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Objective
To
provide a single source for the best available estimates of the US
prevalence of and number of individuals affected by arthritis overall,
rheumatoid arthritis, juvenile arthritis, the spondylarthritides,
systemic lupus erythematosus, systemic sclerosis, and Sjögren's
syndrome. A companion article (part II) addresses additional conditions.
Methods
The
National Arthritis Data Workgroup reviewed published analyses from
available national surveys, such as the National Health and Nutrition
Examination Survey and the National Health Interview Survey (NHIS). For
analysis of overall arthritis, we used the NHIS. Because data based on
national population samples are unavailable for most specific rheumatic
conditions, we derived estimates from published studies of smaller,
defined populations. For specific conditions, the best available
prevalence estimates were applied to the corresponding 2005 US
population estimates from the Census Bureau, to estimate the number
affected with each condition.
Results
More
than 21% of US adults (46.4 million persons) were found to have
self-reported doctor-diagnosed arthritis. We estimated that rheumatoid
arthritis affects 1.3 million adults (down from the estimate of 2.1
million for 1995), juvenile arthritis affects 294,000 children,
spondylarthritides affect from 0.6 million to 2.4 million adults,
systemic lupus erythematosus affects from 161,000 to 322,000 adults,
systemic sclerosis affects 49,000 adults, and primary Sjögren's syndrome
affects from 0.4 million to 3.1 million adults.
Conclusion
Arthritis
and other rheumatic conditions continue to be a large and growing
public health problem. Estimates for many specific rheumatic conditions
rely on a few, small studies of uncertain generalizability to the US
population. This report provides the best available prevalence estimates
for the US, but for most specific conditions, more studies
generalizable to the US or addressing understudied populations are
needed.
In adults, arthritis is the leading cause of disability (1) and is among the leading conditions causing work limitations (2).
Over the next 25 years the number of people affected and the social
impact of doctor-diagnosed arthritis are projected to increase by 40% in
the US (3).
Estimating the burden in the US population of the various rheumatic
conditions that comprise arthritis is important for understanding their
current and potential future impact on the health care and public health
systems. Equally important is identifying the gaps in our understanding
of burden.
This and a companion article (4)
update the National Arthritis Data Workgroup (NADW) reports of
arthritis prevalence, our measure of burden, from 1989 and 1998 (5, 6).
Sjögren's syndrome and carpal tunnel syndrome have been included for
the first time, and additionally, the common symptoms of neck and back
pain are addressed.
METHODS
The
term “prevalence” has been defined and used in conflicting ways. In
these 2 articles, we use prevalence to mean “prevalence proportion”
(incorrectly called “prevalence rate” at times), meaning the proportion
of persons in the population with the condition. We use the phrase
“number affected” to refer to the absolute number of people affected in
the population.
US estimates of disease
prevalence were usually based on data from published national or local
population-based studies from the US and, if no accurate US data were
available, from international studies. For overall arthritis, the number
affected was based on the population sampled in the 2003–2005 National
Health Interview Survey (NHIS). For other conditions, the best available
prevalence estimates were applied to the corresponding July 1, 2005
population estimates from the Census Bureau (http://www.census.gov/popest/national/asrh/NC-EST2005-sa.html)
to estimate the number affected. Some of the US population-based
studies were special studies in small areas that may not reflect the
racial and ethnic profile of the US or of those affected by the illness.
Caveats accompany the estimates presented, when there are concerns
about generalizability.
Several estimates
came from 2 National Center for Health Statistics surveys: the NHIS and
the National Health and Nutrition Examination Survey (NHANES). Both use
probability samples of the US civilian, noninstitutionalized population
to generate national health estimates. The NHANES uses interviews and
examinations (e.g., physical examinations, laboratory tests, and
radiographs) from ∼5,000 respondents annually. The much larger NHIS uses
an annual cross-sectional, in-person interview survey of ∼106,000
respondents in 43,000 households to collect self-reported health status
information. Estimates for overall arthritis obtained using the NHIS
were age adjusted to the projected 2000 population age ≥18 years by 3
age groups (18–44 years, 45–64 years, and ≥65 years) to allow better
comparison of demographic groups (available at http://www.cdc.gov/nchs/data/statnt/statnt20.pdf [used .530458, .299194, and .170271 from distribution 9, for ages 18–44, 45–64, and ≥65, respectively]).
