Alpha-thalassemia (α-thalassemia) has two clinically significant forms: hemoglobin Bart hydrops fetalis (Hb Bart) syndrome (caused by deletion/inactivation of all four alpha globin [α-globin] genes; --/--), and hemoglobin H (HbH) disease (most frequently caused by deletion/inactivation of three α-globin genes; --/-α). Hb Bart syndrome, the more severe form, is characterized by prenatal onset of generalized edema and pleural and pericardial effusions as a result of congestive heart failure induced by severe anemia. Extramedullary erythropoiesis, marked hepatosplenomegaly, and a massive placenta are common. Death usually occurs in the neonatal period. HbH disease has a broad phenotypic spectrum: although clinical features usually develop in the first years of life, HbH disease may not present until adulthood or may be diagnosed only during routine hematologic analysis in an asymptomatic individual. The majority of individuals have enlargement of the spleen (and less commonly of the liver), mild jaundice, and sometimes thalassemia-like bone changes. Individuals with HbH disease may develop gallstones and experience acute episodes of hemolysis in response to infections or exposure to oxidant drugs. [from GeneReviews]