21.3 Membranopathies, Enzymopathies, PNH & Sickle Cell Disease

5. Red Cell Membranopathies & Enzymopathies

Hereditary Spherocytosis (HS)

• Genetics & Pathophysiology: Autosomal dominant in 75% of cases (remainder autosomal recessive or de novo). Mutations in red cell membrane cytoskeletal proteins: ankyrin-1 (ANK1, 50%), band 3 (SLC4A1, 20%), and beta-spectrin (SPTB, 20%). Disruption of vertical membrane-cytoskeleton tethering leads to progressive shedding of lipid membrane vesicles during circulation, reducing the surface-area-to-volume ratio. Erythrocytes assume a spherical morphology (microspherocytes), losing cellular deformability and becoming entrapped and destroyed within the narrow inter-endothelial slits of the splenic red pulp cords (extravascular haemolysis).
• Clinical Presentation: Family history of anaemia, early-onset pigmented (calcium bilirubinate) gallstones, neonatal jaundice, and persistent splenomegaly.
• Aplastic Crisis: Infection with Parvovirus B19 selectively infects and destroys erythroid progenitor cells, causing a sudden, temporary cessation of erythropoiesis. In patients with HS (where red cell lifespan is shortened to 10–20 days), this causes a catastrophic, precipitous drop in haemoglobin accompanied by profound reticulocytopenia (<0.1% or <10 x 10^9/L). In contrast, a haemolytic crisis features high reticulocytes.
• Diagnostics: High Mean Corpuscular Haemoglobin Concentration (MCHC >360 g/L; hallmark of cellular dehydration), negative DAT (distinguishing HS from warm AIHA), and the eosin-5-maleimide (EMA) binding test by flow cytometry (diagnostic gold standard; demonstrates reduced fluorescence due to deficient band 3/CD47). The acidified glycerol lysis test and osmotic fragility test are classical alternatives.
• Management: Daily folic acid supplementation (5 mg/day). Splenectomy is curative of haemolysis and anaemia (though spherocytes persist on blood film); indicated for moderate-to-severe disease. Post-splenectomy management requires lifelong prophylactic oral penicillin V and immunization against encapsulated bacteria (Streptococcus pneumoniae, Neisseria meningitidis, Haemophilus influenzae type b).

Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency

• Genetics & Pathophysiology: X-linked recessive enzymopathy affecting over 500 million people worldwide (predominantly individuals of Mediterranean, African, Middle Eastern, and Southeast Asian ancestry). G6PD catalyzes the first step of the hexose monophosphate (HMP) shunt, generating NADPH. In erythrocytes (which lack mitochondria and nuclei), NADPH is the sole source of reducing equivalents required by glutathione reductase to regenerate reduced glutathione (GSH). GSH neutralizes reactive oxygen species and hydrogen peroxide. Deficient G6PD renders erythrocytes exquisitely vulnerable to oxidative stress.
• Oxidative Triggers:
  • Ingestion of fava beans (Vicia faba, favism);
  • Acute systemic infections (most common clinical trigger);
  • Medications: Antimalarials (primaquine), antibiotics (ciprofloxacin, nitrofurantoin, sulfamethoxazole / co-trimoxazole, dapsone), and rasburicase.
• Blood Film Morphology: Oxidative stress causes crosslinking of hemoglobin sulfhydryl groups, precipitating insoluble aggregates termed Heinz bodies (visualized on supravital stains with methyl violet). When Heinz-body-laden red cells pass through the splenic cords, splenic macrophages pluck out the precipitates, producing signature "bite cells" (degmacytes) and "blister cells" (cells with hemoglobin pulled to one side under a blistered membrane).
• CRITICAL EXAM TRAP: Red cell G6PD enzyme activity assays can be falsely normal during an acute haemolytic crisis! Older, enzyme-deficient erythrocytes have already lysed, leaving behind young, newly released reticulocytes that possess relatively normal or elevated G6PD enzyme levels. The quantitative assay must be repeated 8 to 12 weeks after the acute haemolytic episode has fully resolved to establish the true baseline enzyme level.

