Thalassemia

01Pathophysiology

Normal adult hemoglobin (HbA) is a tetramer composed of two alpha-globin chains and two beta-globin chains (${\alpha }_2{\beta }_2\backslash alpha\_2\backslash beta\_2$). Thalassemia is a quantitative defect: the globin chain itself is structurally normal, but production is reduced or absent. This is the key conceptual distinction from sickle cell disease, which is a qualitative defect (abnormal hemoglobin structure with normal quantity).

In alpha thalassemia, there are deletions in the alpha-globin gene cluster on chromosome 16. Each person normally carries four copies of the alpha-globin gene (two per chromosome). The clinical severity directly correlates with the number of genes deleted: one gene deleted is clinically silent, while four genes deleted is lethal in utero. When alpha chains are deficient, the excess unpaired beta chains or gamma chains aggregate into abnormal tetramers. In adults, excess beta chains form HbH (${\beta }_4\backslash beta\_4$), which is unstable and precipitates within the red cell. In fetuses, excess gamma chains form Hb Bart (${\gamma }_4\backslash gamma\_4$), which has an extremely high oxygen affinity and fails to deliver oxygen to tissues, resulting in severe tissue hypoxia, hydrops fetalis, and intrauterine death.

In beta thalassemia, point mutations (not deletions) in the beta-globin gene on chromosome 11 lead to either reduced (${\beta }^{+}\backslash beta^+$) or absent (${\beta }^0\backslash beta^0$) beta chain production. The clinical consequence is an excess of unpaired alpha chains, which are insoluble and precipitate inside developing erythroid precursors in the bone marrow. This causes oxidative damage to the red cell membrane, leading to two simultaneous problems. First, erythroid precursors are destroyed before they leave the marrow, a process called ineffective erythropoiesis. Second, the few red cells that do reach the circulation are fragile and undergo premature hemolysis in the spleen.

The reason beta thalassemia major does not present at birth is critical to understand. During fetal life, the predominant hemoglobin is HbF (${\alpha }_2{\gamma }_2\backslash alpha\_2\backslash gamma\_2$), which does not require beta chains. The switch from gamma to beta chain production occurs gradually over the first six months of life. Therefore, the child appears well at birth and becomes symptomatic between 6 and 12 months as HbF declines and beta chain deficiency becomes apparent.

Chronic hemolytic anemia drives a compensatory increase in erythropoiesis, which expands the bone marrow cavities. This marrow expansion is responsible for the characteristic skeletal deformities: frontal bossing, maxillary hyperplasia (chipmunk facies), and the "crew-cut" or "hair-on-end" appearance on skull radiographs, where vertical trabecular striations radiate from the outer table. This same mechanism causes cortical thinning, leading to pathologic fractures.

A final critical concept: iron overload in thalassemia major results from two sources. The first is chronic transfusion therapy. The second, which students often miss, is increased intestinal iron absorption driven by suppressed hepcidin levels due to ineffective erythropoiesis. This means iron overload occurs even in non-transfused patients with thalassemia intermedia.

02Classification and Clinical Manifestation

Alpha Thalassemia

The cis deletion ($--\mathrm{/}\alpha \alpha --/\backslash alpha\backslash alpha$) is common in Southeast Asian populations and carries the risk of producing offspring with Bart hydrops fetalis when both parents carry it. The trans deletion ($-\alpha \mathrm{/}-\alpha -\backslash alpha/-\backslash alpha$) is common in African populations and does not produce hydrops fetalis offspring because each chromosome retains at least one functional alpha gene. This distinction is heavily tested in genetics-related questions.

Beta Thalassemia

03Diagnostic Workup

The best initial test for any patient presenting with microcytic anemia is the CBC with red cell indices combined with iron studies. The exam will test your ability to distinguish thalassemia from iron deficiency anemia because both present with microcytic hypochromic anemia. The two critical discriminators at the CBC level are:

1. RBC count: In thalassemia, the bone marrow compensates by producing more (albeit small and defective) red cells, so the RBC count is often elevated or inappropriately normal for the degree of anemia. In iron deficiency, the RBC count is low.

2. RDW (Red Cell Distribution Width): In thalassemia, all red cells are uniformly small, so the RDW is normal or low. In iron deficiency, there is a mixed population of normal-sized and microcytic cells, so the RDW is elevated.

The Mentzer Index ($\frac{MCV}{RBCcount}\backslash frac\{MCV\}\{RBC\backslash \; count\}$) is a rapid bedside calculation. A value less than 13 suggests thalassemia, while a value greater than 13 suggests iron deficiency. This is a commonly tested screening tool but is not definitive on its own.

Iron studies must be ordered next. Normal iron studies in a patient with microcytic anemia effectively rule out iron deficiency and should prompt suspicion for thalassemia or sideroblastic anemia.

The most accurate (confirmatory) test for beta thalassemia is hemoglobin electrophoresis. It quantifies the relative proportions of HbA, HbA2, and HbF. An elevated HbA2 above 3.5% is the hallmark of beta thalassemia trait. In beta thalassemia major, HbF is markedly elevated (60 to 90%) with absent or minimal HbA.

For alpha thalassemia, hemoglobin electrophoresis is often normal in the silent carrier and trait states because the abnormal tetramers (HbH, Hb Bart) may not be present in sufficient quantities in adults. The definitive confirmatory test for alpha thalassemia is DNA analysis (PCR-based genetic testing) to quantify the number of deleted alpha genes. HbH disease can be identified by supravital staining with brilliant cresyl blue, which reveals characteristic golf-ball-like HbH inclusions within red cells.

A peripheral blood smear is supportive and reveals target cells, microcytes, and basophilic stippling. In severe beta thalassemia major, nucleated red blood cells may be seen, reflecting the intensity of the erythropoietic drive.

