Scleroderma epidemiology.
Evidence from multiple sources indicates that SSc does not occur randomly in the population; there are particular groups who are at greater risk. The overall incidence rate of SSc in the adult population of the United States is approximately 20 per million per year; this rate has increased from 1944 to 1973, but has been relatively stable since that time. The prevalence of SSc in the United States also seems to be stable over the past two decades with a prevalence estimate for adults of 240 per million. Recent population studies suggest that SSc occurs more frequently in the United States than in continental Europe, the United Kingdom, and in some areas in Asia. Overall survival has improved over the past few decades; mean survival is approximately 12 years from diagnosis. Renal disease accounts for some of the early mortality, but pulmonary disease has emerged as a major cause of death. Cardiac disease is also correlated with a poorer prognosis. Gastrointestinal involvement contributes to morbidity and, indirectly, to mortality, but the magnitude of this contribution is difficult to assess. Women are affected more frequently than men but the factors that are responsible for this are not apparent. Studies of reproductive history have provided conflicting data; some studies suggested that the number of pregnancies may influence later disease expression, whereas other studies found no correlation. Racial factors seem to play a role in disease susceptibility, as well as disease expression. Age-specific incidence rates are higher in black women than in white women; the greatest difference occurs in the young to middle adult age group (less than 54 years of age). Diffuse disease also seems to occur more commonly in the black population than in the white. Age at onset of diffuse disease is younger, on average, than the age at onset of limited disease; age-adjusted survival is worse in black women than in white women because of their predilection for diffuse disease. Reports of geographic clustering are intriguing; the reported clusters in London and Italy have not resulted in the identification of potential causal factors. The Choctaw Native American cluster suggests that genetic factors that have not been identified may play an important role. Familial clustering of SSc has been demonstrated in the United States and in Australia, which provides additional evidence of a genetic component. It is likely that there is a strong interplay among genetic factors, hormonal or reproductive-related events, and an external trigger that must interact to result in clinical disease.