Measuring
the prevalence of arthritis poses many challenges. From study to study,
the distinction between point prevalence and cumulative (i.e.,
lifetime) prevalence is not always clear. Prevalence is difficult to
determine for conditions that are episodic. Some conditions have no
standard case definition, whereas others have competing or evolving case
definitions based on different symptoms, signs, radiographic findings,
or laboratory data. Estimates vary depending on the inclusion or
exclusion of asymptomatic, mild, or early disease and the aggressiveness
of case finding. Symptomatic individuals in the community who do not
seek treatment may go uncounted. Furthermore, individuals frequently do
not know what specific rheumatic disease they have, so self-reported
data cannot be used for estimates of specific conditions.
RESULTS
Overall arthritis.
The case definition used to identify persons with arthritis has changed since our last report (6).
In 1997 the NHIS stopped using condition lists and International
Classification of Diseases, Ninth Revision, Clinical Modification
(ICD-9-CM) codes, the basis of our previous method, and instead adopted
new surveillance questions. Based on cognitive and validation studies (7, 8),
“self-reported doctor-diagnosed arthritis” is thought to provide the
most credible estimate of overall arthritis prevalence, with acceptable
sensitivity and specificity for surveillance purposes. Respondents were
defined as having doctor-diagnosed arthritis if they answered “yes” to
the question, “Have you EVER been told by a doctor or other health
professional that you have some form of arthritis, rheumatoid arthritis,
gout, lupus, or fibromyalgia?” Among those with doctor-diagnosed
arthritis, activity limitation attributable to arthritis was defined by a
“yes” answer to the question, “Are you now limited in any way in any of
your usual activities because of arthritis or joint symptoms?”
The
prevalence of self-reported doctor-diagnosed arthritis among adults age
≥18 years, estimated using the annual average from the 2003–2005 NHIS
surveys, was 21.6%, or 46.4 million (9) (Table 1).
Although arthritis prevalence was higher in older age groups, with half
of adults age ≥65 years being affected, nearly two-thirds of the adults
reporting doctor-diagnosed arthritis were younger than 65 (Table 1).
More than 60% were women. Age-adjusted arthritis prevalence was higher
for women than for men (24% versus 18%) but was similar for non-Hispanic
whites and African Americans (∼22%), whose rates were higher than those
for Hispanics (16.5%). The number of persons with doctor-diagnosed
arthritis is projected to increase to nearly 67 million by 2030 (3)—an increase of ∼40%.
Using the same report as was used to determine prevalence (9),
we found that an estimated 8.8% of all US adults, or nearly 19 million
persons, had arthritis-attributable activity limitations (Table 1).
The prevalence of activity limitations was higher in older age groups
(affecting >22% of all adults age ≥65 years), higher among women, and
lower among Hispanics. Arthritis or joint symptoms led to activity
limitation in >40% of adults with doctor-diagnosed arthritis. This
outcome is projected to increase to 25 million (9.3% of the adult
population) by 2030 (3).
The
high population prevalence of arthritis and of arthritis-related
activity limitations translates into an immense personal and societal
burden, often differing by race/ethnicity (10). This situation results in “arthritis and rheumatism” being the leading cause of physical disability in the US (1) and causes affected persons to have a substantially worse health-related quality of life (11).
Among various other impact/burden measures, arthritis and other
rheumatic conditions in 1997 were the underlying cause of death in 9,367
persons in the US (12), were present in 300,000 nursing home residents (19%) (13), and resulted in 744,000 hospitalizations (14) and 36.5 million ambulatory care visits (15). Costs of arthritis and other rheumatic conditions in 2003 were $128 billion (16).
Rheumatoid arthritis (RA).
RA
is a multisystem disorder of unknown etiology, characterized by chronic
destructive synovitis. Our previous national prevalence estimates for
RA (6)
were derived from the NHANES I, which used a case definition based on
the clinical diagnosis by the examining physician. Since that time,
classification criteria for RA have been revised (17–19).
