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Risk factors for functional limitations in patients with long-standing ankylosing spondylitis.

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Arthritis & Rheumatism (Arthritis Care & Research)
Vol. 53, No. 5, October 15, 2005, pp 710 –717
DOI 10.1002/art.21444
© 2005, American College of Rheumatology
ORIGINAL ARTICLE
Risk Factors For Functional Limitations in Patients
With Long-Standing Ankylosing Spondylitis
MICHAEL M. WARD,1 MICHAEL H. WEISMAN,2 JOHN C. DAVIS, JR,3
AND
JOHN D. REVEILLE4
Objective. To identify risk factors for functional limitations in patients with ankylosing spondylitis (AS) of at least 20
years’ duration.
Methods. Patients with AS for >20 years were enrolled in the cross-sectional component of the Prospective Study of
Outcomes in AS. All patients had clinical evaluations and completed questionnaires on functional limitations and
potential risk factors. Functional limitations were assessed using the Bath Ankylosing Spondylitis Functional Index
(BASFI; score range 0 –100, higher scores indicate more limitations) and the Health Assessment Questionnaire for the
Spondylarthropathies (HAQS). Risk factors included demographic characteristics, duration of AS, smoking status,
number of comorbid medical conditions, recalled level of recreational activity in teens and twenties, occupational
physical activity throughout life (rated 1 ⴝ little, 2 ⴝ moderate, 3 ⴝ heavy, and weighted by the number of years in each
job), and history of AS in a first-degree relative.
Results. The 326 patients (74% men) had a mean ⴞ SD age of 55.0 ⴞ 10.7 years, a mean duration of AS symptoms of
31.7 ⴞ 10.2 years, and a mean BASFI score of 40.7 ⴞ 25.6. BASFI scores increased with higher lifetime occupational
physical activity (r ⴝ 0.31; P < 0.0001), the number of comorbid conditions (r ⴝ 0.25; P < 0.0001), and the duration of
AS (r ⴝ 0.12; P ⴝ 0.04). BASFI scores were higher among current smokers compared with former/nonsmokers (55.5
versus 38.9; P ⴝ 0.0002), and among nonwhites compared with whites (49.9 versus 39.3; P ⴝ 0.02). In multivariable
analyses, lifetime occupational physical activity, current smoking, education level, number of comorbid conditions, and
family history were significantly associated with BASFI scores. The same risk factors were associated with the HAQS.
Conclusion. Functional limitations in patients with AS for >20 years are greater among those with a history of more
physically demanding jobs, more comorbid conditions, and among smokers, and are less severe among those with higher
levels of education and a family history of AS.
KEY WORDS. Ankylosing spondylitis; Disability; Functional limitations; Work.
INTRODUCTION
Functional limitations and resultant disability are major
consequences of ankylosing spondylitis (AS). The degree
Supported in part by US Public Health Service grants
AR-48465, AR-46208, M01-RR000079, M01-RR02558, and
M01-RR00425, and by the Intramural Research Program,
National Institute of Arthritis and Musculoskeletal and Skin
Diseases.
1
Michael M. Ward, MD, MPH: National Institute of Arthritis and Musculoskeletal and Skin Diseases, National
Institutes of Health, US Department of Health and Human
Services, Bethesda, Maryland; 2Michael H. Weisman, MD:
Cedars-Sinai Medical Center, Los Angeles, California; 3John
C. Davis, Jr, MD, MPH: University of California-San Francisco, San Francisco; 4John D. Reveille, MD: University of
Texas Health Sciences Center, Houston.
Address correspondence to Michael M. Ward, MD, MPH,
NIH/NIAMS/IRP, Building 10 CRC, Room 4-1339, 10 Center
Drive, MSC 1468, Bethesda, MD 20892-1468. E-mail:
wardm1@mail.nih.gov.
Submitted for publication December 22, 2004; accepted in
revised form April 30, 2005.
710
of functional limitation is an important determinant of
health-related quality of life, influences the likelihood of
work disability, and is the major predictor of medical costs
in patients with AS (1– 6). Identification of risk factors for
functional limitations is important because it would enable better understanding of how these limitations develop, would help identify high-risk groups of patients,
and may indicate interventions to prevent functional limitations and improve health outcomes.
