Sickle Cell and Thalassaemia (Haemoglobinopathies) are autosomal recessive inherited conditions that affect haemoglobin.
Inheritance of one altered gene results in a healthy carrier. If a couple are both carriers, they have a 1 in 4 chance in each pregnancy of having an affected child, and a 2 in 4 chance of having a child who is a carrier.
The aim of the antenatal screening programme is to facilitate informed choices in screening, and identify ‘at risk’ women/couples so that prenatal diagnosis may be offered before 12 weeks of pregnancy, with continuation of pregnancy or termination, according to the woman’s choice.
Sickle Cell
People with Sickle Cell Disease produce a structural variant of the haemoglobin protein (HbS). This affects the normal oxygen carrying capacity of red blood cells. When the cells are de-oxygenated and under stress in sickle cell conditions, they change shape from round flexible discs to elongated, or crescent moon shape. These sickled cells cluster together and block blood vessels. This blockage prevents oxygenation of the tissues, resulting in pain from tissue hypoxia (sickle cell crisis), and also severe anaemia, susceptibility to infections, and damage to major organs such as the spleen.
Usually people with Sickle Cell Disease are homozygous for the HbS gene and produce only HbSS, but other haemoglobins in combination with HbS can sometimes cause a sickle cell disorder, for example HbSC, HbS / β thalassaemia.
There are approximately 240,000 healthy carriers and more than 12500 people affected with Sickle Cell in the UK. The prevalence is higher in some regions than others. In high prevalence areas, all women are offered screening for sickle cell; in low prevalence areas, a family origin questionnaire is used to identify those women who, because of their ancestry, should be offered screening for sickle cell disorders.
Within the Northern Region, women living within the Newcastle area are deemed high prevalence; the rest of the Northern Region is deemed low prevalence.
Every year about 300 babies are born in England with sickle cell disorders (and 7000 healthy carriers). These babies are at high risk of death or complications, often arising from treatable infections in the first few years of life.
Thalassaemias
Thalassaemia is the name given to a number of different inherited conditions that are due to an abnormality in the quantitative production of haemoglobin. Haemoglobin is made up of two alpha and two beta chains. If there is a problem with the alpha chains, it is called alpha thalassaemia, and similarly if there is a problem with the beta chains, it is called beta thalassaemia.
Beta thalassaemia major is the result of haemoglobin with few or no beta chains. Problems occur postnatally. People with this will have a severe life threatening anaemia. They will need frequent blood transfusions which bring with it other problems with iron overload.
Alpha thalassaemia major is the result of haemoglobin chains that have no alpha chains. It is invariably fatal. An alternative name is Hb Bart’s hydrops fetalis.
Alpha or beta thalassaemia minor are both mild types of thalassaemia. People with these conditions have reduced amounts of either alpha or beta chains, but sufficient to make enough haemoglobin with the normal chains. They are carriers and may have mild anaemia.
Important Points
- When a pregnant woman is found to be a carrier, the baby’s father should be offered carrier testing as soon as possible.
- Early is best. The earlier in pregnancy an ‘at risk’ couple (when both parents are carriers) are identified, the more time they have to be referred and to make choices about the pregnancy.
- If a previous pregnancy has been affected by a major form of haemoglobinopathy, the parents will need referral to a haematologist and offered genetic counselling to discuss options for future pregnancies.
- Do not assume that sickle cell and thalassaemia are confined to particular ethnic groups. They may occur in any ethnic group.
- Antenatal screening for thalassaemia uses blood indices. A low mean cell haemoglobin (MCH) detected on a full blood count could indicate an iron deficiency, or thalassaemia carrier status, or both. It may be appropriate to simultaneously investigate pregnant women for iron deficiency using ferritin or zinc protoporphyrin, but there is no justification for delaying the investigation of haemoglobinopathies whilst treating iron deficiency, as this will delay the process of identifying ‘at risk’ carrier couples who could be offered prenatal diagnosis.
- Never accept a diagnosis (e.g carrier or not a carrier) on hearsay. All decisions should be based on evidence from seeing the test result.
