Objective: To study the incidence of systemic connective tissue diseases (CTD) and of systemic onset juvenile arthritis (S-JA) in children aged 0 to 15 years.
Methods: A nationwide, prospective, hospital based series of new patients from Finland collected during a 4-year period and supplemented with data from the National Hospital Discharge Register. The population at risk was 1.02 million. Classification criteria developed for adult CTD and the American Rheumatism Association criteria for JA were used.
Results: The annual incidence rates found were as follows: systemic lupus erythematosus 0.37, polymyositis/dermatomyositis (PM/DM) 0.30, mixed connective tissue disease 0.10, scleroderma 0.05, and S-JA 0.47/100,000. Girls outnumbered boys in the whole series of CTD (24:9) and also in S-JA (11:8), but in PM/DM the sex distribution was even.
Conclusion: CTD and S-JA are rare before the age of 16 years. From the present incidence figures and those recently published for JA the incidence of CTD is estimated to be between 4 and 7% of that of JA, and S-JA accounts for only 2 to 4% of all JA.