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 ) 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.
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- PATIENTS AND METHODS
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.