Adult-Onset Still's Disease Mimicking Myositis: A Case Report

The combination of severe myalgia, progressive weakness, and blood in the urine often leads a neurologist to consider myositis. Accordingly, reddish urine may be linked to urine myoglobinuria brought about by muscle destruction. Nevertheless, in a young patient with normal creatine kinase complaining of immobility, adult-onset Still's disease (AOSD) should be one of the top differentials. We discuss a case of a 25-year-old Filipino male who presented to our clinic with a month's history of progressive, generalized weakness, joint pains, and rashes, accompanied initially by undocumented fever, hematuria, and loss of appetite. C-reactive protein (CRP; >5 mg/L) and erythrocyte sedimentation rate [ESR; 21 mm/hr, normal value: <10 mm/hr] were elevated pre- and post-methylprednisolone pulse therapy (MPPT) (CRP: 211 mg/L; ESR: 125 mm/hr). The extremely high serum ferritin levels (1675.56 ug/L, above the machine detection limit) clinched the diagnosis of AOSD. Early detection of AOSD is cost-effective and highly beneficial, as further workup for myositis involves costly antibody testing and unnecessary invasive muscle biopsies.