Genetic Disease Profile: Sickle Cell Anemia

For more about the gene that causes sickle cell anemia, see the HBB Gene Profile. The following was adapted from NIH Publication No. 96-4057.

Disease Description

Sickle cell anemia is an inherited blood disorder characterized primarily by chronic anemia and periodic episodes of pain. The underlying problem involves hemoglobin, a component of red blood cells. Hemoglobin molecules in each red blood cell carry oxygen from the lungs to body organs and tissues and bring carbon dioxide back to the lungs.

In sickle cell anemia, the hemoglobin is defective. After hemoglobin molecules give up their oxygen, some may cluster together and form long, rod-like structures. These structures cause red blood cells to become stiff and assume a sickle shape.

Unlike normal red cells, which are usually smooth and donut-shaped, sickled red cells cannot squeeze through small blood vessels. Instead, they stack up and cause blockages that deprive organs and tissues of oxygen-carrying blood. This process produces periodic episodes of pain and ultimately can damage tissues and vital organs and lead to other serious medical problems. Normal red blood cells live about 120 days in the bloodstream, but sickled red cells die after about 10 to 20 days. Because they cannot be replaced fast enough, the blood is chronically short of red blood cells, a condition called anemia.


Inheritance

Sickle cell anemia is an autosomal recessive genetic disorder caused by a defect in the HBB gene, which codes for hemoglobin. The presence of two defective genes (SS) is needed for sickle cell anemia. If each parent carries one sickle hemoglobin gene (S) and one normal gene (A), each child has a 25% chance of inheriting two defective genes and having sickle cell anemia; a 25% chance of inheriting two normal genes and not having the disease; and a 50% chance of being an unaffected carrier like the parents.


Incidence

Sickle cell anemia affects millions throughout the world. It is particularly common among people whose ancestors come from sub-Saharan Africa; Spanish-speaking regions (South America, Cuba, Central America); Saudi Arabia; India; and Mediterranean countries such as Turkey, Greece, and Italy. In the Unites States, it affects around 72,000 people, most of whose ancestors come from Africa. The disease occurs in about 1 in every 500 African-American births and 1 in every 1000 to 1400 Hispanic-American births. About 2 million Americans, or 1 in 12 African Americans, carry the sickle cell trait.


Symptoms

The clinical course of sickle cell anemia does not follow a single pattern; some patients have mild symptoms, and some have very severe symptoms. The basic problem, however, is the same: the sickle-shaped red blood cells tend to get stuck in narrow blood vessels, blocking the flow of blood. This results in the following conditions:

Hand-foot syndrome. When small blood vessels in hands or feet are blocked, pain and swelling can result, along with fever. This may be the first symptom of sickle cell anemia in infants.

Fatigue, paleness, and shortness of breath. These are all symptoms of anemia or a shortage of red blood cells.

Pain that occurs unpredictably in any body organ or joint. A patient may experience pain wherever sickled blood cells block oxygen flow to tissues. The frequency and amount of pain vary. Some patients have painful episodes (also called crises) less than once a year, and some have as many as 15 or more episodes in a year. Sometimes pain lasts only a few hours; sometimes it lasts several weeks. For especially severe ongoing pain, the patient may be hospitalized and treated with painkillers and intravenous fluids. Pain is the principal symptom of sickle cell anemia in both children and adults.

Eye problems. The retina, the "film" at the back of the eye that receives and processes visual images, can deteriorate when it does not get enough nourishment from circulating red blood cells. Damage to the retina can be serious enough to cause blindness.

Yellowing of skin and eyes. These are signs of jaundice, resulting from rapid breakdown of red blood cells.

Delayed growth and puberty in children and often a slight build in adults. The slow rate of growth is caused by a shortage of red blood cells.


Complications

Infections. In general, both children and adults with sickle cell anemia are more vulnerable to infections and have a harder time fighting them off. This is the result of spleen damage from sickled red cells, thus preventing the spleen from destroying bacteria in the blood. Infants and young children, especially, are susceptible to bacterial infections that can kill them in as little as 9 hours from onset of fever. Pneumococcal infections used to be the principal cause of death in children with sickle cell anemia until physicians began routinely giving penicillin on a preventive basis to those who are diagnosed at birth or in early infancy.

