Alpha-Thalassemia

Article written and reviewed by Dr Luca Moretti

Alpha-thalassemia is an inherited hemoglobin disorder characterized by reduced or absent synthesis of alpha-globin chains, which are components of Hb Gower II, HbF and the adult hemoglobins HbA and HbA₂. The clinical spectrum is extremely heterogeneous, ranging from asymptomatic carriers to severe fetal manifestations that are generally lethal without intrauterine treatment, but are now compatible in a proportion of cases with survival through intrauterine transfusions and postnatal treatment. Its epidemiology reflects evolutionary history: the higher prevalence in populations from sub-Saharan Africa, Southeast Asia, India, China and the Mediterranean basin reflects the selective pressure exerted by malaria, which favored persistence of thalassemic variants.

Although less widely recognized than beta-thalassemia, alpha-thalassemia is now a major challenge in transfusion medicine, genetics and obstetrics, particularly because of the dramatic impact of its most severe forms on pregnancy and neonatal health. Early identification of carriers and molecular diagnosis of variants enable effective prevention strategies in high-risk populations.

Etiology and Genetics

Alpha-thalassemia is caused in most cases by abnormalities of the alpha-globin genes HBA1 and HBA2, located on the short arm of chromosome 16; more rarely, it may result from deletions or variants of regulatory elements upstream of the alpha-globin cluster. Unlike beta-thalassemia, each healthy individual normally has four alpha genes, two on each parental chromosome, whose integrity is essential for balanced synthesis of alpha-chain-containing hemoglobins after the transition from zeta-globin to alpha-globin synthesis.

The genetic abnormalities can be divided into:

• Deletional forms: these account for about 90% of cases. Deletion of one or more alpha genes may result from unequal crossing-over in common recurrent deletions or from other rearrangements/deletions of the alpha-globin cluster; it causes reduced (α+) or absent (α0) alpha-chain synthesis from the affected haplotype. Deletions may involve one (−α), two (−−), three or all four genes.
• Nondeletional forms: these are less common and result from point mutations that alter the coding sequence, translation termination (e.g. Hb Constant Spring, Hb Quong Sze) or mRNA stability, producing abnormal or unstable alpha chains.

The phenotypic expression of disease depends on the number of inactive alpha-globin genes:

• Loss of one gene (−α/αα): “silent carrier,” an asymptomatic condition with a normal complete blood count or mild isolated microcytosis and no clinically significant anemia.
• Loss of two genes (−α/−α or −−/αα): “alpha-thalassemia trait,” with microcytosis and absent or mild anemia that is often unrecognized; the cis (−−/αα) or trans (−α/−α) configuration has important implications for offspring.
• Loss of three genes (−−/−α): “HbH disease,” with chronic hemolytic anemia that is generally mild or moderate in deletional forms and more severe particularly in nondeletional forms, with unstable β4 tetramers (HbH).
• Loss of all four genes (−−/−−): “hydrops fetalis due to Hb Bart’s,” a condition that, without fetal treatment, generally causes hydrops and intrauterine or neonatal death because alpha chains cannot be synthesized, with formation of γ4 tetramers (Hb Bart’s) that have extremely high oxygen affinity and severely impaired tissue oxygen release.

Nondeletional variants (for example Hb Constant Spring, Hb Quong Sze, Hb Seal Rock) are often responsible for more severe phenotypes than equivalent deletions because the mutant, unstable protein may precipitate inside red blood cells, worsening hemolysis and ineffective erythropoiesis.

Genetics and inheritance: Inheritance is autosomal recessive; however, the presence of four loci produces a wide range of parental combinations, with sometimes unexpected phenotypes. Molecular diagnosis and determination of the cis or trans configuration are essential in genetic counseling, especially in high-incidence populations.

Overview of molecular physiology: Alpha-globin chains are present in Hb Gower-2 (α2ε2) during embryonic life and become components of HbF (α2γ2) and, after birth, HbA (α2β2) and HbA2 (α2δ2). Alpha chains are therefore essential not only for efficient oxygen transport but also for maintaining the structural stability of the hemoglobin tetramer and regulating oxygen affinity.

