Given the four loci of alpha-globin, four possible alpha-thalassemia syndromes can result depending on what number of loci are affected.
One deletion: Silent Carrier
Patients with only one alpha-globin deletion are asymptomatic and typically do not display anemia or microcytosis.
Two deletions: Alpha-thalasemia Minor
Patients with two alpha-globin deletions are still asymptomatic but typically display a microcytic anemia on laboratory examination.
Three deletions: Hb H Disease
Patients with three alpha-globin deletions display a symptomatic microcytic anemia with moderate extravascular hemolysis that may yield splenomegaly. Most patients survive to adult life and only become transfusion-dependent in their later years.
Four Deletions: Hydrops Fetalis
Deletion of all four alpha-globin chains is not compatible with life and results in hydrops fetalis.