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Abstract

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Objective

To examine the measurement characteristics of the Childhood Myositis Assessment Scale (CMAS) in children with juvenile idiopathic inflammatory myopathy (juvenile IIM), and to obtain preliminary data on the clinical significance of CMAS scores.

Methods

One hundred eight children with juvenile IIM were evaluated on 2 occasions, 7–9 months apart, using various measures of physical function, strength, and disease activity. Interrater reliability, construct validity, and responsiveness of the CMAS were examined. The minimum clinically important difference (MID) and CMAS scores corresponding to various degrees of physical disability were estimated.

Results

The intraclass correlation coefficient for 26 patients assessed by 2 examiners was 0.89, indicating very good interrater reliability. The CMAS score correlated highly with the Childhood Health Assessment Questionnaire (C-HAQ) score and with findings on manual muscle testing (MMT) (rs = −0.73 and 0.73, respectively) and moderately with physician-assessed global disease activity and skin activity, parent-assessed global disease severity, and muscle magnetic resonance imaging (rs = −0.44 to −0.61), thereby demonstrating good construct validity. The standardized response mean was 0.81 (95% confidence interval 0.53, 1.09) in patients with at least 0.8 cm improvement on a 10-cm visual analog scale for physician-assessed global disease activity, indicating strong responsiveness. In bivariate regression models predicting physician-assessed global disease activity, MMT remained significant in models containing the CMAS (P = 0.03) while the C-HAQ did not (P = 0.4). Estimates of the MID ranged from 1.5 to 3.0 points on a 0–52-point scale. CMAS scores corresponding to no, mild, mild-to-moderate, and moderate physical disability, respectively, were 48, 45, 39, and 30.

Conclusion

The CMAS exhibits good reliability, construct validity, and responsiveness, and is therefore a valid instrument for the assessment of physical function, muscle strength, and endurance in children with juvenile IIM. Preliminary data on MID and corresponding levels of disability should aid in the clinical interpretation of CMAS scores when assessing patients with juvenile IIM.

Juvenile idiopathic inflammatory myopathy (juvenile IIM) is a group of rare, presumably autoimmune, systemic connective tissue diseases affecting children. Juvenile dermatomyositis (juvenile DM) is the most common of these disorders, but juvenile polymyositis (juvenile PM) and myositis associated with other connective tissue diseases may represent up to 20% of patients with juvenile IIM (1). The unifying feature of these illnesses is chronic muscle inflammation, which manifests clinically by muscle weakness, reduced endurance, and in the longer term, muscle atrophy and contractures. For this reason, accurate assessment of muscle strength, physical function, and endurance is of critical importance in the determination of clinical status and prediction of outcome in juvenile IIM.

The Childhood Myositis Assessment Scale (CMAS) is a 14-item observational, performance-based instrument that was developed to evaluate muscle strength, physical function, and endurance in children with juvenile IIM (2). It addresses activities that are often used in clinical assessment of these children, such as observation of timed neck flexion duration, of ability to go from a standing to a seated position, and of ability to perform sit-ups. Content validity (referring to the characteristic of a measure such that it has items that adequately cover the domain in question [3]) has been established by choosing items that provide a balance between upper and lower extremity muscle groups and between proximal and axial muscle groups, as well as by including a range of activities, including those that assess endurance. Demonstration of inter- and intrarater reliability and limited testing of construct validity have also been performed, in a single evaluation of 10 patients with juvenile IIM by 12 physician assessors (2).

Many physicians and other health care professionals who provide care for children with juvenile IIM routinely use the CMAS. Despite this, there are no reported data that fully demonstrate construct validity, responsiveness, or clinical meaning of CMAS scores. The goal of the present study was to evaluate the measurement characteristics of the CMAS, including interrater reliability, convergent construct validity, and responsiveness, in a large, multicenter cohort of children with juvenile IIM, in order to fully validate this instrument for use in clinical research and therapy trials. We also assessed the ability of the CMAS to predict disease activity and endeavored to investigate the clinical meaning of the CMAS by estimating the minimum clinically important difference (MID) and by evaluating which scores correspond to various levels of physical dysfunction.

PATIENTS AND METHODS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Patients.