6. Paroxysmal Nocturnal Haemoglobinuria (PNH)

• Pathophysiology: An acquired, clonal haematopoietic stem cell disorder caused by a somatic mutation in the X-linked PIGA gene (phosphatidylinositol glycan class A). This mutation disrupts the biosynthesis of glycosylphosphatidylinositol (GPI) anchors, which normally anchor surface glycoproteins to cell membranes. Erythrocytes and platelets completely lack GPI-anchored complement-regulatory proteins:
  1. CD55 (Decay-Accelerating Factor, DAF): Accelerates decay of C3 and C5 convertases;
  2. CD59 (Membrane Inhibitor of Reactive Lysis, MIRL): Directly blocks incorporation of C9 into the membrane attack complex (MAC, C5b–9).
• In the absence of CD55 and CD59, erythrocytes are continuously vulnerable to uncontrolled, terminal complement-mediated intravascular haemolysis.
• Clinical Presentation:
  • Classical dark red-brown urine (nocturnal haemoglobinuria), occurring predominantly upon waking (relative hypoventilation and mild respiratory acidosis during sleep augment alternative complement pathway activation);
  • Severe smooth muscle dystonias: Esophageal spasms, dysphagia, severe episodic abdominal pain, and erectile dysfunction, caused by free plasma hemoglobin scavenging and depleting nitric oxide (NO);
  • Thrombosis at Atypical Venous Sites: Thrombosis is the leading cause of mortality in PNH, occurring classically in the hepatic veins (Budd-Chiari syndrome), mesenteric veins, portal vein, and cerebral venous sinuses (dural sinus thrombosis).
• Diagnosis: High-sensitivity multiparameter flow cytometry on peripheral blood granulocytes and erythrocytes demonstrating the absence of CD55 and CD59, alongside FLAER (fluorescently labeled aerolysin) testing, which binds directly to GPI anchors.
• Management: Monoclonal antibody therapy targeting terminal complement component C5: eculizumab or ravulizumab. These agents abolish intravascular haemolysis, reduce transfusion requirements, and slash thrombotic risk by >85%. Mandatory pre-treatment requirement: Patients receiving terminal complement inhibitors have a 1,000-fold increased risk of invasive meningococcal disease and must receive meningococcal vaccination (MenACWY and MenB) and prophylactic penicillin before drug initiation.

7. Sickle Cell Disease (SCD)

• Molecular Genetics: Autosomal recessive condition caused by a single point mutation (single nucleotide transversion: GAG to GTG) at codon 6 of the beta-globin gene (HBB) on chromosome 11, resulting in substitution of hydrophilic glutamic acid with hydrophobic valine (producing HbS).
• Polymerization Mechanics: Under deoxygenated conditions (hypoxia, acidosis, dehydration, cold), hydrophobic valine interacts with complementary hydrophobic pockets on adjacent beta chains, forming insoluble, rigid crystalline polymers that distort erythrocytes into rigid, sickle-shaped cells. Sickled cells occlude microarterioles and post-capillary venules, inducing localized tissue ischaemia, necrosis, and sterile inflammation.
• Sickle Cell Crises:
  1. Vaso-Occlusive / Painful Crisis: Most frequent complication; microvascular occlusion within bones (femur, humerus, vertebrae) causing excruciating deep pain. In infants (6–18 months), presents as acute, symmetrical, painful swelling of the dorsum of the hands and feet (dactylitis / "hand-foot syndrome"). Management: Aggressive multimodal analgesia (rapid titration of parenteral opioids), hydration, keeping warm, and oxygen supplementation only if hypoxic.
  2. Acute Chest Syndrome (ACS): Leading cause of mortality in adult SCD! Defined by the emergence of a new pulmonary infiltrate on chest radiography accompanied by fever, chest pain, tachypnoea, cough, wheeze, or acute hypoxia. Pathophysiology involves a vicious cycle of pulmonary microvascular sickling, alveolar hypoventilation, bone marrow fat embolism, and pulmonary infection (Chlamydia pneumoniae, Mycoplasma, viral). Management: Urgent broad-spectrum intravenous antibiotics, multimodal analgesia, incentive spirometry, and prompt simple or automated exchange blood transfusion (aiming to reduce HbS <30% while keeping total Hb <=100–110 g/L to avoid hyperviscosity).
  3. Aplastic Crisis: Caused by Parvovirus B19 infection selectively destroying erythroid precursors; acute drop in haemoglobin accompanied by reticulocytopenia (<0.1%). Requires supportive red cell transfusion; recovery usually occurs within 1–2 weeks with lifelong immunity.
  4. Splenic Sequestration Crisis: Sudden, massive pooling of erythrocytes within the splenic sinusoids, predominantly in infants and toddlers (before autosplenectomy). Characterized by rapid splenic enlargement, hypovolaemic shock, and profound anaemia. Management requires urgent fluid resuscitation and cautious red cell transfusion.
  5. Priapism: Prolonged, painful, non-sexual erection caused by stasis and sickling within the corpora cavernosa; urological emergency requiring penile aspiration and alpha-agonist instillation if >4 hours.
• Functional Hyposplenism: Recurrent microvascular infarction of splenic sinusoids during early childhood leads to progressive splenic fibrosis, atrophy, and complete autosplenectomy by age 5–6 years. Peripheral blood film exhibits pathognomonic Howell-Jolly bodies (nuclear chromatin remnants), target cells, and Pappenheimer bodies. Patients are at catastrophic risk of fatal sepsis caused by encapsulated bacteria (Streptococcus pneumoniae, Neisseria meningitidis, Haemophilus influenzae, and non-typhoidal Salmonella osteomyelitis). Mandatory prophylaxis: Lifelong daily oral phenoxymethylpenicillin (penicillin V), pneumococcal (conjugate and polysaccharide), meningococcal ACWY/MenB, and Hib immunizations.
• Disease-Modifying Therapy: Hydroxycarbamide (hydroxyurea) stimulates fetal haemoglobin (HbF) synthesis by erythroid progenitors, lowers intracellular HbS concentration, and reduces sickle polymer formation. Clinical trials demonstrate a >50% reduction in painful vaso-occlusive crises, acute chest syndrome episodes, and transfusion requirements. Folic acid supplementation (5 mg daily) supports brisk chronic erythropoiesis.