Epidemiology of Fibromyalgia
Chronic Widespread Pain Population-based studies of chronic widespread
pain in most industrialized countries suggest that 10% to 11% of the population
has this symptom at any given point in time.[4,5] Chronic regional pain is found
in 20% to 25% of the population. Both chronic widespread and regional pain occur
about 1.5 times more often in women than in men.
Fibromyalgia As noted previously, the 1990 ACR criteria for fibromyalgia
require that an individual have both a history of chronic widespread pain and
the finding of at least 11 of 18 possible tender points on examination. Tender
points represent 9 paired regions of the body, such as the lateral epicondyle
and mid-trapezius regions, where all individuals are more sensitive to pain than
in surrounding tissues. If an individual reports pain when a region is palpated
with 4 kg of pressure, this is considered a positive response. Using the ACR
criteria, the population prevalence of fibromyalgia in industrialized countries
has been reported to range from 0.5% to 4%.[6,7] The prevalence of fibromyalgia
is just as high in rural or nonindustrialized societies as it is in countries
such as the United States.
Significance of tender points. At the time the ACR criteria were published, it
was thought that there may be some unique significance to the locations of
tender points. In fact, the term "control points" was coined to describe areas
of the body that should not be tender in fibromyalgia. Since then, we have
learned that the tenderness in fibromyalgia extends throughout the entire body.
This finding means that practitioners can apply pressure wherever they wish to
assess tenderness in clinical practice. As long as they perform this exam with
the same pressure in a series of patients, they can get a good sense of the
overall pain threshold of an individual patient.
The tender point requirement in the ACR criteria not only misrepresented the
nature of the tenderness in this condition (ie, local rather than widespread),
but it also strongly influenced the demographic and psychological
characteristics of fibromyalgia. Women are only 1.5 times more likely than men
to experience chronic widespread pain, but are 11 times more likely than men to
have 11 or more tender points.[4] Because of this difference, women are
approximately 10 times as likely to meet the ACR criteria for fibromyalgia than
men. However, it is likely that most of the men in the population that have
chronic widespread pain but are not tender enough to meet criteria for
fibromyalgia have the same underlying problem as the women who meet the ACR
criteria for fibromyalgia.
Another unintended consequence of diagnosing fibromyalgia on the basis of both
chronic widespread pain and at least 11 tender points is that many individuals
with fibromyalgia will have high levels of distress. Wolfe[8] has described
tender points as a "sedimentation rate for distress" because of population-based
studies showing that tender points are more common in distressed individuals.
Distress is usefully considered a combination of somatic symptoms and symptoms
of anxiety and/or depression. Until recently, many assumed that because tender
points were associated with distress, tenderness (an individual's sensitivity to
mechanical pressure) was associated with distress. However, recent evidence
suggests that this latter association is probably due to the standard tender
point technique, which consists of applying steadily increasing pressure until
reaching 4 kg. In this situation, individuals who are anxious or "expectant"
have a tendency to "bail out" and report
tenderness. More recent, sophisticated measures of tenderness, which give
stimuli in a random, unpredictable fashion, have demonstrated that tenderness is
independent of psychological status.[9,10]
Defining the fibromyalgia syndrome. Population-based studies suggest that the
primary symptom of fibromyalgia, chronic widespread pain, is only modestly
associated with distress, and distress is only weakly associated with the
subsequent development of chronic widespread pain.[11,12] There are far more
psychologically "normal" individuals who develop chronic widespread pain than
distressed or depressed people that do, and most individuals with chronic
widespread pain do not have or subsequently develop distress or depression.
In summary, although many clinicians uniquely associate fibromyalgia with
women who display high levels of distress, much of this is an artifact of 2
contributing factors:

the ACR criteria that require 11 tender points, and


the fact that most studies of fibromyalgia have originated from clinical
samples from tertiary care centers, where psychological and psychiatric
comorbidities are much higher.[13]
When these biases are eliminated by examining chronic widespread pain in
population-based studies, a clearer picture of fibromyalgia can be gleaned, and
chronic widespread pain becomes much like chronic musculoskeletal pain in any
region of the body.
Etiology of Fibromyalgia Genetic Factors Research has indicated a strong
familial component to the development of fibromyalgia. First-degree relatives of
individuals with fibromyalgia display an 8-fold greater risk of developing
fibromyalgia than those in the general population.[14] The same high family
history of these related conditions has been noted in most conditions that fall
within this spectrum. This coaggregation of conditions, which includes
fibromyalgia, chronic fatigue syndrome, migraine headache, and mood disorders,
was originally collectively termed "affective spectrum disorder,"[15] and more
recently has been referred to as "central sensitivity syndromes" and "chronic
multisymptom illnesses."
Recent studies have begun to identify specific genetic polymorphisms that are
associated with a higher risk of developing fibromyalgia. To date, the serotonin
5-HT2A receptor polymorphism T/T phenotype, serotonin transporter, dopamine 4
receptor, and COMT (catecholamine o-methyl transferase) polymorphisms have all
been noted to be seen in higher frequency in fibromyalgia.[16-18] All of these
polymorphisms involve the metabolism or transport of monoamines, compounds that
play a critical role in the activity of the human stress response.
Environmental Factors As with most illnesses that may have a genetic
underpinning, environmental factors may play a prominent role in triggering the
development of fibromyalgia and related conditions. Environmental "stressors"
temporally associated with the development of either fibromyalgia or chronic
fatigue syndrome include physical trauma (especially involving the trunk);
certain infections such as hepatitis C, Epstein-Barr virus, parvovirus, or Lyme
disease; and emotional stress (Table 1).
Table 1. "Stressors" Capable of Triggering Fibromyalgia and Related Conditions
Peripheral pain syndromes Infections (eg, parvovirus, Epstein-Barr virus,
Lyme disease, Q fever, uncommon upper respiratory infections) Physical trauma
(automobile accidents) Psychological stress/distress Hormonal alterations
(eg, hypothyroidism) Drugs Vaccines Certain catastrophic events (war,
but not natural disasters)
The disorder is also associated with other regional pain conditions or
autoimmune disorders (Figures 1 and 2).[19,20]
Figure 1. Regional or localized syndromes that overlap with fibromyalgia in
prevalence, mechanisms, and treatment.



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