One hundred eight consecutive patients with disease that met the Bohan and Peter classification criteria for definite or probable DM or PM (4), who were under 18 years of age at the time of diagnosis and had complete data for the CMAS, were included in the present study. These patients were enrolled in a study investigating disease activity measures in juvenile IIM, conducted by the Juvenile Dermatomyositis Disease Activity Collaborative Study Group at 11 participating centers between July 1994 and August 1997 (5, 6). Ninety-eight (90.7%) had juvenile DM, 6 (5.6%) had juvenile PM, and 4 (3.7%) had myositis associated with an underlying connective tissue disease. Subjects entered the study a median of 24 months after diagnosis (range 0–132 months, interquartile range [IQR] 12 months–48 months). The median age at study enrollment was 9 years (range 3.1–18.8 years, IQR 6.3–12.8). Other demographic features have been described previously (5).

Procedures.

Approval from the Institutional Review Board at each participating center was obtained. Informed consent was obtained from all subjects' parents/legal guardians. Children were assessed at baseline (at any point in their disease course) and again 7–9 months later. At each assessment, a structured history was obtained, and physical examination, laboratory investigations, and measurements of disease activity and damage and physical function were performed. One parent/legal guardian completed self-administered instruments.

Measures.

At 10 centers, the CMAS was administered by a pediatric rheumatologist; at 1 center it was administered by a pediatric physiotherapist. The scores for the 14 items are summed to yield a total score ranging from 0 (very poor physical function and strength) to 52 (normal physical function and strength) (2). The CMAS was performed according to the scoring and instructions provided on the American College of Rheumatology Web site (http://www.rheumatology.org/sections/pediatric/tools.asp?aud = mem). The CMAS scoring sheet differed from that presented by Lovell et al (2) in item 4 (the patient is given a maximum score of 3 if able to move from a supine to a prone position without any difficulty), item 8 (the patient is given a maximum score of 4 if able to maintain the arm raise for at least 60 seconds), and item 12 (the patient is given a maximum score of 4 if able to rise from a chair without any difficulty). The complete CMAS instrument is shown in Appendix A.

Rennebohm et al (7) have used data from a cohort of healthy children ages 4–9 years to generate age- and sex-specific normal data for 9 items of the CMAS (neck flexion, arm raise and leg lift durations, performance of sit-ups, and the ability to achieve the following changes in position: supine to prone, supine to sitting, standing to sitting on the floor, sitting on the floor to standing, and sitting in a chair to standing). These 9 items were used to form the CMAS9, which has a potential range of 0 (very poor physical function and strength) to 37 (normal physical function and strength), in order to compare CMAS scores of juvenile IIM patients with those of healthy children.

The Childhood Health Assessment Questionnaire (C-HAQ) (5, 8), manual muscle testing (MMT) (5), physician global assessments of disease activity and damage and skin activity and damage, and parent global assessment of disease severity were obtained as previously described (6). All global assessments used 10-cm visual analog scales. Serum levels of at least 2 muscle-associated enzymes were measured (creatine kinase, alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, and/or aldolase). To allow comparison between centers, enzyme levels were divided by the upper limit of normal for the laboratory in which the test was performed.

Thirty-six children at 5 centers had magnetic resonance imaging (MRI) studies of thigh and lower pelvic musculature, obtained using STIR or T2-weighted fat suppression techniques, as well as T1 imaging. A single radiologist, blinded to the status of the patient, scored these. Using a reference atlas, an MRI muscle edema score (range 0–4, where 0 = inactive and 4 = extremely active) and an MRI muscle damage score (range 0–4, where 0 = no damage and 4 = very severe damage) were obtained (9–11).

Analysis.

Interrater reliability is an estimate of the tendency of a measurement instrument to give the same result when it is administered by different individuals (or raters) (3). To assess interrater reliability, subjects were administered the CMAS by 2 independent, blinded raters on the same day, using form (3, 1) of the intraclass correlation coefficient (12). A value in excess of 0.9 was considered indicative of excellent reliability. In keeping with the use of nonparametric statistics throughout this work, values of the CMAS were replaced by ranks for this calculation.

