Thalassemia occurs in a broad geographic belt stretching from the Mediterranean basin (Southern Europe — Greece, Italy, Turkey) through the Middle East, Indian subcontinent, and into Southeast Asia (including Southern China, Thailand, Vietnam, Cambodia, Malaysia, Indonesia) ^[2].

• Why this distribution? The geographic overlap with historical malaria endemicity is not coincidental. Thalassemia carriers enjoy a heterozygote survival advantage against Plasmodium falciparum malaria — the parasite grows poorly in microcytic, low-haemoglobin red cells. Natural selection has therefore maintained thalassemia alleles at high frequency in these populations over millennia ^[2].

• α-thalassemia carrier rate: ~5.0% (1997 data) ^[1][2]
• β-thalassemia carrier rate: ~3.5% (1997 data) ^[1][2]
• Most common α-thalassemia deletion in HK: Southeast Asian (SEA) type deletion (--^SEA) — this is a cis deletion removing both α genes on chromosome 16 ^[1][2]
• Most common β-thalassemia mutations in HK ^[2]:
  • Codon 41/42 (~40%, β⁰)
  • Intron 2 (~15.7%, β⁰)
  • Codon 71/72 (~12.4%, β⁰)
  • -28 (~11.6%, β⁺)
  • Codon 17 (~10.5%, β⁰)
• The majority of β-thalassemia patients in HK are compound heterozygotes (carrying two different mutations, one on each chromosome 11) ^[2]

Given carrier rates of ~5% and ~3.5%, a significant number of couples are at risk of having severely affected offspring. This underpins the rationale for:

• Pre-marital, pre-pregnancy, and pre-natal counselling ^[3]
• Newborn screening programmes
• Genetic counselling when both partners are carriers

Key points:

• There are 4 α-globin genes (2 copies of HBA1 + 2 copies of HBA2) per diploid cell — they produce identical α-globin protein ^[2]
• There are 2 β-globin genes (one HBB per chromosome 11) per diploid cell ^[2]

^[2]

Each haemoglobin molecule is a tetramer of 4 globin chains, each carrying a haem group with an iron atom at its centre that reversibly binds O₂. The cooperative binding kinetics (sigmoid O₂ dissociation curve) depend on normal tetramer assembly. Abnormal tetramers (e.g., β₄ = HbH, γ₄ = Hb Bart's) have very high O₂ affinity — they bind oxygen tightly and fail to release it to tissues, functioning essentially as useless oxygen sinks.

• Genetics: Autosomal recessive, gene deletions on chromosome 16 (2 pairs of α-genes) ^[1]

• Most common deletion in HK is the SEA (Southeast Asian) deletion ^[1][2]

• Majority are deletions (~90%) among Asians ^[2]

• The SEA deletion (--^SEA) removes both α-globin genes on one chromosome 16 in cis configuration. This is critically important because:

  • A person can be --^SEA/αα (2 gene deletion, trait) and appear clinically well
  • If two carriers of --^SEA marry, they can produce offspring with --^SEA/--^SEA = 0 α-genes → Hb Bart's hydrops fetalis (lethal)
  • Compare this to the African/Black pattern (α-/α-) where deletions are in trans — you can never get 0 α-genes from two trans carriers

• Non-deletional defects (~10%): include Hb Constant Spring (HbCS) and Hb Quong Sze — these are point mutations in the α-globin gene that produce an elongated or unstable α-chain ^[2]

  • HbCS is relatively common in SE Asia
  • Non-deletional HbH disease (e.g., --^SEA/α^CSα) tends to be more severe than deletional HbH (--^SEA/α-) because the abnormal chain is produced but dysfunctional, causing more membrane damage

• Genetics: Autosomal recessive, point mutations on chromosome 11 (1 pair of β-genes) ^[1]
• >100 different mutations worldwide; each racial group has its own unique set ^[2]
• HK common mutations ^[2]:

• β⁰ = zero β-globin output from that allele (nonsense, frameshift, or severe splicing mutations)
• β⁺ = reduced but not absent β-globin output (promoter mutations, mild splicing defects)
• The majority in HK are compound heterozygotes ^[2]

• δβ-thalassemia: deletion of both δ and β genes → compensatory ↑HbF
• Hereditary Persistence of Fetal Haemoglobin (HPFH): high HbF production that ameliorates β-thalassemia severity ^[2]
• HbE/β-thalassemia: HbE (a structural variant with β⁺ characteristics, common in SE Asia) combined with a β-thalassemia allele → can produce a thalassemia intermedia or major phenotype ^[2]

