Haemolytic anaemia

Haemolysis indicates that there is shortening of the normal red cell lifespan of 120 days. To compensate, the bone marrow may increase its output of red cells six- to eightfold by increasing the proportion of red cells produced, expanding the volume of active marrow, and releasing reticulocytes prematurely. Anaemia only occurs if the rate of destruction exceeds this increased production rate.

The appearances of the red cells may give an indication of the likely cause of the haemolysis: • Spherocytes are small, dark red cells which suggest autoimmune haemolysis or hereditary spherocytosis. • Sickle cells suggest sickle-cell disease. • Red cell fragments indicate microangiopathic haemolysis The compensatory erythroid hyperplasia may give rise to folate deficiency, with megaloblastic blood features.

Extravascular haemolysis Physiological red cell destruction occurs in the reticuloendothelial cells in the liver or spleen, so avoiding free haemoglobin in the plasma. In most haemolytic states, haemolysis is predominantly extravascular. To confirm the haemolysis, patients’ red cells can be labelled with 51chromium. When re-injected, they can be used to determine red cell survival; when combined with body surface radioactivity counting, this test may indicate whether the liver or the spleen is the main source of red cell destruction

Intravascular haemolysis Less commonly, red cell lysis occurs within the blood stream due to membrane damage by complement (ABO transfusion reactions). When intravascular red cell destruction occurs, free haemoglobin is released into the plasma. Free haemoglobin is toxic to cells and binding proteins have evolved to minimise this risk. Haptoglobin is an α2-globulin produced by the liver, which binds free haemoglobin, resulting in a fall in its levels during active haemolysis. Once haptoglobins are saturated, free haemoglobin is oxidised to form methaemoglobin, If all the protective mechanisms are saturated, free haemoglobin may appear in the urine (haemoglobinuria). When fulminant, this gives rise to black urine, as in severe falciparum malaria infection

Red cell membrane defects Hereditary spherocytosis This is usually inherited as an autosomal dominant condition, although 25% of cases have no family history and represent new mutations. The most common abnormalities are deficiencies of beta spectrin or ankyrin . . The severity of spontaneous haemolysis varies. Most cases are associated with an asymptomatic compensated chronic haemolytic state with spherocytes present on the blood film, a reticulocytosis and mild hyperbilirubinaemia. Pigment gallstones are present in up to 50% of patients and may cause symptomatic cholecystitis

The clinical course may be complicated by crises: • A haemolytic crisis occurs when the severity of haemolysis increases; this is rare, and usually associated with infection. • A megaloblastic crisis follows the development of folate deficiency; this may occur as a first presentation of the disease in pregnancy. • An aplastic crisis occurs in association with parvovirus B19 infection

Investigations: An osmotic fragility test may show increased sensitivity to lysis in hypotonic saline solutions but is limited by lack of sensitivity and specificity. More specific flow cytometric tests, detecting binding of eosin-5-maleimide to red cells, are recommended in borderline cases.

Management Folic acid prophylaxis, 5 mg daily, should be given for life. Consideration may be given to splenectomy, which improves but does not normalise red cell survival, although splenectomy should be delayed until after 6 years of age in view of the risk of sepsis.

Red cell enzymopathies Glucose-6-phosphate dehydrogenase deficiency The enzyme glucose-6-phosphate dehydrogenase (G6PD) is pivotal in the hexose monophosphate shunt pathway. Deficiencies result in the most common human enzymopathy, affecting 10% of the world’s population. The enzyme is encoded by a gene on the X chromosome. The deficiency therefore affects males and rare homozygous females

Haemoglobinopathies: These diseases are caused by mutations affecting the genes encoding the globin chains of the haemoglobin molecule. Normal haemoglobin is comprised of two alpha and two non-alpha globin chains, fetal haemoglobin (HbF-αα/γγ) has two gamma chains, while the predominant adult haemoglobin (HbA-αα/ββ) has two beta chains.

