A strong familiality of ankylosing spondylitis through several generations

To elucidate the familiality of ankylosing spondylitis (AS) in Iceland.

The Icelandic genealogy database and population-wide data on all living Icelanders diagnosed as having AS (n=280), who previously had taken part in an epidemiological study on the prevalence of AS in Iceland, were included in the study. Identification of all interpatient relationships in the genealogy database allowed calculation of estimates of the RR for AS in the first-degree relatives (FDRs) to fourth-degree relatives of patients. For each AS proband, 1000 sets of matched Icelandic subjects in the genealogy database were used as controls.

FDRs, second-degree and third-degree relatives had RRs of 75.5, 20.2 and 3.5, respectively (all p values <0.0001), indicating a significantly increased risk for relatives of the patients with AS to develop AS, suggesting a strong heritable factor, while the fourth-degree relatives had a RR of 1.04 (p=0.476) for having AS.

Patients with AS in Iceland are significantly more related to each other than to randomly sampled control subjects. This is in agreement with previous reports on the familiality of AS, but the present study has more power and extends over larger familiar cohorts than previously reported.