Sickle Cell Disease
Sickle cell disease is an inherited condition characterized by sickle (crescent)-shaped red blood cells and chronic anemia caused by excessive destruction of red blood cells.
Sickle cell disease affects blacks almost exclusively. About 10% of blacks in the United States have one copy of the gene for sickle cell disease (that is, they have sickle cell trait); they do not develop sickle cell disease, although rarely they may notice blood in their urine. About 0.3% of blacks have two copies of the gene; they develop the disease.
 See the figure Red Blood Cell Shapes.
In sickle cell disease, the red blood cells contain an abnormal form of hemoglobin (the protein that carries oxygen) that reduces the amount of oxygen in the cells. The reduced oxygen causes some of the red blood cells to become sickle-shaped. Because these deformed cells are fragile, they break apart as they travel through the smallest blood vessels, causing severe anemia, blocked blood flow, and reduced oxygen supply. The deformed cells also damage the spleen, kidneys, brain, bones, and other organs.
Symptoms and Complications
People who have sickle cell disease always have some degree of anemia and mild jaundice, but they may have few other symptoms. Anything that reduces the amount of oxygen in their blood, such as vigorous exercise, mountain climbing, flying at high altitudes without sufficient oxygen, or an illness, may bring on a sickle cell "crisis." A sickle crisis may consist of a sudden worsening of anemia, pain (often in the abdomen or long bones of the arms and legs), fever, and sometimes shortness of breath. Abdominal pain may be severe, and vomiting may occur.
In children, sickle cell crisis may take the form of a chest syndrome, characterized by severe chest pain and difficulty breathing. The exact cause of the chest syndrome is unknown but may be related to or produced by an infection or a blocked blood vessel resulting from a blood clot or an embolus (a piece of a blood clot that has broken off and lodged in a blood vessel).
Most people who have sickle cell disease develop an enlarged spleen during childhood. By the time the person reaches adolescence, the spleen is often so badly injured that it shrinks and no longer functions. Because the spleen helps fight infection, people with sickle cell disease are more likely to develop pneumococcal pneumonia and other infections. Viral infections, in particular, can decrease blood cell production, so anemia becomes more severe. The liver becomes progressively larger throughout life, and gallstones often form from the pigment of broken-apart red blood cells. The heart usually enlarges, and heart murmurs are common.
Children who have sickle cell disease often have a relatively short torso but long arms, legs, fingers, and toes. Changes in the bones and bone marrow may cause bone pain, especially in the hands and feet. Episodes of joint pain with fever may occur, and the hip joint may become so damaged that it eventually needs to be replaced.
Poor circulation to the skin may cause sores on the legs, especially at the ankles. Young men may develop persistent, often painful erections (priapism). Blocked blood vessels may cause strokes that damage the nervous system. In older people, lung and kidney function may deteriorate.
Diagnosis
Doctors recognize anemia, stomach and bone pain, and nausea in a young black person as signs of a sickle cell crisis. Sickle-shaped red blood cells and fragments of destroyed red blood cells can be seen in a blood sample examined under a microscope.
A blood test called electrophoresis can detect abnormal hemoglobin and indicate whether a person has sickle cell trait or sickle cell disease. Discovering the trait may be important for family planning, to determine the risk of having a child with sickle cell disease.
Treatment and Prevention
Sickle cell disease can be a relatively mild disease that requires little treatment or a severe, recurring disease that causes enormous disability and early death. Rarely, a person who has sickle cell trait dies suddenly while undergoing very strenuous exercise that has caused severe dehydration, such as during military or athletic training.
Sickle cell disease cannot be cured, so treatment is aimed at preventing crises, controlling the anemia, and relieving symptoms. People who have this disease should try to avoid activities that reduce the amount of oxygen in their blood and should seek prompt medical attention for even minor illnesses, such as viral infections. Because they are at increased risk of infection, they should be immunized with pneumococcal and Haemophilus influenzae vaccines.
Sickle cell crisis may require hospitalization. The person is given large amounts of fluid intravenously and drugs to relieve pain. Blood transfusions and oxygen may be given if a doctor suspects that anemia is severe enough to pose a risk of stroke, heart attack, or lung damage. Conditions that may have caused the crisis, such as an infection, are treated.
Drugs can help control sickle cell disease. Hydroxyurea increases the production of a form of hemoglobin found predominantly in fetuses, which decreases the number of red blood cells becoming sickle-shaped. Therefore, it reduces the frequency of sickle cell crises. Bone marrow or stem cells from a family member or other donor who does not have the sickle cell gene may be transplanted in a person with the disease. Although such transplantation may be curative, it is risky, and recipients must take drugs that suppress the immune system for the rest of their lives. Gene therapy, a technique in which normal genes are implanted in precursor cells (cells that produce blood cells), is under study.
Prenatal diagnosis and counseling are available for couples who know they are at risk for having a child with sickle cell disease (see Section 22, Chapter 256). Fetal cells obtained through amniocentesis can be directly examined, and the presence of one or two copies of the sickle cell gene can be accurately determined.
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