Anaemia, haemolytic

A form of anaemia caused by premature destruction of red cells in the bloodstream (haemolysis). Haemolytic anaemias can be classified according to whether the cause of haemolysis is inside or outside the red cells. When haemolysis is due to a defect inside the red cells, the underlying problem is abnormal rigidity of the cell membrane. This causes the cells to become trapped, at an early stage of their life-span, in the small blood vessels of the spleen, where they are destroyed by macrophages (cells that ingest foreign particles). Abnormal rigidity may result from an inherited defect of the cell membrane (as in hereditary spherocytosis), a defect of the haemoglobin in the cell (as in sickle-cell anaemia), or a defect of one of the cell’s enzymes. An inherited deficiency of the glucose-6phosphate dehydrogenase enzyme (see G6PD deficiency) may result in episodes of haemolytic anaemia since the red cells are prone to damage by infectious illness or certain drugs or foods. Haemolytic anaemias due to defects outside the red cells fall into 3 main groups. First are disorders in which red cells are destroyed by buffeting (by artificial surfaces such as replacement heart valves, abnormal blood-vessel linings, or a blood clot in a vessel, for example). In the 2nd group, the red cells are destroyed by the immune system. Immune haemolytic anaemias may occur if foreign blood cells enter the bloodstream, as occurs in an incompatible blood transfusion, or they may be due to an autoimmune disorder. In haemolytic disease of the newborn, the baby’s red cells are destroyed by the mother’s antibodies crossing the placenta. Thirdly, the red cells may be destroyed by microorganisms; the most common cause is malaria. People with haemolytic anaemia may have symptoms common to all types of anaemia, such as fatigue and breathlessness, or symptoms specifically due to haemolysis, such as jaundice. Diagnosis is made by examination of the blood (see blood film). Some inherited anaemias can be controlled by removing the spleen (see splenectomy). Others, such as G6PD deficiency, can be prevented by avoiding the drugs or foods that precipitate haemolysis. Anaemias due to immune processes can often be controlled by immunosuppressant drugs. Transfusions of red cells are sometimes needed for emergency treatment of life-threatening anaemia.