Thalassemias

Mechanism of the primary genetic defect

• α-thalassemia — gene deletions: The α-globin locus is duplicated (four genes total), so most disease arises from large deletions rather than point mutations. Severity tracks gene dosage: one gene lost (silent carrier), two (trait), three (HbH disease, β4 tetramers), four (Hb Bart hydrops fetalis, γ4 tetramers).
• Cis vs trans configuration is the counseling point: Southeast Asian ancestry favors cis deletions (αα/--), which can generate a four-gene-deletion fetus; African ancestry favors trans deletions (-α/-α), which essentially cannot.
• Non-deletional α variants: Hb Constant Spring (stop-codon readthrough producing an elongated, unstable α chain) causes disease that is often more severe than an equivalent deletion.
• β-thalassemia — point mutations: A single β gene per chromosome means promoter, splice-site, and nonsense mutations predominate. Complete absence of chain output is β⁰; reduced output is β⁺, which explains the continuum from major to intermedia.
• Compound heterozygosity: HbE/β-thalassemia (Southeast Asia) and sickle-β-thalassemia produce clinically significant disease in patients who are not homozygous for either allele.
• Genetic modifiers: Variants that sustain HbF production, or coinherited α-thalassemia, reduce β-thalassemia severity because the toxic species is the unpaired excess α chain.

Non-modifiable risk factors

• Ancestry: Mediterranean, Middle Eastern, South and Southeast Asian, and African populations — geographic overlap with historical Plasmodium falciparum endemicity (heterozygote advantage).
• Both parents carriers: autosomal recessive transmission; consanguinity and a positive family history raise pretest probability sharply.

Modifiable contributors to severity

• Transfusion burden and empiric iron therapy: Iron overload is largely iatrogenic; giving iron to a microcytic patient assumed to be iron-deficient is the classic avoidable harm.
• Cofactor and infectious exposures: Folate deficiency, hepatitis B/C, and alcohol accelerate anemia and iron-related liver injury.
• Screening gaps: ACOG recommends CBC-based screening in pregnancy with hemoglobin electrophoresis when MCV is low or ancestry confers risk, plus partner testing and offered genetic counseling.

• Defective globin chain synthesis: Point mutations, deletions, or splice site abnormalities in β-globin (β-thalassemia) or α-globin (α-thalassemia) genes reduce or eliminate production of the affected chain. β-thalassemia results from mutations in the single β-globin gene on chromosome 11; α-thalassemia results from deletions of one or more of the four α-globin genes on chromosome 16.

• Imbalanced globin chain ratios and precipitate formation: The unaffected globin chains accumulate and form insoluble aggregates and precipitates within erythroid precursors and mature red blood cells. In β-thalassemia, excess α chains precipitate; in α-thalassemia, excess β chains (forming β4-tetramers or "hemoglobin H") precipitate. These precipitates damage the red cell membrane and cause intramedullary hemolysis.

• Ineffective erythropoiesis and intravascular hemolysis: Massive destruction of erythroid precursors in the bone marrow (intramedullary hemolysis) occurs as the reticuloendothelial system attempts to clear precipitate-laden cells. Surviving red blood cells are destroyed both in the spleen (extravascular hemolysis) and circulation (intravascular hemolysis), resulting in severe chronic hemolytic anemia despite increased erythropoietin levels and marked bone marrow hyperplasia.

• Secondary iron overload: Chronic transfusions (in transfusion-dependent thalassemia) and increased intestinal iron absorption (from ineffective erythropoiesis-driven erythropoietin signaling) lead to iron deposition in myocardium, liver, endocrine glands, and pituitary. Iron accumulation generates reactive oxygen species via Fenton chemistry, causing fibrosis, cirrhosis, cardiomyopathy, and endocrine dysfunction.

• Reactive oxygen species (ROS) and oxidative stress: Precipitated globin chains generate oxidative stress; iron overload exacerbates this through Fenton-catalyzed free radical production. ROS damages cell membranes, proteins, and DNA, amplifying hemolysis and contributing to organ damage.

