Hereditary Spherocytosis

• Membrane Protein Defects: HS most commonly results from mutations affecting spectrin (25–30% of cases), ankyrin (25–30%), band 3 (20–25%), protein 4.2 (3–5%), and RhAG (rare). These proteins form the vertical linkages anchoring the lipid bilayer to the underlying spectrin-actin-based cytoskeleton. Defects disrupt this critical interaction, causing progressive detachment of membrane lipid from the cytoskeleton. The inheritance pattern depends on the affected protein: ankyrin and spectrin mutations are typically autosomal dominant, while band 3 and protein 4.2 mutations show variable inheritance.

• Membrane Instability and Vesiculation: The membrane skeletal defect renders RBCs mechanically unstable. As cells circulate, they sustain microtrauma in the splenic vasculature and capillaries. Rather than remaining biconcave, defective RBCs progressively lose membrane fragments through microvesiculation—a process accelerated by oxidative stress and mechanical shear. Each vesiculation event removes surface area while preserving hemoglobin content, progressively shifting cell morphology from biconcave discs toward smaller, denser spheres. This spheroidization is the hallmark morphologic change and explains both the disease name and the osmotic fragility findings.

• Splenic Trapping and Hemolysis: The spleen serves as the primary site of hemolysis through two mechanisms. First, the splenic microvasculature (particularly the splenic sinusoids with 2–3 μm apertures) mechanically traps spherocytes, which are less deformable than normal biconcave RBCs. Second, splenic macrophages recognize the exposed band 3 molecules (made available by spectrin-ankyrin defects) as senescent or opsonized cells, promoting antibody-independent cellular phagocytosis. The combination of mechanical entrapment and immune recognition results in selective RBC destruction, typically reducing RBC lifespan from the normal 120 days to 10–30 days in moderate disease or even shorter intervals in severe HS.

• Osmotic Vulnerability: The spherical morphology and reduced surface-area-to-volume ratio render HS RBCs exquisitely sensitive to hypotonic stress. Normal RBCs can expand to 150% their original volume before osmotic lysis occurs; spherocytes begin lysis at 120% of normal volume. This occurs because spherical cells have already maximized their volume relative to surface area—they cannot accommodate further fluid influx without rupturing. The osmotic fragility test, wherein RBCs are exposed to progressively hypotonic solutions, reveals lysis at higher saline concentrations in HS compared to normal controls, forming the basis of a classical diagnostic test.

• Metabolic Stress and Dehydration: HS RBCs exhibit increased intracellular calcium and loss of intracellular potassium, leading to a "desiccated" state. The ATP-dependent Na⁺-K⁺-ATPase pump becomes hyperactive attempting to restore ionic balance, consuming glucose at elevated rates and predisposing to metabolic exhaustion. Oxidative stress from sustained hemolysis and splenic inflammation generates free radicals, which further damage membrane proteins and accelerate hemolysis. This creates a vicious cycle of hemolysis accelerating RBC destruction.

• Compensatory Erythropoiesis: The persistent hemolytic stimulus triggers compensatory bone marrow response with marked reticulocytosis (typically 5–25% of RBCs) and erythroid hyperplasia. While this response partially compensates for shortened RBC survival, it cannot fully match the degree of hemolysis in moderate to severe disease, resulting in anemia. The marked erythroid hyperplasia predisposes to complications including aplastic crisis (when parvovirus B19 transiently suppresses erythropoiesis) and folate depletion.

• Spectrin Mutations: Spectrin forms the primary structural scaffold of the RBC cytoskeleton, providing mechanical strength and deformability. Mutations in SPTA1 (encoding α-spectrin) or SPTB (β-spectrin) account for 25–30% of HS cases. These are typically autosomal dominant; some autosomal recessive cases result from homozygous spectrin deficiency. Spectrin defects produce particularly severe hemolysis in some families, correlating with degree of spectrin deficiency measured on RBC membranes.

