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Minimal difference in pain associated with change in quality of life in children with rheumatic disease.

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Arthritis & Rheumatism (Arthritis Care & Research)
Vol. 47, No. 5, October 15, 2002, pp 501–505
DOI 10.1002/art.10661
© 2002, American College of Rheumatology
ORIGINAL ARTICLE
Minimal Difference in Pain Associated With
Change in Quality of Life in Children With
Rheumatic Disease
SONNY DHANANI,1 JOSEE QUENNEVILLE,1 MEGAN PERRON,2 MOHAMED ABDOLELL,1
3
AND BRIAN M. FELDMAN
Objective. To establish the minimal change on a pain visual analog scale (VAS) associated with change in self-reported
quality of life in pediatric rheumatology patients.
Methods. Subjects were a cohort of 533 pediatric rheumatology patients in Toronto. Pain and perceived quality of life
were measured at 2 consecutive visits to a clinic.
Results. Among patients who rated quality of life at the second visit as “a little better” and “much better,” the mean
change in pain score on a 10-cm VAS was reduced by 0.82 and 1.45 cm, respectively. For those whose quality of life
changed to “a little worse” and “much worse,” the pain scores increased by 1.90 and 3.69 cm.
Conclusion. Our results suggest that future studies of the assessment and treatment of pain in this population should aim
for a minimum reduction in pain score of 0.82 cm on a 10-cm VAS to achieve clinical improvement in quality of life.
KEY WORDS. Minimal difference; Pain; Quality of life; Pediatrics; Rheumatic disease.
INTRODUCTION
Rheumatic diseases include both common and rare pediatric chronic diseases, such as juvenile rheumatoid arthritis, systemic lupus erythematosus, dermatomyositis, and
the spondylarthropathies. These diseases can affect many
aspects of life, such as pain and physical function, which
in turn can have profound effects on a child’s daily living
and psychosocial well being. Thus, the clinical assessment
of children with rheumatic diseases, such as juvenile rheumatoid arthritis, is commonly based on a diverse set of
measures (1,2).
In particular, pain is an important but poorly underDr. Feldman’s work is supported by a Career Scientist
Award from the Ontario Ministry of Health.
1
Sonny Dhanani, MD, BSc, Josee Quenneville, MSc, Mohamed Abdolell, MSc: The Hospital for Sick Children and
the University of Toronto, Toronto, Ontario, Canada;
2
Megan Perron, RN: The Bloorview-MacMillan Centre, Toronto, Ontario, Canada; 3Brian M. Feldman, MD, MSc,
FRCPC: The Hospital for Sick Children, the University of
Toronto, and The Bloorview-MacMillan Centre, Toronto,
Ontario, Canada.
Address correspondence to Brian M. Feldman, MD, MSc,
FRCPC, Associate Professor, Pediatrics and Public Health
Sciences, The Hospital for Sick Children, University of Toronto, 555 University Avenue, Toronto, Ontario, M5G 1X8,
Canada. E-mail: brian.Feldman@sickkids.on.ca.
Submitted for publication August 8, 2000; accepted in
revised form February 3, 2002.
stood aspect of rheumatic disease in children (3), perhaps
because its measurement is also not yet well understood.
Varni and Thompson were the first to develop a standardized pain assessment tool for children: the Pediatric Pain
Questionnaire (PPQ), which established the use of a visual
analog scale (VAS) to measure pain intensity (4,5). This
scale is now accepted along with other aspects of the PPQ
as a valid and reliable method of assessing pain in the
pediatric population (6,7), and has been used as part of
several comprehensive assessment tools, such as the
Childhood Health Assessment Questionnaire (CHAQ)
(2,8).
Although the importance of pain has long been accepted, methods for assessing it in the pediatric population are only now being devised. Similarly, quality of life
has been recognized as one of the most important clinical
outcomes in rheumatology, but it too is not well understood (2,9,10). Some studies have suggested that pain may
correlate with clinical disease activity and functional status (6,11–13), whereas others have examined the relevance
of pain to the general health and quality of life of both
children and adults (8,11,14,15). Although a relationship
between pain and quality of life has been established,
studies have not been done to determine the minimal
clinically important change in pain, as measured by VAS,
that will lead to change in quality of life.
