Longitudinal follow-up of mixed connective tissue disease and overlapping autoimmune diseases of childhood onset in the Afro-descendant population of the French West Indies.

Introduction: Overlap autoimmune syndromes (OAS) and mixed connective tissue disease (MCTD) are rare in children. We performed a retrospective, longitudinal and descriptive study of Afro-Caribbean patients from the French West Indies followed for MCTD and OAS to describe their characteristics and outcomes during childhood.

Methods: Retrospective study from January 2000 to 2023. Listings of patients were obtained from multiple sources: computerized hospital archives and national hospital-based surveillance system, registry of pediatricians and adult specialists in internal medicine and the national registry for rare diseases. MCTD was defined according to Kasukawa's criteria. OAS was defined as overlapping features of systemic lupus erythematosus (SLE), systemic sclerosis (SSc), and dermatomyositis/autoimmune myositis (DM/AM).

Results: Sixteen patients were included over a 23-year period (10 MCTD and 6 OAS). The incidence was 0.23 per 100,000 children-years. The mean age at diagnosis was 11.9 years old (2.4-17) with median follow up of 7.9 years (2.1-19.6). SLE phenotype was present in the highest, followed by SSc and DM/AM. Patients had an average of three flares during childhood (1-7). A quarter (25%) had symptomatic pulmonary arterial hypertension (PAH). Ninety-four percent received steroids during follow-up and 88% required a corticosteroid-sparing therapy. Three patients (19%) developed SLE after more than 10y of follow-up. There were no death and no chronic organ failure.

Conclusion: This is the largest pediatric cohort of MCTD and OAS in Afro-descendant patients treated in a country with a high standard of care. The clinical evolution did not differ between MCTD and OAS. The main complication was PAH, more frequent in our cohort.