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

OBJECTIVE: To determine the frequency with which scleroderma (systemic sclerosis; SSc) recurs in families and the familial relative risk (lambda) 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 (lambda1) and siblings (lambdas) 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%).