The primary defect is decreased or absent production of one type of globin chain:

• α-thalassemia → ↓α-globin
• β-thalassemia → ↓β-globin

• The unpaired excess chains cannot form stable tetramers on their own
• Excess chains polymerise into unstable tetramers or aggregate and precipitate inside the cell ^[2]:

^[1][2]

Iron Overload — The Central Complication

Iron overload in thalassemia occurs through THREE mechanisms ^[1][4]:

1. Increased GI iron absorption: Ineffective erythropoiesis → erythroid precursors produce erythroferrone → suppresses hepatic hepcidin → ↑ferroportin activity on enterocytes → ↑dietary iron absorption (can be 2–5× normal)
2. Repeated blood transfusions: Each unit of packed red cells contains ~200–250 mg of iron. A regularly transfused patient may accumulate 5–10 g/year of iron. The body has no physiological mechanism to excrete iron — what comes in, stays in.
3. Release from ineffective erythropoiesis: Premature destruction of iron-laden erythroid precursors releases their iron back into the plasma

Pathophysiology of iron overload damage ^[4]:

• Excessive iron stores → gradual overwhelming of plasma transferrin binding capacity
• Iron bound to other molecules (albumin, citrate, acetate) = non-transferrin-bound iron (NTBI)
• NTBI taken up by liver, heart, endocrine glands, and pancreatic islets
• Excessive iron reacts with H₂O₂ to catalyse formation of reactive oxygen species (Fenton reaction)
• → Multiple organ damage: cirrhosis, cardiomyopathy, diabetes mellitus, hypogonadism

^[1][2]

^[1][2]

Other sources of variability in β-thalassemia clinical severity ^[2]:

• Genetic determinant of HbF level: ↑HbF compensates partly for ↓HbA (e.g., HPFH)
• Configuration of α cluster: triplicated or quadruplicated α genes → ↑imbalance → ↑severity
• Concomitant α-thalassemia: ↓excess α-chains → ↓imbalance → ↓severity (ameliorating)

Triggers for laboratory diagnosis of thalassaemia: Low MCV ± clinical features (pallor, splenomegaly, failure to thrive) ^[7][8]

^[1][2][9]

When thalassemia presents with overt haemolysis (jaundice, splenomegaly, ↑LDH, ↑unconjugated bilirubin, ↓haptoglobin), the differential includes other causes of inherited haemolytic anaemia:

Inherited causes of anaemia — Pathophysiology categories: (1) Inherited bone marrow failure syndromes (pancytopenia), (2) Disorders of red cell membrane, (3) Disorders of haemoglobin (haemoglobinopathy and thalassaemia), (4) Disorders of RBC metabolism ^[5]

Inherited haemolytic anaemia classification: Membrane (hereditary spherocytosis), Metabolism (G6PD deficiency, pyruvate kinase deficiency), Haemoglobin (HbS, HbH, unstable Hb) ^[5]

^{[1][2][5][10]}

When Hb Bart's hydrops fetalis presents in utero, the differential includes other causes of non-immune hydrops fetalis:

• Immune hydrops: Rh/ABO incompatibility (DAT +ve)
• Cardiac: structural heart defects, arrhythmias
• Chromosomal: Turner syndrome (45,X), Down syndrome
• Infection: parvovirus B19 (aplastic crisis in fetus), CMV, toxoplasmosis, syphilis
• Other haematological: severe congenital anaemias (e.g., homozygous pyruvate kinase deficiency, congenital dyserythropoietic anaemia)

The key distinguishing feature: Hb Bart's shows Hb Bart's (γ₄) on electrophoresis/HPLC and family history of SEA-type α-thalassemia carriers ^[1][2].

Triggers for laboratory diagnosis of haemoglobinopathy: Clinical features (pallor, jaundice/haemolysis, splenomegaly, plethora/erythrocytosis, cyanosis/metHb/low SaO₂) + Laboratory findings ^[7][8]

This GC lecture slide emphasises that the diagnostic workup for haemoglobinopathy is triggered by a combination of clinical features AND laboratory findings — not just microcytosis alone. In thalassemia specifically, the triggers are low MCV ± pallor, splenomegaly, failure to thrive ^[7][8].

What it tells you: The first-line screening tool. Identifies anaemia and suggests the morphological category.

