A case of Schnitzler syndrome complicated by rheumatoid arthritis treated with methotrexate.

Schnitzler syndrome (SchS) is a rare autoinflammatory disorder characterised by recurrent urticaria, monoclonal gammopathy, fever, and arthralgia. We herein report a case of SchS complicated by rheumatoid arthritis (RA). A 78-year-old man presented to our hospital with fever, myalgia, and urticaria, each lasting for ~1 week over the past 8 years. He was diagnosed with SchS based on monoclonal immunoglobulin G gammopathy, chronic urticaria, an intermittent fever, arthritis, high inflammatory markers, and neutrophil infiltration in the dermis on skin biopsy. Although colchicine improved the symptoms slightly, the patient subsequently developed arthritis and was diagnosed with RA based on elevated anti-cyclic citrullinated peptide antibody and rheumatoid factor levels. Methotrexate (MTX), a first-line therapy for RA, was initiated, resulting in a remarkable improvement in both RA and SchS symptoms. This case highlights the rare coexistence of SchS and RA as well as the efficacy of MTX in treating both conditions. Although interleukin (IL)-1 inhibitors are considered the most effective treatment for SchS, they are not approved for SchS in Japan and are expensive. The efficacy of IL-6 inhibitors in SchS has also been reported, and MTX suppresses inflammatory cytokines, including IL-1 and IL-6. As shown in our case, drugs that modulate IL-6 levels, such as MTX, may be a viable treatment option for SchS.