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Sickle Cell Disease

(Hemoglobin S Disease)

(See also Overview of Anemia.)

Sickle cell disease is an inherited genetic abnormality of hemoglobin (the oxygen-carrying protein found in red blood cells) characterized by sickle (crescent)-shaped red blood cells and chronic anemia caused by excessive destruction of the abnormal red blood cells.

  • People always have anemia and sometimes jaundice.

  • Worsening anemia, fever, and shortness of breath with pain in the long bones, abdomen, and chest can indicate sickle cell crisis.

  • A special blood test called electrophoresis can be used to determine whether people have sickle cell disease.

  • Avoiding activities that may cause crises and treating infections and other disorders quickly can help prevent crises.

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). People who have sickle cell trait do not develop sickle cell disease, but they do have increased risks of some complications such as blood in their urine. About 0.3% of blacks have two copies of the gene. These people develop the disease.

Red Blood Cell Shapes

Normal red blood cells are flexible and disk-shaped, thicker at the edges than in the middle. In several hereditary disorders, red blood cells become spherical (in hereditary spherocytosis), oval (in hereditary elliptocytosis), or sickle-shaped (in sickle cell disease).

In sickle cell disease, the red blood cells contain an abnormal form of hemoglobin (the protein that carries oxygen). The abnormal form of hemoglobin is called hemoglobin S. When red blood cells contain a large amount of hemoglobin S, they can become deformed into a sickle shape and less flexible. Not every red blood cell is sickle-shaped. The sickle-shaped cells become more numerous when people have infections or low levels of oxygen in the blood.

The sickle cells are fragile and break apart easily. Because the sickle cells are stiff, they have difficulty traveling through the smallest blood vessels (capillaries), blocking blood flow and reducing oxygen supply to tissues in areas where capillaries are blocked. The blockage of blood flow can cause pain and, over time, cause damage to the spleen, kidneys, brain, bones, and other organs. Kidney failure and heart failure may occur.

Symptoms

People who have sickle cell disease always have some degree of anemia (often causing fatigue, weakness, and paleness) and may have mild jaundice (yellowing of the skin and eyes). Some people have few other symptoms. Others have severe, recurring symptoms that cause enormous disability and early death.

Sickle cell trait

In people with sickle cell trait, red blood cells are not fragile and do not break easily. Sickle cell trait does not cause painful crises, but rarely, people die suddenly while undergoing very strenuous exercise that causes severe dehydration, such as during military or athletic training.

People with sickle cell trait are at increased risk of chronic kidney disease and pulmonary embolism. Rarely, they may notice blood in their urine. People with sickle cell trait are also at risk for an extremely rare form of kidney cancer.

Sickle cell crisis

Anything that reduces the amount of oxygen in the blood, such as vigorous exercise, mountain climbing, flying at high altitudes without sufficient oxygen, or an illness, may bring on a sickle cell crisis (also called an exacerbation). A sickle cell pain (vaso-occlusive) crisis is an episode of increased symptoms and can 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. Sometimes, additional complications occur with a pain crisis, including

  • Aplastic crisis: Production of red blood cells in the bone marrow stops during infection with some viruses

  • Acute chest syndrome: Caused by blockage of capillaries in the lungs

  • Hepatic (liver) sequestration: Rapid enlargement of the liver

Acute chest syndrome can occur in people of all ages, but it is most common among children. Chest syndrome is characterized by severe chest pain and difficulty breathing. Repeated episodes of chest syndrome predispose to pulmonary hypertension (high pressure in the vessel that carries blood from the heart to the lungs). Acute chest syndrome can be fatal.

Complications

Most people who have sickle cell disease develop an enlarged spleen during childhood because sickled cells become trapped in the spleen. 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 red blood cell production, so anemia becomes more severe.

The liver can become progressively larger throughout life (causing upper abdominal fullness), and gallstones often form from the pigment of broken-apart red blood cells.

The heart usually enlarges, and an enlarged heart is less effective in pumping blood to the body, possibly leading to heart failure. 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). Episodes of priapism may permanently damage the penis so that the man can no longer have erections. Blocked blood vessels may cause strokes that damage the nervous system. In older people, lung and kidney function may deteriorate.

Diagnosis

  • Blood tests

  • Hemoglobin electrophoresis

  • Prenatal testing

Doctors recognize anemia, stomach and bone pain, and nausea in a young black person as possible signs of a sickle cell crisis. When doctors suspect sickle cell disease, they do blood tests. Sickle-shaped red blood cells and fragments of destroyed red blood cells can be seen in a blood sample examined under a microscope.

Hemoglobin electrophoresis, another blood test, is also done. In electrophoresis, an electrical current is used to separate the different types of hemoglobin and thus detect abnormal hemoglobin.

Further testing may be needed, depending on the specific symptoms the person experiences during the crisis. For example, if the person has difficulty breathing or a fever, a chest x-ray may be done.

Screening

Blood tests are done on relatives of people with the disorder because they also may have sickle cell disease or trait. Discovering the trait in people may be important for family planning, to determine their risk of having a child with sickle cell disease.

Newborns are routinely screened with a blood test in the United States. Tests can be done during early pregnancy to screen the fetus and allow prenatal counseling for couples who are at risk of having a child with sickle cell disease. Fetal cells obtained through amniocentesis or chorionic villus sampling are tested for the presence of the sickle cell gene.

Treatment

  • Treatments aimed at preventing crises

  • Treatment of crises and conditions that cause them

Treatment is aimed at

  • Preventing crises

  • Controlling the anemia

  • Relieving symptoms

Stem cell transplantation may cure sickle cell disease. 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 so it is not often done. Recipients must take drugs that suppress the immune system for the rest of their life.

Gene therapy, a technique in which normal genes are implanted in precursor cells (cells that produce blood cells), is being studied.

Preventing sickle cell crisis

People who have sickle cell 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 people are at increased risk of infection, they should receive vaccines for pneumococcal, meningococcal, influenza, and Haemophilus influenzae type b infections. Children typically take penicillin by mouth from age 4 months until age 6 years.

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 and acute chest syndrome.

Controlling anemia

People are given folate, a vitamin that helps the body make new red blood cells.

Blood transfusions may be given to correct the anemia.

Treating sickle cell crisis

Sickle cell crisis may require hospitalization. People are given fluid by vein (intravenously) and drugs to relieve pain. Blood transfusions and oxygen are given for anemia that 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.

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