• Thalassemia major (β⁰/β⁰ or severe β⁺/β⁺ genotypes): Severe microcytic hemolytic anemia presents in infancy (typically 6-12 months) when fetal hemoglobin production declines. Patients develop failure to thrive, hepatosplenomegaly (often massive), jaundice, pallor, and growth retardation. Severe bone pain from marrow hyperplasia and pathologic fractures occur. Frontal bossing, maxillary prominence, and dental malocclusion result from craniofacial remodeling from intense marrow expansion.

• Thalassemia intermedia: Moderate anemia that may not require regular transfusions; patients present later (after age 2) with milder symptoms. Splenomegaly and hepatomegaly present but less severe than in major disease. Patients tolerate moderate anemia and may have spontaneous fetal hemoglobin elevation.

• Thalassemia minor (trait): Asymptomatic carriers with microcytosis and mild anemia (hemoglobin 10-12 g/dL). Often detected incidentally on CBC during routine screening; many are unaware they are carriers. Disproportionate microcytosis relative to the degree of anemia is a classic finding (low MCV with relatively preserved hemoglobin).

• α-thalassemia presentation variants:
• Hemoglobin H disease (3-gene deletion): Moderate hemolytic anemia with splenomegaly; β4-tetramers visible on supravital stain as "Heinz bodies"
• Hydrops fetalis (4-gene deletion): Intrauterine severe anemia, heart failure, and fetal death; incompatible with life without intrauterine transfusion

• Complications from chronic hemolysis: Gallstones (pigmented), leg ulcers, and splenic sequestration crises with acute severe anemia and abdominal pain.

• Complete blood count (CBC) showing microcytic, hypochromic anemia: Hemoglobin typically 6-9 g/dL in thalassemia major; MCV markedly low (60-75 fL). Reticulocyte count paradoxically HIGH despite severe anemia (>2-3%), reflecting the compensatory response to hemolysis and ineffective erythropoiesis. Peripheral smear shows target cells, nucleated RBCs, polychromasia, and in α-thalassemia, small refractile inclusions (Heinz bodies on supravital stain).

• Hemoglobin electrophoresis/HPLC (high-performance liquid chromatography): Gold standard for diagnosis. In β-thalassemia major: elevated HbA2 (2-5%), elevated HbF (10-90%), and reduced or absent HbA. In β-thalassemia minor: mild elevation of HbA2 (3.5-5.5%) and normal or slightly elevated HbF. α-thalassemia cannot be diagnosed by hemoglobin electrophoresis because the imbalance is between normal hemoglobins; diagnosis requires DNA testing or other specialized assays.

• Serum ferritin and iron studies: Elevated ferritin (>1000 ng/mL in transfusion-dependent disease) suggests iron overload. However, ferritin is an acute phase reactant, so it can be falsely elevated with concurrent infection or inflammation. Transferrin saturation and serum iron provide additional context.

• DNA sequencing and genetic testing: Identifies specific mutations in β-globin or α-globin genes; essential for counseling and determining prognosis. Southern blot or PCR-based deletion analysis specifically detects α-globin gene deletions.

• Bone marrow biopsy: Not routinely needed for diagnosis but may show erythroid hyperplasia (often 50-90% cellularity with erythroid predominance). Used to evaluate for myelodysplasia or other concurrent disorders if clinical features are atypical.

• Important diagnostic consideration: Thalassemia trait must be distinguished from iron deficiency anemia, as both present with microcytosis. Elevated ferritin and normal iron studies (in thalassemia) versus low ferritin and low serum iron (in iron deficiency) help differentiate. Hemoglobin electrophoresis is definitive. The Mentzer index (MCV/RBC count <13) favors thalassemia trait over iron deficiency.

• Chronic transfusion therapy (first-line for thalassemia major): Maintain hemoglobin 9-10 g/dL via regular RBC transfusions (typically monthly) to suppress ineffective erythropoiesis, reduce compensatory marrow hyperplasia, and improve quality of life and growth. Transfusion goals: improve oxygen delivery, suppress erythropoietin (reducing intestinal iron absorption), and minimize hepatosplenomegaly progression. Chelation therapy is initiated in parallel.