Classification of anaemia based on laboratory findings (MCV) ^[11]:

^[11]

Key CBC findings in thalassemia ^[1][2][12]:

Mentzer Index = MCV / RBC count: > 13 = IDA, < 13 = thalassemia trait ^[2]

Why the Mentzer Index works: In thalassemia, the marrow pumps out many small cells (↓MCV, ↑RBC) → low ratio. In IDA, the marrow is iron-starved and cannot produce cells efficiently (↓MCV, ↓RBC) → high ratio. It's a quick bedside calculation but has limited sensitivity/specificity (~80%) — use it as a guide, not a definitive test.

What it tells you: Morphological clues that narrow the differential and suggest severity.

Key PBS findings in thalassemia ^[1]:

Compare with:

• IDA PBS: pencil cells (elongated elliptocytes), anisopoikilocytosis ^[1]
• G6PD deficiency: bite cells, Heinz bodies ^[1]
• HS/AIHA: spherocytes ^[1]

^[1]

Why this is essential: To exclude IDA (by far the most common cause of microcytic anaemia) and to assess for iron overload in known thalassemia patients ^[1].

Workup for microcytic anaemia: peripheral blood smear, Fe profile (serum Fe, TIBC) + ferritin, Hb pattern, clotting profile ^[1]

Laboratory findings comparison ^[12]:

^[12]

Interpretation pearls:

• Ferritin is the most sensitive and specific marker for iron deficiency — a low serum ferritin is diagnostic of IDA ^[9][13]
  • But: ferritin is a positive acute phase reactant → can be falsely elevated in inflammation (false negative for IDA). Use higher thresholds in inflammatory states (e.g., < 100 μg/L in hospitalised/elderly rather than < 15 μg/L) ^[9][13]
• In thalassemia patients on regular transfusion: ferritin and TSAT will be elevated (iron overload), not low ^[1]
• If a thalassemia patient also has IDA (e.g., menorrhagia in a woman with β-thal trait): ferritin will be lower than expected, and the anaemia/microcytosis will be disproportionately severe

Why it is essential: This is the definitive diagnostic modality, especially for:

• α-thal minima and minor (1–2 gene deletions): Hb pattern is completely normal → only way to diagnose ^[2]
• Precise mutation identification for genetic counselling and prenatal diagnosis
• Distinguishing cis (--/αα) from trans (α-/α-) deletions — critical for predicting risk to offspring
• Identifying non-deletional variants (HbCS, Hb Quong Sze)
• Confirming β-thal mutations for prognostic and counselling purposes

DNA-based genotyping for precise diagnosis (esp α-thal minor/minima) and genetic counselling ^[2]

Methods used:

Interpretation for genetic counselling ^[1][2]:

In moderate-to-severe thalassemia (HbH, intermedia, major), markers of haemolysis help confirm ongoing red cell destruction and monitor disease activity ^[1][14]:

^[1][14]

This transitions from diagnosis to monitoring, but is tested frequently.

^[1][14]

XR skull: hair-on-end appearance, thinning of cortex ^[1]

Not routinely required for thalassemia diagnosis (which is made by Hb analysis + genotyping). However, may be done if:

• Diagnostic uncertainty persists
• Need to assess other concurrent pathology (e.g., MDS, aplastic anaemia)
• Iron staining to differentiate IDA from ACD (iron present in macrophages but absent from erythroblasts in ACD; absent from both in IDA) ^[2]

Findings in thalassemia (when performed):

• Erythroid hyperplasia: markedly increased erythroid:myeloid ratio (can be 5:1 or higher, normal ~3:1)
• Dyserythropoiesis: megaloblastoid changes, nuclear-cytoplasmic asynchrony
• Iron stain: increased iron stores (unless concurrent IDA)

Applies to: α-thal minima (1 deletion), α-thal minor (2 deletions), β-thal minor (β/β⁰ or β/β⁺)

What to do:

• Reassurance — these patients are NOT sick, they have a carrier state
• Genetic counselling and partner screening — the clinical importance is reproductive, not personal health ^[3]
• Avoid unnecessary iron supplementation — a common mistake is to prescribe iron for the microcytic anaemia of thalassemia trait. This is futile (the problem is globin, not iron) and can cause iatrogenic iron overload
• Low iron diet ^[2] is generally not necessary for carriers unless iron studies show genuine overload, but they should be counselled to avoid over-supplementation

Folate supplementation for all thal major and thal intermedia with chronic haemolysis ^[2] Dose: 1–2 mg/day ^[2]

Why? Chronic haemolysis and ineffective erythropoiesis create a state of accelerated red cell turnover → increased folate consumption for DNA synthesis in rapidly dividing erythroid precursors → risk of folate deficiency → megaloblastic crisis superimposed on existing anaemia.

