A Case of Synovitis Acne Pustulosis Hyperostosis Osteitis (SAPHO) Syndrome With Myeloperoxidase Anti-neutrophil Cytoplasmic Antibody: Exploring an Association or Coincidence.

We report the first documented case of synovitis, acne, pustulosis, hyperostosis, and osteitis (SAPHO) syndrome in a Japanese male with positive serum myeloperoxidase antineutrophil cytoplasmic antibodies (MPO-ANCA). The patient, who initially presented with a history of hypertension and cerebral hemorrhage at age 73, was admitted for respiratory symptoms and gastrointestinal issues. Laboratory tests revealed elevated MPO-ANCA levels (34.7 IU/mL), leading to a diagnosis of interstitial pneumonia (IP) based on chest imaging. Conservative management was initiated, and oral prednisolone (PSL) therapy (initially 20 mg/day) was introduced at age 75 due to increased C-reactive protein (CRP) levels. However, the MPO-ANCA levels recorded two and five months after therapy were 52.8 and 71.8 IU/mL, respectively. At age 76, he developed weakness in the right lower limb and gait disturbance following a stroke. Admission findings included elevated CRP levels (3.70 mg/dL) and a fresh infarction in the left corona radiata. Despite persistently elevated CRP, rehabilitation commenced. Follow-up imaging two months post-stroke showed new interstitial changes consistent with usual interstitial pneumonia (UIP) and sternoclavicular joint abnormalities suggestive of arthritis. The patient later developed a fever and a markedly high CRP level (19.91 mg/dL), prompting a resumption of PSL therapy (initially 60 mg/day). Post-treatment, interstitial pneumonia activity was controlled, and MPO-ANCA levels decreased to 3.2 IU/mL. The final diagnosis of SAPHO syndrome was established based on sternoclavicular arthritis and inflammatory changes. While MPO-ANCA is primarily linked to autoimmune vasculitis, which can occasionally be accompanied by IP, the presence of MPO-ANCA in this case of SAPHO syndrome raises questions about its chance occurrence or potential association. This case highlights the first reported occurrence of SAPHO syndrome associated with MPO-ANCA positivity and underscores the need for further research to explore the relationship between autoimmune markers like MPO-ANCA and SAPHO syndrome.