Our objective was to determine the minimal change in
the pediatric pain VAS that is associated with change in
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502
Dhanani et al
scale their pain level in the past week. The number of
centimeters marked is recorded as a score from 0 –10, with
0 ⫽ no pain and 10 ⫽ very severe pain. The categorical
change in quality of life score was obtained from the Quality of My Life scale (10), a 5-category scale that asks “Since
my last visit my life is. . . .” Possible answers are “much
worse,” “a little worse,” “the same,” “a little better,” or
“much better.” Patients were asked to circle the category
that best described their quality of life compared with the
previous visit. If patients were too young to complete the
questionnaires themselves, parents helped complete these
scales. The pain VAS and the scales in the CHAQ have
been developed for use in children as young as 1 year old.
Figure 1. Outcome measures.
self-reported quality of life in pediatric rheumatology patients, using a retrospective cohort design. Establishment
of this minimal important change in the perception of pain
may enhance our understanding of its impact in rheumatic
disease and help define a standard for studies dealing with
the treatment of pain.
PATIENTS AND METHODS
Patients. Subjects were pediatric patients attending the
rheumatology clinic at The Hospital for Sick Children and
the Bloorview-MacMillan Centre in Toronto, Ontario, Canada from August 1995 to February 1999. These facilities
are tertiary care centers serving metropolitan Toronto as
well as central and northern Ontario. All patients, regardless of type of rheumatic disease, age, race, or sex, are
routinely asked to answer a standard battery of questions.
These questionnaires are completed by either the patient
or parent on repeated visits to the clinic. Parents are asked
to complete or help complete the questionnaires if patients
are too young to respond adequately themselves.
Our study included all those who completed pain scales
and change in quality of life assessments on 2 consecutive
visits. Patients were excluded if they made only 1 visit to
the clinic or if they returned for a second visit but did not
complete pain scales or quality of life assessments.
Measurements. The main instruments used for this
study were a pain VAS and a categorical scale measuring
change in quality of life (Figure 1). These measures formed
part of a larger standard battery of questionnaires that
included scales for quality of life, coping, global disease
activity, global disease impact, health-related quality of
life, and the CHAQ (2). These scales were also used in our
study and have been previously validated in various pediatric rheumatic diseases (2,8,10).
The pain VAS used in the CHAQ is a double-anchored
10-cm analog scale with anchors of “no pain” and “very
severe pain.” Patients were asked to indicate on the linear
Data collection and statistical analysis. Patients completed questionnaires at each clinic visit between August
1995 and February 1999. Data from the pain VAS and the
quality of life scale were compiled, entered into a computer database, and analyzed using Datadesk 6 (Data Description Inc., Ithaca, NY).
The mean change in pain score associated with each
category of quality of life assessment between 2 consecutive visits was established with 95% confidence intervals
(95% CI). We calculated change in pain scores by subtracting the pain score at the second visit from the pain score at
the first visit.
Many patients had more than 2 visits. For the initial
analysis, the first 2 visits for each subject were used to see
whether there were noticeable changes in quality of life
associated with change in pain. To increase the precision,
we performed a subsequent analysis that calculated the
mean change in pain score for all patients who at any 2
consecutive visits scored the change in quality of life for
the second as “a little better.” Separate analyses were
likewise done for each of the other 4 possible responses to
the change in quality of life scale. For example, if patient
A from the first to second visit was “much better,” then his
change in pain score was used in the calculation of mean
pain change for the category of “much better.” Subsequently, if patient A from the third to fourth visit was
“much worse,” then this change in pain score was used in
calculating the mean pain change for the category “much
worse.” But, if patient A was again “much better” from the
third to fourth visit, then the data from this visit was not
used. In this way, some subjects contributed data to the
mean change in pain score for 1 or more categories of
change in quality of life. However, the calculation of mean
pain change associated with any one category of change in
quality of life did not use any subject more than once.
Analysis of variance (ANOVA) or chi-square tests were
used where appropriate to compare the demographic and
clinical profiles of subjects who chose each category of
change in quality of life. P values ⬍ 0.05 were considered
statistically significant. Subsequent pair-wise comparisons
were corrected for multiple statistical testing by the least
significant difference (LSD) test. General linear models
were also used to determine whether such factors as sex,
age, parent or child responder, type of rheumatic disease,
coping, disease severity, and overall disability confounded
the relationship of change in pain and change in quality of
Minimal Difference in Pain Needed to Change Quality of Life
life. For these models, change in quality of life was recoded as a number from –2 to ⫹2. Because change in
quality of life was an ordinal variable in its original form,
we used standard regression diagnostics to ensure that the
assumptions underlying multiple regression were met. Regression analysis was carried out to examine the contribution of change in pain to perceptions of change in quality
of life; the R2 from the ANOVA model was used to estimate
the amount of variability accounted for by change in pain.