Functional limitations in AS increase with age and with
the duration of symptoms (1,7–13). Additional risk factors
noted in prior studies included the severity of pain and
stiffness, peripheral arthritis, total hip arthroplasty, and
smoking (2,7,8,12–17). Results of studies differ on whether
functional limitations are more severe among women or
men, and on whether the age at onset of AS, familial AS, or
unsupervised exercise are associated with functional limitations (6 –10,12–14,18,19). Few studies have examined
possible associations with education level or comorbid
medical conditions, even though these are important determinants of functional limitations in other rheumatic
Functional Limitations in AS
diseases (1,8,20 –22). In addition, occupational characteristics have been largely overlooked as potential risk factors, despite strong associations between functional limitations and work disability, and between physically
demanding jobs and work disability (3– 8,23,24).
A potential reason for discrepant results among prior
studies may be heterogeneity in the patients examined. In
particular, risk factors for functional limitation may differ
with the duration of AS. Functional limitations in early AS
may be closely related to symptom severity and factors
that modify symptoms, whereas functional limitations in
long-standing AS may be more closely related to factors
whose effects cumulate over time and that reflect spinal
fusion and long-term structural damage. In this cross-sectional study, we examined demographic and clinical risk
factors for functional limitations in a large group of patients with AS symptoms for ⱖ20 years.
PATIENTS AND METHODS
Patients. Patients were participants in the cross-sectional component of the Prospective Study of Outcomes in
Ankylosing Spondylitis, an observational study whose
main aim is to investigate genetic markers of AS severity.
Patients were recruited from the clinics of the investigators
or local rheumatologists, from patient support and advocacy groups, and from the community by advertisement. A
total of 62 participants were drawn from a study of patients with familial AS (25). Enrollment occurred in 2002–
2004. Inclusion criteria were a diagnosis of AS by the
modified New York criteria (26) and duration of AS for
ⱖ20 years, dated from the onset of persistent musculoskeletal symptoms. All participants had a clinical evaluation
by one of the study rheumatologists, had pelvic and spinal
radiographs to confirm their diagnosis, and completed
questionnaires about their personal and medical history
and functional status.
Measures of functional limitation. Two measures of
functional limitation were used: the Bath Ankylosing
Spondylitis Functional Index (BASFI) and the Health Assessment Questionnaire modified for the Spondylarthropathies (HAQS) (27,28). The BASFI is a 10-item scale
that asks respondents to rate the degree of difficulty they
have performing tasks, using visual analog scales labeled
from 0 (easy) to 100 (impossible). The BASFI score is
calculated as the mean of the 10 responses. It has been
demonstrated to have good reliability and construct validity (2,11,14,27,29 –31). The HAQS is a 25-item scale that
asks respondents to rate the degree of difficulty they have
performing tasks in 10 functional areas (dressing, arising,
eating, walking, hygiene, reach, grip, errands and chores,
bending, and driving). Responses to each question can
range from 0 (no difficulty) to 3 (unable to do), and the
HAQS score is the average of the highest score in each of
the 10 function categories. The HAQS has also been demonstrated to have good reliability and validity (11,28,32).
Risk factors. The risk factors considered for association
with functional limitations were patient age, sex, ethnicity
711
(white versus other), education level, smoking status (current, former, or nonsmoker), pack-years of smoking, and
number of comorbid medical conditions (by patient self
report). In addition, we examined associations with the
duration of AS, age at onset of AS, history of AS in a
first-degree relative (by patient report), and history of iritis
(by patient report). We also asked participants to report
their level of recreational physical activity in their teens
and twenties, relative to their same-sex peers (1 ⫽ less
than peers, 2 ⫽ same as peers, 3 ⫽ more than peers).
Because previous studies demonstrated associations between functional limitations in AS and occupational physical activity (7), we also computed an occupational physical activity score for each patient. Each patient was asked
to report each paid job they had in their lifetime, and to
rate the level of physical activity in each job (1 ⫽ little, 2 ⫽
moderate, 3 ⫽ heavy) (5). The occupational physical activity score was then computed as the mean of these ratings, weighted by the number of years spent in each job
(possible range 1–3). For example, the occupational physical activity score of a patient who reported working as a
carpenter with heavy activity for 10 years, and then as a
supervisor with moderate activity for 15 years would be
calculated by the equation [(10 ⫻ 3) ⫹ (15 ⫻ 2)]/25 ⫽ 2.4.
