thalassemia, thalassanemia

Any of a group of inherited disorders of hemoglobin metabolism in which there is impaired synthesis of one or more of the polypeptide chains of globin; several genetic types exist, and the corresponding clinical picture may vary from barely detectable hematologic abnormality to severe and fatal anemia. [G. thalassa, the sea, + haima, blood]

- α t. t. due to one of two or more genes that depress (severely or moderately) synthesis of α-globin chains by the chromosome with the abnormal gene. Heterozygous state : severe type, t. minor with 5–15% of Hb Barts at birth, only traces of Hb Barts in adult; mild type, 1–2% of Hb Barts at birth, not detectable in adult. Homozygous state : severe type, erythroblastosis fetalis and fetal death, only Hb Barts and Hb H present; mild type not clinically defined. SEE ALSO: hemoglobin H.

- A₂ t. β t., heterozygous state.

- β t. t. due to one of two or more genes that depress (partially or completely) synthesis of β-globin chains by the chromosome bearing the abnormal gene. Heterozygous state (A₂ t.): t. minor with Hb A₂ increased, Hb F normal or variably increased, Hb A normal or slightly reduced. Homozygous state : t. major with Hb A reduced to very low but variable levels, Hb F very high level.

- β-δ t. t. due to a gene that depresses synthesis of both β- and δ-globin chains by the chromosome bearing the abnormal gene. Heterozygous state : t. minor with Hb F comprising 5–30% of total hemoglobin but distributed unevenly among cells, Hb A₂ reduced or normal. Homozygous state : moderate anemia with only Hb F present, no Hb A or Hb A₂. SYN: F t..

- F t. SYN: β-δ t..

- t. intermedia a clinical variant of t. characterized by an intermediate degree of severity. These patients have severe anemia but usually do not require regular blood transfusions. Intermedia disorders represent a heterogeneous group of genetic disorders and may include cases with homozygous or heterozygous abnormalities in the β-globin chain gene.

- α t. intermedia hemoglobin H.

- Lepore t. [MIM*142000.0020 and others] t. syndrome due to production of abnormally structured Lepore hemoglobin. Heterozygous state : t. minor with about 10% of Hb Lepore, Hb F moderately increased, Hb A₂ normal. Homozygous state : t. major with only Hb F and Hb Lepore produced, no Hb A or Hb A₂.

- t. major [MIM*141800–142310 passim] the syndrome of severe anemia resulting from the homozygous state of one of the t. genes or one of the hemoglobin Lepore genes with onset, in infancy or childhood, of pallor, icterus, weakness, splenomegaly, cardiac enlargement, thinning of inner and outer tables of skull, microcytic hypochromic anemia with poikilocytosis, anisocytosis, stippled cells, target cells, and nucleated erythrocytes; types of hemoglobin are variable and depend on the gene involved. SYN: Cooley anemia, primary erythroblastic anemia.

- t. minor [MIM*141800–142310 passim] the heterozygous state of a t. gene or a hemoglobin Lepore gene; usually asymptomatic and quite variable hematologically, with target cells, mild hypochromic microcytosis, and often slightly reduced hemoglobin level with slightly increased erythrocyte count; types of hemoglobin are variable and depend on the gene involved.