Sickle-cell anaemia(Qualitative abnormalities – abnormal haemoglobins) Sickle-cell disease results from a single glutamic acid to valine substitution at position 6 of the beta globin polypeptide chain. It is inherited as an autosomal recessive trait (p. 53). Homozygotes only produce abnormal beta chains that make haemoglobin S (HbS, termed SS), and this results in the clinical syndrome of sickle-cell disease. Heterozygotes produce a mixture of normal and abnormal beta chains that make normal HbA and HbS (termed AS), and this results in the clinically asymptomatic sickle-cell trait.

When haemoglobin S is deoxygenated, the molecules of haemoglobin polymerise to form pseudocrystalline structures known as ‘tactoids’. These distort the red cell membrane and produce characteristic sickle-shaped cells. The polymerisation is reversible when re-oxygenation occurs. The distortion of the red cell membrane, however, may become permanent and the red cell ‘irreversibly sickled’. The abnormal haemoglobin C variant participates in the polymerisation more readily than haemoglobin A, whereas haemoglobin F strongly inhibits polymerisation.

Clinical features Sickling is precipitated by hypoxia, acidosis, dehydration and infection. Irreversibly sickled cells have a shortened survival and plug vessels in the microcirculation. This results in a number of acute syndromes, termed ‘crises’, and chronic organ damage. 1. Painful vaso-occlusive crisis. Plugging of small vessels in the bone produces acute severe bone pain. This affects areas of active marrow: the hands and feet in children (so-called dactylitis) or the femora, humeri, ribs, pelvis and vertebrae in adults.

2. Sickle chest syndrome. This may follow a vasoocclusive crisis and is the most common cause of death in adult sickle disease. Bone marrow infarction results in fat emboli to the lungs, which cause further sickling and infarction, leading to ventilatory failure if not treated 3. Sequestration crisis. Thrombosis of the venous outflow from an organ causes loss of function and acute painful enlargement. In children, the spleen is the most common site. Massive splenic enlargement may result in severe anaemia, circulatory collapse and death

4. Aplastic crisis. Infection with human parvovirus B19 results in a severe but self-limiting red cell aplasia. This produces a very low haemoglobin, which may cause heart failure. Unlike in all other sickle crises, the reticulocyte count is low.

Investigations Patients with sickle-cell disease have a compensated anaemia, usually around 60–80 g/L. The blood film shows sickle cells, target cells and features of hyposplenism. A reticulocytosis is present. The definitive diagnosis requires haemoglobin electrophoresis to demonstrate the absence of HbA, 2–20% HbF and the predominance of HbS. Both parents of the affected individual will have sickle-cell trait

Management All patients with sickle-cell disease should receive prophylaxis with daily folic acid, and penicillin V to protect against pneumococcal infection, which may be lethal in the presence of hyposplenism. These patients should be vaccinated against pneumococcus, meningococcus, Haemophilus influenzae B, hepatitis B and seasonal influenza. A regular transfusion programme to suppress HbS production and maintain the HbS level below 30% may be indicated in patients with recurrent severe complications

The thalassaemias (Quantitative abnormality): Thalassaemia is an inherited impairment of haemoglobin production, in which there is partial or complete failure to synthesise a specific type of globin chain. In alphathalassaemia, disruption of one or both alleles on chromosome 16 may occur, with production of some or no alpha globin chains. In beta-thalassaemia, defective production usually results from disabling point mutations causing no (β0 ) or reduced (β– ) beta chain production.

Beta-thalassaemia Failure to synthesise beta chains (beta-thalassaemia) is the most common type of thalassaemia, most prevalent in the Mediterranean area. Heterozygotes have thalassaemia minor, a condition in which there is usually mild anaemia and little or no clinical disability, which may be detected only when iron therapy for a mild microcytic anaemia fails. Homozygotes (thalassaemia major) either are unable to synthesise haemoglobin A or, at best, produce very little; after the first 4–6 months of life, they develop profound hypochromic anaemia