An alternative occupational physical activity score was
also calculated, using the activity ratings for each job assigned by the US Department of Labor in the Dictionary of
Occupational Titles (1 ⫽ sedentary, 2 ⫽ light, 3 ⫽ medium, 4 ⫽ heavy, 5 ⫽ very heavy) (33), and also weighted
by the number of years in each job (possible range 1–5).
The correlation between the 2 occupational physical activity scores was moderately high (r ⫽ 0.45; P ⬍ 0.0001).
Statistical analysis. Associations between potential risk
factors and the BASFI were tested using t tests (for categorical variables) or Pearson’s correlations (for continuous
variables). Because values of the HAQS were not normally
distributed, nonparametric rank sum tests and Spearman’s
correlations were used to test associations with this
measure. Multivariable linear regression models were developed to examine the independent association of risk
factors with the BASFI or HAQS. A square-root transformation of the HAQS was used as the dependent variable in
the regression model for this measure, to make the distribution of this variable more normal (Shapiro-Wilk W ⫽
0.985; skewness ⫽ ⫺0.04). To determine if risk factors for
functional limitations differed between men and women,
sex-specific models were also developed. All hypotheses
were 2-tailed, and P values less than 0.05 were considered
significant. Analyses were performed using SAS version
8.2 programs (SAS Institute, Cary, NC).
RESULTS
The study included 326 patients, with a mean ⫾ SD age of
55.0 ⫾ 10.7 years and a mean duration of AS symptoms of
31.7 ⫾ 10.2 years (Table 1). The patients were mainly
white and well educated, and women comprised 26% of
the group. The most common comorbid conditions were
hypertension (35.6%), peptic ulcer disease (19.3%), de-
712
Ward et al
Table 1. Characteristics of the study patients (N ⴝ 326)*
Age, mean ⫾ SD years
Male
Race/Ethnicity
White
African American
Asian/Pacific Islander
Native American
Hispanic
Other
Employment status
Employed
Retired
Professional occupation
Education level, mean ⫾ SD
years
Lifetime occupational physical
activity score, mean ⫾ SD
(range 1–3)
Recreational activity in teens
and twenties, mean ⫾ SD
(range 1–3)
Smoking status
Nonsmoker
Former smoker
Current smoker
Number of comorbid
conditions
None
1
2
3
ⱖ4
Inflammatory bowel disease
History of iritis
Duration of AS, mean ⫾ SD
years
Age at onset, mean ⫾ SD years
History of AS in a first-degree
relative
BASFI score, mean ⫾ SD
HAQS score, mean ⫾ SD
55.0 ⫾ 10.7
241 (73.9)
284 (87.1)
12 (3.7)
7 (2.1)
4 (1.2)
17 (5.2)
2 (0.6)
189 (58.0)
70 (21.5)
212 (68.0)
15.9 ⫾ 3.0
1.8 ⫾ 0.7
2.1 ⫾ 0.6
145 (44.5)
146 (44.8)
35 (10.7)
association was present when measured both by the patients’ subjective ratings of job activity and by the standard
ratings of the Dictionary of Occupational Titles.
Scores for the BASFI and HAQS were similar in men
and women, in patients with and those without a family
history of AS, and in those with and those without a
history of iritis (Table 3). However, among men, patients
with a family history of AS tended to have lower BASFI
scores than those without a family history (30.3 versus
38.2; P ⫽ 0.11). Nonwhites had more functional limitations than whites, and current smokers had much higher
scores on the BASFI and HAQS than nonsmokers or
former smokers. Scores were comparable among nonsmokers and former smokers (mean BASFI 36.9 and 40.8, respectively; median HAQS 0.6 and 0.6, respectively).
Among current and former smokers, there was no association between BASFI or HAQS scores and the number of
pack-years of smoking (BASFI r ⫽ 0.12, P ⫽ 0.12; HAQS
r ⫽ 0.09; P ⫽ 0.24).
