What is sickle cell anaemia?
Sickle cell disorders are caused by inherited abnormalities of the blood protein haemoglobin A (Hb A). Haemoglobin is found in red blood cells. It carries oxygen molecules from the lungs to the rest of the body. In sickle cell anaemia the haemoglobin A molecule is abnormal and the abnormal variant is called haemoglobin ‘S’ or Hb S and this affects the ability of red cells to travel through the circulation picking up and releasing oxygen. New red blood cells are a normal disc shape; because the sickle haemoglobin protein does not work properly they become deformed. The deformed red cells are sickle shaped when examined under the microscope hence the name ‘sickle cell anaemia’.
Abnormally shaped sickle cells are removed from the circulation quickly as they do not function properly once damaged. Normal red blood cells last for about 120 days in the circulation, in sickle cell anaemia, the cells are damaged quite quickly and the average life span of red blood cells is much lower than normal. The reduced life span of red blood cells in sickle cell anaemia leads to two effects: one is anaemia – the haemoglobin count is lower than normal and the other is increased production of cells. High rates of production are needed to try to catch up with destruction of abnormal cells. This can lead to iron deficiency because iron is required by the body to make haemoglobin.
The sickle cells have two main effects -:
- The red cells are more fragile than normal and tend to disintegrate more easily than normal cells. This is particularly true if oxygen levels become abnormally low. When the red cells disintegrate in the circulation they literally break apart releasing their contents in to the blood plasma (called haemolysis). When many cells do this at once it is called a haemolytic crisis and can be life threatening.
- The second problem relates to the shape of the sickle cells. Once a cell has ‘sickled’, it is less elastic and is unable to squeeze through small spaces in to the capillary beds where oxygen is exchanged. The rigid sickle cells tend to get stuck in very small vessels and block them off. This cuts off the oxygen supply to capillary beds and causes cell death in the affected areas. Over time as more small areas infarct (die due to lack of oxygen) measurable loss of function develops. Examples of organs typically affected by this process include the bones, nervous system, lungs and kidneys.
Sickle cell disease is an inherited or genetic disorder
The production of haemoglobin in the body is controlled by genes inherited from both parents. In sickle cell disorders an abnormal gene causes the production of abnormal ‘sickle’ haemoglobin in red blood cells. It is a recessive condition – this means that a person must inherit one abnormal gene from each parent to have the condition. If they inherit one normal gene and one abnormal gene the normal gene will enable them to produce enough normal haemoglobin not to have the condition – this is called having sickle cell trait and means the person is carrier of the sickle cell gene. Under very low oxygen conditions a person with sickle cell trait may develop sickling of their red cells. Such conditions include diving underwater at depth, flying at high altitude and general anaesthesia. Precautions against ‘sickling’ include supplemental oxygen under these conditions – life is otherwise normal with no disabling effects.
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. This is because their red cells will have a strong tendency to ‘sickle’ and this is likely to cause painful tissue damage and anaemia.
What are the 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 children 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)
- sickle beta plus Thalassaemia (milder disease)
Incidence & 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.
