4. Haemolytic anaemias (inherited and acquired)

Last updated on November 21, 2019 at 00:23

Haemolytic anaemias

Haemolytic anaemias are a group of conditions characterized by the pathologically increased breakdown of RBCs. The RBC lifespan is shorter than the normal 120 days.

To compensate for increased turnover of RBCs the bone marrow can increase the output of RBCs 6 – 8-fold. Anaemia only occurs if the rate of destruction exceeds this increased production rate.

Intravascular and extravascular haemolysis:

We distinguish between intravascular haemolysis and extravascular haemolysis depending on where the haemolysis happens. In intravascular haemolysis pathological haemolysis occurs in vessels while in extravascular haemolysis the physiological haemolysis in the MPS is pathologically increased.

RBC metabolism:

RBCs are broken down by macrophages of the mononuclear phagocyte system (MPS), also called the reticuloendothelial system (RES). The majority of the breakdown occurs in the spleen. Haemoglobin is split into the globin part and the heme part. The globin part is broken down to amino acids while the heme part is converted to indirect bilirubin. Indirect bilirubin is then converted to direct bilirubin in the liver and so on.

Diagnostics of haemolysis:

A protein called haptoglobin is important in the diagnosis of haemolysis. Haptoglobin is a plasma protein which binds to free haemoglobin in the plasma. When there is intravascular haemolysis, more haemoglobin is released into the plasma. Haptoglobin in the plasma will bind to the released haemoglobin. This decreases the amount of free circulating haptoglobin, which is what’s measured in the lab.

Lactate dehydrogenase (LDH) is an enzyme found in virtually all cells. Its level in the plasma increases when there is increased breakdown of any cell, not just RBCs.

When there is haemolysis erythropoiesis is increased, as explained earlier. This causes the bone marrow to release more reticulocytes into the blood.

  • Laboratory tests
    • In both types of haemolysis
      • Haptoglobin ↓
      • Lactate dehydrogenase ↑
      • Indirect bilirubin ↑
      • Reticulocytes ↑
      • Urinary urobilinogen ↑
    • Only in intravascular haemolysis
      • Free haemoglobin in plasma ↑
        • Only in severe cases
      • Brown-coloured urine
        • Due to haemoglobinuria or haemosiderinuria
  • Peripheral blood smear
    • Spherocytes – Small, spherical RBCs with no central pallor

It can be difficult to differentiate intravascular and extravascular haemolysis on a lab test as no one parameter is different in the two. In extravascular haptoglobin can be normal, and there is rarely free haemoglobin in the plasma.

The so-called Coombs test is essential in the diagnosis of antibody-mediated anaemias. There are two types of Coombs test, the direct type and the indirect type. The direct Coombs test is positive if there are autoantibodies against the patient’s own RBCs bound to the RBCs in the patient’s blood. The direct Coombs test is positive in immune-mediated haemolytic anaemias.

The indirect Coombs test is positive if there are autoantibodies against foreign (not the patient’s) RBCs in the patient’s blood. The indirect Coombs is used to check if the patient’s blood contains anti-D antibodies which would cause haemolytic disease of the newborn.

Congenital haemolytic anaemias:

The most important congenital haemolytic anaemias are:

  • Haemoglobinopathies
    • Sickle cell anaemia
    • Thalassaemia
  • Membranopathies – diseases characterised by abnormal RBC membrane
    • Hereditary spherocytosis
    • Hereditary elliptocytosis
  • Enzymopathies
    • Glucose 6-phosphate dehydrogenase deficiency
    • Pyruvate kinase deficiency

Haemoglobinopathies are characterised by defects of haemoglobin. These defects often cause haemoglobin to precipitate in the cell, which alters its shape.

Acquired haemolytic anaemias:

  • Immune-mediated haemolytic anaemias
    • Warm antibodies
    • Cold antibodies
  • Non-immune-mediated haemolytic anaemias
    • Microangiopathic haemolytic anaemias (described in topic 5)
    • Macroangiopathic haemolytic anaemias
      • Prosthetic heart valves
      • Severe aortic stenosis
    • Drugs
    • Valvular haemolysis
    • Infection
    • Paroxysmal nocturnal haemoglobinuria

Corpuscular and extracorpuscular haemolysis:

We can classify haemolytic anaemias as corpuscular and extracorpuscular based on whether the cause of haemolysis is inside or outside the RBCs.