Indications: All TDT (thal major), all thal intermedia with evidence of chronic haemolysis, HbH disease ^[1]

Contraindications: None significant. Folate is safe and well-tolerated.

Hb Bart's: in-utero transfusion ^[1]

• Historically considered uniformly lethal → pregnancies were terminated
• Current approach (at specialised centres): intrauterine transfusion (IUT) starting from ~18–20 weeks → can sustain fetus to delivery → postnatal management with regular transfusion → definitive cure with HSCT or gene therapy
• Maternal risks must be carefully counselled: pre-eclampsia, antepartum haemorrhage (mirror syndrome)
• Long-term neurodevelopmental outcomes of surviving Hb Bart's patients are still being studied

Monitoring schedule (from paediatric protocol) ^[2]:

^[2]

High mortality without treatment (85% by 5 years, majority from CVS complications) ^[2]

Cardiac iron overload (T2 < 20 ms) is the leading cause of death in inadequately chelated β-thalassemia major. T2 < 10 ms = severe, requiring urgent intensification of chelation** ^[2]

Dilated cardiomyopathy as a cause of DCMP: infiltrative, eg. iron overload ^[16]

Why liver fibrosis from iron? Hepatic stellate cells are activated by ROS from iron-laden hepatocytes → they transform into myofibroblasts → deposit excessive collagen → fibrosis → cirrhosis. The liver also experiences direct hepatocyte apoptosis from iron-catalysed oxidative damage ^[15].

Endocrine glands are exquisitely vulnerable to iron deposition because they are highly vascularised and express transferrin/NTBI uptake mechanisms.

S/S of iron overload: hypogonadism, DM ^[1]

Bronze/leaden-grey skin pigmentation — due to excessive melanin deposition stimulated by iron. The classic "bronze diabetes" appearance when combined with DM ^[1][4].

Iron-overloaded patients are susceptible to siderophilic organisms — bacteria that depend on iron for growth:

This is why dietary advice includes avoiding uncooked seafood and why clinicians must have a low threshold for cultures and empiric antibiotics in febrile thalassemia patients ^[15].

Extramedullary haematopoiesis (prevented by early transfusion) ^[1]:

• Hepatosplenomegaly ^[1]
• BM expansion: Cooley's facies (frontal bossing, maxillary overgrowth) ^[1]
• XR skull: hair-on-end appearance, thinning of cortex ^[1]

These occur because chronic severe anaemia → ↑EPO → massive marrow expansion (up to 30× normal). In flat bones (skull, facial bones), the expanding marrow pushes outward → characteristic deformities. In the skull, new bone forms perpendicular to the outer table ("hair-on-end"). Medullary cavities of long bones also expand → cortical thinning → osteopenia → pathological fractures.

Extramedullary haematopoietic masses can develop in the liver, spleen, paravertebral region, and even the thorax → can cause spinal cord compression (a rare but serious complication requiring urgent radiation or transfusion to suppress erythropoiesis).

These skeletal complications are prevented by adequate early transfusion (maintaining Hb ≥ 9–10 g/dL suppresses endogenous EPO drive) ^[1].

Jaundice, gallstones (↑bilirubin) ^[1]

• Chronic haemolysis + ineffective erythropoiesis → ↑unconjugated bilirubin → hepatic conjugation capacity exceeded → ↑bilirubin in bile → precipitation as calcium bilirubinate → pigmented (black) gallstones ^[6]
• Present in up to 50–70% of thalassemia intermedia/major patients by adulthood
• Can cause biliary colic, acute cholecystitis, choledocholithiasis, cholangitis, gallstone pancreatitis
• Management: cholecystectomy (laparoscopic) for symptomatic stones

The spleen enlarges from two processes:

1. Extramedullary haematopoiesis — the spleen resumes fetal function of red cell production
2. Work hypertrophy — increased clearance of damaged/abnormal red cells

Progressive splenomegaly → hypersplenism (the spleen traps and destroys not just red cells but also white cells and platelets) → pancytopaenia → further ↑transfusion requirement (a vicious cycle).

↑transfusion requirement or cytopaenias due to hypersplenism is an indication for splenectomy ^[2].

This is an underappreciated but clinically significant complication, especially in thalassemia intermedia and post-splenectomy patients.

