Hemolytic anemia

Hemolytic anemias are a group of conditions characterized by the breakdown of red blood cells. Hemolysis is caused by either abnormalities of the RBCs themselves (abnormalities in hemoglobin, the RBC membrane or intracellular enzymes), also called corpuscular anemia, or by external causes (immune-mediated or mechanical damage), which is referred to as extracorpuscular anemia. All hemolytic anemias feature varying degrees of fatigue, pallor, and weakness (from asymptomatic disease to life-threatening hemolytic crisis), although some diseases have more specific findings (e.g., venous thrombosis in paroxysmal nocturnal hemoglobinuria). They also share common laboratory findings, such as elevated indirect bilirubin and lactate dehydrogenase, reticulocytosis, and decreased haptoglobin levels. The Coombs test helps to distinguish autoimmune (positive Coombs test) from non-autoimmune anemias (negative Coombs test). Treatment includes RBC transfusions as required but otherwise depends on the specific type of hemolytic anemia and its causes.

Laboratory signs of hemolysis

• ↓ Haptoglobin
  • Only free and unbound haptoglobin is measured
  • Hemoglobin released from broken down erythrocytes binds to haptoglobin; . → ↓ free, circulating haptoglobin
  • Occurs mostly in intravascular hemolysis
  • Haptoglobin can be increased or normal as a reaction to an infectious process, thereby masking hemolysis
• ↑ Lactate dehydrogenase (LDH): nonspecific parameter that simply indicates increased cellular breakdown
• ↑ Indirect/unconjugated bilirubin
  • Hemoglobin released from erythrocytes is turned into indirect/unconjugated bilirubin.
  • Increased bilirubin levels can result in icterus.
  • Urobilinogenuria
• ↑ Reticulocytes: loss of erythrocytes → reactive increase in erythropoiesis → increase in reticulocytes in the peripheral blood
• In extreme hemolysis: ↑ free hemoglobin
  • Brown-colored urine due to hemoglobinuria

Coombs test

This test detects antibodies and/or complement proteins on RBCs surface; (direct test) or in patient's serum (indirect test). The test uses a special Coombs serum that contains anti-human globulins. A positive result in a patient with hemolysis supports the diagnosis of antibody-mediated, extracorpuscular anemia.

• Direct Coombs test
  • After taking a patient's blood sample, it is purified so that only the erythrocytes remain.
  • Coombs serum containing anti-human globulins (antigens) is added.
  • The examiner visually analyzes the sample and looks for erythrocyte agglutination.
    • In the case of erythrocyte agglutination → positive test → confirmation of preexisting erythrocytes coated with autoantibodies
    • In the case of absent erythrocyte agglutination → negative test
• Indirect Coombs test
  • After taking a patient's blood sample, it is purified so that only the serum remains.
  • A donor's blood sample containing erythrocytes is added.
  • Coombs serum containing anti-human globulins (antigens) is added as well.
  • The examiner visually analyzes the sample and looks for erythrocyte agglutination.
    • In the case of erythrocyte agglutination → positive test → confirmation of preexisting circulating, free antibodies within patient's serum that bound to donor's RBC surfaces
    • In the case of absent erythrocyte agglutination → negative test

References:^[1][2]

Erythrocytic membrane defects

• Hereditary spherocytosis
• Paroxysmal nocturnal hemoglobinuria (PNH)
  • Description: : an acquired genetic defect characterized by a triad of hemolytic anemia, pancytopenia, and thrombosis
  • Pathophysiology
    • Physiologically, a membrane-bound glycosylphosphatidylinositol (GPI) anchor protects RBCs against complement-mediated hemolysis
    • Acquired mutation on the PIGA gene located on the X chromosome → GPI anchor loses its protective effect → RBC destruction by complement and reticuloendothelial system → intravascular and extravascular hemolysis
    • Can occur in ∼ 10% patients with aplastic anemia
  • Clinical symptoms
    • Pallor, excessive fatigue; , weakness
    • Intermittent jaundice and episodes of red/dark urine
    • Venous thrombosis in unusual locations (e.g., hepatic, cerebral, abdominal veins)
    • Vasoconstriction: headache, pulmonary hypertension
    • Abdominal pain, dysphagia, erectile dysfunction
  • Diagnosis
    • Anemia or pancytopenia → infections and bleeding
    • Signs of hemolysis
    • Flow cytometry: absence of CD55 and CD59 on the surface of RBCs
    • Coombs test: negative .
  • Therapy
    • Eculizumab: Anti-C5 antibodies
    • Bone marrow transplantation in the case of bone marrow hypoplasia
    • Allogeneic stem cell transplantation
    • Iron and folate supplementation
  • Complications
    • Complications related to vasoconstriction and thrombotic emboli (e.g., infarctions, Budd-Chiari syndrome)
    • ↑ Risk of acute leukemias

Think of PNH if a patient presents with hemolytic anemia, venous thrombosis, and pancytopenia!