  • Corpuscular haemolytic anaemias
    • RBC membrane defects
      • Paroxysmal nocturnal haemoglobinuria
      • Hereditary spherocytosis
    • Enzymopathies
      • G6PD deficiency
      • Pyruvate kinase deficiency
    • Haemoglobinopathies
      • Sickle cell disease
      • Thalassaemia
  • Extracorpuscular haemolytic anaemias
    • Immune-mediated haemolytic anaemias
      • Autoimmune haemolytic anaemia
        • Cold agglutinin disease
        • Warm agglutinin disease
      • ABO incompatibility
      • Rhesus incompatibility
    • Non-immune-mediated haemolytic anaemias
      • Microangiopathic haemolytic anaemias
        • Haemolytic uraemic syndrome
        • Thrombotic thrombocytopaenic purpura
        • Malignant hypertension
        • SLE
      • Macroangiopathic haemolytic anaemias
        • Valvular haemolysis
Sickle cell anaemia

Sickle cell anaemia is a condition which is characterised by the RBCs changing shape to that of a sickle. It’s more frequent in Africa and the Mediterranean.

Pathophysiology:

Sickle cell anaemia is caused by a point mutation in the β-globin gene, causing a glutamate to be replaced by a valine. The mutated β-globin gene causes an abnormal form of haemoglobin called haemoglobin S (HbS) to form.

People who only have the mutation on one allele are heterozygotes and is said to have sickle cell trait. In these people haemoglobin S accounts for only 40% on all haemoglobin, which is often not enough to be symptomatic. People who have the mutation of both alleles are homozygotes and have manifest sickle cell anaemia. These people have 75 – 95% haemoglobin S, the remaining being foetal haemoglobin (HbF).

The problem with HbS is that, when deoxygenated, the HbS molecules polymerize and form a gelatinous network inside the RBC. This causes the RBC to change shape to the characteristic sickle-shape, forming a stiff, viscous sickle cell. Several factors can induce this sickling, like:

  • Hypoxia
  • Low pH
  • Fever
  • Infection
  • Exercise
  • Dehydration
  • Abrupt temperature changes
  • Stress

The sickle cells cause two problems. Because they are stiff, they can’t circulate properly through capillaries, causing capillary occlusion with microinfarctions and ischaemia, especially of the spleen. Second, they’re more prone to haemolysis in the spleen. The increased RBC turnover that occurs with the haemolysis may increase the demand for folate, causing macrocytic anaemia.

Clinical features:

Most people with sickle cell anaemia experience symptoms within the first years of life.

The most common symptom is the occurrence of so-called painful crises, which are characterised by episodes of splenic vaso-occlusion which causes acute left upper quadrant pain and anaemia. Another common symptom is the painful swelling of the hands and feet.

Due to chronic microinfarctions of the spleen these patients lose splenic function early. This makes them more susceptible to infections by encapsulated bacteria, like S. pneumoniae, H. influenzae and salmonella. Chronic microinfarctions also cause chronic pain and can cause infarction of virtually any organ.

Diagnosis:

  • Laboratory results corresponding to haemolytic anaemia
  • Peripheral blood smear
    • Sickle cells
    • Target cells
    • Howell-Jolly bodies
  • Haemoglobin electrophoresis

Treatment:

  • Avoiding painful crises
    • Avoid triggers
    • Pneumococcal and H. flu vaccines
    • Antibiotic prophylaxis during invasive procedures
    • Vigorous oral hydration during or in anticipation of periods of exercise, stress
    • Treatment with hydroxyurea
  • Management of acute crises
    • Vigorous hydration
    • Treatment of underlying cause
    • Analgesia
    • Blood transfusion – in extreme cases

Hydroxyurea increases the amount of foetal haemoglobin which reduces the proportion of HbS.

The only curative treatment is allogenic haematopoietic stem cell transplantation. This is only indicated in severe cases and only in children.

Thalassaemia

Thalassaemia is a group of diseases characterised by one or more defective globin genes. Like sickle cell anaemia it is more common in Africa and the Mediterranean, but also south-east Asia.

Pathophysiology:

We distinguish alpha and beta thalassaemia, based on whether the alpha or beta globin genes are defective. Alpha globin chain production is controlled by two genes, so there are four alleles that can be defective. Beta globin chain production is controlled by only one gene so there are only two alleles that can be defective.