Multivariable analysis. Based on the results of the univariable analyses, age, ethnicity, education level, number
of comorbid conditions, smoking status, lifetime occupa-
33 (10.1)
71 (21.8)
84 (25.8)
50 (15.3)
88 (27.0)
10 (3.1)
134 (41.1)
31.7 ⫾ 10.2
23.2 ⫾ 7.5
101 (31.0)
40.7 ⫾ 25.6
0.8 ⫾ 0.6
* Values are the number (percentage) unless otherwise indicated.
AS ⫽ ankylosing spondylitis; BASFI ⫽ Bath Ankylosing Spondylitis Functional Index; HAQS ⫽ Health Assessment Questionnaire
modified for the Spondylarthropathies.
pression (16%), osteoporosis (12.6%), and asthma
(10.1%). BASFI scores were distributed over a broad range,
with 75% of scores falling between 20 and 61 (Figure 1).
HAQS scores were generally low, with a median of 0.7,
and positively skewed.
In univariable analyses, scores for both the BASFI and
HAQS increased with older age, longer duration of AS,
and the number of comorbid conditions (Table 2). The
BASFI and HAQS scores were lower among patients with
higher levels of education, but scores were not associated
with either the age at onset of AS or with recalled levels of
recreational physical activity in young adulthood. However, both the BASFI and HAQS were strongly associated
with lifetime occupational physical activity scores. Patients who had more physically demanding jobs throughout their lifetimes had more functional limitations. This
Figure 1. Distribution of scores of the A, Bath Ankylosing Spondylitis Functional Index (BASFI) and B, Health Assessment Questionnaire for the Spondylarthropathies (HAQS).
Functional Limitations in AS
713
Table 2. Correlations of the Bath Ankylosing Spondylitis Functional Index (BASFI)
scores and Health Assessment Questionnaire for the Spondylarthropathies (HAQS)
scores with continuous risk factors*
BASFI
HAQS
Risk factor
r
P
r
P
Age
Duration of AS
Age at onset
Education level
Number of comorbid conditions
Recreational activity in teens and twenties
Lifetime occupational physical activity score
Lifetime DOT occupational physical activity score
0.11
0.12
⫺0.01
⫺0.24
0.25
⫺0.03
0.31
0.19
0.05
0.04
0.96
⬍ 0.0001
⬍ 0.0001
0.61
⬍ 0.0001
0.0007
0.06
0.06
⫺0.01
⫺0.29
0.21
⫺0.01
0.32
0.16
0.24
0.23
0.97
⬍ 0.0001
0.0001
0.99
⬍ 0.0001
0.004
* Pearson correlations for BASFI; Spearman correlations for HAQS. Positive correlations indicate greater
functional limitation on the BASFI and HAQS with higher levels of the risk factor, and negative
correlations indicate less functional limitation with higher levels of the risk factor. AS ⫽ ankylosing
spondylitis; DOT ⫽ Dictionary of Occupational Titles.
lower in patients with a family history of AS. Also, the
BASFI score increased by 8.9 points with each additional
point in the lifetime occupational physical activity score,
meaning that patients whose past jobs all involved moderate physical activity had BASFI scores that were on
average 8.9 points higher than those whose past jobs all
involved little physical activity. Similarly, patients whose
past jobs all involved heavy physical activity had BASFI
scores that were on average 8.9 points higher than those
whose past jobs all involved moderate activity. When the
job activity ratings of the Dictionary of Occupational Titles
(33) were used in place of patients’ own ratings, BASFI
scores were also significantly associated with occupational
physical activity, increasing on average by 3.5 points with
each grade in the 5-grade scale (P ⫽ 0.05). There was no
evidence of a threshold effect for occupational physical
activity or the number of comorbid medical conditions.
There were no associations with age, sex, or ethnicity in
the multivariate analysis.
Results for the HAQS were similar to those for the
BASFI, with significant associations with education level,
the number of comorbid conditions, smoking status, fam-
tional physical activity score, and family history of AS
were considered as potential risk factors in the multivariable models. Sex was also included because it was a potential confounder of the association between functional
limitations and age and occupational physical activity.
Age and duration of AS were highly correlated (r ⫽ 0.70)
and could not be included simultaneously in the models
because of collinearity. Age was used in the primary analysis rather than duration of AS because age was more
highly correlated with education level (r ⫽ 0.12, P ⫽ 0.03)
than was duration of AS (r ⫽ 0.07, P ⫽ 0.21), and thus
provided better adjustment for the association of education level with functional limitation scores.
