Objective: To quantify the frequency of siblings of patients with psoriatic arthritis (PsA) having psoriasis and/or inflammatory arthritis. To describe the similarity or otherwise of patterns of arthritis in siblings.
Methods: Available and consenting index cases with PsA and one or more siblings living locally were assessed. Mean sibling concordance rates and Weinberg's segregation analysis were calculated. Heritability was also estimated. To assess whether the same type of arthritis occurred within the same sibship, the dually affected sibships were then classified for type of arthritis according to methods suggested by Moll, Helliwell, Veale and McGonagle.
Results: Eighty index cases and 112 siblings were assessed. The median age of index cases was 49 yr (range 24-80 yr) and for siblings 46 yr (range 18-79 yr). The concordance rate for all types of PsA was 14% (9% if enthesitis is excluded) and for psoriasis 21%. There was no difference in the two methods used to calculate concordance rates. Sixteen dually affected sib pairs were found. Four of the 16 sibships (25%) had the same pattern of joint involvement (Moll and Wright classification). The most frequent pattern seen was joint involvement identical to rheumatoid arthritis (3/5). The most common symptom in affected siblings was enthesitis (approximately 5%). When the dually affected sibships were analysed using the other classifications, the simpler the classification the greater the concordance for joint pattern.
Conclusion: The concordance for psoriasis is greater than for PsA, but the concordance rate for PsA was similar to that in HLA identical siblings with rheumatoid arthritis. There was discordance in pattern of arthritis for most sib pairs. There is no support for the use of more complex classifications of PsA.