Familial occurrence frequencies and relative risks for systemic sclerosis (scleroderma) in three United States cohorts

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Abstract
Objective To determine the frequency with which scleroderma (systemic sclerosis; SSc) recurs in families and the familial relative risk (λ) in the US. Methods Family histories of SSc were prospectively surveyed in 3 large US cohorts of SSc patients, 2 in Texas and 1 in Michigan. Diagnoses of familial SSc were verified by rheumatologist evaluation and/or review of medical records. Familial relative risks for first‐degree relatives (λ1) and siblings (λs) were calculated using actual reported counts of first‐degree relatives in 2 cohorts and recent estimates of SSc prevalence in the US. Results Compared with the estimated prevalence of SSc in the US (2.6 cases/10,000 population [0.026%]), the disease occurred in 1 or more first‐degree relatives in 1.5–1.7% of SSc families in the 3 cohorts (or 11 of 703 families [1.6%]), a significant increase. Familial relative risks in first‐degree relatives in the 3 cohorts ranged from 10 to 16 (13 combined), and in siblings they ranged from 10 to 27 (15 combined). Conclusion SSc occurs significantly more frequently in families with scleroderma (1.6%) than in the general population (0.026%). A positive family history of SSc is the strongest risk factor yet identified for SSc; however, the absolute risk for each family member remains quite low (<1%).