In the multivariate analysis, BASFI scores were significantly associated with education level, the number of comorbid medical conditions, smoking status, lifetime occupational physical activity, and family history of AS (Table
4). On average, the BASFI score decreased by 1.1 points
(out of 100) with each additional year of education, increased by 3.1 points with each additional comorbid condition, was 11.8 points higher among current smokers than
among nonsmokers or former smokers, and was 6.7 points
Table 3. Associations of the Bath Ankylosing Spondylitis Functional Index (BASFI)
scores and Health Assessment Questionnaire for the Spondylarthropathies (HAQS)
scores with categorical risk factors*
BASFI
HAQS
Risk factor
Mean ⴞ SD
P
Median (IQR)
P
Men
Women
White
Nonwhite
Family history of AS
No family history of AS
History of iritis
No history of iritis
Current smoker
Nonsmoker or former smoker
40.5 ⫾ 25.5
41.0 ⫾ 25.8
39.3 ⫾ 24.6
49.9 ⫾ 30.0
38.0 ⫾ 24.6
41.8 ⫾ 26.0
38.0 ⫾ 24.2
42.5 ⫾ 26.4
55.5 ⫾ 27.4
38.9 ⫾ 24.8
0.89
0.6 (0.3–1.1)
0.7 (0.4–1.4)
0.6 (0.3–1.2)
0.9 (0.5–1.5)
0.6 (0.3–1.1)
0.7 (0.1–1.2)
0.6 (0.3–1.1)
0.7 (0.3–1.25)
1.1 (0.7–1.4)
0.6 (0.3–1.1)
0.19
* IQR ⫽ interquartile range; AS ⫽ ankylosing spondylitis.
0.02
0.22
0.13
0.0002
0.01
0.25
0.15
⬍ 0.0001
714
Ward et al
Table 4. Multivariable regression analyses of risk factors associated with the Bath Ankylosing Spondylitis Functional Index
(BASFI) scores and Health Assessment Questionnaire for the Spondylarthropathies (HAQS) scores, using age as the measure of
time*
BASFI
HAQS
Risk factor
␤
t
P
␤
t
P
Age, years
Male
White
Education level, years
Number of comorbid conditions
Current smoker
Lifetime occupational physical activity score
Family history of AS
0.1
⫺0.6
⫺5.6
⫺1.1
3.1
11.8
8.9
⫺6.7
0.87
⫺0.2
⫺1.38
⫺2.42
4.45
2.77
4.42
⫺2.38
0.39
0.85
0.17
0.02
⬍ 0.0001
0.006
⬍ 0.0001
0.02
0.002
⫺0.6
⫺0.07
⫺0.02
0.04
0.18
0.12
⫺0.08
1.18
⫺1.34
⫺1.24
⫺3.29
3.82
2.85
3.93
⫺1.95
0.25
0.19
0.22
0.002
0.0002
0.005
0.0001
0.06
* All independent variables were included simultaneously in the models. The dependent variable in the HAQS model was the square root of the
HAQS. AS ⫽ ankylosing spondylitis.
ily history, and occupational physical activity (Table 4).
Results were also similar when duration of AS was used
instead of age as the measure of time (Table 5).
0.17) or a parent with AS (2.9 points lower versus men
without a family history, P ⫽ 0.62).
Sex-specific analyses. Because risk factors for functional limitations may differ between men and women, we
also performed analyses stratified by sex (Table 6). BASFI
scores were associated with smoking status, the number of
comorbid conditions, and lifetime occupational physical
activity in both men and women. P values for these associations were higher among women, and were likely due to
the smaller number of women in the study. The association with education level was also similar in both sexes.
However, a family history of AS was more strongly associated with lower BASFI scores in men than in women.
White ethnicity was marginally associated with lower
BASFI scores only among men. Sex-specific analyses of
the HAQS demonstrated similar results (data not shown).
To explore further the association of BASFI scores with
a family history of AS among men, we categorized patients
by the type of relative affected. In adjusted analyses,
BASFI scores were lower among men with a child affected
with AS (10.7 points lower versus men without a family
history, P ⫽ 0.20) than among those with a sibling with AS
(5.5 points lower versus men without a family history, P ⫽
DISCUSSION
In this study, occupational physical activity, smoking status, comorbidity, level of formal education, and family
history of AS were important predictors of the degree of
functional limitation in patients with long-standing AS.