Several
studies have provided estimates of the prevalence of RA in defined
populations. Although these studies had a number of methodologic
limitations (20),
the remarkable finding was the uniformity of prevalence estimates in
populations from different developed countries: ∼0.5%–1% of the adult
population. However, studies from the Pima Indian population showed
significantly higher incidence and prevalence estimates (21).
A
study from Rochester, Minnesota showed a prevalence of RA in 1985 of
1.07% (95% confidence interval [95% CI] 0.94–1.20) among adults ≥35
years of age (22);
this fell to 0.85% in 1995 (95% CI 0.75–0.95) (Gabriel S, et al:
unpublished data). The prevalence among women in 1995 was approximately
double that in men (1.06% versus 0.61%) (Gabriel S, et al: unpublished
data).
Trends in RA prevalence in
Rochester, Minnesota by age and calendar year show increasing prevalence
with older age and decreasing prevalence for most age groups in more
recent time periods (Figure 1). These trends, by calendar year, age, and sex, have also been demonstrated in numerous other populations (21–26).
In particular, the temporal decline in RA prevalence is consistent with
studies showing a progressive decline in RA incidence since the early
1960s (21, 27–30).
Also, the average age of persons with prevalent RA has increased
steadily over time, from 63.3 years in 1965 to 66.8 years in 1995,
suggesting that RA is becoming a disease of older adults. This
observation, along with the expected rapid growth in the proportion of
Americans age >60 years, suggests that RA-associated morbidity,
mortality, and disability are likely to increase among older adults.
Figure 1. Prevalence of rheumatoid arthritis (adjusted to the 2000 white US population) among female residents (A) and male residents (B) of Rochester, Minnesota at 4 time points (1965, 1975, 1985, and 1995 [January 1 of each year]), by age group.
Using
the 1995 Rochester, Minnesota age/sex-specific prevalence and the
corresponding 2005 population estimates from the Census Bureau, we
estimated that 1,293,000 American adults age ≥18 years (0.6%) have RA.
This is lower than the previous estimate of 2,100,000 (6)
because of the decline in RA prevalence. These Rochester estimates are
likely to be generalizable to the white US population, but their
generalizability to other racial/ethnic populations is uncertain.
Juvenile arthritis.
The
prevalence of chronic, inflammatory arthritis in children is difficult
to estimate because of differences in nomenclature (e.g., “juvenile
rheumatoid arthritis” [JRA], “juvenile chronic arthritis” [JCA], and
most recently “juvenile idiopathic arthritis” [JIA]) and classification
criteria (1977 American College of Rheumatology [ACR; formerly, the
American Rheumatism Association] [31], 1978 European League Against Rheumatism [32], and 1997 International League of Associations for Rheumatology [33] with a revision published in 2004 [34]), and the heterogeneity of the diseases and their subtypes encompassed under this rubric (35).
In addition, variability in disease course among the subtypes of JIA
may make it difficult to compare prevalence estimates for this condition
across different study settings. In some types of the disease extended
remissions occur, so that prevalence estimates include individuals who
were ever affected, but are not currently affected.
Prevalence
reported in a comprehensive review ranged from 7 to 401 per 100,000
children across a broad diversity of geographic regions (35).
Data from Rochester, Minnesota suggested declining prevalence, from
9.43 per 100,000 children in 1980 to 8.61 per 100,000 children in 1990 (36).
These prevalences were lower than previous estimates from the same
population, owing, in part, to differences in assignment of case
definition.
The combined incidence of JRA
and juvenile spondylarthritis (“spondylarthritis” being a more
contemporary term for what is synonymously referred to in many earlier
publications as “spondylarthropathy” [see below]) from other recent US
and Canadian studies consistently ranges from 4.1 to 6.1 per 100,000,
with the incidence of juvenile spondylarthritis ranging from 1.1 to 2
per 100,000 (37–39).
These studies have encompassed a number of diverse regions including
New England; Manitoba, Canada; and 13 other centers across Canada. The
prevalence of JCA from 2 Canadian studies was 3.2 and 4.0 per 100,000
children (40).
