What is sickle cell disease?
Sickle cell disease is an inherited blood disorder characterized by defective hemoglobin (a protein in red blood cells that carries oxygen to the tissues of the body).
Sickle cell disease involves the red blood cells, or hemoglobin, and their ability to carry oxygen. Normal hemoglobin cells are smooth, round, and flexible, like the letter "O," so they can move through the vessels in our bodies easily. Sickle cell hemoglobin cells are stiff and sticky, and form into the shape of a sickle, or the letter "C," when they lose their oxygen. These sickle cells tend to cluster together, and cannot easily move through the blood vessels. The cluster causes a blockage in small arteries or capillaries and stops the movement of healthy, normal oxygen-carrying blood. This blockage is what causes the painful and damaging complications of sickle cell disease.
Sickle cells only live for about 10 to 20 days, while normal hemoglobin can live up to 120 days. Also, sickle cells risk being destroyed by the spleen because of their shape and stiffness. The spleen is an organ that helps filter the blood of infections and sickled cells get stuck in this filter and die. Due to the decreased number of hemoglobin cells circulating in the body, a person with sickle cell disease is chronically anemic. The spleen also suffers damage from the sickled cells blocking healthy oxygen carrying cells and typically infants in the first few years of life. Without a normal functioning spleen, these individuals are more at risk for infections. Infants and young children are at risk for life-threatening infections.
The most common variations of the sickle cell gene are:
- Sickle cell trait. The child is carrying a single defective gene. Some of their hemoglobin is the destructive HbS, but they also have some normal hemoglobin, HbA. This is referred to as HbAS. Children with sickle cell trait are usually without symptoms of the disease. Mild anemia may occur and red cells tend to be small. Under intense stressful conditions, exhaustion, hypoxia (low oxygen), and/or severe infection, the sickling of the defective hemoglobin may occur and result in some complications associated with the sickle cell disease. Most children with the sickle cell trait lead normal lives.
- Sickle cell anemia. The child has most or all of the normal hemoglobin (HbA) replaced with the sickle hemoglobin (HbS). This is referred to as HbSS. It is the most common and most severe form of the sickle cell variations. These children suffer from a variety of complications due to the shape and thickness of the sickled cells. Severe and chronic anemia is also a common characteristic for children with HbSS.
- Sickle cell-hemoglobin SC disease. The child has one copy of both HbS and HbC. This is often referred to as HbSC. Hemoglobin C causes red blood cells, called target cells, to develop. Having just some hemoglobin C and normal hemoglobin, a person will not have any symptoms of anemia, but can develop it and eye and hip complications later in life. However, if the sickle hemoglobin S is combined with the target cell, some mild to moderate anemia occurs. These children often suffer some of the complications associated with HbSS, sickle cell disease, but to a milder degree. Vaso-occlusive crises (the flow of blood is blocked because the sickled cells have become stuck in the blood vessels), organ damage from repeated sickling and anemia, and high risk for infection are all similar traits for HbSS and HbSC.
- Sickle cell-hemoglobin E disease. This variation is similar to sickle cell-C disease except that an element has been replaced in the hemoglobin molecule. This variation is most often also seen in Southeast Asia populations. Some children with hemoglobin E disease are without symptoms. However, under certain conditions, such as exhaustion, hypoxia, severe infection, and/or iron deficiency, some mild to moderate anemia may occur.
- Hemoglobin S-beta-thalassemia. This involves an inheritance of both the thalassemia and sickle cell genes. The disorder produces symptoms of moderate anemia and many of the same conditions associated with sickle cell disease. While this disorder more often has milder symptoms than sickle cell disease, it may also produce exacerbations as severe as those of sickle cell disease.
All forms of sickle cell disease can exhibit the complications associated with the disease.
Who is affected by sickle cell disease?
Sickle cell disease primarily affects those of African descent and Hispanics of Caribbean ancestry, but the trait has also been found in those with Middle Eastern, Indian, Latin American, American Indian, and Mediterranean heritage.
It has been estimated that 90,000 to 100,000 people in the U.S. are affected by sickle cell anemia and that approximately 3 million people, and 10 percent of African-Americans, have sickle cell trait. Millions worldwide suffer complications from sickle cell disease.