Ordinal logistic regression was used to model the probability of “worse” quality of life as a function of pain
change. A proportional odds model was employed. The
proportional odds assumption was investigated using
plots of the cumulative logits to confirm that no crossover
effects were indicated. The effect of the predictor variable
on the odds of being in a “worse” quality of life category is
reported as an odds ratio with 95% CI. Values of the
change in pain that are associated with a 50% chance of
reporting that they are “much worse,” “much worse or a
little worse” and “much worse or a little worse or the
same” were obtained from the ordinal logistic regression
model.
RESULTS
Data from a total of 533 patients were available for analysis
between August 1995 and February 1999. The sex distribution was 69.6% female and 30.4% male. The mean age
was 11.3 years (range 1.3–19.2 years). The median time
between clinic visits was 87 days. The most common
diagnosis was juvenile rheumatoid arthritis (41.4%); however, a full spectrum of illnesses was encountered, including systemic lupus erythematosus (14.3%), nonarticular
rheumatism (8.5%), and spondylarthritis (4.7%). Other
diagnoses (31.1%) included pain related to myositis-, orthopedic-, psoriatic-, infectious-, vasculitic-, and inflammatory bowel-related illnesses. The age and sex statistics
of the study group were representative of the rheumatology subspecialty population in general.
Questionnaires were completed by the patient 81.8% of
the time (parents assisted in 11.5%) and by parents in
18.2% of cases. We had some concern that questionnaires
completed by parents might differ systematically. However, when the data from parent-completed forms were
removed from the analysis, the results were unchanged.
Thus, further analysis of data included all the questionnaires.
We calculated the average change in pain score associated with each category in quality of life between the first
2 visits. For patients whose quality of life was rated as “the
same” between the first 2 visits, the mean difference in
pain score was 0.07 cm (95% CI – 0.39, 0.24; n ⫽ 156).
Change in quality of life to “a little better” showed that the
mean change in pain score was reduced by 0.89 cm (95%
CI –1.29, – 0.49; n ⫽ 156). In the group of patients whose
quality of life changed to “a little worse,” average pain
scores increased by 2.15 cm (95% CI 1.33, 2.98; n ⫽ 50).
The difference in pain scores observed for changes in
quality of life to “much better” and “much worse” between
the first 2 visits were –1.22 cm (95% CI –1.68, – 0.75; n ⫽
503
151) and ⫹3.29 cm (95% CI 1.77, 4.82; n ⫽ 12), respectively.
There was a significant difference in the amount of
change in pain among the patients in different categories
of change in quality of life (n ⫽ 533; F ⫽ 24.99, degrees of
freedom ⫽ 4, P ⱕ 0.0001). Pair-wise comparisons revealed
that patients who rated their quality of life as “a little
better” had a significantly different change in pain as compared with those who rated their quality of life as “the
same” (P ⫽ 0.005, LSD). Likewise, those whose quality of
life was “a little worse” had a significantly different
change in pain compared with those whose quality of life
was “the same” (P ⬍ 0.0001).
After analyzing data from the first 2 visits alone, we
calculated the average change in pain score associated
with each category of change in quality of life between any
2 consecutive visits to increase the power of our estimation. For patients whose quality of life was rated as “the
same” between any 2 visits, the mean difference in pain
score was 0.06 cm (95% CI – 0.34, 0.22; n ⫽ 228). This
change in pain score was significantly different from patients who rated quality of life as “a little better:” In this
category, the mean change in pain score was reduced by
0.82 cm (95% CI –1.15, – 0.49; n ⫽ 234). In the group of
patients whose quality of life changed to “a little worse,”
average pain scores increased by 1.90 cm (95% CI 1.30,
2.51; n ⫽ 92). The difference in pain scores observed for
changes in quality of life to “much better” and “much
worse” were –1.45 cm (95% CI –1.83, –1.07; n ⫽ 210) and
⫹3.69 cm (95% CI 2.59, 4.80; n ⫽ 26), respectively.
Table 1 shows the characteristics of the subjects in each
category of change in quality of life. The group that scored
quality of life as “much better” was significantly younger
than the rest. There were minor, but statistically significant differences in sex as well. Fewer patients with juvenile rheumatoid arthritis, but more with systemic lupus
erythematosus, rated their quality of life as the same between 2 visits. Patients who scored their quality of life as
“a little worse” or “much worse” had significantly worse
global disease severity scores and higher scores on measures of global disease impact (2).