The
prevalence of JRA in the US in different published reports ranged from
1.6 to 86.1 per 100,000. Data from the NHIS suggested a prevalence of
150 per 100,000 for all types of childhood arthritis, including JRA,
juvenile spondylarthritis, Lyme disease, arthritis associated with the
less common pediatric connective tissue diseases, and other types of
childhood arthritis. The prevalence of JCA (the name for JRA outside the
US) found in a population-based study in Australia, in which
respondents were surveyed door to door (41), was far higher (400 per 100,000) than has been found in other studies.
In
summary, there are very wide variations in the reported prevalences of
chronic inflammatory arthritides of childhood, such as JRA and juvenile
spondylarthritis. The lack of comparable prevalence estimates across
different regions in the US makes it difficult to estimate the total
number affected. Perhaps the best prevalence estimates come via a novel
approach using data from pediatric ambulatory care visits recorded in
the 2001–2004 National Ambulatory and Medical Care Survey and the NADW
ICD-9-CM case definition for adults (6)
modified to reflect pediatric conditions, by which it was estimated
that 294,000 children ages 0–17 years (95% CI 188,000–400,000) were
affected by the broadly defined “arthritis or other rheumatic
conditions” (42).
Spondylarthritides.
The
spondylarthritides (more contemporary term for what is synonymously
referred to in many earlier publications as “spondylarthropathies”) are a
family of diseases that includes ankylosing spondylitis (AS), reactive
arthritis (formerly known as Reiter's syndrome), psoriatic arthritis,
enteropathic arthritis (associated with ulcerative colitis or Crohn's
disease), juvenile spondylarthritis, and undifferentiated
spondylarthritis, which encompasses disorders expressing elements of but
failing to fulfill criteria for the above diseases. The prevalence of
AS and other spondylarthritides parallels the frequency of the genotype
HLA–B27.
Ankylosing spondylitis.
Among
studies of white Europeans and East Asians, the reported prevalence of
AS has varied between 30 per 100,000 and 900 per 100,000 (reflecting
differences in HLA–B27 frequency and in patient referral and disease
ascertainment) (43–45). In the US, a 1979 study from Rochester, Minnesota showed a prevalence of 129 per 100,000 in a Caucasian population (46). Prevalence data suggest that AS occurs less frequently in African Americans than in whites (47).
The
overall prevalence of severe or moderate radiographic sacroiliitis on
pelvic radiographs in men ages 25–74 years in the NHANES I was 730 per
100,000; among women ages 50–74 years, the prevalence was 300 per
100,000 (48) (Table 2).
Of those with moderate to severe radiographic sacroiliitis, only 7.6%
were currently experiencing “significant pain in their lower backs on
most days for at least one month.” Since questions regarding
inflammatory back pain (49) were not asked in this survey, the prevalence of AS cannot be ascertained.
Reactive arthritis.
The prevalence of reactive arthritis appears to be decreasing in developed countries (50). One study in Rochester, Minnesota investigated incidence (51),
but prevalence in the general US population is unknown. Studies of
American Indian groups have shown frequencies of 300 per 100,000 among
Navajos (52) and 200–1,000 per 100,000 among Alaskan Yupik and Inupiat Eskimos (53),
2 groups with a high frequency of HLA–B27. Because many persons with
reactive arthritis have remissions, prevalence estimates include
individuals who were ever affected but are not currently affected.
Psoriatic arthritis.
Enteropathic arthritis.
The prevalence of inflammatory bowel disease (IBD) in the US has been estimated to be 500 per 100,000 (55).
However, the prevalence of enteropathic arthritis/spondylitis has not
been determined. The self-limited and nondestructive nature of
peripheral enteropathic arthritis complicates calculations of its
prevalence (56), although it has been reported to occur in up to 13% of patients with IBD (57–59). Although inflammatory back pain occurs in up to 50% of patients with IBD (58, 59), AS occurs in <10% (57).
Applying these percentages (13% for peripheral arthritis and 10–50% for
spinal arthritis) to the prevalence of 0.5% for IBD, the estimated US
prevalence of enteropathic peripheral arthritis is 65 per 100,000 and
that of enteropathic spinal arthritis ranges from 50 to 250 per 100,000
(Table 2).