Pathogenesis and Pathophysiology

The normal balance between production of alpha and non-alpha chains (beta, gamma, delta) is essential for formation of functional hemoglobin. With alpha-chain deficiency, a relative excess of non-alpha chains develops and these cannot be incorporated into normal hemoglobin tetramers. This causes formation of abnormal tetramers: during fetal life γ4 tetramers (Hb Bart’s), characterized by high oxygen affinity, predominate, whereas after birth β4 tetramers (HbH) are unstable and may precipitate in red blood cells.

The main pathophysiologic steps are:

• Formation of abnormal hemoglobins: reduced alpha-chain synthesis limits formation of HbF, HbA and HbA2; with functional absence of all four alpha copies, these hemoglobins cannot be assembled and Hb Bart’s (γ4) predominates, whereas after birth, in forms with three inactive alpha genes, HbH (β4) forms alongside a residual proportion of HbA; both tetramers have abnormal functional properties and stability.
• Intramedullary and peripheral hemolysis: HbH and unstable alpha variants may precipitate in erythroid precursors and mature red blood cells, forming inclusion bodies that can be visualized with brilliant cresyl blue. This causes membrane injury, red blood cell rigidity and premature destruction, predominantly extravascular; Hb Bart instead mainly causes severe inefficiency of oxygen delivery to fetal tissues.
• Ineffective erythropoiesis: in clinically significant forms, especially severe HbH disease, premature destruction of erythroid precursors in the marrow may cause expansion of erythropoietic mass, extramedullary hematopoiesis and bone hyperplasia.
• Hypochromic microcytic picture: reduced hemoglobin production causes microcytosis and hypochromia; chronic anemia, reticulocytosis and increased indirect bilirubin characterize mainly HbH disease and severe forms.
• Secondary siderosis: in HbH disease and severe forms, increased intestinal iron absorption and any transfusions may cause progressive iron accumulation in tissues, with a risk of multiorgan damage.

Pathophysiologic features of the most severe forms: In hydrops fetalis due to Hb Bart’s, functional absence of alpha chains prevents formation of HbF and HbA; Hb Bart’s (γ4) has extremely high oxygen affinity and releases oxygen poorly to fetal tissues, contributing, together with severe anemia, to systemic hypoxia, massive edema (hydrops), high-output heart failure and intrauterine or early neonatal death. Anemia and hypoxia strongly stimulate erythropoietin, producing hepatomegaly, splenomegaly and placentomegaly.

In HbH disease, production of HbH (β4) only partially compensates for the deficiency of functional hemoglobin, but this molecule is unstable, oxidizes readily and precipitates in red blood cells, causing chronic hemolysis, Heinz-body formation and acute hemolytic crises under oxidative stress.

Relationship with clinical expression: Clinical variability reflects the combination of genetic abnormalities, including the number of inactive genes and type of mutation, and the presence or absence of modifying factors such as nondeletional variants and compensatory mutations. Forms that appear genetically similar may show very different phenotypes, even within the same family.

Clinical Features

The clinical expression of alpha-thalassemia is a continuum that depends on the number of inactive alpha-globin genes, the presence of nondeletional mutations and modifying factors.

Silent carrier (−α/αα): The condition is completely asymptomatic; the only laboratory finding may be isolated microcytosis with a normal or mildly increased red blood cell count and normal hemoglobin. Recognition usually occurs during family genetic studies or prenatal screening.

Alpha-thalassemia trait (−α/−α or −−/αα): Patients typically have mild, well-tolerated microcytic anemia that does not respond to iron therapy, without specific symptoms. The trait may go unrecognized and be mistaken for iron-deficiency anemia or constitutional microcytosis.

HbH disease (−−/−α): This form is the most typical symptomatic manifestation of alpha-thalassemia in children and adults. The clinical picture is characterized by:

• Chronic anemia of variable severity, generally milder in deletional forms and on average more severe in nondeletional forms, with possible acute hemolytic crises triggered by infections, oxidant drugs or pregnancy.
• Fluctuating hyperbilirubinemia, mild jaundice and scleral or cutaneous jaundice during crises.
• Splenomegaly and sometimes hepatomegaly.
• Growth delay, fatigue, reduced exercise tolerance and chronic pallor.
• Extramedullary hematopoiesis in severe cases (paraspinal masses, lymphadenopathy, skeletal deformities, early osteoporosis).
• Formation of Heinz bodies (HbH inclusions) visible on the peripheral smear with specific staining.
• Gallstones due to increased indirect bilirubin.

