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Juvenile dermatomyositis (JDM) is both the most common inflammatory myopathy of childhood and a rare disease. The investigation and management of which has slowly evolved over the last two decades, necessitating a rethink of diagnostic criteria. The Children's Arthritis and Rheumatology Research Alliance (CARRA) and the United Kingdom Juvenile DM Cohort are leading data generators in this field, supplemented in this issue of the Journal by two smaller cohorts of JDM from the diverse APLAR region.[1, 2]

Prasad et al.[1] from India and Gowdie et al.[2] from Australia report a prevalence of muscle weakness, Gottron's papules, and heliotrope rash not so greatly different from the initial 1975 descriptions by Bohan and Peter,[3, 4] and very similar to the 2011 description of European and Latin American patients with JDM,[5] in spite of different time period and sociocultural diversities. This two cohorts provide useful insights into the diverse clinical manifestations over and above those currently used for diagnostic classification, and both emphasise dysphagia and dysphonia. Disease manifestations may change between early and late childhood, with the UK JDM cohort reporting that children with disease onset before age 5 years were more likely to present with oedema and ulcerative skin disease.[6] Gowdie et al.[2] found nail fold changes in 68% of their cohort unlike the finding of reduced nailfold capillary density virtually in all JDM patients in a longitudinal study by Schmeling et al.[7] Although capillaroscopic change seems to be a marker of both skin and muscle disease activity,[8] and has been suggested as a diagnostic criterion, it requires further refinement and precision to become a clinically useful tool in JDM.[9]

The dreadful complication of calcinosis cutis occurs in 20% to 40% of cases and more so with increasing disease duration.[10, 11] Delayed or inadequate therapy and persistent skin inflammation are thought to be predisposing factors.[12] None of the children in the Australian series [2] had calcinosis at diagnosis, though 18% had developed this probably in the more chronic phase. The Indian series[1] had 27% of their children with calcinosis at presentation or during follow up, which has been reported by other Indian studies, and is higher than reports from other countries [13] possibly due to a delayed diagnosis and initiation of treatment, thereby a higher cumulative period of active disease and accrual of damage. Indeed, the median duration of symptoms prior to diagnosis in the Indian study was 9.25 months [1] as compared to 2.8 months in the Australian report [2] and 5 months in the cohort of 384 children of United states pooled from 55 paediatric rheumatology clinics.[13] The factors influencing the variation in time to diagnosis and initiation of therapy which favourably impacts on both mortality and morbidity warrant further study.

Magnetic resonance imaging (MRI) is a new tool to identify muscle inflammation and subcutaneous edema and it correlates well with other measures of disease activity.[14] The original Bohan and Peter criteria require at least two of elevated muscle enzymes, myopathic EMG, or muscle biopsy – the latter two being relatively invasive within a juvenile population. Both cohorts [1, 2] comment on their marked reduction in undertaking muscle biopsy in the last decade. The Australian cohort has also ceased EMG testing in favour of MRI. The CARRA registry reports MRI as the most commonly performed study in nearly all enrollees, and was more likely (91%) than EMG (50%) or muscle biopsy (76%) to reveal abnormalities consistent with JDM.[13] Gowdie et al.[2] performed MRI in 50% of children with JDM and in 97% showed evidence of myositis. MRI has rapidly becoming the preferred non-invasive test indicating muscle inflammation, displacing muscle biopsy and EMG in the diagnosis of JDM and it is heartening to see the CARRA registry including MRI evidence of myositis as a fifth diagnostic/classification criterion for definite diagnosis of JDM.[13]

Management of JDM with corticosteroids in conjunction with weekly methotrexate as the mainstay of therapy is based on consensus opinions rather than randomized trials due to the rarity of this serious illness.[15, 16] Use of methotrexate was 100% in the 2002–2011 series of Prasad et al., as compared to only 63% in Gowdie's series which increased to 86% amongst their post year 2000 patients. A CARRA treatment survey indicated that more than 80% of North American paediatric rheumatologists would use methotrexate as part of initial therapy for moderate JDM.[17] Comparably over 90% of the UK JDM group patients received treatment with methotrexate and corticosteroids,[6] similar to the CARRA group in whom 95% had been treated with corticosteroids and 92% with methotrexate.[13] Furthermore, all three CARRA JDM consensus treatment protocols include methotrexate.[15, 16]

Both the JDM cohorts published in this issue portray changing pattern of diagnostic (use of MRI) and therapeutic approach (use of methotrexate) over the years, apart from highlighting similarities and differences across ethnicities. However, such cohorts are neither designed, nor powered to assess treatment outcome of JDM. The rarity and low incidence of JDM precludes RCTs in the acute management of JDM even with collective cohorts such as CARRA and the UK JDM group. Alternatives to the RCT model of evidence such as comparative effectiveness research are eagerly awaited in this movement sapping disease.

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