Clinical features:

The clinical features of each type depend on how many alleles are defective:

Alpha-thalassaemia:

Genotype Name Clinical features
αα/αα Normal Asymptomatic
-α/αα Silent carrier Asymptomatic
-α/-α or – -/αα Alpha thalassaemia minor Mild haemolytic normocytic anaemia
– – /-α Hb H disease Jaundice and anaemia at birth, chronic haemolytic anaemia
– – / – – Alpha thalassaemia major / hydrops foetalis Incompatible with life or death right after birth

Beta-thalassaemia:

Genotype Name Clinical features
β/β Normal Asymptomatic
β/- Beta thalassaemia minor Asymptomatic
-/- Beta thalassaemia major/Cooley anaemia Severe haemolytic anaemia, skeletal deformities (especially face) due to bone marrow hyperplasia

Diagnosis:

  • Laboratory results
    • Corresponding to haemolytic microcytic anaemia
    • RDW normal or ↑
  • Peripheral blood smear
    • Target cells
    • Teardrop cells
  • Bone marrow biopsy
    • Reactive hyperplasia
  • Haemoglobin electrophoresis
  • DNA analysis (PCR)

Treatment:

The thalassaemia minor variants rarely require treatment. The major variants get all the treatment.

  • Lifelong blood transfusions
  • Iron chelating agents

The iron chelating agents are necessary to prevent iron overload and secondary haemochromatosis due to the many blood transfusions.

The only curative treatment is allogenic haematopoietic stem cell transplant.

Enzymopathies

Glucose 6-phosphate dehydrogenase deficiency:

Also called G6PD deficiency, this disease is X-linked recessively inherited. It’s the most common enzyme deficiency worldwide. Like the haemoglobinopathies it’s most common in Africa, Asia and the Mediterranean. The enzyme G6PD is important to regenerate glutathione, an important antioxidant.

The disease is usually asymptomatic until the person experiences a sudden increase in oxidative stress. This can be triggered by infection, fava beans or drugs like antimalarial drugs, sulpha drugs, nitrofurantoin, NSAIDs, etc. This triggers a potentially life-threatening haemolytic crisis with abdominal pain, jaundice and haemoglobinuria.

Pyruvate kinase deficiency:

RBCs only produce energy by glycolysis, which depends on pyruvate kinase (PK). When PK is deficient will RBCs be deficient in ATP, which can lead to haemolysis. In contrast with G6PD deficiency, PK deficiency causes chronic haemolytic anaemia.

Autoimmune haemolytic anaemia

Autoimmune haemolytic anaemia (AHIA) is a haemolytic anaemia that occurs due to the body producing autoantibodies against its own RBCs. We distinguish two types of AHIA based on which temperature the autoantibodies best bind to the RBCs. In warm AIHA, also called warm agglutinin disease, the autoantibodies bind more strongly to the RBCs at body temperature (37°C). In cold AIHA, also called cold agglutinin disease, the autoantibodies bind to the RBCs more strongly in lower temperatures. The autoantibodies are called warm agglutinins and cold agglutinins, respectively.

Cold agglutinin disease:

Cold agglutinin disease can be idiopathic (especially in elderly women) or it can be secondary to lymphoma, CLL or mycoplasma infection. The cold agglutinins are of IgM type and bind most strongly to RBCs at 4°C, but that doesn’t mean that they won’t bind at higher temperatures too. The warmest temperature the antibodies will bind to RBCs at varies from person to person; most clinically significant cold agglutinins bind to RBCs at around 28 degrees.

The periphery of the body can easily reach 28 degrees and so patients can experience haemolysis all the time. Cold obviously makes the symptoms worse as more autoantibodies bind to RBCs.

Warm agglutinin disease:

Warm agglutinin disease can be idiopathic but is secondary in 2/3 of cases. It can be secondary to lymphoid neoplasms, solid tumors, autoimmune diseases and certain drugs, especially methyldopa. The warm agglutinins are of IgG type.

Pathophysiology:

When autoantibodies bind to the RBCs the complement system is activated. This forms the membrane attack complex (MAC), which causes intravascular haemolysis. The opsonized RBCs are haemolysed in the spleen, which causes extravascular haemolysis. Intravascular haemolysis is only characteristic for cold agglutinin disease while extravascular haemolysis occurs in both.

Clinical features:

Cold agglutinin disease:

  • Acrocyanosis
  • Splenomegaly
  • Features of anaemia
    • Especially pallor

Warm agglutinin disease:

  • Features of anaemia
    • Especially pallor
  • Jaundice
  • Splenomegaly

Diagnosis:

  • Laboratory findings
    • Signs of haemolysis
  • Peripheral blood smear
    • Normochromic anaemia
    • Anisocytosis
    • Autoagglutination
    • Polychromasia
      • RBCs that stain both basophilic and eosinophilic
  • Positive direct Coombs test
  • Detection of cold agglutinins
    • Only in cold agglutinin disease of course

Treatment:

The underlying cause, if any, should be treated. Corticosteroids or rituximab can improve the symptoms. People with cold agglutinin disease should about the cold.