In general, validity is defined as the ability of an instrument to actually measure what it is intended to measure (3). Under ideal circumstances, the instrument would be compared against a gold standard. However, there is no gold standard against which to test the validity of the CMAS. For this reason, convergent construct validity of the CMAS was investigated. Construct validity is a form of validation that seeks to examine whether the construct in question, in this case the CMAS, is related to other measures in a manner consistent with a priori predictions. If the relationships are similar to what was predicted, the measure is said to exhibit construct validity. It should be noted that there is no “right answer”; the predictions are based on expert opinion and knowledge of the various measures to be considered. In this study, a series of a priori predictions about expected correlations of the CMAS with other measures of physical function, muscle strength, and disease activity were made. Correlations were assessed using Spearman's rank correlation. For the purpose of this analysis, correlations >0.7 were considered high, correlations ranging from 0.4 to 0.7 were considered moderate, and correlations <0.4 were considered low. Agreement between predicted and observed correlations was taken as evidence of construct validity.

Given that the CMAS is considered to be a measure of physical function and muscle strength, it was predicted that the correlations between scores on the CMAS and those on the C-HAQ and MMT, which measure related constructs, would be high. Correlations between the CMAS and physician global assessments of disease activity and skin activity, as well as the parent global assessment of disease severity, were predicted to be moderate, since physical function and strength are thought to be important components of disease activity (13). Correlations between the CMAS and measures of disease damage (physician global assessments of disease damage and skin damage) were predicted to be low, and correlations between the CMAS and MRI scores were predicted to be moderate. Correlations between the CMAS and serum levels of muscle-associated enzymes were predicted to be low.

In order to determine whether the CMAS exhibited different characteristics in mildly and more severely affected subjects, the group of patients with moderate-to-severe disease was identified as those with a score of ≥2 on the physician global assessment of disease activity (5-point Likert scale). Key correlations were then recalculated, and compared with those for the complete population (CMAS versus C-HAQ, CMAS versus MMT, and CMAS versus physician global assessment of disease activity [visual analog scale]).

Responsiveness (or sensitivity to change) refers to the ability of a measurement tool to detect change over time (3). The standardized response mean (SRM) was used to assess responsiveness (14). This was calculated by dividing the mean change between the 2 assessments by the standard deviation of the change scores. A 95% confidence interval (95% CI) for the SRM was determined by the method described by Beaton et al (15). The SRM was calculated for all patients with CMAS results at both assessments, and for the subset of patients who met an a priori established external standard of change (≥0.8 cm change in physician global assessment of disease activity). SRM values of 0.8 were considered large, 0.5 moderate, and 0.2 small (15).

The relative ability of the CMAS, C-HAQ, and MMT to predict physician global assessment of disease activity was assessed by linear regression modeling using data from the baseline visit. Univariate models were compared by considering the proportion of variance in physician global assessment of disease activity explained by the model, as estimated by the adjusted model R2. Bivariate and multivariate models were used to assess which combinations of measures contributed the most to explanation of the variance in physician global assessment of disease activity. To allow comparison, beta coefficients for all models were standardized by dividing each beta coefficient by its standard error.

Clinical meaning of the CMAS was assessed in several ways. First, an estimate of the minimum clinically important change was obtained through comparison with variables that have previously established estimates of MID. These variables were the C-HAQ, with an estimate of 0.13 (16), the physician global assessment of disease activity, with an estimate of 0.8 cm (17), and the parent global assessment of disease severity, also with an estimate of 0.8 cm (17). Univariate regression models were used to determine the relationship between the CMAS and each of these variables. The resulting linear regression equations were then used to calculate the change in CMAS that corresponds to the MID in these measures. These calculations accounted for sampling variability in both the regression coefficients and the MID estimates for each of these variables, resulting in very wide confidence intervals. Only children with scores recorded for all 3 measures were used in this analysis (n = 55). Because of the inherent error in these methods, this analysis was considered to be a preliminary exploration of this issue.

The clinical significance of an abnormal CMAS score was then investigated by comparing scores of juvenile IIM patients with those of healthy control subjects. Using the age- and sex-specific normal values for the CMAS9 (7), juvenile IIM patients between 4 and 9 years old (n = 55) were determined to have either a normal or an abnormal CMAS9 score. Median C-HAQ, MMT, physician global assessment of disease activity, and parent global assessment of disease severity scores in children with normal and those with abnormal CMAS9 scores were then compared using the Mann-Whitney U test.