• Ankyrin Mutations: Ankyrin functions as the primary attachment point linking band 3 (a chloride-bicarbonate antiporter) to the spectrin skeleton via a vertical linkage. Mutations in ANK1 are the most common genetic cause of autosomal dominant HS (25–30% of cases). Ankyrin deficiency leads to secondary loss of band 3, heightening membrane instability. Ankyrin-deficient RBCs show profound osmotic fragility and typically manifest moderate-to-severe hemolysis.

• Band 3 Mutations: Band 3 (SLC4A1 gene) serves dual roles as both a structural protein in the vertical linkage and as the primary anion transporter (exchanging Cl⁻ for HCO₃⁻). Mutations account for 20–25% of HS cases and show variable inheritance (both autosomal dominant and recessive). Band 3 defects disrupt both structural integrity and potentially anion transport capacity, though the hemolytic severity is often less severe than ankyrin deficiency.

• Protein 4.2 Mutations: Protein 4.2 functions as a linker stabilizing band 3 and protein interactions. Mutations in EPB42 account for only 3–5% of HS cases, typically showing autosomal recessive inheritance and predominantly affecting populations of Asian and African descent. Protein 4.2 deficiency causes mild HS with less severe hemolysis.

• RhAG Mutations: RhAG (Rh-associated glycoprotein) is involved in ammonia transport and membrane protein organization. Rare mutations cause a unique phenotype combining HS features with Rh antigen abnormalities, important for transfusion considerations in affected patients.

• Genetic Background and Penetrance: Despite identical mutations, clinical severity varies significantly among families, suggesting modifying genetic factors influence disease expression. Environmental stressors (infection, oxidative stress) and spleen size modulate symptom severity. Approximately 10–15% of HS cases result from de novo mutations with no family history, important for counseling.

• Jaundice and Hyperbilirubinemia: Visible jaundice typically appears in infancy or early childhood as hemolysis drives unconjugated (indirect) hyperbilirubinemia. The degree of jaundice correlates with hemolysis severity; severe HS may present with neonatal jaundice requiring phototherapy or exchange transfusion. In adults, persistent mild jaundice (bilirubin 2–4 mg/dL) is common in moderate disease. Jaundice reflects the hemolytic burden and indicates that bilirubin production exceeds hepatic conjugation and excretion capacity. The unconjugated nature of hyperbilirubinemia distinguishes HS from hepatic causes.

• Fatigue and Dyspnea on Exertion: These symptoms result from the anemia of chronic hemolysis. Moderate HS typically produces hemoglobin levels of 8–12 g/dL; severe disease may fall below 7 g/dL. The chronicity allows for partial compensation through increased cardiac output and 2,3-DPG production, partially offsetting hypoxemia. Nevertheless, exertional dyspnea, fatigue, palpitations, and reduced exercise tolerance commonly occur, particularly during hemolytic crises.

• Abdominal Pain and Splenomegaly: Most HS patients develop significant splenomegaly (splenic enlargement 2–10-fold normal), which patients may describe as left upper quadrant fullness, discomfort, or pain. The enlarged spleen is the primary site of hemolysis, and its size often correlates with disease severity and hemolysis rate. Splenomegaly results from both splenic hyperplasia (increased reticuloendothelial tissue response to hemolysis) and vascular congestion from the heavy RBC destruction burden. Left upper quadrant pain or fullness is a classic presenting complaint in childhood HS.

• Gallstones (Cholelithiasis): Chronic hemolysis generates sustained high bilirubin levels, predisposing to pigmented gallstone (bilirubin stone) formation. HS patients develop cholelithiasis at rates 50–75 times higher than the general population, with 5–15% symptomatic by adulthood. Gallstones may remain clinically silent or precipitate acute cholecystitis, choledocholithiasis, or biliary pancreatitis. The risk increases with disease duration and severity, making screening ultrasound reasonable in adolescents and young adults.

