Sex ratio in the spondyloarthropathies and its relationship to phenotypic expression, mode of inheritance and age at onset.

OBJECTIVE: To investigate the interrelated effect of phenotypic expression [i.e., primary ankylosing spondylitis (1 degree AS) or disease secondary to psoriasis (Ps) AS or inflammatory bowel disease (IBD)AS], age at onset, sex and inheritance of responsible genes in AS.
METHODS: Three studies were performed to evaluate 1949 subjects with AS. Subgroups of the patients were formed for each study depending on disease type (1 degree AS = 1695; Ps AS = 173; IBD AS = 81), nature of inheritance or age at onset of AS symptoms. These groups were further subdivided to assess the effect of sex.
RESULTS: The sex ratio of the entire group was 2.6:1 in favor of men. However, IBD AS had an equal sex distribution as does IBD alone. By contrast, Ps, which has an equal sex ratio as a lone event or in association with arthritis, resulted in a male dominance of 4.1:1 when it occurred as Ps AS. Women with IBD AS had a significantly younger onset compared to women with 1 degree AS [mean onset 21.7 years (SD 6.65) vs mean onset 24.4 years (SD 9.79), respectively; p = 0.019]. A younger age at onset was found in women with familial disease [mean 22.2 years (SD 7.55)] compared with the mean onset of sporadic disease in women [24.5 years (SD 10.0); p = 0.0059]. There was a progressive fall in the sex ratio as the age at onset increased (p = 0.053). For example: M:F ratio of < 20 years old was 3:1 compared to 1.8:1 for those with an onset of > 40 years.
CONCLUSION: Sex ratio and age at onset are influenced both by each other and such factors as disease type and familial versus sporadic occurrence. These data help provide a predictable pattern of disease in spondyloarthropathy.