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Anaemia is a syndrome characterised by a lack of healthy red blood cells or haemoglobin deficiency in the red blood cells, resulting in inadequate oxygen supply to the tissues. The condition can be temporary, long-term or chronic, and of mild to severe intensity. There are many forms and causes of anaemia. Normal blood consists of three types of blood cells: white blood cells (leucocytes), platelets and red blood cells (erythrocytes). The first generation of erythrocyte precursors in the developing foetus are produced in the yolk sac. They are carried to the developing liver by the blood where they form mature red blood cells that are required to meet the metabolic needs of the foetus. Until the 18th week of gestation, erythrocytes are produced only by liver after which the production shifts to the spleen and the bone marrow. The life of a red blood cell is about 127 days or 4 months (Shemin and Rittenberg, 1946; Kohgo et al., 2008). The main causes of anaemia are blood loss, production of too few red blood cells by the bone marrow or a rapid destruction of cells.
Haemoglobin, a protein, present in the red blood cells is involved in the transport of oxygen from the lungs to all the other organs and tissues of the body. Iron is an important constituent of the haemoglobin protein structure which is intimately involved in the transport of oxygen. Anaemia is generally defined as a lower than normal haemoglobin concentration. The normal blood haemoglobin concentration is dependent on age and sex, and, according to the World Health Organisation (WHO) Expert Committee Report, anaemia results when the blood concentration of haemoglobin falls below 130 g/L in men or 120 g/L in non-pregnant women (WHO, 1968). However, the reference range of haemoglobin concentration in blood could vary depending on the ethnicity, age, sex, environmental conditions and food habits of the population analysed. According to Beutler and Warren (2006), more reasonable benchmarks for anaemia are 137 g/L for white men aged between 20 and 60 years and 132 g/L for older men. The value for women of all ages would be 122 g/L. Also, the lower limit of normal of haemoglobin concentrations of African Americans are appreciably lower than that of Caucasians (Beutler and Warren, 2006).
Besides the well recognised iron deficiency anaemia, several inherited anaemias are also known. These are mostly haemoglobinopathies. Adult haemoglobin is a tetrameric haeme-protein. Abnormalities of beta-chain or alpha-chain produce the various medically significant haemoglobinopathies. The variations in amino acid composition induced genetically impart marked differences in the oxygen carrying properties of haemoglobin. Mutations in the haemoglobin genes cause disorders that are qualitative abnormalities in the synthesis of haemoglobin (e.g., sickle cell disease) and some that are quantitative abnormalities that pertain to the rate of haemoglobin synthesis (e.g., the thalassemias) (Weatherall., 1969). In SCD, the missense mutation in the Î²-globin gene causes the disorder. The mutation causing sickle cell anemia is a single nucleotide substitution (A to T) in the codon for amino acid 6. The substitution converts a glutamic acid codon (GAG) to a valine codon (GTG). The form of haemoglobin in persons with sickle cell anemia is referred to as HbS. Also, the valine for glutamic acid replacement causes the haemoglobin tetramers to aggregate into arrays upon deoxygenation in the tissues. This aggregation leads to deformation of the red blood cell making it relatively inflexible and restrict its movement in the capillary beds. Repeated cycles of oxygenation and deoxygenation lead to irreversible sickling and clogging of the fine capillaries. Incessant clogging of the capillary beds damages the kidneys, heart and lungs while the constant destruction of the sickled red blood cells triggers chronic anaemia and episodes of hyperbilirubinaemia.
Fanconi anaemia (FA) is an autosomal recessive condition, and the most common type of inherited bone marrow failure syndrome. The clinical features of FA are haematological with aplastic anaemia, myelodysplastic syndrome (MDS), and acute myeloid leukaemia (AML) being increasingly present in homozygotes (Tischkowitz and Hodgson, 2003). Cooley's anaemia is yet another disorder caused by a defect in haemoglobin synthesis.
Autoimmune haemolytic anaemia is a syndrome in which individuals produce antibodies directed against one of their own erythrocyte membrane antigens. The condition results in diminished haemoglobin concentrations on account of shortened red blood cell lifespan (Sokol et al., 1992).
Megaloblastic anaemia is a blood disorder in which anaemia occurs with erythrocytes which are larger in size than normal. The disorder is usually associated with a deficiency of vitamin B12 or folic acid . It can also be caused by alcohol abuse, drugs that impact DNA such as anti-cancer drugs, leukaemia, and certain inherited disorders among others (Dugdale, 2008).
Malaria causes increased deformability of vivax-infected red blood cells (Anstey et al., 2009). Malarial anaemia occurs due to lysis of parasite-infected and non-parasitised erythroblasts as also by the effect of parasite products on erythropoiesis (Ru et al., 2009).