• Iron chelation therapy (essential in transfusion-dependent disease):
• Deferasirox (oral, first-line): 20-30 mg/kg/day; well-tolerated and convenient
• Deferoxamine (IV/subcutaneous, older agent): 20-50 mg/kg/day 5 nights per week or continuous infusion; requires pump compliance but effective
• Deferiprone (oral): 75-

Complications of the disease itself

• Iron-overload cardiomyopathy: Non-transferrin-bound iron enters myocytes via L-type calcium channels; Fenton-generated radicals cause dilated cardiomyopathy and arrhythmia. This is the leading cause of death in transfusion-dependent thalassemia. Signaled by falling ejection fraction or a shortened cardiac T2* on MRI — ferritin correlates poorly with cardiac iron. Acute decompensated heart failure or ventricular arrhythmia is an emergency.
• Endocrinopathy from anterior pituitary and gland siderosis: Hypogonadotropic hypogonadism (delayed puberty is often the first sign), diabetes, hypothyroidism, hypoparathyroidism, and short stature.
• Hepatic iron loading: Fibrosis and cirrhosis, with hepatocellular carcinoma risk; once cirrhosis is established, AASLD recommends semiannual ultrasound surveillance.
• Extramedullary hematopoiesis: Marrow expansion produces crew-cut skull, chipmunk facies, pathologic fractures, and paraspinal masses. Paraspinal masses causing cord compression are an emergency (urgent MRI, steroids, transfusion/radiation).
• Chronic hemolysis sequelae: Pigment gallstones, osteoporosis, leg ulcers, pulmonary hypertension, and a hypercoagulable state that is worst in non-transfused thalassemia intermedia after splenectomy.
• Acute anemic crises (emergencies): Splenic sequestration (sudden splenomegaly, shock) and parvovirus B19 aplastic crisis — the tell is a reticulocyte count that falls to near zero in a patient whose baseline retic is high.

Complications of treatment

• Transfusion-related: RBC alloimmunization and delayed hemolytic transfusion reaction (hemoglobin falls days after transfusion), TACO, and transfusional hemosiderosis.
• Post-splenectomy sepsis: Overwhelming encapsulated-organism infection is an emergency; ACIP recommends pneumococcal, meningococcal, and Hib vaccination in advance of elective splenectomy.
• Deferiprone: Agranulocytosis — an emergency requiring drug cessation and neutrophil monitoring; FDA labeling carries a boxed warning and mandates ANC surveillance.
• Deferasirox: Renal and hepatic injury and GI hemorrhage (boxed warning); monitor creatinine and transaminases.
• Deferoxamine: Ototoxicity, retinopathy, growth retardation in children, and predisposition to Yersinia enterocolitica infection.

• Microcytosis out of proportion to anemia is the opening move: A near-normal hemoglobin with a strikingly low MCV, a high-normal or elevated RBC count, and a normal RDW points to thalassemia trait; iron deficiency gives a low RBC count and a high RDW. The Mentzer index (MCV/RBC <13) favors thalassemia.

• Single best next step in suspected trait: Check iron studies first, then hemoglobin electrophoresis/HPLC. Never treat empirically with iron — the classic distractor — because a normal or high ferritin plus ongoing iron loading makes supplementation harmful.

• HbA2 is the β-thalassemia minor marker: An elevated HbA2 (δ chains substituting for absent β) confirms β-trait. Concurrent iron deficiency can falsely normalize HbA2, so repeat electrophoresis after iron repletion.

• α-thalassemia trait has a normal electrophoresis: The imbalance is among structurally normal hemoglobins, so a microcytic patient with normal iron studies and normal HbA2/HbF is α-thalassemia by exclusion — confirm with DNA-based α-globin gene analysis.

• Timing separates α from β: β-thalassemia major surfaces at 6–12 months as HbF (α2γ2, which needs no β chain) declines; α-thalassemia is symptomatic in utero or at birth because α chains are required for fetal hemoglobin.

• Tetramer identity is a favorite one-liner: γ4 = Hb Bart (hydrops fetalis, four-gene deletion); β4 = HbH (three-gene deletion, inclusions on supravital stain).

• The association examiners test: Heterozygosity confers relative protection against Plasmodium falciparum malaria, explaining the geographic distribution.

• Skeletal buzzwords: Crew-cut skull on radiograph and chipmunk facies reflect marrow expansion — expect them in an under-transfused child, and expect them to disappear with adequate transfusion.

• Cause of death and monitoring: Iron-overload cardiomyopathy, tracked by cardiac T2* MRI rather than ferritin alone. Do not accept "normal ferritin" as evidence of a safe cardiac iron burden.