The course is chronic, but quality of life is good in most cases; hemolytic crises may be severe and require occasional transfusions. Forms associated with nondeletional mutations, such as Hb Constant Spring, tend to have a more severe course, with greater transfusion requirements and earlier complications.

Hydrops fetalis due to Hb Bart’s (−−/−−): This is an extremely severe fetal disorder, almost invariably associated with intrauterine or early neonatal death in the absence of fetal treatment. The clinical picture is dominated by:

• Massive edema (fetal anasarca)
• Ascites and pleural/pericardial effusions
• Marked placental enlargement (placentomegaly)
• Hepatosplenomegaly and fetal growth restriction
• High-output heart failure
• Absence of HbF and HbA, with predominance of Hb Bart’s and possible presence of small amounts of Hb Portland

This condition, in addition to being lethal for the fetus, also poses serious risks to the mother, including severe preeclampsia, hemorrhage and coagulopathy.

Nondeletional forms: Point mutations, such as Hb Constant Spring, produce unstable alpha chains that worsen the severity of HbH disease, with presentations often more similar to thalassemia intermedia or a transfusion-dependent syndrome.

Investigations and Diagnosis

The diagnostic work-up for alpha-thalassemia is based on an integrated approach that includes clinical assessment, basic hematologic tests, advanced laboratory studies and molecular diagnostics.

The complete blood count shows microcytosis of variable degree (low MCV), hypochromia (reduced MCH) and a normal or increased red blood cell count; in silent carriers, indices may be normal or only mildly abnormal, whereas chronic reticulocytosis may be evident in HbH disease.

The biochemical profile is generally normal in carriers and individuals with trait unless concomitant conditions are present; in HbH disease and transfused forms, ferritin may increase because of enhanced intestinal absorption or transfusional iron. Indirect bilirubin and LDH may increase in hemolytic forms.

The peripheral blood smear shows microcytosis, hypochromia, poikilocytosis, target cells and, in HbH disease, Heinz bodies/HbH inclusions after staining with brilliant cresyl blue.

Hemoglobin electrophoresis or HPLC may be normal in carrier and trait forms, whereas it reveals HbH (approximately 3-30%) in HbH disease. Hb Bart’s can be detected in newborns and fetuses.

Molecular diagnostics: Genetic analysis (PCR, MLPA, Southern blot, NGS) is essential for diagnostic confirmation, characterization of the mutation as deletional or nondeletional and definition of the cis/trans configuration. In pregnancies at risk of hydrops fetalis, prenatal diagnosis uses analysis of chorionic villi or amniotic fluid.

Fetal diagnosis: Obstetric ultrasound documents signs of hydrops, while prenatal confirmation is molecular on chorionic villi or amniotic fluid. In selected cases, hemoglobin analysis may be performed on fetal blood obtained by cordocentesis.

Treatment and Prognosis

Silent carrier and alpha-thalassemia trait: They require no therapy, apart from correct identification and education (avoid unnecessary iron supplementation, provide targeted genetic counseling before reproduction, and explain the inherited nature of the condition).

HbH disease: Management is individualized and multidisciplinary:

• Regular hematologic follow-up with hematologic and iron-status monitoring
• Folic acid as continuous supplementation to support increased erythropoiesis
• Transfusions occasionally during severe hemolytic crises, acute infections, pregnancy or surgery
• Prevention and management of infections (vaccinations, antibiotics when necessary)
• Splenectomy in severe hypersplenism, only after adequate vaccination; during post-splenectomy follow-up, thrombotic risk should be assessed and, in appropriate patients, an antiplatelet or antithrombotic prophylactic strategy should be considered
• Prevention and treatment of iron overload: in non-transfusion-dependent HbH disease, ferritin should be monitored and liver iron quantified by MRI when indicated; iron chelation is generally recommended with LIC ≥5 mg/g dry weight or ferritin ≥500 ng/mL.