Finally, the clinical meaning of the CMAS was investigated by estimating the range of CMAS scores that would correspond to various degrees of physical dysfunction, by comparison with data reported by Dempster et al (16) for the C-HAQ in children with chronic arthritis. The mean C-HAQ scores and 95% CIs for children with chronic arthritis and no, mild, mild-to-moderate, and moderate physical disability were 0 (−0.072, 0.072), 0.24 (0.14, 0.35), 0.71 (0.52, 0.91), and 1.53 (0.96, 2.1), respectively. Linear regression modeling was used to define the relationship between the CMAS and the C-HAQ, and using the resulting regression equation (adjusted R2 = 0.64), corresponding CMAS scores and 95% CIs were calculated for each level of physical disability. These values were assumed to be an estimate of the median CMAS score that would be seen at each level of physical disability. These calculations accounted for both the sampling variability in the mean C-HAQ scores and the regression coefficients. Once again, because of the inherent error in these methods, this analysis was considered to be a preliminary exploration of this issue.

RESULTS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Outcome measures.

The median CMAS score was 44 at baseline (interquartile range [IQR] 37–48] and 47 at the followup assessment [IQR 41–50]). The mean ± SD CMAS score was 40 ± 13.2 at baseline and 44 ± 7.6 at the followup assessment. Other outcome measures at baseline and followup were similar to those presented previously (5). The median MRI muscle edema score was 2 at baseline (range 0–4; n = 36) and 0 at followup (range 0–4; n = 22). The median MRI muscle damage score was 0.5 at baseline (range 0–2.5; n = 35) and 0.5 at followup (range 0–1.5; n = 23).

Interrater reliability.

For the 26 patients who had been assessed by 2 investigators, the intraclass correlation coefficient was 0.89, indicating very good interrater reliability.

Construct validity.

Spearman's correlation coefficients used to assess convergent construct validity of the CMAS are summarized in Table 1.

As predicted, the Spearman's correlations of the CMAS with the C-HAQ and MMT were high (rs = −0.73 and 0.73, respectively, P < 0.0001) (Figure 1A and B). Also as predicted, correlations with the physician global assessment of disease activity, physician global assessment of skin activity, and parent global assessment of disease severity were moderate (rs = −0.44 to −0.61 P < 0.0001). Correlations with physician global assessment of disease damage and physician global assessment of skin damage were low (rs = −0.15 and −0.02, respectively, P ≥ 0.1), while correlations with MRI scores of muscle edema and muscle damage were moderate, as predicted (rs = −0.57 and −0.48, respectively, P ≤ 0.004). Correlations with levels of serum muscle-associated enzymes were low (rs < −0.4, P = 0.002–0.02 [except P = 0.3 for creatine kinase]).

Table 1. Construct validity of the CMAS in juvenile IIM, determined using Spearman's correlations of the CMAS with other outcome measures*
Assessment variable (potential minimum–maximum)Median (IQR)nSpearman's correlation (rs)
  • *

    All data are from the baseline assessment. The median Childhood Myositis Assessment Scale (CMAS) score was 44 (interquartile range [IQR] 37–48). Median (IQR) values for serum muscle-associated enzymes represent laboratory values divided by the upper limit of normal for the laboratory in which the test was performed. Juvenile IIM = juvenile idiopathic inflammatory myopathy; C-HAQ = Childhood Health Assessment Questionnaire; MMT = manual muscle testing; MRI = magnetic resonance imaging.

  • All P < 0.0001 except as indicated otherwise.