Because the sample was limited to patients with AS duration of at least 20 years, these factors may be more closely
associated with functional limitations due to spinal fusion
or permanent structural damage, and less associated with
limitations due to inflammatory symptoms. The restricted
nature of the sample was also likely to be the reason that
the association of age and duration of AS with functional
limitations was less pronounced in this study compared
with other studies, which included patients with AS of
any duration (1,8 –10). In this sample, we did not find the
degree of functional limitation to be associated with age at
onset of AS, sex, ethnicity, history of iritis, or recreational
activity in young adulthood.
Occupational physical activity, assessed over the patient’s working life, was strongly associated with func-
Table 5. Multivariable regression analyses of risk factors associated with the Bath Ankylosing Spondylitis Functional Index
(BASFI) scores and Health Assessment Questionnaire for the Spondylarthropathies (HAQS) scores, using duration of AS as the
measure of time*
BASFI
HAQS
Risk factor
␤
t
P
␤
t
P
Duration of AS, years
Male
White
Education level, years
Number of comorbid conditions
Current smoker
Lifetime occupational physical activity score
Family history of AS
0.2
⫺0.4
⫺5.6
⫺1.1
3.0
12.1
9.0
⫺6.9
1.23
⫺0.1
⫺1.39
⫺2.36
4.47
2.82
4.46
⫺2.44
0.22
0.89
0.17
0.02
⬍ 0.0001
0.005
⬍ 0.0001
0.02
0.001
⫺0.05
⫺0.07
⫺0.02
0.04
0.17
0.12
⫺0.08
0.61
⫺1.20
⫺1.14
⫺3.24
4.12
2.80
3.97
⫺2.02
0.55
0.23
0.26
0.002
⬍ 0.0001
0.006
⬍ 0.0001
0.05
* All independent variables were included simultaneously in the models. The dependent variable in the HAQS model was the square root of the
HAQS. AS ⫽ ankylosing spondylitis.
Functional Limitations in AS
715
Table 6. Multivariable regression analyses of risk factors associated with the Bath Ankylosing Spondylitis Functional Index
(BASFI) scores in men and women*
Men
Women
Risk factor
␤
t
P
␤
t
P
Age, years
White
Education level, years
Number of comorbid conditions
Current smoker
Lifetime occupational physical activity score
Family history of AS
0.1
⫺8.6
⫺0.9
2.9
10.8
9.0
⫺8.5
0.61
⫺1.76
⫺1.70
3.51
2.22
3.79
⫺2.48
0.54
0.08
0.09
0.0005
0.03
0.0002
0.02
0.2
⫺0.8
⫺1.7
3.8
14.4
8.6
⫺3.6
0.66
⫺0.1
⫺1.65
2.81
1.56
2.19
⫺0.7
0.51
0.92
0.11
0.007
0.13
0.04
0.50
* All independent variables were included simultaneously in the models. AS ⫽ ankylosing spondylitis.
tional limitations. In this sample, scores on the BASFI
were estimated to be, on average, ⬃18 points higher in
patients who had always worked in physically demanding
jobs, compared with those who worked in sedentary jobs.
When computed using the standard job activity ratings of
the Dictionary of Occupational Titles, BASFI scores were
estimated to be 14 points higher in patients who consistently worked in jobs involving heavy labor, compared
with those with sedentary jobs. Although the nature of
work differs between men and women, similar associations were found in both sexes. These associations were
present in analyses that adjusted for the level of formal
education, suggesting that the association between occupational activity and functional limitation was not likely
due to confounding by socioeconomic status. However, we
cannot be certain that residual confounding by socioeconomic status was not present. Patients with severe functional limitations may have overestimated the physical
demands of their past jobs, resulting in information bias,
but the results were similar when standard ratings of occupational activity from the Dictionary of Occupational
Titles were used. It is important to note that the association here was between a cumulative measure of occupational activity and functional limitations, which by the
restricted nature of the sample would also be more likely
to represent cumulative effects. In early AS, this association would not be expected, because physically demanding jobs would more likely be held by individuals with few
or no functional limitations. Studies of groups of patients
that are heterogeneous in the duration or stage of AS may
mask such associations.