Mechanisms:

1. Abnormal red cell membranes expose phosphatidylserine (normally on inner leaflet) → provides a procoagulant surface for coagulation factor assembly
2. Splenectomy removes the splenic filter → abnormal red cells and their procoagulant fragments circulate freely → ↑thrombin generation
3. Thrombocytosis post-splenectomy → additional prothrombotic risk
4. Endothelial dysfunction from chronic anaemia and iron overload
5. ↑Platelet activation from free haemoglobin and arginase release (↓NO bioavailability)

Clinical manifestations: Deep vein thrombosis, pulmonary embolism, portal vein thrombosis, stroke (especially silent cerebral infarcts), pulmonary hypertension ^[2].

Risks of splenectomy: ↑thromboembolism, ↑life-threatening infection, ↑pulmonary hypertension ^[2]

Found in 10–75% of thalassemia intermedia patients (depending on screening methodology) and in post-splenectomy TDT patients.

Mechanisms:

• Chronic haemolysis → free Hb scavenges NO → ↓NO bioavailability → pulmonary vasoconstriction
• Red cell microparticles (procoagulant) → chronic pulmonary microthrombi
• Iron deposition in pulmonary vasculature
• Splenectomy removes the filter for red cell fragments → more fragments reach pulmonary circulation

Screening: Echocardiography (TRJV > 2.5 m/s suggestive); right heart catheterisation for confirmation.

Parvovirus B19 has tropism for erythroid precursors (binds to P antigen/globoside on erythroblasts) → temporarily halts red cell production for ~7–10 days. In a normal person, this is subclinical (RBCs live 120 days, so a 10-day pause causes only minor Hb drop). In thalassemia, where RBC lifespan is already shortened and the marrow is working at maximum capacity, a 10-day production halt → precipitous Hb drop → potentially life-threatening anaemia.

Features: Sudden worsening of anaemia, absent reticulocytes (reticulocytopaenia), ± fever, rash. Management: Transfusion support; usually self-limiting; IVIg if immunocompromised.

Chronic haemolysis → ↑RBC turnover → ↑folate consumption for DNA synthesis → if dietary intake insufficient → folate deficiency → megaloblastic erythropoiesis superimposed on thalassemic erythropoiesis → worsening anaemia with macrocytic change.

Prevention: Folate supplementation 1–2 mg/day for all thal major and intermedia ^[2].

Growth retardation ^[1]

Multifactorial:

1. Chronic anaemia → chronic tissue hypoxia → impaired growth
2. Iron overload → GH/IGF-1 deficiency (pituitary damage), hypogonadism (delayed puberty)
3. Chelation toxicity (deferoxamine at high doses can affect epiphyseal growth plates)
4. Chronic disease burden, suboptimal nutrition, recurrent infections

Management: Adequate transfusion, optimal chelation, GH replacement if deficient, sex hormone replacement for delayed puberty.

Complications after transfusion ^[18]

^[1][2][18]

^[2][14]

Specific complications of splenectomy ^[19]:

Immediate:

• Bleeding: slipped ligature, haematemesis from gastric mucosal damage
• Injury to surrounding structures: pleural effusion, left basal atelectasis, stomach, pancreas (abscess, fistula)

Early:

• Post-operative thrombocytosis: prophylactic aspirin if platelet > 1000 ^[19]
• Post-splenectomy septicaemia ^[19]

Late — Overwhelming post-splenectomy infection (OPSI) ^[19]:

Asplenic patients are unable to mount immunological response against encapsulated organisms ^[19]

Mnemonic — "Some Nasty Killers Have Some Capsule Protection": Streptococcus pneumoniae, Neisseria meningitidis, Klebsiella pneumoniae, Haemophilus influenzae, Salmonella typhi, Cryptococcus neoformans, Pseudomonas aeruginosa ^[19]

Prevention of OPSI ^[19]:

• Vaccination: PCV13 + PPSV23 (repeat Q5 years), Hib vaccine, meningococcal ACWY vaccine, influenza vaccine
• Penicillin V prophylaxis (lifelong in children; practice varies in adults) ^[19]
• Patient education: seek immediate medical attention for fever > 38°C; carry medical alert card
• Malaria prophylaxis if travelling to endemic areas

Complications of HSCT ^[20]:

^[20]

• Risk of Hb Bart's hydrops fetalis in offspring (if both carry --^SEA)
• Mother may develop polyhydramnios-related complications or even mirror syndrome (maternal pre-eclampsia associated with fetal/placental hydrops) ^[2]

• ↑Anaemia during pregnancy (dilutional + ↑demand) → ↑transfusion requirement
• ↑Thrombotic risk (already hypercoagulable + pregnancy is prothrombotic)
• Iron overload-related cardiac dysfunction may decompensate during pregnancy
• Fertility issues from hypogonadotropic hypogonadism (may need assisted reproduction)