• Hemolytic Crises: Acute hemolytic crises manifest as sudden worsening of jaundice, dark urine (hemoglobinuria/myoglobinuria from severe intravascular hemolysis), increased fatigue, dyspnea, and sometimes fever. These are typically triggered by infections, oxidative stressors, or rarely by transfusion of incompatible blood. Acute crises represent dramatic acceleration of the baseline hemolytic process, sometimes revealing previously undiagnosed or mild HS. Laboratory findings show profound anemia, markedly elevated reticulocyte counts (>25%), elevated indirect bilirubin, elevated LDH, and decreased haptoglobin.

• Aplastic Crisis: A life-threatening complication occurring when parvovirus B19 infects and transiently destroys erythroid progenitor cells in the bone marrow. Patients develop sudden severe anemia with inappropriate reticulocytopenia (the hallmark distinguishing feature from hemolytic crisis). Presents with profound fatigue, dyspnea, syncope, and hemodynamic instability. Aplastic crisis typically lasts 7–10 days before spontaneous recovery; management includes supportive transfusion. This complication is particularly common in children and may be the presenting event for previously undiagnosed HS.

• Megaloblastic Crisis: Prolonged hemolysis and compensatory erythropoiesis create marked folate demands; folate deficiency may develop insidiously, producing a megaloblastic anemia superimposed on HS. Patients develop glossitis, peripheral neuropathy, or cognitive changes from B₁₂ deficiency (when concurrent). The MCV typically normalizes after folate supplementation.

• Physical Examination Findings: Beyond splenomegaly, physical exam may reveal jaundice (scleral icterus), pallor from anemia, tachycardia (compensatory response to anemia), and occasional leg ulcers in severe chronic hemolysis (similar to sickle cell disease). Growth and pubertal delay may occur in severe childhood HS from chronic disease and metabolic demands of compensatory erythropoiesis.

• Asymptomatic Discovery: A significant proportion of HS patients (particularly those with mild disease) are discovered incidentally on routine blood smears showing spherocytes, family screening in relatives of diagnosed patients, or during evaluation for unrelated conditions. Asymptomatic patients require no treatment but need counseling regarding complications, infection prevention, and activity restrictions.

• Complete Blood Count and Blood Smear Morphology: The CBC reveals variable anemia (depending on disease severity), elevated reticulocyte count (typically 5–25% in compensated disease, higher during crises), and normal or elevated platelet and WBC counts. The peripheral blood smear shows the characteristic finding of spherocytes—small, dense, darkly-staining RBCs lacking the normal biconcave pallor. Spherocytes appear particularly dense compared to surrounding RBCs. While spherocytes are the hallmark finding, they may be subtle in mild disease or present in other conditions (autoimmune hemolytic anemia, severe burns, transfused RBCs). Polychromasia reflects reticulocytosis from compensatory erythropoiesis.

• Osmotic Fragility Test: This classical test exposes RBCs to progressively hypotonic saline solutions and measures the osmolarity at which hemolysis begins (measured by hemoglobin release and optical density changes). Normal RBCs begin hemolysis at approximately 0.48% NaCl and complete hemolysis by 0.30% NaCl. HS RBCs show a leftward shift of the hemolysis curve, beginning lysis at higher saline concentrations (e.g., 0.64%) due to their reduced surface-area-to-volume ratio and inability to accommodate further fluid influx. The osmotic fragility test has sensitivity and specificity exceeding 95% for HS when abnormal. An incubated osmotic fragility test (incubating RBCs at 37°C for 24 hours before testing) increases sensitivity by allowing continued in vitro hemolysis and spheroidization, potentially revealing HS in borderline cases. This test has declining use with advent of flow cytometry but remains a classic teaching tool.

• Flow Cytometry and Eosin-5′-Maleimide (EMA) Binding Assay: Modern laboratories increasingly use EMA binding flow cytometry as the primary diagnostic test. EMA is a fluorescent dye that binds to band 3 and other RBC membrane proteins; reduced band 3 expression (from membrane skeletal defects) results in decreased fluorescence. HS RBCs show decreased EMA binding intensity compared to normal controls. This test has sensitivity >95% and specificity >99%, making it the gold standard for HS diagnosis. Flow cytometry can also assess RBC volume distribution (RDW); HS shows increased RDW reflecting the mixed population of normal and spheroidal RBCs. Flow cytometry is less affected by in vitro hemolysis than osmotic fragility testing and requires smaller sample volumes.

