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

Objective. To determine the frequency with which scleroderma (systemic scle rosis; SSc) recurs in families and the familial relative risk (lambda) in t he 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 familia l SSc were verified by rheumatologist evaluation and/or review of medical r ecords. Familial relative risks for first-degree relatives (lambda1) and si blings (lambdas) were calculated using actual reported counts of first-degr ee 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-degre e relatives in 1.5-1.7% of SSc families in the 3 cohorts (or 11 of 703 fami lies [1.6%]), a significant increase. Familial relative risks in first-degr ee relatives in the 3 cohorts ranged from 10 to 16 (13 combined), and in si blings they ranged from 10 to 27 (15 combined). Conclusion. SSc occurs significantly more frequently in families with scler oderma (1.6%) than in the general population (0.026%). A positive family hi story of SSc is the strongest risk factor yet identified for SSc; however, the absolute risk for each family member remains quite low (<1%).