Large amounts of iron are needed for haemoglobin synthesis by erythroblasts in the bone marrow. Transferrin receptor 1 (TfR1) expressed highly in erythroblasts plays an important role in extracellular iron uptake (Kohgo et al., 2008). Inside the erythroblasts, iron transported into the mitochondria gets incorporated into the haeme ring in a multistep pathway. Genetic abnormalities in this pathway cause the phenotype of ringed sideroblastic anemias (Fleming, 2002). The sideroblastic anemias are a heterogeneous group of acquired and inherited bone marrow disorders, characterised by mitochondrial iron overload in developing red blood cells. These conditions are diagnosed by the presence of pathologic iron deposits in erythroblast mitochondria (Bottomley, 2006).
2. Classification of anaemia
Anaemia can be generally classified based on the morphology of the red blood cells, the pathogenic spectra or clinical presentation (Chulilla et al., 2009). The morphological classification is based on mean corpuscular volume (MCV) and comprises of microcytic, macrocytic and normocytic anaemia. Microcytic anaemia refers to the presence of RBCs smaller than normal volume, the reduced MCV (< 82 fL) reflecting decreased haemoglobin synthesis. Thus, it is usually associated with hypochromic anaemia. Microcytic anaemia can result from defects either in iron acquisition or availability (Iolascon et al., 2009), or disorders of haeme metabolism or globin synthesis (Richardson, 2007). The main diagnostic possibilities with microcytic anaemia include iron deficiency anaemia (IDA), thalassaemia, anaemia of chronic disorders (ACD), and rarely sideroblastic anaemia (Chulilla et al., 2009). Microcytosis without anaemia is characteristic of thalassaemia trait. The red blood cell distribution width (RDW) obtained with haematological analysers provides the index of dispersion in the erythrocyte distribution curve and complements MCV values. RDW is helpful to differentiate between thalassaemia and IDA. RDW is normal in thalassemia; on the contrary, microcytic anemia with RDW > 15 would probably indicate IDA (Chulilla et al., 2009).
In macrocytic anaemia, erythrocytes are larger (MCV > 98 fL) than their normal volume (MCV = 82-98 fL). Vitamin B12 deficiency leads to delayed DNA synthesis in rapidly growing hematopoietic cells, and can result in macrocytic anemia. Drugs that interfere with nucleic acid metabolism, such as.hydroxyurea increases MCV (> 110 fL) while alcohol induces a moderate macrocytosis (100-110 fL). In the initial stage, most anaemias are normocytic. The causes of normocytic anaemia are nutritional deficiency, renal failure and haemolytic anemia (Tefferi, 2003). The most common normocytic anaemia in adults is anaemia of chronic disease (ACD) (Krantz, 1994). Common childhood normocytic anaemias are, besides iron deficiency anaemia, those due to acute bleeding, sickle cell anemia, red blood cell membrane disorders and current or recent infections especially in the very young (Bessman et al., 1983). Homozygous sickle cell disease is the most common cause of haemolytic normocytic anemias in children (Weatherall DJ, 1997).
In practice, the morphological classification is quicker and therefore, more useful as a diagnostic tool. Besides, MCV is also closely linked to mean corpuscular haemoglobin (MCH), which denotes mean haemoglobin per erythrocyte expressed in picograms (Chulilla et al., 2009). Thus, MCV and MCH decrease simultaneously in microcytic, hypochromic anemia and increase together in macrocytic, hyperchromic anemia.
Pathogenic classification of anaemia is based on the production pattern of RBC: whether anaemia is due to inadequate production or loss of erythrocytes caused by bleeding or haemolysis. This approach is useful in those cases where MCV is normal. Pathogenic classification is also essential for proper recognition of the mechanisms involved in the genesis of anaemia. Based on the pathogenic mechanisms, anaemia is further divided into two types namely, (i) hypo-regenerative in which the bone marrow production of erythrocytes is decreased because of impaired function, decreased number of precursor cells, reduced bone marrow infiltration, or lack of nutrients; and (ii) regenerative: when bone marrow upregulates the production of erythrocytes in response to the low erythrocyte mass (Chulilla et al., 2009). This is typified by increased generation of erythropoietin in response to lowered haemoglobin concentration, and also reflects a loss of erythrocytes, due to bleeding or haemolysis. The reticulocyte count is typically higher.
Sickle cell disease is characterised by sickled red cells. The molecular nature of sickle haemoglobin (Hb S) in which valine is substituted for glutamic acid at the sixth amino acid position in the beta globin gene reduces the solubility of Hb, and causing red cells to sickle. The abnormal red cells break down, leading to anaemia, and clog blood vessels with aggregates, leading to recurrent episodes of severe pain and multiorgan ischaemic damage (Creary et al., 2007). The high levels of inflammatory cytokines in SCD may promote retention of iron by macrophage/reticuloendothelial cells and/or renal cells. SCD care commonly depends on transfusion that results in iron overload (Walter et al., 2009).