C-HAQ (0–3)0.25 (0–1.13)106−0.73
Total MMT (0–100)90.3 (81.3–95.3)560.73
 Proximal MMT (0–100)89.6 (80.5–94.3)560.78
 Distal MMT (0–100)93.8 (82.4–100)560.47
 Axial MMT (0–100)87 (73–96.5)560.65
 Upper extremity MMT (0–100)91.2 (83.7–97.2)560.70
 Lower extremity MMT (0–100)91.6 (81.6–94.7)560.70
Physician global assessment of disease activity (0–10)2.1 (0.6–4.4)108−0.61
Physician global assessment of skin activity (0–10)1.6 (0.3–3.3)106−0.44
Parent global assessment of disease severity (0–10)0.8 (0.2–4.4)101−0.49
Physician global assessment of disease damage (0–10)0.5 (0–1.7)108−0.15 (P = 0.1)
Physician global assessment of skin damage (0–10)0.5 (0–1.5)105−0.02 (P = 0.9)
MRI muscle edema score (0–4)2 (0–3)36−0.57 (P = 0.0003)
MRI muscle damage score (0–4)0.5 (0–1)35−0.48 (P = 0.004)
Creatine kinase (0–no maximum)0.3 (0.2–0.6)106−0.11 (P = 0.3)
Alanine aminotransferase (0–no maximum)0.5 (0.4–0.9)96−0.26 (P = 0.01)
Aspartate aminotransferase (0–no maximum)0.9 (0.7–1.1)101−0.24 (P = 0.01)
Lactate dehydrogenase (0–no maximum)1 (0.8–1.3)72−0.36 (P = 0.002)
Aldolase (0–no maximum)0.9 (0.6–1.5)80−0.27 (P = 0.02)
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Figure 1. Relationship of scores on the Childhood Myositis Assessment Scale (CMAS) to scores on the Childhood Health Assessment Questionnaire (C-HAQ) (A) and to results of manual muscle testing (B), with superimposed regression lines. All data are from the baseline assessment.

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When only the patients with moderate-to-severe disease activity were analyzed, convergent construct validity was similar to that for the entire population. Correlations of the CMAS with the C-HAQ (rs = −0.73; n = 35), MMT (rs = 0.78; n = 18), and physician global assessment of disease activity (rs = −0.66; n = 35) were high in this subset of patients with more active disease.

Responsiveness.

For the study population with data for both assessments (n = 90), the SRM for the CMAS was 0.42 (95% CI 0.21, 0.63). When only those subjects who met the external standard of change were considered (≥0.8-cm improvement in physician global assessment of disease activity; n = 49), the SRM for the CMAS was 0.81 (95% CI 0.53, 1.09). This represents strong responsiveness, given that the latter value is an indicator of the ability of the CMAS to detect change in those subjects who have in fact experienced change in their status.

Prediction of disease activity.

Results of univariate and multivariate regression analyses to investigate the ability of the CMAS, C-HAQ, and MMT to predict disease activity, as measured by the physician global assessment of disease activity, are summarized in Table 2. In univariate models, the CMAS appeared to be the strongest predictor of global disease activity, based on having the highest model-adjusted R2, although all 3 measures were significant disease activity predictors. In bivariate and multivariate models, addition of the MMT score to the CMAS appeared to provide some additional information, whereas the C-HAQ was not statistically significant in models that included the CMAS.

Table 2. Linear regression analyses of the ability of the CMAS, C-HAQ, and MMT to predict physician global assessment of disease activity*
Predictor variableStandardized parameter estimatePModel- adjusted R2
  • *

    All data are from the baseline assessment (n = 55). See Table 1 for definitions.

Univariate   
 CMAS−7.37<0.0010.51
 C-HAQ6.46<0.0010.43
 MMT−6.64<0.0010.44
Bivariate   
 CMAS−3.350.0020.50
 MMT−2.290.03
 CMAS−2.790.0070.53
 C-HAQ0.920.4
 C-HAQ2.570.010.50
 MMT−2.840.006
Multivariate   
 CMAS−2.040.04 
 MMT−2.100.040.53
 C-HAQ0.390.7 

Clinical significance of the CMAS.

The estimates for the MID of the CMAS ranged from 1.5 to 3.0 points on a 0–52-point scale. A change of 1.5 points in the CMAS (95% CI −12, 15.2) corresponded to a 0.13-point change in the C-HAQ, a change of 3.0 points in the CMAS (95% CI −16, 22) corresponded to a 0.8-cm change in physician global assessment of disease activity, and a change of 1.8 points in the CMAS (95% CI −9.6, 13.2) corresponded to a 0.8-cm change in parent global assessment of disease severity.

