Objectives Chronic Recurrent Multifocal Osteomyelitis (CRMO), also known as chronic nonbacterial osteomyelitis (CNO), is a rare autoinflammatory condition affecting the bones in children and teenagers. The actual incidence of CRMO remains uncertain. The objective of this study is to identify the incidence of CRMO in children and young people under the age of 16 years in the United Kingdom (UK) and Republic of Ireland (ROI). We also aim to delineate the demographics, clinical presentation, investigations, initial management and healthcare needs for children and adolescents with CRMO. Methods We conducted monthly surveys among all paediatric consultants and paediatric orthopaedic surgeons to identify patients newly diagnosed with CRMO between October 2020 and November 2022. A standardised questionnaire was sent to reporting clinicians to collect further information. Results Over the surveillance period, 288 patients were reported, among which, 165 confirmed and 20 probable cases were included in the analysis. The highest incidences were among 8-10 year-olds. A two-to-one female-to-male difference in incidence was observed, and male patients were more likely to present with multifocal disease. A negative correlation was observed between reporting clavicular and leg pain. Investigation-wise, 80.0% of patients were reported to have undergone whole-body MRI and 51.1% had bone biopsies. The most common initial treatments were NSAIDs (93.9%) and bisphosphonates (44.8%). Conclusion This study estimates an average annual CRMO incidence of 0.65 cases per 100,000 children and adolescents in the UK and ROI. These findings establish a crucial baseline for ongoing research and improvement in the care of individuals with CRMO.

Athimalaipet Ramanan has received Speaker fees/Honoraria/Consultancy from Abbvie, Eli Lilly, Pfizer, Novartis, SOBI and UCB. Sandrine Compeyrot-Lacassagne has received Consultancy fees from Galapagos/AlfaSigma.

This work was supported by the British Paediatric Surveillance Unit Sir Peter Tizard Research Bursary and Addenbrookes Charitable Trust. C.S. is supported by an NIHR clinical lectureship.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

This study was approved by the British Paediatric Surveillance Unit Scientific Committee. Ethical approval was gained from London - Central Research Ethics Committee (reference: 20/LO/0195); Health Research Authority Confidentiality Advisory Group (reference: 20/CAG/0029); Public Benefit and Privacy Panel for Health and Social Care (reference: 1920-0202).

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All data produced in the present study are available upon reasonable request to the corresponding authors.