A Case of Polyarteritis Nodosa with Positive Anti-Phospholipid Antibodies Presenting with Multifocal Myositis

Abstract Background: Polyarteritis nodosa (PAN) is a rarely necrotizing vasculitis in childhood and is characterized by the inflammation of small and medium vessels affecting multiple organs. Although the presence of antiphospholipid (aPL) antibodies in PAN has been documented, there is limited data on the prevalence and understanding of pathogenesis and management for such co-incidence. Case presentation: We herein reported that a 7-year-old boy without significant past medical history presented initially with high-grade fever, painful subcutaneous nodules, and ankle arthritis for 14 days prior to hospitalization. Biological findings revealed increased acute inflammatory biomarkers and no evidence of infection. The deep skin biopsy taken from the subcutaneous nodule proved leukocytoclastic medium-sized vasculitis suggestive of the PAN diagnosis. He eventually achieved a complete response with the use of intravenous corticosteroids (2 mg/kg/day) and subcutaneous methotrexate (15 mg/m2 body surface area (BSA)/week). However, the patient developed painfulness of bilateral proximal muscles and new-onset subcutaneous nodules of lower limbs after 1-week maintenance. Though the creatine phosphokinase (CPK) level was normal, the magnetic resonant imaging (MRI) showed multiple foci myositis of bilateral gluteal and femur regions. The serum level of D-dimer was remarkably elevated, and the lupus anticoagulant was positive. A combination of subcutaneous enoxaparin and pulsed cyclophosphamide (500 mg/m2 BSA) and three-day methylprednisolone (30 mg/kg/day) have contributed to a favorable outcome in this case. She further sustained remission on maintenance of gradually tapering doses of oral prednisolone and methotrexate. Conclusions: We describe a case of PAN with the presence of aPL antibody manifesting with multifocal myositis. It is recommended that general testing for aPL antibodies should be undergone in patients with PAN, as well as other systemic vasculitis. Despite unusual co-incidence, aPL antibodies might worsen systemic vasculitis through thrombotic events, which clinicians should consider adequate coagulant therapeutics besides immunosuppressors.