Enzyme defects

• Glucose-6-phosphate dehydrogenase deficiency
• Pyruvate kinase deficiency
  • Pathophysiology: autosomal recessive defect of pyruvate kinase
  • Etiology: : Glucose is the only energy source for RBCs. In glycolysis, pyruvate kinase converts phosphoenolpyruvate into pyruvate irreversibly. This step is required in the degradation of glucose into pyruvate to generate ATP; (via the Embden-Meyerhof pathway, the major anaerobic glycolytic pathway). In the absence of pyruvate kinase; , RBCs are deficient in ATP; . This deficiency leads to a disturbed cation gradient among the RBC membrane, resulting in hemolysis.
  • Clinical symptoms: In contrast to G6PD deficiency, the majority of patients with pyruvate kinase deficiency display clinical symptoms.
    • Splenomegaly
    • Icterus
    • Pallor, fatigue, weakness
    • In rare cases: lethal hydrops fetalis
  • Diagnosis
    • Signs of hemolysis
    • Measure pyruvate kinase enzyme activity
    • Detection of pyruvate kinase, liver, and red blood cells (PKLR) gene mutation
  • Therapy
    • Phototherapy and/or exchange transfusions
    • In case of severe anemia or excessively enlarged spleen: splenectomy

Hemoglobinopathies

• Sickle cell disease
• Thalassemia
• Hemoglobin C disease
  • Pathophysiology:
    • β-globin mutation (glutamate replaced by lysine)
    • HbC is less soluble than HbA → hexagonal crystal formation and RBC dehydration (↑ MCHC)
    • RBCs have reduced oxygen binding capacity and a shorter lifespan
  • Clinical symptoms: hemolytic anemia (usually mild), jaundice, splenomegaly
  • Blood smear: target cells, hemoglobin crystals within RBCs
  • Therapy: Folic acid supplements

References:^{[3][4][5][6][7][8][9][10][11][12][13][14][15][16]}

Isoimmune hemolytic anemia

• ABO incompatibility
• Rhesus incompatibility

Autoimmune hemolytic anemia (AIHA)

• Cold agglutinin disease
  • Occurs in lower temperatures (0–5°C/32–41°F) and usually affects elderly women (≥ 70 years)
  • Etiology: mostly IgM antibodies (bound to erythrocytes) that can cause both intravascular and extravascular hemolysis
    • Idiopathic (esp. in the elderly)
    • Secondary
      • Mycoplasma or EBV infection
      • Malignancy (e.g. non-Hodgkin lymphoma, chronic lymphocytic leukemia)
  • Clinical features
    • Pallor, fatigue, weakness
    • Cyanosis of the extremities (acrocyanosis)
  • Diagnosis
    • Signs of hemolysis
    • Peripheral blood smear: spherocytes, polychromasia
    • Coombs test: positive
    • ↑ Cold agglutinins titer
    • ↓C3 and ↓ C4; (due to complement activation)
  • Prognosis: spontaneous remission within a few weeks is common
  • Therapy
    • Mild disease: avoid cold exposure
    • Moderate/severe disease: immunosuppressive therapy; with rituximab alone or in combination with interferon, fludarabine, or prednisone
• Warm agglutinin disease
  • Occurs at body temperature (37°C/98.6° F)
  • Etiology: mostly IgG antibodies, that result in extravascular hemolysis mediated by the reticuloendothelial system of the spleen and liver
    • Idiopathic
    • Secondary: due to lymphoma, chronic lymphocytic leukemia, systemic lupus erythematosus, or a drug reaction (e.g., cephalosporins, penicillins, α-methyldopa) ^[17]
  • Clinical features:
    • Often more severe and more acute than cold agglutinin disease
    • Signs of anemia (pallor, fatigue, weakness)
  • Diagnosis
    • Signs of hemolysis
    • Peripheral blood smear: spherocytes, polychromasia
    • Coombs test: positive
  • Therapy
    • Initial treatment of choice: glucocorticoids
    • If glucocorticoids are unsuccessful
      • Adults: splenectomy
      • Children: rituximab

Microangiopathic hemolytic anemia

• Hemolytic uremic syndrome (HUS)
• Thrombotic thrombocytopenic purpura (TTP)
• Malignant hypertension
• Systemic lupus erythematosus (SLE)

Macroangiopathic hemolytic anemia

• Etiology
  • Moderate and severe aortic stenosis; : heart valve replacement resolves the anemia
  • Prosthetic heart valve
  • Extracorporeal circulation (e.g., dialysis)
  • Exertional hemoglobinuria (“March hemoglobinuria”): destruction of RBCs in the feet during strenuous exercise (especially running and walking on hard surfaces)
• Pathophysiology
  • RBC destruction in the systemic circulation due to mechanical forces applied to the erythrocyte membrane
• Clinical features
  • Features of anemia (e.g., pallor, fatigue)
  • Features of intravascular hemolysis

References:^{[3][4][18][19]}

	Intravascular hemolysis	Extravascular hemolysis
Coombs test	Positive in cold agglutinin disease	Positive in cold and warm agglutinin disease
Haptoglobin	↓	Usually normal
LDH	↑	↑
Urine hemosiderin	Present	Usually absent
Urine hemoglobin	Present	Usually absent
Peripheral smear	Usually schistocytes Heinz bodies and bite cells in G6PD deficiency Reticulocytes	Spherocytes in spherocytosis and possibly in cold and warm agglutinin disease Sickle cells in sickle cell disease
Examples	Cold agglutinin disease Microangiopathic anemia Macroangiopathic anemia G6PD deficiency PNH	Cold agglutinin disease Warm agglutinin disease Sickle cell disease Spherocytosis Pyruvate kinase deficiency PNH

References:^[20]