Haemolytic Anaemia and Haemoglobinopathies

Types of haemolytic anaemia

Haemolytic anaemia occurs when red blood cells only have a short lifespan. Haemolytic anaemia is often associated with increased bilirubin (a product of the breakdown of haem), increased lactate dehydrogenase (an enzyme found within red blood cells), and reduced haptoglobins (a protein that normally binds to haemoglobin following cell death, but can get used up when large numbers of cells are dying). A blood film may also show spherocytes (round cells) and schistocytes (fragmented cells). There may also be an increase in immature red blood cells as the body attempts to create more cells to counteract the anaemia.

Intravascular and extravascular

Red blood cell death can occur either in the macrophages of the reticuloendothelial system (i.e. liver, spleen, etc.) or in the blood vessels. If cell death occurs in the reticuloendothelial system (i.e. if it is extravascular), the globin is broken down to its amino acids, the iron is released and recycled, and the haem is metabolised to bilirubin (which may result in jaundice). There may also be increased lactate dehydrogenase. If cell death occurs in the blood vessels (i.e. if it is intravascular), the haemoglobin is released in the plasma and can find its way into the urine, resulting in dark plasma and brown urine.

Inherited and acquired: membrane, enzyme, environment

Haemolytic anaemia can be due to an inherited condition, such as a defect in the red blood cell membrane, enzyme production, or globin chains. Haemolytic anaemia may also be due to an acquired condition, such as an immune disorder, liver disease, or infections.

Membrane defects

The red blood cell membrane must be stable yet able to deform in order to squeeze through small blood vessels. If there is a defect in the membrane, the cell may be more prone to breaking.

Hereditary spherocytosis

Hereditary spherocytosis is the most common inherited haemolytic anaemia. It is an autosomal dominant disorder in a structural membrane protein. It usually affects peripheral proteins such as spectrin and actrin, but it can affect integral proteins such as Band 3 as well. In hereditary spherocytosis, the red blood cells lose their membrane as they pass through the spleen, causing them to become rigid and spherical. Ultimately, the red blood cells are destroyed. Hereditary spherocytosis can have variable severity, not only between patients but within the same patient (anaemia might normally be compensated for, but all that goes out the window once the patient gets an infection).

Hereditary spherocytosis may be associated with splenomegaly (since the spleen needs to work overtime to get rid of the damaged cells) and gallstones (from the high levels of bilirubin). On a blood film, there may be increased reticulocytes (as the body is trying to increase RBC production) and/or spherical cells. The EMA test, which is a flow cytometry test looking at levels of Band 3, can also be used for diagnosis. Treatment involves giving folic acid to help with making new blood cells, and may also involve getting rid of an enlarged spleen or the gallbladder.

Hereditary elliptocytosis

Hereditary elliptocytosis, just like hereditary spherocytosis, is an autosomal dominant disorder. However, hereditary elliptocytosis is milder and is mainly associated with mutations in spectrin. As the elliptocytes are less prone to destruction than spherocytes, most people with hereditary elliptocytosis are asymptomatic. Symptomatic hereditary elliptocytosis is also known as hereditary pyro-poikilocytosis.

Enzyme defects

Just like every other cell in the body, red blood cells need certain enzymes to carry out the processes that keep them healthy. When there are defects in these enzymes, the red blood cells may be more prone to damage and haemolysis.

Glucose-6-phosphate dehydrogenase (G6PD) deficiency

Glucose-6-phosphate dehydrogenase (G6PD) is an enzyme that converts glucose-6-phosphate to 6-phosphogluconate, while at the same time reducing NADP+ to NADPH. NADPH can then act as a reducing agent in other reactions, such as synthesis reactions. Therefore, people with a G6PD deficiency are more prone to oxidant stress, particularly when exposed to triggers such as certain drugs, hypoxia, infection, or even certain foods such as fava beans. Oxidised haemoglobin gives the appearance of a white cap on the red blood cell, and when this oxidised haemoglobin is removed, "bite" cells can result.

G6PD deficiency is an X-linked recessive disorder, meaning that males are more prone to this disorder. There is no cure for G6PD deficiency. Treatment mainly involves avoidance of triggers of oxidant haemolysis as when the patient is not in an oxidant crisis, their blood count is normal. If it is absolutely necessary, transfusion might be considered, but it's important to remember that transfusion is a last case resort (remember, transfusing someone else's blood is basically just a transplant and comes with risks).

Pyruvate kinase deficiency

Pyruvate kinase is the last enzyme in the glycolysis pathway (see here if you want to punish yourself with a bunch of enzyme names). Without pyruvate kinase, not enough ATP is made, resulting in a rigid cell membrane (since red blood cell shape is controlled by ATP) and premature cell death. Red blood cells may appear "prickle-shaped" on a blood film. It is autosomal recessive and presentations can range from mild to severe. Pyruvate kinase deficiency can be diagnosed via a pyruvate kinase assay.

