Objective: The rarity of reports on extended multiplex families points out that the genetic component in juvenile idiopathic arthritis (JIA) might not be particularly strong. Our objective was to determine the frequency of chronic inflammatory rheumatic diseases among the parents who had two or more offspring affected by JIA.
Methods: During the last 17 years patients with JIA treated at the Rheumatism Foundation Hospital in Heinola and their parents have been systematically asked about the familial occurrence of rheumatic diseases. A total of 45 families with more than one sibling affected by JIA were found among about 2,300 JIA cases. In these "multicase families", 9 parents from 8 families also had a diagnosis of chronic inflammatory rheumatic disease. Their case histories were studied.
Results: Four of the parents had had JIA (one subsequently developed ankylosing spondylitis), and 4 had rheumatoid factor-negative chronic arthritis (one had also had chronic iritis since the age of 10, resembling that seen in JIA). Three of them had features of JIA and only one met the classification criteria for rheumatoid arthritis. One had ankylosing spondylitis.
Conclusions: Since the expected number of JIA cases among the 90 parents was about 0.2, there was drastic increase in JIA frequency among the parents in families with multiple offspring also affected by JIA. These results suggest that JIA susceptibility genes may likely be clustered in these families.