A71: Localized Myositis as a Manifestation of Systemic-Onset Juvenile Idiopathic Arthritis: A Case Series From a Tertiary-Care Hospital
Authors: Maleewan Kitcharoensakkul, Andrew J. White, Anthony R. French, Megan A. Cooper, Kevin W. Baszis
Journal: Arthritis & Rheumatology (2014)

Background/Purpose:

Localized myositis is rarely reported in patients with systemic-onset JIA (sJIA), despite common symptoms of myalgia. We describe sJIA patients with the manifestation of localized myositis.



Methods:

We reviewed the charts of 6 pediatric patients with sJIA who developed localized myositis during their disease course.



Results:

The age at presentation with myositis ranged from 5 to 19 years. 4 were males and 2 were females. Races included Caucasian (4), Caucasian/Mayan (1), and African-American (1). Three patients had localized myositis with initial presentation of sJIA, and 3 patients developed localized myositis during disease flares. For the disease flare group, the time from diagnosis of sJIA until development of myositis ranged from 3 to 6 years, and 2 of 3 patients were on immunosuppressive agents: methotrexate (MTX) (1), anakinra and MTX (1). All 6 patients complained of localized swelling and pain in a localized muscle group. Accompanying symptoms included arthralgia, arthritis, fever and rash. Three patients had localized erythema of the skin overlying myositis areas on exam. Sites of myositis included unilateral biceps (3), bilateral biceps (1), quadriceps (1), and flexor pollicis longus and flexor digitorum superficialis and profundus (1). All patients had at least one muscle enzyme obtained, and all were normal. Five patients had imaging studies demonstrating evidence of myositis: 2 by ultrasound, 2 by ultrasound and MRI, and 1 by MRI. All 6 patients had laboratory evidence of systemic inflammation, including elevated ESR (41–172 mm/hr) and CRP (54–269 mg/L). Four patients had thrombocytosis (462,000–834,000/μL). Two patients transferred care to another hospital, thus we had no information on follow-up. The remaining 4 patients responded well to prednisone, with resolution of their symptoms within 4 weeks. One patient underwent incision and drainage for a fluid collection and myositis of biceps muscle. He was initially treated with antibiotics and started on anakinra after negative cultures. Duration of follow-up since onset of myositis was 6–14 months, and no patients had recurrent myositis. The current treatment of the 4 patients includes infliximab (1), MTX (1), canakinumab and leflunomide (1), and canakinumab and MTX (1).



Conclusion:

Localized myositis is a rare extraarticular manifestation of sJIA and can present either as an initial manifestation or during a disease flare. Patients usually present with unilateral or bilateral localized swelling, overlying warmth and erythema, and pain of one muscle group, most commonly the biceps in our series of patients. Our subjects' muscle enzymes were normal, likely secondary to the focal nature of muscle inflammation. Imaging studies are helpful in establishing the diagnosis. To our knowledge, this study represents the largest series to date of sJIA patients with localized myositis.