Undifferentiated spondylarthritis.
Overall spondylarthritis.
The
prevalence of spondylarthritis in the US is unknown. In studies of
European whites, the reported prevalence has varied widely, from 470 per
100,000 (60) to 1,900 per 100,000 (44). Higher prevalences in Eskimos from Siberia and Alaska have been reported (53).
The prevalence of overall spondylarthritis in the US can be roughly
estimated by summing either low or high prevalence estimates of its
component subtypes, resulting in a range of 346–1,310 per 100,000 among
those age ≥25 years (Table 2).
Using this range of prevalence and the corresponding 2005 population
estimates from the Census Bureau, we estimated that between 639,000 and
2,417,000 adults age ≥25 years have spondylarthritis.
Systemic lupus erythematosus (SLE).
SLE
is a multisystem autoimmune disorder of unknown etiology, with disease
manifestations that vary over time. The 1982 ACR criteria for the
classification of SLE (62),
which are the most widely used, rely on signs and symptoms present at
any time during a person's illness. Patients with early or atypical
disease often have not accumulated enough manifestations to meet
criteria, and may not be counted.
Studies
of SLE prevalence have been performed in different regions of the
country and have used varying methods of case identification, including
screening of inpatient and outpatient records (63, 64) and inferring prevalence on the basis of cases identified using multiple outpatient and hospital sources (65).
In studies from a San Francisco, California health maintenance
organization (HMO) and from Rochester, Minnesota, both involving
predominantly white populations, SLE prevalence was estimated to be 44
per 100,000 whites (63) and 40 per 100,000 (mostly whites) (64, 66), respectively. In a study from Nogales, Arizona, prevalence in Hispanic women was estimated to be 103 per 100,000 (67).
A study from Hawaii showed a prevalence of 50 per 100,000 among whites
and persons of Japanese descent, versus 100 per 100,000 among persons of
Chinese descent (68).
In all of these studies, prevalence estimates of SLE among nonwhites
were based on a limited number of cases, resulting in wide confidence
intervals and limiting the precision of results.
The
estimated prevalence of SLE from the NHANES III was 53.6 per 100,000
among adults age ≥18 years and 100 per 100,000 among adult women, based
on self-reported physician diagnosis and current prescription of
medications used for SLE treatment (69).
Among both whites and blacks, the prevalence of SLE is higher in women than in men. Using data from the San Francisco study (63),
the prevalences in whites and African Americans among those ages 15–64
years were as follows: 100 per 100,000 white women, 400 per 100,000
black women, 10 per 100,000 white men, and 50 per 100,000 black men.
Findings of one study suggest that the prevalence of suspected SLE is similar to that of definite SLE (66).
For estimating SLE prevalence, we used a range that included the number
of persons with definite SLE at the low end and double that number at
the high end, to include patients with suspected disease who do not meet
strict ACR criteria for disease. Our reason was that the latter
patients, like those who do meet the classification criteria, consume
health resources and must cope with their illness, and many of them meet
criteria later in their disease course (63, 64, 67–69).
Using the San Francisco sex/race prevalence among persons ages 15–64
and the corresponding 2005 population estimates from the Census Bureau,
we estimated that as few as 161,000 and as many as 322,000 persons in
the US have SLE (161,000 definite SLE [11,000 white men, 80,000 white
women, 7,000 African American men, 56,000 African American women, and
7,000 people of other races]; 322,000 definite or suspected SLE),
although the generalizability of the San Francisco HMO data to the US
population has not been determined.
Systemic sclerosis (SSc; scleroderma).
There
are 2 forms of SSc: a systemic form, which can have limited or diffuse
skin involvement, and a localized form, which is confined to the skin
and surrounding tissue. This report addresses only the systemic form.
In
a population-based study of SSc in southeast Michigan, prevalence was
ascertained from multiple sources, including hospital discharge data,
outpatient data from 2 academic centers, private-practice
rheumatologists, and the local chapter of a scleroderma support group.
Cases were defined as persons age ≥18 years who met the 1980 ACR
preliminary criteria for the classification of SSc (70).