04Management and Treatment

For Beta Thalassemia Minor (Trait): No treatment is needed. The single most important point is to avoid giving iron supplements to these patients. Because the anemia is microcytic, it is often mistakenly treated as iron deficiency. Unnecessary iron supplementation can lead to iatrogenic iron overload. Genetic counseling is essential, particularly if both partners carry thalassemia trait, as there is a 25% chance of having a child with thalassemia major.

For Beta Thalassemia Major (Acute Stabilization and Chronic Management):

The cornerstone of management is chronic transfusion therapy aimed at maintaining a pre-transfusion hemoglobin of 9 to 10 g/dL. This level is critical: it suppresses the patient's own ineffective erythropoiesis, thereby reducing marrow expansion, skeletal deformities, and intestinal iron absorption. Transfusions are typically administered every 2 to 4 weeks using leukoreduced, phenotype-matched packed red blood cells to minimize alloimmunization.

The inevitable consequence of chronic transfusion is iron overload, which deposits in the heart, liver, and endocrine organs. Iron chelation therapy must begin after approximately 10 to 20 transfusions or when serum ferritin exceeds 1,000 ng/mL. Three chelating agents are available:

• Deferoxamine (Desferal): The oldest agent; administered as a slow subcutaneous infusion over 8 to 12 hours, 5 to 7 days per week, at a dose of 25 to 50 mg/kg/day. Its main limitation is adherence due to the inconvenient route of administration. Side effects include ototoxicity and retinal toxicity (requires annual eye and hearing exams).

• Deferasirox (Exjade/Jadenu): An oral chelator dosed at 20 to 40 mg/kg/day, taken once daily. It is the most commonly used agent due to oral bioavailability and once-daily dosing. Monitor for renal toxicity (elevated creatinine) and hepatotoxicity. Contraindicated in patients with severe renal impairment (creatinine clearance <40 mL/min).

• Deferiprone (Ferriprox): An oral chelator dosed at 75 to 100 mg/kg/day in three divided doses. Uniquely effective at chelating cardiac iron and often used in combination with deferoxamine for patients with cardiac siderosis. The major adverse effect is agranulocytosis (requires weekly CBC monitoring).

Folic acid at 1 mg/day is given to support the high erythropoietic demand.

Splenectomy is indicated when transfusion requirements increase by more than 50% over baseline or when annual transfusion volume exceeds 200 to 220 mL/kg/year of packed RBCs. Pre-splenectomy vaccinations against encapsulated organisms (pneumococcus, meningococcus, and Haemophilus influenzae type b) are mandatory, followed by post-splenectomy prophylactic penicillin.

Hematopoietic stem cell transplantation (HSCT) is the only curative therapy. The best outcomes are in patients classified as Pesaro Class I (young age, no hepatomegaly, no portal fibrosis, regular chelation), with disease-free survival exceeding 90% when an HLA-matched sibling donor is available. For patients without a matched sibling, haploidentical and matched unrelated donor transplants are increasingly explored.

Luspatercept (Reblozyl) is a newer agent approved for adults with transfusion-dependent beta thalassemia. It is an erythroid maturation agent that reduces transfusion burden by approximately one-third.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common trap on this topic is the student who sees "microcytic anemia" and reflexively selects iron deficiency anemia as the diagnosis without checking iron studies. The exam will present a patient, often a child of Southeast Asian or Mediterranean descent, with a microcytic anemia and then provide normal ferritin and normal TIBC somewhere in the lab values. If you miss those normal iron studies, you will incorrectly prescribe iron and select the wrong answer. Always check the iron panel before concluding the etiology of any microcytic anemia.

A second common error involves timing. Students often wonder why a child with beta thalassemia major appears healthy at birth. The answer is the fetal-to-adult hemoglobin switch: HbF (${\alpha }_2{\gamma }_2\backslash alpha\_2\backslash gamma\_2$) does not require beta chains, so the deficit only manifests once gamma chain production declines around 6 months of age. If the vignette describes a newborn with severe anemia and hydrops, that is not beta thalassemia major; that is Bart hydrops fetalis (four alpha gene deletions).

A third trap concerns the alpha thalassemia workup. Students expect hemoglobin electrophoresis to always be diagnostic, but in alpha thalassemia silent carrier and trait, the electrophoresis is normal in adults. The confirmatory test is genetic (DNA) analysis. The exam may present a patient with microcytic anemia, normal iron studies, and a normal hemoglobin electrophoresis, and the correct next step is genetic testing for alpha-globin gene deletions.

Fourth, be alert to the iron overload question. When the exam asks about the cause of heart failure or endocrine dysfunction (diabetes, hypogonadism, hypothyroidism) in a chronically transfused thalassemia patient, the answer is secondary hemochromatosis from transfusional iron overload, and the next best step is initiating or intensifying iron chelation. The most dangerous consequence is dilated cardiomyopathy, which remains the leading cause of death in inadequately chelated patients.

Finally, remember the distinction between cis and trans deletions in alpha thalassemia for genetic counseling questions. Two parents with the trans deletion ($-\alpha \mathrm{/}-\alpha -\backslash alpha/-\backslash alpha$, common in African populations) cannot have a child with hydrops fetalis. Two parents with the cis deletion ($--\mathrm{/}\alpha \alpha --/\backslash alpha\backslash alpha$, common in Southeast Asian populations) can produce a child with four-gene deletion. The exam tests this concept in the context of prenatal counseling and population genetics.

The core competency being assessed across all thalassemia questions is your ability to differentiate microcytic anemias using iron studies and hemoglobin electrophoresis, correctly identify the clinical severity based on genotype, and anticipate the complications of chronic hemolysis and transfusion therapy.