Immune haemolytic anaemia

Immune haemolytic anaemia is, as the name suggests, haemolytic anaemia triggered by an immune process. There are two main types of immune haemolytic anaemia: auto-immune (where antibodies are directed against own blood cells) and allo-immune (where antibodies are directed against blood cells from another person).

Auto-immune haemolytic anaemia is mostly idiopathic, but some conditions such as B-cell lymphoma can cause it. It can be diagnosed via the Direct Antiglobulin Test (DAT), which looks for antibodies against red blood cells. Auto-immune haemolytic anaemia can be treated by treating the cause (if known) or by immunosuppressive drugs (e.g. corticosteroids). Features of the blood film include spherocytes, polychromasia, and increased reticulocytes (as the body is trying to make more cells in order to compensate for the cell destruction).

Allo-immune haemolytic anaemia includes transfusion reactions and haemolytic disease of the newborn. Haemolytic disease of the newborn occurs when an Rh(D) negative mother is pregnant with an Rh(D) positive fetus. At birth, some of the blood from the fetus may mix with the blood from the mother, causing the mother to produce anti-D antibodies. If the mother becomes pregnant with another Rh(D) positive fetus, her newly-formed anti-D antibodies might attack the fetus' blood (since the new antibodies are IgG and IgG can cross the placenta). In order to prevent haemolytic disease of the newborn, Rh(D) negative mothers are given exogenous anti-D so that the Rh(D) antigen is cleared before the mother can make her own antibodies and memory cells against it.

Fragmentation haemolysis

Fragmentation haemolysis is an acquired cause of haemolytic anaemia. It is also known as "micro-angiopathic haemolytic anaemia." In fragmentation haemolysis, there is mechanical damage to the red blood cells due to exposure to an abnormal surface, such as damaged blood vessels or fibrin strands in the vasculature. Damage to the red blood cells results in the formation of schistocytes (fragmented cells).

Liver disease

Liver disease can also cause haemolytic anaemia. There wasn't too much detail on this during the lecture, other than that severe liver disease can alter the red blood cell membrane, resulting in spiky cells called acanthocytes. (Severe renal dysfunction results in similar-looking cells called echinocytes.)

Infections

Several different infections can cause damage to red blood cells, causing haemolytic anaemia. Two examples of infections that can lead to haemolytic anaemia include malaria (caused by Plasmodium protozoa) and Clostridium welchii. Severe bacterial sepsis with disseminated intravascular coagulation can also cause severe sepsis.

Explain the biology and clinical consequences of inherited disorders of haemoglobin

Globin chain defects, or haemoglobinopathies, can affect the structure and function of haemoglobin and can lead to microcytic (small) red cells. Haemoglobinopathies are inherited, rather than acquired. The main types of haemoglobinopathies are structural haemoglobinopathies (amino acid substitution leading to abnormal chain structure) and thalassaemias (reduced production of chains). Haemoglobinopathies can be variable in their presentation, because haemoglobin doesn't read textbooks. Sigh.

Structural haemoglobinopathies

Structural haemoglobinopathies, as mentioned earlier, are abnormal globin chains (usually beta-globin chains) due to amino acid substitution. There are three main structural haemoglobinopathies to know about. HbS is the structural haemoglobinopathy that results in sickle cell anaemia, in which the red cells are sickle-shaped and can occasionally get stuck, leading to pain and decreased blood flow to certain areas. HbE is a structural haemoglobinopathy commonly found in Thailand and HbC is a structural haemoglobinopathy commonly found in West Africa.

Thalassaemia

Thalassaemia is a condition in which there is reduced production of globin chains due to a mutation. The most common types are alpha- and beta-thalassaemia, which are characterised by reduced production in alpha- and beta-globin chains, respectively.

Alpha-thalassaemia, which involves mutations in the alpha-globin gene, is most common in southeast Asia. We have four alpha-globin genes (two from each parent), so the severity of alpha-thalassaemia depends on how many genes have been affected. If only two genes have been affected, the patient will have thalassaemia minor. If three genes have been affected, the patient will have Haemoglobin H disease. Finally, if all four genes are affected, the patient will have Hb Barts hydrops fetalis, which may result in death in utero (however this paper mentions a patient who was 31 in 2017).

Beta-thalassaemia is most common in the Mediterranean or in southeast Asia. It involves mutations in the beta-globin gene, resulting in reduced beta-chain production. Since fetal haemoglobin has alpha and gamma chains, and adult haemoglobin (with alpha/beta chains) doesn't fully take over until around 6 months of age, symptoms of homozygous beta-thalassaemia (a.k.a. thalassaemia major) may not present until 3-6 months of age. (Heterozygous beta-thalassaemia, or thalassaemia minor, may remain asymptomatic.) Thalassaemia major results in severe anaemia that requires regular transfusions. The only cure for thalassaemia major is a bone marrow transplant.