Considering only 4–9-year-old children with juvenile IIM, for whom corresponding normative CMAS data were available, 19 of 55 (35%) had a normal CMAS9 score at baseline and 23 of 48 (48%) had a normal CMAS9 score at followup. The Spearman correlation coefficient of the CMAS9 with the CMAS was 0.99 (P < 0.0001). Median values for C-HAQ, MMT, physician global assessment of disease activity, and parent global assessment of disease severity in juvenile IIM study patients with normal and those with abnormal CMAS9 scores are summarized in Table 3. Scores on all measures were worse in juvenile IIM patients who had abnormal baseline CMAS9 scores compared with those who had normal baseline CMAS9 scores.

Table 3. Comparison of outcome measures in juvenile IIM patients ages 4–9 years with normal versus those with abnormal CMAS9 scores*
MeasureNormal CMAS9 score, rangeAbnormal CMAS9 score, rangeP
  • *

    All data are from the baseline assessment. Normal scores on the CMAS9 (a score based on 9 items of the CMAS for which normal data were available) were determined using CMAS data acquired from a healthy cohort of children ages 4–9 years, and then applied to the juvenile IIM study population to identify those with normal and those with abnormal CMAS9 scores. See Table 1 for definitions.

  • By Mann-Whitney U test.

C-HAQ0 (0–0.3) (n = 18)0.6 (0.1–1.2) (n = 36)0.001
MMT95 (90.5–95) (n = 9)85 (78.5–92.6) (n = 17)0.02
Physician global assessment of disease activity0.8 (0–2.5) (n = 19)2.6 (1.7–5.1) (n = 36)0.001
Parent global assessment of disease severity0.5 (0.2–1.7) (n = 17)1.6 (0.5–4.2) (n = 33)0.07

In the linear regression model relating the C-HAQ and CMAS scores, the adjusted R2 was 0.64. CMAS scores (95% CI) corresponding to no, mild, mild-to-moderate, and moderate physical disability were 48 (47.2, 48.8), 45 (43.7, 46.3), 39 (36.4, 41.6), and 30 (22.8, 37.2), respectively.

DISCUSSION

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

We have demonstrated that the CMAS exhibits very good interrater reliability, good convergent construct validity, and moderate-to-strong responsiveness in a large cohort of children with juvenile IIM. These data greatly extend the preliminary validation of the CMAS by Lovell et al (2). By documenting these key measurement characteristics, we have shown that the CMAS is a valid instrument for the assessment of muscle strength, physical function, and endurance in this population, a finding that will have potential applicability in both clinical and research contexts.

There are other tools that could potentially be used to evaluate muscle strength and/or physical function in children with juvenile IIM, including MMT, the C-HAQ, and the Myositis Function Index (18). MMT has previously been shown to exhibit good inter- and intrarater reliability (19), as well as good responsiveness in children with moderate muscle weakness (20, 21). However, it does not address issues of function or endurance, and may be difficult to assess in young children (9). The C-HAQ, in contrast, does address physical function, but has a significant floor effect (i.e., all children with mild or no disease have a score of 0, making it impossible to detect changes in physical function as the child's status approaches normal). The Myositis Function Index is not currently a viable alternative for several reasons, including the need for specialized equipment (manometer and peak flow meter), the inability to perform some maneuvers in young children (i.e., pulmonary items), the time required to perform the evaluation (30–60 minutes), and the lack of validation in the pediatric population.

For the above reasons, we believe the CMAS is the best currently available single measure for the combined assessment of muscle strength, physical function, and endurance in children with juvenile IIM. It exhibits good measurement characteristics, is easy to administer, and is rapid, taking only 10–15 minutes to perform. It possesses both face and content validity, is already widely used in pediatric rheumatology clinics, and is acceptable to both children with juvenile IIM and their caregivers. Several of the authors have found it to be a useful and practical clinical tool, particularly in situations in which there is a dilemma regarding therapy for a child with juvenile IIM. However, this does not necessarily mean that the CMAS should be the only tool used for assessment of muscle strength or physical function. When linear regression models were used to predict physician global assessment of disease activity, the CMAS, MMT, and C-HAQ were all significant predictors of disease activity, although the CMAS appeared to be the best. More importantly, MMT remained significant in a bivariate disease prediction model containing the CMAS. This suggests that MMT provides additional, important information contributing to the assessment of disease activity, beyond that provided by the CMAS.