Deletional HbH generally causes milder anemia, lower transfusion requirements and less intensive follow-up than nondeletional HbH, in which severe anemia, hemolytic crises, transfusion requirements and iron overload are more frequent. Surveillance and treatment should therefore be tailored to the molecular subtype and clinical phenotype.

In more severe cases involving nondeletional mutations, management approaches that of thalassemia intermedia, with more frequent transfusions and iron chelation therapy. In adults with anemia associated with alpha-thalassemia, mitapivat is authorized in the European Union for both transfusion-dependent and non-transfusion-dependent forms; the specific indication and continuation of treatment must follow the product information and specialist assessment.

Hydrops fetalis due to Hb Bart’s: This condition is generally lethal without intensive intrauterine or perinatal therapy: treatment requires intrauterine transfusions of compatible red blood cells, delivery planning at a tertiary center according to obstetric indications and advanced neonatal care. Survivors may require regular lifelong transfusions and consequent iron chelation. In appropriate candidates, allogeneic hematopoietic stem-cell transplantation is currently the only curative option and should be discussed early. Prevention through parental screening and prenatal diagnosis is crucial.

Prognosis: Excellent in carriers and individuals with trait; in HbH disease, quality of life is good, but risks of hemolytic crises, infections, gallstones and, in patients with iron overload, including non-transfusion-dependent individuals, systemic siderosis and organ dysfunction persist. Hydrops fetalis is almost invariably fatal without intrauterine treatment and specialist perinatal care.

Complications

Complications of alpha-thalassemia depend on the severity of alpha-globin deficiency, transfusion requirements and the presence of nondeletional mutations:

• Secondary siderosis (iron accumulation in the liver, heart, pancreas and endocrine glands) due to repeated transfusions or increased intestinal absorption
• Gallstones (pigment stones) and chronic jaundice due to persistent hemolysis
• Massive splenomegaly and risk of hypersplenism
• Extramedullary hematopoiesis (paraspinal masses, skeletal deformities in more severe forms)
• Post-splenectomy complications (severe infections, increased thromboembolic risk)
• Fertility and pregnancy: risk of hydrops fetalis due to Hb Bart when both parents carry alpha-zero alleles in cis (−−/αα) or other variants capable, in combination, of inactivating all four alpha genes in the offspring
• Maternal complications in pregnancies with hydrops fetalis (preeclampsia, hemorrhage, coagulopathy)

Appropriate management and multidisciplinary surveillance can substantially reduce the incidence of severe complications and improve life expectancy.

References
1. Higgs DR, Engel JD, Stamatoyannopoulos G. Thalassaemia. Lancet. 2012;379(9813):373–383.
2. Steinberg MH, Forget BG, Higgs DR, Nagel RL. Disorders of Hemoglobin: Genetics, Pathophysiology, and Clinical Management. Cambridge University Press. 2nd ed. 2009.
3. Vichinsky EP. Alpha thalassemia major: new mutations, intrauterine management, and outcomes. Hematology Am Soc Hematol Educ Program. 2009:35–41.
4. Chui DH, Waye JS. Hydrops fetalis caused by alpha-thalassemia: an emerging health care problem. Blood. 1998;91(7):2213–2222.
5. Galanello R, Cao A. Gene test review: alpha-thalassemia. Genet Med. 2011;13(2):83–88.
6. Harteveld CL, Higgs DR. Alpha-thalassemia. Orphanet J Rare Dis. 2010;5:13.
7. Tamary H, Greenberg-Kushnir N, Dgany O. Alpha-Thalassemia. GeneReviews®. Updated April 23, 2026.
8. Lo L, Singer ST. Thalassemia: current approach to an old disease. Pediatr Clin North Am. 2002;49(6):1165–1191.
9. Higgs DR. The molecular basis of α-thalassemia. Cold Spring Harb Perspect Med. 2013;3(1):a011718.
10. Amid A, Lal A, Coates TD, Fucharoen S, eds. Guidelines for the Management of α-Thalassaemia. Nicosia: Thalassaemia International Federation; 2023.
11. European Medicines Agency. Pyrukynd (mitapivat): EPAR and product information. 2022, updated 2026.