The R2 from the ANOVA model was used to estimate the
amount of variability accounted for by change in pain. The
result yielded an R2 of 0.159 (P ⬍ 0.0001), suggesting that
change in pain alone explained about 16% of the variance
in quality of life. Furthermore, ordinal logistic regression
was used to estimate the probability of worse quality of life
for any pain change. This suggested that a 1-cm increase in
change in pain on a 10-cm scale increased the odds of a
worse quality of life by a factor of 1.3 with a 95% CI of
1.22, 1.38. Another way to determine the minimal clinically important change in pain is to determine the threshold value at which 60% of the subjects reported that their
life was “a little better” or “much better.” From the ordinal
logistic regression model, this threshold value was – 0.7
cm.
We constructed general linear models to examine the
influence of other disease and demographic factors on
change in quality of life measured at the second visit. The
best predictor remained change in pain scores, though
other factors were also significant (Table 2).
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Dhanani et al
Table 1. Characteristics of the 533 study subjects by change in quality of life at second clinic visit*
Variable
Much better
Subjects
Age, years (SD)
Female/male
Diagnosis, n (%)
JRA
Lupus
Nonarticular rheumatism
Seronegative spondylarthropathy
Other
Days since previous visit (SD)
Global disease severity score from
CHAQ (SD)
Global disease impact score from
CHAQ (SD)
151
9.9 (4.4)
107/44
Little better
Same
Little worse
Much worse
P
156
11.5 (4.2)
92/64
164
12.1 (4.4)
126/38
50
11.9 (4.3)
39/11
12
11.9 (4.4)
7/5
ⱕ0.0001 (ANOVA)
ⱕ0.005 (␹2)
76 (50.3)
12 (7.9)
9 (6.0)
3 (2.0)
51 (33.8)
96 (200)
0.3 (0.5)
67 (42.9)
16 (10.3)
14 (9.0)
13 (8.3)
46 (29.5)
83 (260)
0.6 (0.6)
62 (37.8)
40 (24.4)
15 (9.1)
5 (3.0)
42 (25.6)
86 (79)
0.5 (0.6)
23 (46.0)
6 (12.0)
5 (10.0)
3 (6.0)
13 (26.0)
83 (54)
1.2 (0.8)
5 (41.7)
1 (8.3)
1 (8.3)
1 (8.3)
4 (33.3)
76 (82)
1.7 (0.8)
0.4 (0.5)
0.7 (0.7)
0.6 (0.7)
1.4 (0.7)
2.0 (0.7)
ⱕ0.007 (␹2)
0.57 (ANOVA)
ⱕ0.0001 (ANOVA)
ⱕ0.0001 (ANOVA)
* CHAQ ⫽ Childhood Health Assessment Questionnaire; JRA ⫽ juvenile rheumatoid arthritis; ANOVA ⫽ analysis of variance; ␹2 ⫽ chi-squared test.
DISCUSSION
In addition to reemphasizing that pain alters quality of life,
this study identified the minimal clinically important difference leading to significant improvement in quality of
life in pediatric rheumatology patients. This difference can
be defined as the smallest difference in the domain of
interest (level of pain) that patients perceive as beneficial;
it is thus influential in their management (16). In our
study, this average reduction of pain score associated with
a minimal improvement in quality of life was 0.82 cm on
a 10-cm VAS. Defining the minimal score may set a standard for future studies that involve the treatment of pain;
in other words, a reduction of 0.82 cm may be used in
other studies in this population to show significant change
in clinical care.
Studies in adults have suggested that pain has a profound effect on a person’s quality of life (14,15). Our results show that changes in pain scores are associated with
changes in quality of life. We showed that every 1-cm
increase in pain score increased the odds of a worse quality of life by a factor of 1.3. Furthermore, the threshold
value of clinically important change in pain associated
with a 60% chance of patients reporting they are “much
better” or “a little better” was – 0.7 cm. This was similar to
the mean pain change of – 0.82 cm associated with “a little
better” quality of life. Patients who judged their quality of
Table 2. General linear model (multiple linear
regression) in which change in quality of life is the
dependent variable
Independent variable
Pain change (10-cm scale)
Global disease severity
(scale 0–3)
Age (years)
Sex
Health-related quality of
life (scale 0–3)
Quality of life (scale 0–3)
Regression (␤)
t
coefficient statistic
P
0.116
0.272
7.77 ⱕ0.0001
3.62 ⱕ0.0003
0.029
0.102
⫺0.173
3.17 ⱕ0.0016
2.35 ⱕ0.0192
⫺2.19 ⱕ0.0290
⫺0.228
⫺2.71 ⱕ0.0069
life as “the same” showed little or no difference in pain
scores—a result that suggests that no change in pain is
associated with none in quality of life. Furthermore, lower
pain scores were associated with improved quality of life
and increased pain scores with worsening quality.
