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How hot are you on... sickle cell anaemia?

Test your knowledge for the nMRCGP with this little GEM from GPnotebook

Test your knowledge for the nMRCGP with this little GEM from GPnotebook

Helen is a 28-year-old white woman who lives in rural Devon. During the second trimester of her pregnancy, the midwife completed a family origins questionnaire and noted that her family originated from Africa. She was due to have a screening test to see if she carried a thalassaemia or sickle cell disorder but the request form was misplaced. Her partner, Steve, is of Jamaican origin. A week after the heel prick test, the preliminary results suggested their baby had a sickle cell disorder. The health visitor informs you of the results and visits the couple with the local specialist haemoglobinopathy counsellor to explain the results. An urgent referral to the paediatric haematologist is organised.

Q What are the sickle cell disorders?

This is a group of blood disorders that cause red blood cells to sickle when they are short of oxygen. This leads to anaemia, risk of life-threatening infections and unpredictable attacks of severe pain (crisis) that can be fatal. The red blood cells sickle because of changes in the structure of the haemoglobin. The commonest sickle cell disorder is due to the structure of the usual haemoglobin A changing to haemoglobin S. However, other haemoglobins in combination with haemoglobin S can also cause sickle cell disorders.

Q How are the conditions managed?

It is important to provide families with information on risk factors for sickling (dehydration, heat, cold, infection and stress). The child should be offered prophylactic antibiotics from an early age and attend specialist clinics regularly. It is also important they have rapid access to medical help in an emergency.

Q How are sickle cell disorders inherited?

Sickle cell disorders are autosomal recessive conditions. If the child inherited one copy of the gene variant that leads to change in the haemoglobin structure, the child will be a healthy carrier. However, if the child inherits two copies (one from each parent) they can have the disease.

When only one partner carries a gene variant, they can pass on either the usual gene or the gene variant. Each child therefore has a one in two (50%) chance of inheriting the variant and being a healthy carrier. None of the children can have a serious haemoglobin disorder. Each child also has a 50% chance of carrying the usual gene. Healthy carriers can pass on haemoglobin gene variants for many generations without anyone realising that a variant is in the family.

When both partners carry a gene variant, each can pass on to a child either the usual gene or the gene variant. There are four possible combinations of genes. At each conception there is a 25% (one in four) probability of a child with usual haemoglobin, a 50% (one in two) probability of a healthy carrier, and a 25% (one in four) probability of inheriting two copies of the gene variant leading to a child with a sickle cell disorder.

Q What does it mean to carry a sickle cell disorder?

A carrier of a recessive condition is someone with one usual and one altered copy of the gene concerned (gene variant). The single copy of the usual gene is sufficient for normal function, and so carriers do not have symptoms of the condition.

Everyone is a carrier for one or two recessive disorders, but it matters only if both partners are carriers for the same condition.

Although a person who is a carrier of an autosomal recessive condition is healthy, they can have an ill child affected by the condition if their partner is also a carrier for the same recessive condition.

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