• Hemolysis Markers and Indirect Evidence of Hemolysis: Laboratory tests corroborating hemolysis include:
• Elevated indirect (unconjugated) bilirubin (typically 1–3 mg/dL, higher during crises)
• Elevated lactate dehydrogenase (LDH) (>250 IU/L, often 500–1000+ in active hemolysis)
• Decreased haptoglobin (<10 mg/dL, often <5 mg/dL in moderate-to-severe disease), reflecting consumption by binding hemoglobin released from lysed RBCs
• Elevated reticulocyte count and reticulocyte index >2 (indicating compensatory erythropoiesis)
• Normal coagulation studies (PT, PTT) and normal platelet count (distinguishing from thrombotic microangiopathies)
• Negative direct Coombs (antiglobulin) test, distinguishing HS from autoimmune hemolytic anemia where antibodies coat RBCs

• Membrane Protein Quantification: Ektacytometry measures RBC deformability under increasing shear stress and can detect membrane protein deficiencies; RBCs from HS patients show reduced deformability. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) quantifies RBC membrane proteins and reveals deficiencies in spectrin, ankyrin, band 3, or protein 4.2. These tests are specialized and not routinely performed outside reference laboratories but can confirm specific protein deficiencies.

• Genetic Testing: DNA sequencing of genes encoding membrane proteins (*SPTA1, SPTB, ANK1, SLC4A1, EPB

Immediate stabilisation

• Packed RBC transfusion: reserved for symptomatic anemia, hemodynamic compromise, or aplastic crisis from parvovirus B19. Reticulocytopenia with falling hemoglobin — not the hemoglobin value alone — drives the decision, since the marrow cannot compensate for 7–10 days.
• Neonatal hyperbilirubinemia: phototherapy, escalating to exchange transfusion at threshold bilirubin levels, per the AAP clinical practice guideline on hyperbilirubinemia in infants ≥35 weeks. Hemolysis is a recognized neurotoxicity risk factor and lowers the treatment threshold.

Chronic supportive therapy

• Folate supplementation (folic acid 1 mg daily): sustained erythroid hyperplasia consumes folate; replacement prevents superimposed megaloblastic crisis. Most useful in moderate–severe disease.
• Erythropoiesis-stimulating agents (epoetin alfa) are sometimes used in severe neonatal/infantile HS to bridge past the physiologic nadir and reduce transfusion exposure.

Definitive management

• Splenectomy is curative of the anemia — it removes the site of entrapment and phagocytosis, so RBC survival normalizes even though the membrane defect and spherocytes persist. Reserved for severe disease, or moderate disease with growth failure, transfusion dependence, refractory symptoms, or recurrent crises. Laparoscopic approach is standard; concomitant cholecystectomy is performed when symptomatic pigment stones are present.
• Subtotal (partial) splenectomy is an option in young children, preserving splenic immune function while blunting hemolysis.

Peri-splenectomy prophylaxis (ACIP/AAP)

• Vaccination against encapsulated organisms — pneumococcal (PCV20, or PCV15 followed by PPSV23), Haemophilus influenzae type b, and meningococcal ACWY plus MenB — given at least two weeks before elective surgery.
• Oral penicillin V prophylaxis in children post-splenectomy, plus a written fever action plan and standby antibiotics for life.

Avoid

• Splenectomy in mild, asymptomatic disease, and in children under roughly 5–6 years, because of overwhelming post-splenectomy infection risk.
• Empiric iron supplementation — iron stores are typically normal or high; give only for documented deficiency.

