Objective. To assess the reliability and validity of the Disease Activity Score (DAS), an instrument used to evaluate children with juvenile dermatomyositis (JDM). Methods. Psychometric study of internal consistency, reliability, rater agreement, and the relationship with measures of muscle strength and disability was conducted. Results. The DAS ratings are internally consistent (reliability ‫؍‬ 0.89) and describe a wide range of disease activity. The pediatric rheumatologists in this study agree on the presence of most of the disease indicators. Their disagreements tend to cancel each other, resulting in highly correlated (r ‫؍‬ 0.79) overall measures across raters. Estimates of muscle weakness using the DAS and ratings of muscle strength obtained independently from therapists are highly related (r ‫؍‬ ؊0.77), but estimates of disease activity and disability are weakly related (r ‫؍‬ 0.20). Conclusion. The DAS exhibits evidence of good reliability and validity. The combination of skin and muscle strength assessments makes this easily administered instrument a useful addition in the evaluation of children with JDM. KEY WORDS. Juvenile dermatomyositis; Disease activity; Reliability; Validity. INTRODUCTIONJuvenile dermatomyositis (JDM) is the most common of the pediatric inflammatory myopathies, with an annual incidence of 3.2 per 1 million children in the US (1) and a mean age at diagnosis of 6.3 years (SD ϭ 3.4 years) (2,3).There are few validated assessments of either clinical (4) or laboratory-based (5) data that reliably evaluate the disease activity in children diagnosed with JDM. The assessments that are available focus on muscle strength and do not quantify levels of cutaneous involvement. Because the measurement of muscle strength is problematic in children who are younger than 4 years, such assessments are typically used for examinations of children who are older. As a result of this dearth of information, recent publications have focused on identifying the important variables in assessing disease activity as well as tissue damage (6 -9). Recently, we have documented that the cutaneous manifestations in JDM are strongly associated with evidence of systemic derangement of blood vessel morphology, thus strengthening the importance of the inclusion of the extent and severity of skin involvement in the assessment of disease activity (10). Several laboratory-based methods are now available as markers for disease activity, but no single test has yet proven to be a reliable indicator (5,8,11). Thus, there is an urgent need for a disease activity assessment that can measure more than muscle strength and be used to evaluate children of all ages who are diagnosed with JDM.The Juvenile Myositis Clinic at Northwestern University Medical School's Children's Memorial Hospital (Chicago, Illinois) has evaluated more than 210 children with JDM during the past 2 decades. In this time, a rapid assessment of each child's clinical status has evolved and been described as a measure of disease activity (to differentiate it fro...

Little is known concerning factors associated with the outcome of juvenile dermatomyositis (JDM), which can be variable and lethal. Previous work has documented that the association of DQA1*0501 with JDM is higher than in control groups and that the first symptoms (rash and weakness) of JDM appear to follow evidence of an infectious process--most frequently upper respiratory in nature. Preliminary data show that a long period of symptoms being left untreated before starting therapy and the TNF alpha-308A allele are associated with prolonged JDM symptoms requiring > or = 36 months of immunosuppressive therapy. A short duration of untreated disease is associated with a relative increase in CD8(+) T cells and CD56(+) natural killer (NK) cells in the untreated JDM muscle biopsy compared with a longer duration of untreated disease. The TNF alpha-308A allele is overrepresented in white children with JDM. In addition, it is associated with pathologic calcifications, increased production of TNF alpha by peripheral blood mononuclear cells in vitro and JDM muscle fibers in vivo, and occlusion of capillaries, which may be mediated in part by elevated circulating levels of thrombospondin-1, a potent anti-angiogenic factor. We speculate that DQA1*0501 is associated with JDM susceptibility to an infectious process, eliciting and activating NK cells early in the disease course. We conclude that the TNF alpha-308A allele indicates directly (or is a surrogate marker of) children with JDM who produce higher concentrations of TNF alpha in response to this undefined inflammatory stimulus, as well as increased concentrations of TSP-1 with resultant small vessel occlusion, contributing to subsequent disease chronicity.

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