Macroangiopathic haemolytic anaemias

Macroangiopathic haemolytic anaemias are characterised by the intravascular haemolysis of RBCs due to mechanical forces in the large blood vessels (macroangio-). The most important causes are severe aortic stenosis and prosthetic heart valves.

In severe aortic stenosis the lumen of the aortic valve is so narrow that the blood that flows through it reaches very high velocities. This rapid, turbulent transvalvular blood flow damages the RBC membrane, causing them to haemolyse. Turbulent blood flow through prosthetic heart valves cause similar damage to the RBCs.

The peripheral blood smear will show fragmented red blood cells (schistocytes).

Paroxysmal nocturnal haemoglobinuria

Paroxysmal nocturnal haemoglobinuria (PNH) has an unfortunate name. Its name implies that the main clinical feature is the presence of haemoglobin in the urine in the night, but this symptom isn’t present in everyone with PNH. Also, the name doesn’t explain the most important clinical features of the disease: Haemolysis, venous thrombosis and pancytopaenia.

Pathophysiology:

All cells of the blood have a so-called membrane bound glycosylphosphatidylinositol (GPI) anchor. Many proteins are connected to this anchor, the most important being CD55 and CD59. The function of CD55 and CD59 is to act as a shield against complement-mediated haemolysis, protecting the blood cells, especially the RBCs, from getting destroyed. This anchor is encoded by the PIGA gene.

In PNH a haematopoietic stem cell acquires a mutation in PIGA, which causes them to lose the GPI anchor. As such they can’t express CD55 and CD59 either, making them susceptible to complement-mediated haemolysis. This mutated stem cell, and all the cells which will derive from it, are known as PNH clones. The RBCs derived from the PNH clone are not protected against complement-mediated haemolysis and are therefore constantly haemolysed.

The severity of the PNH depends on how much of the bone marrow is comprised of PNH clones. Many healthy people have some PNH clones in their bone marrow but are asymptomatic (subclinical PNH). However, if a person with a few PNH clones in their bone marrow develops aplastic anaemia the PNH clone will have more space to proliferate. As such there is a hypothesis that clinical PNH arises from the expansion of a PNH clone in an injured bone marrow.

Approximately half of all patients with PNH also have aplastic anaemia, and approximately half of all patients with aplastic anaemia have PNH.

Venous thrombosis and vasoconstriction are important features of PNH. The current hypothesis is that free haemoglobin produced by constant haemolysis scavenges and inactivates NO in the serum, and that free haemoglobin activates the endothelium. The loss of the potent vasodilator and the activation of the endothelium causes thrombosis.

Classification:

We distinguish three types of PNH, based on the context under which it was diagnosed:

  • Classic PNH
    • People with established PNH and symptoms but without another bone marrow disorder
  • PNH in the setting of another bone marrow disorder
    • People with established PNH and symptoms and with aplastic anaemia, MDS, etc.
  • Subclinical PNH
    • People with the presence of PNH clones but without clinical symptoms

Clinical features:

  • Haemolytic anaemia
  • Pancytopaenia
    • Due to the high occurrence of simultaneous aplastic anaemia
  • Venous thrombosis
    • Often in unusual locations, like hepatic, cerebral or abdominal veins
    • Can cause Budd-Chiari syndrome
  • Abdominal pain
  • Dark urine in the morning
    • Due to increased haemolysis during the night and concentration of urine during the night
    • Not seen in all patients
  • Renal failure
    • Due to chronic haemoglobinuria
  • Pulmonary hypertension
    • Due to vasoconstriction in the pulmonary circulation

Diagnosis:

  • Laboratory
    • Haemolytic anaemia
    • Pancytopaenia
  • Negative Coombs test (as the haemolysis is mediated by complement and not antibodies)
  • Flow cytometry
    • Absence of CD55 and CD59 on blood cells

Flow cytometry is the gold standard for the diagnosis of PNH.

Therapy:

There are only two established therapies for PNH. The first is eculizumab, an anti-C5 antibody. It inhibits complement factor C5, thereby inhibiting complement-mediated haemolysis. The second is allogenic stem cell transplantation.


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2. Iron deficiency anaemia

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5. Microangiopathic haemolytic anaemias (TTP, HUS)

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