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Sickle Cell Anemia

A congenital hemolytic anemia that occurs primarily, but not exclusively, in blacks, sickle cell anemia results from a defective hemoglobin (Hb) molecule (hemoglobin S) that causes red blood cells (RBCs) to roughen and become sickle-shaped. Such cells impair circulation, resulting in chronic ill health (fatigue, dyspnea on exertion, swollen joints), periodic crises, long-term complications, and premature death.

Sickle cell anemia is most common in tropical Africans and in people of African descent; about 1 in 10 blacks carries the abnormal gene. If two such carriers have offspring, there is a 1 in 4 (25%) chance that each child will have the disease. Overall, 1 in every 400 to 600 black children has sickle cell anemia.

This disease also occurs in Puerto Rico, Turkey, India, the Middle East, and the Mediterranean, as well as in other populations. The defective Hb gene may have persisted in areas where malaria is endemic because the heterozygous sickle cell trait provides resistance to malaria and is actually beneficial.

Penicillin prophylaxis can decrease morbidity and mortality from bacterial infections. Half of such patients die by their early twenties; few live to middle age.

Causes

Sickle cell anemia results from homozygous inheritance of the Hb S gene, which causes substitution of the amino acid valine for glutamic acid in the B Hb chain. Heterozygous inheritance of this gene results in sickle cell trait, usually an asymptomatic condition.

Altered cells

The abnormal Hb S found in RBCs of patients with sickle cell anemia becomes insoluble whenever hypoxia occurs. As a result, these RBCs become rigid, rough, and elongated, forming a crescent, or sickle, shape. Such sickling can produce hemolysis (cell destruction).

In addition, these altered cells tend to pile up in capillaries and smaller blood vessels, making the blood more viscous. Normal circulation is impaired, causing pain, tissue infarctions, and swelling. Such blockage causes anoxic changes that lead to further sickling and obstruction.

Signs and symptoms

Symptoms of sickle cell anemia include paleness of skin as the anemia worsens, especially on the insides of eyelids, under fingernails and in the creases of the palm of the hand. The skin may also turn yellow (jaundiced). If the anemia is severe, the shortage of normal red blood cells may cause weakness, shortness of breath, or even heart failure. The symptoms of shock caused by heart failure are low blood pressure, rapid pulse, and decreasing consciousness.

Diagnosis

A positive family history and typical clinical features suggest sickle cell anemia. Hb electrophoresis showing Hb S or other hemoglobinopathies can confirm it. Electrophoresis should be done on umbilical cord blood samples at birth to provide sickle cell disease screening for all neonates at risk.

Additional laboratory studies show a low RBC count, elevated white blood cell and platelet counts, decreased erythrocyte sedimentation rate, increased serum iron level, decreased RBC survival, and reticulocytosis. Hb may be low or normal.

During early childhood, palpation may reveal splenomegaly, but as the child grows older, the spleen shrinks.

Treatment

Prophylactic penicillin is given before age 4 months. If the patient's Hb drops suddenly or if his condition deteriorates rapidly, a transfusion of packed RBCs is needed.

In a sequestration crisis, treatment may include sedation, administration of analgesics, blood transfusion, oxygen administration, and large amounts of oral and I.V. fluids. A good antisickling agent isn't available yet; the most commonly used drug, sodium cyanate, has many adverse effects.

Prevention

If you carry the sickle cell trait, you may wish to see a genetic counselor before trying to conceive a child. A genetic counselor can help you understand your risk of having a child with sickle cell anemia. He or she can also explain possible treatments, preventive measures and reproductive options.

There is an in vitro fertilization procedure that improves the chances that parents who both carry the sickle cell gene will have a child with normal hemoglobin. This procedure is known as preimplantation genetic diagnosis. First, eggs are taken from the mother. Then, sperm is taken from the father. In a laboratory, the eggs are fertilized with the sperm. The fertilized eggs are then tested for the presence of the sickle cell gene. Fertilized eggs free of the sickle cell gene can be implanted into the mother for normal development. However, this procedure is expensive and not always successful.