Antenatal Diagnosis of Thalassemia
Primary Objectives
- Identify at-risk carrier couples.
- Provide periconceptional and antenatal counseling.
- Achieve optimal maternal and neonatal outcomes.
- Establish precise genotype diagnosis.
- Facilitate appropriate clinical management.
Pathophysiological Basis for Antenatal Screening
Alpha Thalassemia Syndromes
- Alpha-globin gene deletions reduce alpha-chain synthesis.
- Four-gene deletion causes hemoglobin Barts hydrops fetalis.
- Zero alpha-to-beta synthesis ratio occurs.
- Hemoglobin Barts (gamma-chain tetramers) constitutes 80-100% of fetal hemoglobin.
- Results in death in utero or shortly after birth.
- Antenatal diagnosis enables life-saving fetal packed red blood cell transfusions.
Beta Thalassemia Syndromes
- Impaired beta-globin chain production causes alpha-to-beta chain imbalance.
- Absent beta-chain synthesis leads to beta0-thalassemia.
- Reduced beta-chain synthesis leads to beta1-thalassemia.
- Homozygous states cause severe anemia, ineffective erythropoiesis, and extramedullary hematopoiesis.
- Early diagnosis prepares families for lifelong transfusion and chelation therapy.
Stepwise Antenatal Screening Algorithm
Maternal Initial Assessment
- Full medical and family history required.
- Complete blood count with erythrocyte indices evaluated.
- Mean corpuscular volume less than 80 fl indicates microcytosis.
- Mean corpuscular hemoglobin less than 27 pg indicates hypochromia.
- Peripheral blood smear evaluates for target cells and basophilic stippling.
Secondary Maternal Evaluation
- Serum ferritin evaluates concurrent iron deficiency.
- High-performance liquid chromatography functions as primary antenatal screening tool.
- Hemoglobin A2 greater than 4% signifies beta-thalassemia trait.
- Normal hemoglobin A2 with microcytosis suggests alpha-thalassemia trait.
Paternal Screening
- Abnormal maternal high-performance liquid chromatography mandates paternal screening.
- Identifies specific parental genotype combinations.
- Two alpha-thalassemia trait parents risk hemoglobin Barts hydrops fetalis.
- Two beta-thalassemia trait parents risk beta-thalassemia major.
Fetal Sampling Techniques
- Exact in utero diagnosis requires mutation analysis of fetal deoxyribonucleic acid.
| Modality | Gestational Age | Sample Source | Current Status |
|---|---|---|---|
| Chorionic villus sampling | 10-14 weeks | Chorionic villus biopsy | Established standard. |
| Amniocentesis | 15-20 weeks | Fetal fibroblasts | Established standard. |
| Noninvasive prenatal testing | Not specified | Cell-free deoxyribonucleic acid | Under investigation. |
Molecular Diagnostic Modalities
- Genomic research enables precise mutation detection.
- Mutation screening establishes definitive fetal genotype.
| Target Syndrome | Molecular Technique | Diagnostic Target |
|---|---|---|
| Beta thalassemia | End point polymerase chain reaction | Common specific mutations. |
| Beta thalassemia | Sanger sequencing | Full beta-globin gene analysis. |
| Alpha thalassemia | Multiplex ligation probe amplification | Alpha-globin gene deletions and mutations. |
Specific Targeted Mutations
- Five common mutations dominate beta-thalassemia genetics.
- Targets include intervening sequence 1-5 mutations, intervening sequence 1-1 mutations, and 619 base pair deletions.
- Alpha-thalassemia testing identifies single, double, triple, or quadruple gene deletions.
Genetic Counseling and Interventions
- Mandatory for identified structural hemoglobin variants and thalassemias.
- Explains autosomal codominant transmission of traits.
- Clarifies 25% risk of major disease in offspring of two trait-positive parents.
- Details 50% risk of carrier status in offspring.
- Prepares parents for potential intrauterine fetal transfusions if hydrops fetalis identified.
- Guides postnatal transition to chronic transfusion therapy or allogeneic hematopoietic stem cell transplantation.