Sickle Cell Anemia

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Overview, Causes, & Risk Factors

Sickle cell anemia is caused by an abnormal type of hemoglobin (oxygen carrying molecule) called hemoglobin S. It is inherited as an autosomal recessive trait -- that is, it occurs in someone who has inherited hemoglobin S from both parents.

Someone who inherits hemoglobin S from one parent and normal hemoglobin (A) from the other parent will have sickle cell trait. Approximately 8% of African Americans have sickle cell trait. Someone who inherits hemoglobin S from one parent and another type of abnormal hemoglobin from the other parent will have another form of sickle cell disease, such as sickle cell-b ⁰ thalassemia, hemoglobin SC disease, or sickle cell-b ⁺ thalassemia. Someone with sickle cell trait or these forms of sickle cell disease will usually have no symptoms or only mild ones. However, some of these conditions can cause symptoms similar to sickle cell anemia.

Sickle cell disease is much more common in certain ethnic groups, affecting approximately one out of every 500 African Americans. Because people with sickle trait were more likely to survive malaria outbreaks in Africa than those with normal hemoglobin, it is believed that this genetically aberrant hemoglobin evolved as a protection against malaria.

Although sickle cell disease is inherited and present at birth, symptoms usually don't occur until after 4 months of age. Sickle cell anemia may become life-threatening when damaged red blood cells break down (hemolytic crisis), when the spleen enlarges and traps the blood cells (splenic sequestration crisis), or when a certain type of infection causes the bone marrow to stop producing red blood cells (aplastic crisis). Repeated crises can cause damage to the kidneys, lungs, bones, eyes, and central nervous system.

Blocked blood vessels and damaged organs can cause acute painful episodes. These painful crises, which occur in almost all patients at some point in their lives, can last hours to days, affecting the bones of the back, the long bones, and the chest. Some patients have one episode every few years, while others have many episodes per year. The crises can be severe enough to require admission to the hospital for pain control and intravenous fluids.

Many manifestations of this disease are a result of the fragility and inflexibility of the sickle red blood cells. When patients experience dehydration, infection, and low oxygen supply, these fragile red blood cells assume a crescent shape, causing red blood cell destruction and poor flow of these blood cells through blood vessels, resulting in a lack of oxygen to the body's tissues.