Persons were also considered to be cases if they had a physician
diagnosis and at least 2 of the 5 features of CREST syndrome
(calcinosis, Raynaud's phenomenon, esophageal dysmotility,
sclerodactyly, telangiectasias) (71).
Seven hundred six SSc cases were identified and extrapolated to the US
population, yielding a prevalence of 24.2 per 100,000 adults (95% CI
21.3–27.4) (72).
Using capture–recapture methods, an estimated number of missing cases
was added, yielding a revised prevalence estimate of 27.6 cases per
100,000 US adults (95% CI 24.5–31.0). Women were affected 4.6 times more
frequently than men. SSc prevalence had a modestly higher prevalence
among African Americans than whites, with an age-adjusted prevalence
ratio of 1.15 (95% CI 1.02–1.30). In addition, African Americans were
significantly younger than whites at the time of diagnosis (mean ± SD
41.0 ± 14.6 years versus 48.1 ± 15.9 years; P < 0.001).
The
highest reported prevalence of SSc has been in a Choctaw Indian group
in Oklahoma (66 cases per 100,000, based on 14 cases) (73). There may be genetic factors that contribute to increased disease susceptibility in this group (74–76).
A
20-year study of hospital-diagnosed scleroderma cases in Allegheny
County, Pennsylvania from 1963 through 1982 suggested that disease
incidence doubled over this period (77). However, recent data do not suggest any continued increase in incidence or prevalence (78).
Using
the southeast Michigan sex/race prevalence and the corresponding 2005
population estimates from the Census Bureau, we estimated that 49,000
Americans age 18 and older have SSc, although the generalizability of
the Michigan data to the US population has not been determined.
Primary Sjögren's syndrome (SS).
SS
may occur alone (primary SS) or with other autoimmune diseases,
including RA or SLE (secondary SS). Prevalence estimates reported herein
are confined to primary SS because there are insufficient data to
evaluate the prevalence of secondary SS.
Primary SS prevalence estimates have ranged from 0.05% to 4.8% across international communities (79–86), but only 3 of these studies (79–81)
were population based. More recently reported prevalence rates have
generally tended to be lower than those in earlier publications, which
could reflect increasing rigor of epidemiologic studies, more
restrictive and objective classification criteria, small sample sizes in
earlier studies, and selection biases. For example, in 1988, a
prevalence of 4.8% (95% CI 3.1–6.5%) was found in an elderly and
institutionalized population (84), and in 1989 a prevalence of 2.7% (95% CI 1.0–4.3%) was found in Swedish adults (85). Subsequent studies provided lower SS prevalence estimates in Greek women (0.6% [95% CI 0.19–1.39%]) (81), in residents of Olmsted County, Minnesota (0.32% cumulative incidence [which approximates prevalence]) (80), and in China (0.77% [95% CI 0.62–0.92%]) (79). The prevalence of SS among women from 2 primary care practices in the UK ranged from <0.1% to 0.4% (86).
Because no prevalence studies have been performed in the US, we used incidence data from Olmsted County, Minnesota (80) and prevalence data from international studies (79, 81)
to infer SS prevalence in the US. The population-based study in Olmsted
County was based on existing records, reflected physician-diagnosed
cases, and included few confirmatory labial salivary gland biopsies. The
Chinese study (79)
examined a substantial population in clinics that served and were
likely to be representative of the general Chinese population. The Greek
study included only women (81).
The cumulative incidence data from Olmsted County and the data from
China and Greece suggest a similar prevalence estimate for SS of ∼0.6%
(600 per 100,000), which may be as low as 0.19% or as high as 1.39%
according to the highest and lowest confidence intervals from the
international studies.
Using the Olmsted
County prevalence estimates and the corresponding 2005 population
estimates from the Census Bureau, we estimated that 1.3 million American
adults (range 0.4–3.1 million) have primary SS. The Olmsted County
estimates are generalizable to the white US population, but their
generalizability to other racial/ethnic populations is uncertain, as is
the generalizability of the data from the international studies.