MMT is a pure measure of muscle strength and provides a more detailed assessment of isolated muscle strength than does the CMAS. The CMAS, while addressing muscle strength, focuses more on functional performance and endurance. Our data suggest that these 2 instruments provide complementary and nonredundant information. For example, it is possible to imagine a child with significant joint contractures secondary to previous juvenile IIM, but who has relatively little disease activity and no muscle weakness. This child may potentially score poorly on the CMAS due to fixed range of motion deficits that limit the performance of certain CMAS maneuvers, but may perform significantly better on MMT when specific muscle groups are assessed. In contrast, the CMAS may demonstrate subtle alterations in muscle function or endurance when muscle strength has already returned to normal. For these reasons, it is our recommendation that both CMAS and MMT evaluations be included in clinical or research studies of children.

Given that the CMAS is already widely used and, as we have demonstrated, possesses measurement characteristics that may lead to its use in future clinical research, we thought it very important to provide some clinical context for CMAS scores. These results must be considered as estimates only, since the data were not generated with these particular questions in mind, are being compared with standards of physical function and global activity for children with juvenile rheumatoid arthritis, and the methods used could be expected to provide only rough estimates. This is reflected in the wide confidence intervals, particularly for the estimates of minimum important change for the CMAS scores. However, these preliminary guidelines for the interpretation of CMAS scores have some value in that they provide a starting point for clinicians and researchers, and will hopefully direct and stimulate future research.

These exploratory investigations provided some interesting results. First, the values that were obtained to estimate the MID were surprisingly consistent, which may imply that the true value of the MID is similar. Second, they also suggest that the minimum clinically important change in the CMAS is a relatively small amount, perhaps on the order of 5–10%. Third, through comparison with C-HAQ values for varying degrees of disability in children with arthritis, we arrived at estimates of CMAS scores corresponding to these degrees of physical disability. Although these values are very preliminary, it appears that children in whom physical function has been affected to a relatively small degree may have CMAS scores that are significantly different from those of children with no physical dysfunction. Each of these lines of investigation will require further prospective investigation and confirmation.

There have been concerns that some of the items in the CMAS may be developmentally inappropriate for very young children, particularly the timed items and sit-ups. It is possible to construct a “core” CMAS score, which consists of the 10 remaining items when the 3 timed items and sit-ups are deleted from the CMAS. This core score has the potential to be superior to the complete CMAS in very young children, but will need to be evaluated in future research. Unfortunately, the data in this study were insufficient to test this hypothesis.

Some concern about the generalizability of these results has been raised, given that the overall disease burden was relatively mild in many participants (as suggested by median CMAS scores of 44 and 47 [of a possible 52] at baseline and followup, respectively). We partially addressed this issue by demonstrating that the correlations of the CMAS with other measures (such as the C-HAQ, MMT, and physician global assessment of disease activity) were similar in children with greater disease activity. As well, responsiveness tends to be underestimated when assessed in a population with relatively little potential for improvement. We believe these results should be widely applicable, although additional data on more severely affected children would be helpful.

In conclusion, we have shown that the CMAS exhibits good measurement properties and is a valid measure of muscle strength, physical function, and endurance in children with juvenile IIM. It is an appropriate tool for research and clinical contexts. The CMAS appears to have significant advantages over the C-HAQ and appears to be most informative when used in conjunction with MMT. Exploratory investigations regarding the clinical meaning of CMAS scores, while in need of confirmation, may enhance the interpretability of CMAS scores in the clinical care of patients with juvenile IIM.

Acknowledgements

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES

We would like to thank Dr. Suzanne Bowyer for her participation in the initial data collection for this project, and Drs. Holly Cintas and Lynn Gerber for critical review of the manuscript.

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REFERENCES

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. REFERENCES
  • 1
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  • 2
    Lovell DJ, Lindsley CB, Rennebohm RM, Ballinger SH, Bowyer SL, Giannini EH, et al, in cooperation with the Juvenile Dermatomyositis Disease Activity Collaborative Study Group. Development of disease activity and damage indices for the juvenile idiopathic inflammatory myopathies. II. The Childhood Myositis Assessment Scale (CMAS): a quantitative tool for the evaluation of muscle function. Arthritis Rheum 1999; 42: 22139.
  • 3
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