Working at physically demanding jobs may affect longterm functional ability by stressing joints in the axial skeleton, thereby perpetuating inflammation. Alternatively,
persons with physically demanding jobs may be less likely
to perform therapeutic or recreational exercises, which
then could lead to greater functional limitations in the
future (34). Both of these factors may contribute to functional limitations. In previous studies, occupational activity was characterized globally as manual or sedentary, and
it was not considered that physical activity can vary
greatly within these categories, or that individuals change
jobs, often moving into less physically demanding positions with time (7,14). The more detailed characterization
of occupational physical activity in this study may have
permitted better detection of associations between work
activity and functional limitations. The association of occupational physical activity with functional limitations
provides a mechanistic link between work demands, functional limitations, and work disability (3– 8,23,24,35).
Current smokers also had much more functional limitation than nonsmokers or former smokers. Smoking status
has rarely been examined as a risk factor for functional
limitation in AS, but smoking has been associated with
more rapid progression of functional disability and with
higher levels of functional limitations in prior studies of
this risk factor (13,14,17). In the study by Doran and colleagues, current and former smokers were examined together, and it was not clear if most or all of the association
was due to current smokers (14). In our study, former
smokers had levels of functional limitation similar to that
of nonsmokers, and only current smokers had increased
functional limitation. This finding, along with the absence
of an association between pack-years of smoking and functional limitations, suggests that the primary association
may not be a direct effect of smoking on functional ability.
Rather, smoking may be a marker of poor health behaviors,
including perhaps lack of exercise, that contribute to functional limitations.
Two previous studies reported that patients with familial AS have similar degrees of functional impairment as
patients with sporadic AS, but a third study reported less
severe functional limitations in patients with familial AS
(12,14,19). Less severe disease in those with familial AS
may be a consequence of genetic effects, but may also
result from earlier diagnosis and better treatment, more
knowledge, or more support regarding AS in affected families. Alternatively, mildly affected individuals in families
with AS may be more likely to be diagnosed than mildly
affected individuals without a family history of AS, in
whom low back pain and stiffness may be attributed to
other conditions. Similar selection factors may also influence which patients elect to participate in research studies, and it may not be possible to know if familial AS
influences severity without a population-based study.
However, our finding that the protective association of
family history was more pronounced among fathers of
children with AS than among brothers or sons of persons
with AS suggests that earlier diagnosis, education, or so-
716
cial support may not be the mechanism underlying this
association.
The strengths of this study include the large, well-characterized sample; examination of a number of risk factors;
and replication of the findings using 2 different measures
of functional limitation. The focus on patients with longstanding AS may have permitted better identification of
risk factors associated with irreversible changes and structural damage. However, the study was cross-sectional, and
causal inferences cannot be drawn. Despite the cross-sectional design, all risk factors were either invariant (e.g.,
ethnicity) or historical (e.g., lifetime occupational physical
activity, education level, comorbid conditions), which
preserved the correct temporal relationship between the
risk factors and the outcome. Information on familial AS
was collected by patient report, and diagnoses in family
members were not verified for all patients. We did not
collect information on symptoms or peripheral arthritis,
and we cannot determine how much of the variation in
functional limitations in these patients may have been
associated with differences in AS activity. We also did not
examine associations with treatment or past medication
use, but few patients had extensive treatment with anti–
tumor necrosis factor ␣ medications. The measures of
functional limitations we used have good reliability and
validity, but the HAQS has previously been demonstrated
to have a floor effect (36). This may limit detection of mild
functional limitations but would not be expected to limit
detection of more severe impairment. Lastly, the patients
represented a volunteer sample, and findings may have
been different had a community-based or populationbased sample been examined.
Our findings indicate that studies of long-term functional outcomes in AS should include assessment of
the patient’s level of education, comorbid conditions,
smoking history, family history, and occupational history.
Identification of the specific work activities and the specific behaviors related to smoking that are associated with
functional limitations in AS will provide guidance for
recommendations that may improve long-term functional
outcomes for patients.
Ward et al
5.
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12.
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14.
15.
16.
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19.
ACKNOWLEDGMENTS
The authors would like to thank Cheryl Kallmann, Lori
Guthrie, Erin Skrok, Stephanie Morgan, and Laura Diekman for their assistance.
20.
21.
22.
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