Alpha Thalassemia and Its Pathophysiology

We turn our attention to alpha thalassaemia, i.e., when there is reduced or absent alpha globin chain synthesis. It is almost always (but not always) due to a deletion of one or more of the (four) alpha globin genes. The resulting blood picture is also hypochromic and microcytic and we use the same investigative approach as we did for beta thalassaemia (same tests + HbH). Go through the pathophysiology, local and global distributions and tests (and expected results) used to investigate them.

Learning objectives for this topic are to be able to:

  1. Describe the inheritance pattern and underlying pathophysiology of:

  1. Alpha thalassaemia trait/minor

  2. HbH disease

  3. Hydrops fetalis

  • List the tests (and expected results) used to investigate and confirm/diagnose them.

    1. FBC and blood film morphology

    2. Detection of HbH

    3. Hb electrophoresis (HbEPP)

    4. High-performanvce liquid chromatography (HPLC)

    5. Molecular studies

  • Describe the principle and rationale of the tests listed above (point 3).

  • Describe the expected clinical features/presentation for the forms listed above (point 2).

Introduction to Alpha Thalassemia

  • Definition: Alpha thalassemia is a blood disorder characterized by reduced or absent synthesis of alpha globin chains, leading to various clinical presentations.

Pathophysiology

  • Key Concept: The underlying cause of alpha thalassemia is related to genetic mutations and deletions in the alpha globin genes.

  • Genetic Variants:

    • Can result from deletions of one to four genes.

    • Comparison with beta thalassemia, where mutations are often due to single base mutations or frame shift mutations instead of deletions.

Global Distribution

  • Overview:

    • Thalassemias are the most common genetic diseases worldwide.

    • There is significant overlap in the distribution of alpha and beta thalassemia, often in Mediterranean and Asian regions.

Alpha Thalassemia Genetic Mechanisms

  • Gene Deletion Patterns:

    • One gene deletion (alpha thalassemia trait/minor).

    • Two gene deletions (alpha thalassemia trait).

    • Three gene deletions (Hemoglobin H disease).

    • Four gene deletions (hydrops fetalis).

  • Nomenclature:

    • Alpha null: No alpha globin production.

    • Alpha plus: Some alpha globin produced.

    • Alpha t: Non-deletional mutation affecting alpha globin.

Clinical Features of Thalassemias

  • Thalassemia Types and Characteristics:

    • Alpha thalassemia trait/minor:

    • May present with normal or reduced hemoglobin.

    • Borderline low MCV/MCH, normal RDW.

    • Generally asymptomatic.

    • Hemoglobin H disease:

    • Chronic hemolytic anemia varying from mild to severe.

    • Splenomegaly and hepatomegaly may occur.

    • Lower hemoglobin (typically 60-100 g/L).

    • MCV low; morphology changes such as spherocytes and target cells observed.

    • Hydrops fetalis:

    • Result of four gene deletions leading to absence of alpha globin chains.

    • Not compatible with life; often leads to severe hemolysis and jaundice.

Laboratory Diagnosis

  • Blood Count Results:

    • Hypochromic, microcytic anemia observed across various forms of thalassemia.

    • Iron studies: Generally normal in asymptomatic individuals; careful monitoring needed in Hemoglobin H disease due to potential iron deficiency from hemolysis.

  • Electrophoresis & HPLC:

    • Alpha thalassemia trait/minor: Normal results.

    • Hemoglobin H disease: Fast moving band observed in electrophoresis.

  • Microscopic Evaluation:

    • Morphological changes in blood film.

    • Identification of hemoglobin H bodies (golf ball cells).

Distinctions Between Thalassemias

  • Comparison of Alpha and Beta Thalassemia:

    • Hemoglobin A2 levels: Increased in beta thalassemia, but normal in alpha thalassemia.

    • Key identifiers:

    • Microcytic and hypochromic cells with target or teardrop shape in thalassemia; pencil cells are absent in this condition.

Clinical Implications of Gene Deletions

  • Importance of identifying gene deletions to inform about potential offspring risk and need for prenatal testing.

  • Migration of at-risk populations leads to increased incidence of thalassemia across different regions, necessitating awareness and genetic counseling.

Study Considerations

  • Key Points to Remember:

    • The level of alpha globin synthesis is crucial for understanding various phenotypic expressions of alpha thalassemia.

    • Clinical implications of three and four gene deletions are significant, affecting patient management and prenatal considerations.

    • The role of molecular testing in diagnosing alpha thalassemia as routine laboratory tests may not reveal all relevant information.

Pathophysiology Questions for Study

  • 1. Describe and explain the pathophysiology of alpha thalassemia.

  • 2. What are the differences between hemoglobin H and hemoglobin Bart's?

  • 3. Why do patients with hemoglobin H disease experience chronic hemolytic anemia?

  • 4. Discuss laboratory findings associated with each type of thalassemia and their significance.

  • 5. When should prenatal testing for thalassemia be considered?

Final Thoughts

  • Understanding the subtleties of laboratory findings and genetic implications is essential for confirming diagnoses of alpha thalassemia and informing management strategies.

  • Practical knowledge of hematological assessments will aid in effective differentiation between iron deficiency anemia and thalassemias.