LAUSR

OBJECTIVES Chronic nonbacterial osteomyelitis (CNO)/chronic recurrent multifocal osteomyelitis (CRMO) is an autoinflammatory disorder characterised by aseptic inflammation of bone tissue. The number of patients and the clinical picture of CNO/CRMO in Japan is unknown; therefore, we conducted the first nationwide epidemiological survey. METHODS In the primary survey, the number of patients diagnosed with CNO/CRMO between 2015 and 2019 in paediatrics, rheumatology, orthopaedic surgery and dermatology departments of various institutions was investigated. In the second survey, the clinical pictures of these patients were assessed. RESULTS The primary survey reported 289 patients, and the estimated number of patients nationwide was 434 (95% confidence interval, 367-501). The second survey analysed 205 patients. The median number of radiological bone lesions was three and the bone affected most frequently was the tibia. Tumour necrosis factor inhibitors showed higher efficacy and lower relapse rates than NSAIDs or bisphosphonates. Cases that responded to tocilizumab and canakinumab were also reported. Earlier diagnosis was associated with higher rates of symptom- or drug-free status at the last follow-up visit. CONCLUSIONS This is the first nationwide study in Japan, and one of the largest multi-centre studies worldwide, to provide comprehensive epidemiological data about CNO/CRMO.