibular joint disorders, major depression, anxiety, panic attack, post-traumatic stress disorder [47].

The prevalence of FM in patients with some types of headache is shown in Table 2. FM is highly prevalent both in migraineurs, with episodic and chronic forms, as well as in patients with tension type headache (TTH). Schur et al. [47] have shown that twins with chronic TTH have 6.6 times more FM than those without it, and patients with FM have 5.0 times more chronic TTH—a bidirectional association. Aaron et al. [56] found five patients (22.7 %) with chronic TTH out of 22 with FM. Marcus et al. [57], reported that 76 out of 100 patients with FM had headaches; 32 had migraines, 18 TTH, 16 combined migraine and TTH, 4 post-traumatic headache, and 6 probable analgesic overuse headache. 84 % of the patients with FM + headache described important or severe impact from their head pain. Ravindran et al. [58] stated that there is a strong association between FM and chronic fatigue syndrome + migraine without aura (47.4 %).

Table 2 Prevalence of fibromyalgia in patients with some types of headache
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Conclusion

The global prevalence of FM, in 26 studies worldwide, is 2.7 %. FM is more prevalent in women, in patients over 50 years of age, in subjects with low education level, with low socioeconomic status, living in rural areas, and possibly in obese women.

FM is comorbid with many diseases, usually called "functional" disorders, such as chronic fatigue syndrome, irritable bowel syndrome, depression, anxiety, panic attacks, and post-traumatic stress disorder.

The association of FM with headache is significant, including episodic and chronic migraine and chronic TTH.

It is desirable and important to have more nationwide epidemiological studies on FM, especially outside of Europe, to have a better view of the prevalence of this disorder worldwide, and to measure the burden of FM on persons, families and society.