Stroke. Defective hemoglobin damages the walls of red blood cells, causing them to stick to blood vessel walls. The resulting narrowed or blocked small blood vessels in the brain can lead to serious, life-threatening strokes, primarily in children.

Acute chest syndrome. Similar to pneumonia, this life-threatening complication is caused by infection or trapped sickled cells in the lung. It is characterized by chest pain, fever, and an abnormal chest X ray.


Diagnosis

Early diagnosis of sickle cell anemia is critical so children who have the disease can receive proper treatment.

Blood test - More than 40 states now perform a simple, inexpensive blood test for sickle cell disease on all newborn infants. This test is performed at the same time and from the same blood samples as other routine newborn-screening tests. Hemoglobin electrophoresis is the most widely used diagnostic test. If the test shows the presence of sickle hemoglobin, a second blood test is performed to confirm the diagnosis. These tests also tell whether or not the child carries the sickle cell trait.


Treatment

Although there is no cure for sickle cell anemia, doctors can do a great deal to help patients, and treatment is constantly being improved. Basic treatment of painful crises relies heavily on painkilling drugs and oral and intravenous fluids to reduce pain and prevent complications.

Blood Transfusions - Transfusions correct anemia by increasing the number of normal red blood cells in circulation. They can also be used to treat spleen enlargement in children before the condition becomes life-threatening. Regular transfusion therapy can help prevent recurring strokes in children at high risk.

Oral Antibiotics - Giving oral penicillin twice a day beginning at 2 months and continuing until the child is at least 5 years old can prevent pneumococcal infection and early death. Recently, however, several new penicillin-resistant strains of pneumonia bacteria have been reported. Since vaccines for these bacteria are ineffective in young children, studies are being planned to test new vaccines.

Hydroxyurea - The first effective drug treatment for adults with severe sickle cell anemia was reported in early 1995, when a study conducted by the National Heart, Lung, and Blood Institute showed that daily doses of the anticancer drug hydroxyurea reduced the frequency of painful crises and acute chest syndrome. Patients taking the drug needed fewer blood transfusions.

Regular health maintenance is critical for people with sickle cell anemia. Proper nutrition, good hygiene, bed rest, protection against infections, and avoidance of other stresses all are important in maintaining good health and preventing complications. Regular visits to a physician or clinic that provides comprehensive care are necessary to identify early changes in the patient's health and ensure immediate treatment.

Today, with good health care, many people with sickle cell anemia are in reasonably good health much of the time and living productive lives. In fact, in the past 30 years, the life expectancy of people with sickle cell anemia has increased.

Organizations and Support Groups

Sickle Cell Disease Association of America (SCDAA)

American Sickle Cell Association

National Heart, Lung, and Blood Institute

The Sickle Cell Information Center

Joint Center for Sickle Cell and Thalassemic Disorders

Diagnostic Testing

Hemoglobin Electrophoresis and Sickle Cell Test - Entries in ADAM Medical Encyclopedia available through MEDLINEplus

Sickle Cell Test - Description of the test provided by Lab Tests Online, a peer-reviewed, non-commercial resource on clinical lab testing

Clinical Trials

Sickle Cell Anemia Studies at ClinicalTrials.gov

Other Sickle Cell Anemia Resources

MedlinePlus Entry for Sickle Cell Anemia

NCBI Genes and Disease: Sickle Cell Anemia

Sickle Cell Anemia from New York Online Access to Health (NOAH)

Sickle Cell Disease Information from the March of Dimes

NIH Publication No. 02-2117: The Management of Sickle Cell Disease (206 page document that you can download or view online as PDF. Printed version also can be ordered.)

Hemoglobin S Allele and Sickle Cell Disease - This HuGE Review is a peer-reviewed publication from the Human Genome Epidemiology Network (HuGENet) at the CDC's Office of Genetics and Disease Prevention. It describes the gene, genetic variants, associated disorders, interactions, as well as laboratory and population testing.