DISCUSSION
The
burden of a chronic condition can be measured in various ways. The NADW
has chosen to focus on national disease prevalence as an important
measure for this report and previous publications (5, 6),
because prevalence includes people with existing disease as well as
those with new disease. Incidence (new cases), a competing measure, can
provide a picture of how a disease is newly affecting a population, but
is very difficult to measure because it requires a record of the date of
disease onset, which is difficult to come by for most of the conditions
of interest, especially in the published literature.
The prevalence of overall arthritis in the US has continued to grow since our last estimate (6),
which is not surprising given that many of these conditions are age
related and the overall population is aging. This increase suggests that
overall arthritis will have a growing impact on the health care and
public health systems in the future, one that needs to be anticipated in
order to provide the early diagnosis and interventions that could help
reduce that impact. Of interest is the decline in the prevalence of RA,
which is consistent with findings of other studies but has no clear
explanation.
We have provided estimates of
prevalence and numbers of persons affected for overall arthritis and
for selected rheumatic conditions in this and the companion article (4)
and given a rough snapshot of current burden. These estimates have been
made by recognized disease experts using the best data available, but,
as noted in many of the sections, must be interpreted with several
limitations in mind.
First, there may be
competing case definitions for each disease, which may vary by symptoms,
signs, laboratory results, and radiographic and other factors used in
various classification criteria. Second, there may be difficulty in
deciding the appropriate measurement interval for a disease (e.g.,
gout), resulting in compromises such as 1-year prevalence and lifetime
prevalence estimates. Third, many of these conditions (e.g., SLE) are
difficult to diagnose even by experienced clinicians, especially early
in the course of disease, which may necessitate the inclusion of
patients with suspected disease as well as those classified as having
definite disease, and may result in missed cases. Fourth, some of these
conditions may not be chronic in the traditional sense but rather may
have extended remissions (e.g., pauciarticular juvenile arthritis,
“burnt out” RA) or be episodic by nature (e.g., gout) or because of good
treatment (e.g., RA). These potential difficulties must be kept in mind
when interpreting prevalence estimates. Fifth, for many conditions the
studies of prevalence have been infrequent, leading to wide variation in
competing estimates (e.g., 8-fold range in estimates for primary SS and
4-fold range in estimates for overall spondylarthritides), or may be
small, leading to wide confidence intervals around the estimate, or may
lack specific data (e.g., information on age, sex, and race) to allow
extrapolation to larger populations. Sixth, some of the estimates are
based on data that are old (e.g., estimates of overall osteoarthritis
rely on data from the 1971–1975 NHANES; estimates of SLE rely on San
Francisco data from the 1970s), meaning that any changes occurring since
those studies were conducted are not taken into account. Seventh, most
of the specific conditions have not been studied from a national
perspective, necessitating assumptions about the generalizability of
available but more localized data in generating national estimates.
Finally,
many of the estimates are based on data from a single study site in
Rochester (Olmsted County), Minnesota and raise the question of just how
representative that site is. The Rochester Epidemiology Project (REP)
represents one of the best data sources for estimating the US prevalence
of any disease (not just arthritis), and because of its expense is
unlikely to be replicated elsewhere. The unique capabilities of the REP
allow enumeration of the entire Olmsted County population over many
years of followup (typically decades) and provide an accurate account of
in- and out-migration as well as deaths. There is no other community in
the US where this is feasible. No single community can be fully
representative of the US population, but this issue has been examined
well in the long-running REP, and the limitations described. Comparisons
of these studies with US Census data and other published literature
have shown that, with the exception of a higher proportion of the
working population being employed in the health care industry, the
demographic characteristics of the Rochester population are similar to
those of the majority of the US (i.e., whites) (87).
Therefore, while the REP is often the only source of relevant data,
these data have uncertain generalizability to nonwhite populations.
We
have presented the best available prevalence estimates and tried to
identify many of the gaps and limitations in their interpretation. Given
the large and growing burden of arthritis and other rheumatic
conditions, we hope this work will inspire studies that better address
these gaps and limitations and provide a better understanding of the
burden of these conditions.
source: http://onlinelibrary.wiley.com/doi/10.1002/art.23177/full
source: http://onlinelibrary.wiley.com/doi/10.1002/art.23177/full
