What are Sickle cell disorders?

Sickle cell disorders are caused by inherited abnormalities of haemoglobin. Haemoglobin is the oxygen carrying protein found in red blood cells, which conveys oxygen from the lungs to all the tissues of the body. In sickle cell anaemia the abnormal sickle haemoglobin is less soluble than normal and causes deformity (sickling) of the red cells. The red cells are more fragile than normal and tend to disintegrate (haemolyse) in the circulation leading to anaemia. When the red blood cells sickle they plug up the small blood vessels depriving the tissues of oxygen. This causes damage in tissues of the body including bones, nervous system, lungs and kidneys.

The configuration of haemoglobin in the body is controlled by genes inherited form both parents. In sickle cell disorders an abnormal gene causes the production of sickle haemoglobin in red blood cells. If a person inherits the abnormal gene from both parents all their red cells contain the abnormal haemoglobin, and they are described as having sickle cell anaemia. Their red cells will have a tendency to sickle and cause painful tissue damage.

When a person inherits the abnormal gene from one parent only, they are able to make normal haemoglobin (Hb A) in addition to sickle haemoglobin (Hb S). In the majority of circumstances they have sufficient Hb A to permit the normal functioning of red cells. Their red cells do not sickle or cause tissue damage. People with this condition are described as having sickle cell trait and as carriers of the sickle cell gene. If two people who are sickle cell carriers have a baby, there is a one in four chance that the child will have sickle cell anaemia.

Sickle cell anaemia

Sickle cell anaemia is an inherited blood disorder characterised by the presence of abnormal haemoglobin, sickle haemoglobin (Hb S), in red blood cells. The abnormal haemoglobin is less soluble than normal haemoglobin and causes deformity (sickling) of the red cells. The red cells are also more fragile than normal with a shortened life span and tend to disintegrate (haemolyse) in the circulation causing anaemia. The sickled red cells block small blood vessels causing damage to the tissues of the body.

Episodes of sickling, called ‘crises’, occur from time to time. Often these occur spontaneously, but they may be precipitated by infections (e.g. upper respiratory tract infections, gastroenteritis), dehydration, cold, menstruation or by administration of a general anaesthetic. Destruction of red cells causes anaemia, which may be severe and of sudden onset. Tissue damage causes pain, interferes with the normal function of the organs of the body and causes long-term complications. Repeated frequent crises may cause progressive deterioration in health and shorten life.

Sickle cell trait

People with sickle cell trait have sufficient normal haemoglobin in their blood such that their red blood cells function normally. The red cells do not normally sickle (haemolyse) or cause tissue damage. The sickle cell carrier does not have anaemia, or any the symptoms of sickle cell anaemia, and leads a normal life. In some exceptional circumstances where there is less ambient oxygen e.g. scuba diving, during a general anaesthetic or at high altitude, the sickle cell carrier may develop problems with the blood.

Other types of sickle cell disorder

There are other types of abnormal inherited haemoglobin such as haemoglobin C (Hb C) which has a generally milder course and Hb D Punjab and HbO Arab which are more severe. Sickle cell genes can also be inherited with beta thalassaemia. Sickle Beta Thalassaemia is a sickle cell anaemia syndrome and patients with this condition may be White or Asian, they will suffer from either severe sickle cell disease or a milder form with less frequent crisis depending on the severity of the beta Thalassaemia gene they have inherited. The type will often be clear from the medical evidence provided, for example, sickle beta zero Thalassaemia (severe) or sickle beta plus Thalassaemia (milder disease). Click on the link for details of Thalassaemia guidance.

Incidence and prevalence

In the UK sickle cell disorders are most commonly found in people of African and Caribbean descent. They are also seen in people from the eastern Mediterranean, the Middle East and India. It is estimated that there are around 10-15000 adults and children with sickle cell disorders in the UK at the present time. About 140 babies with sickle cell anaemia are born in the UK each year.

Amended May 2009