Abstract
An 8-year-old girl developed muscle weakness localized in the thighs and upper extremities and had
difficulty walking. She had a skin rash, severe muscle weakness, elevated serum creatine kinase levels, and a low
score on the Childhood Myositis Assessment Scale. T2-weighted magnetic resonance imaging of the muscles
revealed lesions as signal high intensity consistent with myositis. Therefore, the patient was diagnosed with
juvenile dermatomyositis（ JDM）. She was treated with two courses of methylprednisolone pulse therapy, with no
improvement in symptoms, leading to the diagnosis of JDM-associated macrophage activation syndrome（ MAS）.
Accordingly, she showed elevated serum markers of tissue damage and ferritin with a decreased platelet count.
The low levels of serum interleukin-6 and interleukin-18 were inconsistent with MAS with systemic juvenile
idiopathic arthritis. Eventually, her symptoms alleviated in response to cyclosporine and dexamethasone palmitate
without flare. There have only been a few reports of JDM-associated MAS ; hence, its clinical features have not
been well-understood. For this patient, we had considered anti-melanoma differentiation-associated gene 5
autoantibody-positive JDM to have resulted in elevations of serum myogenic enzyme and ferritin, which might
have caused the delay in the diagnosis of MAS.