Complications of the disease

• Pigment (calcium bilirubinate) gallstones: chronic unconjugated hyperbilirubinemia supersaturates bile. Signalled by right upper quadrant pain, or by stones on ultrasound in an otherwise well adolescent. Emergency variants are ascending cholangitis (fever, jaundice, RUQ pain) and gallstone pancreatitis.
• Aplastic crisis (parvovirus B19): the virus uses the P antigen to infect erythroid progenitors, halting production for 7–10 days. The signal is a sharp hemoglobin drop with an inappropriately low reticulocyte count — the opposite of a hemolytic crisis. This is an emergency; patients can decompensate rapidly and require transfusion and droplet precautions (with attention to exposed pregnant contacts).
• Megaloblastic crisis: folate exhaustion from sustained erythropoiesis; signalled by rising MCV with hypersegmented neutrophils.
• Hemolytic crisis: infection or oxidative stress accelerates splenic clearance; deepening jaundice, dark urine, and a rising reticulocyte count with falling haptoglobin.
• Growth and pubertal delay, leg ulcers, and extramedullary hematopoiesis in severe untreated disease reflect the metabolic cost of chronic marrow expansion.
• Iron overload in transfusion-dependent patients: ferritin and transferrin saturation rise; cardiac and hepatic dysfunction follow.

Complications of splenectomy

• Overwhelming post-splenectomy infection (OPSI): loss of splenic clearance of opsonized encapsulated bacteria — Streptococcus pneumoniae, Haemophilus influenzae type b, Neisseria meningitidis. Presents as fever progressing within hours to fulminant sepsis, purpura fulminans, and DIC. A true emergency: any fever in an asplenic patient warrants immediate blood cultures and empiric broad-spectrum antibiotics before results return.
• Post-splenectomy thrombocytosis and venous thrombosis, including portal and mesenteric vein thrombosis: signalled by post-operative abdominal pain with a markedly elevated platelet count.
• Recurrent hemolysis after splenectomy: an accessory spleen or splenosis; the tip-off is the absence of Howell-Jolly bodies on smear in a patient who should be asplenic.
• Persistent spherocytes on the smear after splenectomy are expected — the membrane defect is not corrected — and do not indicate failure.

• Spherocytes plus a negative direct antiglobulin (Coombs) test = hereditary spherocytosis. The single most tested discriminator is the DAT: warm autoimmune hemolytic anemia produces morphologically identical spherocytes but is Coombs-positive. If a stem shows spherocytes, the best next step is a direct antiglobulin test before any membrane-specific assay.
• Elevated MCHC (>36 g/dL) with elevated RDW is the classic CBC fingerprint — the only common anemia that raises MCHC, because membrane loss concentrates hemoglobin in a smaller cell. Pair it with the loss of central pallor on smear.
• EMA binding flow cytometry is the modern confirmatory test of choice; the osmotic fragility test is the classic exam answer but is normal in a minority of patients and can be falsely abnormal in any spherocytic state.
• Parvovirus B19 → aplastic crisis. Sudden severe anemia with a low reticulocyte count in a child with known hemolytic anemia is the association examiners test most often. Reticulocytosis points to a hemolytic crisis instead.
• Splenectomy cures the anemia but not the defect. Spherocytes remain on the smear afterward; Howell-Jolly bodies should appear, and their absence in a patient with recurrent hemolysis suggests an accessory spleen.
• Vaccinate before you cut. ACIP requires pneumococcal, meningococcal ACWY/B, and Hib vaccination at least two weeks before elective splenectomy, with penicillin prophylaxis in children afterward — a frequent "next best step" answer.
• Common distractors: G6PD deficiency gives bite cells and Heinz bodies after an oxidant exposure, not spherocytes; hereditary elliptocytosis reflects horizontal (spectrin–spectrin) defects, whereas HS reflects vertical membrane–cytoskeleton linkage defects; and splenectomy is contraindicated in hereditary stomatocytosis because of thrombotic risk — do not generalize the HS answer to every membranopathy.
• Gallstones in a young patient with jaundice and splenomegaly should prompt consideration of chronic hemolysis, not primary biliary disease.