Alpha thalassemia spectrum encompasses a group of inherited hemoglobinopathies caused by pathogenic variants in HBA1 and HBA2, the two alpha-globin genes located on chromosome 16. Impaired synthesis of alpha-globin chains leads to a relative excess of beta-globin chains, producing unstable hemoglobin tetramers that result in hemolytic anemia of variable severity. The phenotypic outcome depends directly on the number of inactivated alpha-globin alleles: the normal diploid genome carries four alpha-globin gene copies (two from HBA1 and two from HBA2), and the degree of clinical disease scales with the cumulative extent of allele loss. The two clinically significant forms are hemoglobin Bart hydrops fetalis (Hb Bart) syndrome, arising from deletion or inactivation of all four alleles, and hemoglobin H (HbH) disease, most frequently resulting from deletion or inactivation of three alleles. Silent carrier states and the alpha-thalassemia trait involving one or two deleted alleles are clinically benign.

The clinical presentation of alpha thalassemia spectrum depends on the degree of alpha-globin chain deficiency relative to beta-globin chain production. Hb Bart syndrome is the most severe form. Affected fetuses are typically delivered stillborn between 30 and 40 weeks' gestation or die shortly after birth. Cardinal clinical features include generalized edema, pleural and pericardial effusions, and congestive heart failure produced by severe anemia; red cells containing Hb Bart have extremely high oxygen affinity and are incapable of effective tissue oxygen delivery. Additional findings documented in certified GeneReviews data include marked hepatosplenomegaly, massive placenta, and extramedullary erythropoiesis. Structural anomalies reported in association with Hb Bart syndrome include brain growth retardation, hydrocephalus, cardiovascular deformities, and urogenital defects. Maternal complications during affected pregnancies commonly include preeclampsia, polyhydramnios or oligohydramnios, antepartum hemorrhage, and premature delivery. HbH disease presents considerably more variably; clinical features are often identified incidentally during routine hematologic analysis in an otherwise asymptomatic individual. Most individuals with HbH disease have splenomegaly and, less commonly, hepatomegaly. The phenotypic range of HbH disease extends from mild, clinically compensated hemolytic anemia to forms requiring transfusion support. Non-deletion HbH variants, which cause more severe alpha-globin chain deficiency than deletion forms, are associated with a more severe clinical phenotype. A small number of individuals with Hb Bart syndrome have survived following intrauterine transfusion and postnatal management, as documented in an international registry.

Alpha thalassemia spectrum results from pathogenic variants in HBA1 and HBA2 on chromosome 16. The majority of pathogenic alterations are gene deletions affecting one or more of the four alpha-globin alleles; non-deletion pathogenic variants affecting HBA1 and/or HBA2 also produce disease, typically with greater clinical severity than deletion variants of comparable allele count. The severity of the phenotype correlates with the cumulative degree of alpha-globin chain deficiency relative to beta-globin chain production. The GeneReviews genetic counseling section documents autosomal recessive inheritance for the clinically significant forms (Hb Bart syndrome and HbH disease); the specific phenotypic outcome in offspring depends on the number and type of alpha-globin allele disruptions inherited from each parent.

According to certified GeneReviews data, Hb Bart syndrome is suspected when deletion or inactivation of all four alpha-globin alleles is identified (genotype: --/--). HbH disease is most frequently caused by deletion or inactivation of three alpha-globin alleles (for example, genotype --/-alpha) and can also result from non-deletion variants in combination with deletion alleles. Differentiation of deletion from non-deletion forms of HbH disease by molecular genetic testing of HBA1 and HBA2 is described in the GeneReviews management section, given the difference in clinical severity between these forms. Hemoglobin analysis using high-performance liquid chromatography (HPLC) and electrophoresis identifies hemoglobin H and hemoglobin Bart. Referral for hematologic evaluation and molecular confirmation is described in the GeneReviews chapter as part of initial diagnostic workup. Prenatal diagnosis is available for pregnancies at risk for Hb Bart syndrome or HbH disease.

According to certified GeneReviews data, Hb Bart syndrome was historically considered uniformly fatal, but outcomes have shifted with advances in prenatal testing, intrauterine blood transfusions, improved postnatal transfusion strategies, and, in rare cases, hematopoietic stem cell transplantation. An international registry of affected infants documented 39 of 69 alive following treatment; approximately 40-50% of survivors had growth restriction and approximately 20% had neurodevelopmental delay, with congenital anomalies common. Most surviving individuals with Hb Bart syndrome require lifelong transfusion support. GeneReviews data note that in utero transfusions were associated with earlier resolution of hydrops, delivery closer to term, and better neurodevelopmental outcomes compared to no intrauterine treatment. Individuals with the most severe non-deletional HbH disease who become transfusion dependent are managed similarly to those with thalassemia major per the GeneReviews chapter. The 2017 Thalassemia International Federation guidelines for non-transfusion-dependent thalassemia address management considerations for HbH disease.

Prognosis across the alpha thalassemia spectrum depends on the number and type of alpha-globin allele disruptions. Hb Bart syndrome prognosis is shifting with advances in intrauterine and postnatal therapy; however, among survivors in the international registry, growth restriction, neurodevelopmental delay, and congenital anomalies remain common, and most require lifelong transfusion. HbH disease is generally compatible with survival; clinical severity ranges from mild compensated anemia to transfusion dependence depending on the specific alpha-globin genotype. Non-deletion HbH forms carry a more severe prognosis than deletion forms. No quantitative life expectancy data beyond registry statistics for Hb Bart syndrome are provided in this packet.

Nine active clinical trials are recorded for alpha thalassemia spectrum in this packet. The most advanced investigations involve mitapivat, a pyruvate kinase activator evaluated in a set of Phase 3 trials sponsored by Agios Pharmaceuticals: NCT04770779 (transfusion-dependent thalassemia; active, not recruiting; projected completion June 2029) and NCT04770753 (non-transfusion-dependent thalassemia; active, not recruiting; projected completion December 2028). Two additional Phase 3 pediatric mitapivat trials are not yet recruiting: NCT07506863 (transfusion-dependent; projected start September 2026, completion June 2032) and NCT07517133 (non-transfusion-dependent; projected start September 2026, completion March 2032). NCT04872179 (International Registry of Patients With Alpha Thalassemia; University of California, San Francisco; recruiting; projected completion January 2037) and NCT06213402 (RADeep European epidemiological platform for rare anemia disorders; recruiting; projected completion November 2036) represent ongoing longitudinal registry and epidemiological studies. The classified publication landscape includes 120 publications, with gene therapy research, biomarker studies, and recent trial-linked publications represented.