Although pain is an important factor, it is only one
aspect of quality of life. This likely accounts for the range
of results seen in our study, in which change in pain
accounted for about 16% of the variability in quality of
life. Other factors, such as global disease severity, also had
a significant influence on change in quality of life.
Previous studies established minimally important differences with other disorders, such as asthma and chronic
lung disease (17,18). Although these studies used a 7-point
scale as opposed to a VAS, they were able to consistently
demonstrate a minimal important difference in diseasespecific quality of life assessments. Using a quality of life
questionnaire for asthma and a global rating of change,
they found that the minimal important difference of quality of life score per item was close to 0.5. These authors
suggest that this minimal difference may be generalized to
many areas of health-related quality of life assessments
(17,19). If such a broad applicability can be demonstrated,
these results could help clinicians interpret quality of life
assessments, which are used in many areas of medicine.
As a result, the same standards may eventually be used to
assess the magnitude of changes across a wide variety of
instruments. Determining the minimal important difference may also help clinicians judge the benefit when comparing 2 treatments, calculating sample size, and assessing
therapeutic interventions (20,21).
Interpreting values of minimal clinically important differences is subject to several limitations. First, the term “a
little better” used in our scale may not actually represent
the smallest difference important to that patient’s quality
of life (19); a patient may report change in quality of life in
smaller increments than can be measured with conventional scales. Second, most studies assessing the minimal
important difference depend on patients’ judgments when
they are stable and cooperative. However, results may be
influenced by other factors that may alter interpretation of
Minimal Difference in Pain Needed to Change Quality of Life
quality of life (18). When patients are unstable or more
seriously ill, they may be more heavily influenced by
factors such as mood. A third limitation may be that in our
study we used quality of life as a standard measure. However, smaller changes in pain may still be important to
patients but may not affect quality of life. Another, more
specific factor might have been more sensitive to smaller
changes in pain.
A point of statistical methodology in our study should
be addressed. In our secondary assessment, to increase the
precision of estimation of the mean change in pain scores
for each quality of life category, we used multiple measurements from some subjects. In other words, we used
data from not only the first and second visits but in some
cases also from subsequent visits. Because the perception
of pain and quality of life are highly personal, it is conceivable that some people would consistently associate
only large changes in pain scores with a small change in
quality of life and vice versa. Theoretically this might bias
our results, especially if these patients were repeatedly
used in the data sets. However, in our primary analysis we
used data from only the first 2 visits, i.e., each patient was
used only once. The results were very similar to that of the
subsequent larger data set. For example, change in quality
of life to “a little better” showed a mean change in pain of
– 0.89 cm as compared with – 0.82 cm. Thus, we felt that
there was adequate independence of the responses.
A few other limitations and assumptions of our study in
particular need to be addressed. First, the study was retrospective. However, our patients were selected from a
population in a clinic that kept careful records and ensured that questionnaires were completed in full. Second,
we must be aware of errors in causal inference, assuming
that change in pain score led to change in quality of life.
Changes in quality of life may affect pain perception itself.
Thus, we cannot assume a cause-and-effect relationship
and can only report an association between pain change
and quality of life. Third, there may be confounding factors that may affect both pain and quality of life independently. However, we were unable to find other factors that
statistically confounded the relationship between pain
and quality of life.
In summary, our study showed that pain is an important
factor associated with alteration in quality of life in pediatric rheumatology patients. Reduced pain scores over
time were associated with improved quality of life, and
increased pain scores with worsening quality of life. Furthermore, we were able to define the minimal clinically
important difference associated with significant improvement in quality of life in this population. Establishment of
this minimal important change in the perception of pain
may enhance our understanding of its impact in rheumatic
disease and help define a standard for studies dealing with
the treatment of pain. Our results suggest that future studies of the assessment and treatment of pain in this population should aim for a minimum reduction in pain score
of 0.82 cm on a 10-cm VAS.
ACKNOWLEDGMENTS
We would like to thank the Department of Rheumatology
at The Hospital for Sick Children in Toronto for their
505
cooperation and resources. Special thanks to Mr. Frank
Quinlan of the Department of Medical Editing at The Hospital for Sick Children for his time, effort, and invaluable
suggestions.
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