Anemia is a condition in which the number of red blood cells or the amount of hemoglobin (the protein that carries oxygen in them) is low.

Red blood cells contain hemoglobin, a protein that enables them to carry oxygen from the lungs and deliver it to all parts of the body. When the number of red blood cells is reduced or the amount of hemoglobin in them is low, the blood cannot carry an adequate supply of oxygen. An inadequate supply of oxygen in the tissues produces the symptoms of anemia.

Causes

The causes of anemia are numerous, but most can be grouped within three major mechanisms that produce anemia:

• Blood loss (excessive bleeding)
• Inadequate production of red blood cells
• Excessive destruction of red blood cells

Anemia may be caused by excessive bleeding. Bleeding may be sudden, as may occur in an injury or during surgery. Often, bleeding is gradual and repetitive, typically from abnormalities in the digestive or urinary tract or heavy menstrual periods. Chronic bleeding typically leads to low levels of iron, which leads to worsening anemia.

Anemia may also result when the body does not produce enough red blood cells. Many nutrients are needed for red blood cell production. The most critical are iron, vitamin B₁₂, and folate (folic acid), but the body also needs trace amounts of vitamin C, riboflavin, and copper, as well as a proper balance of hormones, especially erythropoietin (a hormone that stimulates red blood cell production). Without these nutrients and hormones, production of red blood cells is slow and inadequate, or the red blood cells may be deformed and unable to carry oxygen adequately. Chronic disease also may affect red blood cell production. In some circumstances, the bone marrow space may be invaded and replaced (for example, by leukemia, lymphoma, or metastatic cancer), resulting in decreased production of red blood cells.

Anemia may also result when too many red blood cells are destroyed. Normally, red blood cells live about 120 days. Scavenger cells in the bone marrow, spleen, and liver detect and destroy red blood cells that are near or beyond their usual life span. If red blood cells are destroyed prematurely (hemolysis), the bone marrow tries to compensate by producing new cells faster. When destruction of red blood cells exceeds their production, hemolytic anemia results. Hemolytic anemia is relatively uncommon compared with the anemia caused by excessive bleeding and decreased red blood cell production.

Common Causes of Anemia Mechanism Examples Chronic excessive bleeding Bladder tumors Cancer in the digestive tract Heavy menstrual bleeding Hemorrhoids Kidney tumors Nosebleeds Polyps in the digestive tract Ulcers in the stomach or small intestine Sudden excessive bleeding Injuries Childbirth A ruptured blood vessel Surgery Decreased red blood cell production Aplastic anemia Chronic disorders Folate deficiency Iron deficiency Leukemia Lymphoma Metastatic cancer Myelodysplasia (abnormalities in bone marrow tissue) Myelofibrosis Multiple myeloma Vitamin B12 deficiency Vitamin C deficiency Increased red blood cell destruction Autoimmune reactions against red blood cells An enlarged spleen Glucose-6-phosphate dehydrogenase (G6PD) deficiency Hemoglobin C disease Hemoglobin E disease Hemoglobin S-C disease Hereditary elliptocytosis Hereditary spherocytosis Mechanical damage to red blood cells Paroxysmal nocturnal hemoglobinuria Sickle cell disease Thalassemia

More About Some Causes of Anemia Cause Mechanism Treatment Comments Enlarged spleen An enlarged spleen traps and destroys too many red blood cells The disorder that caused the spleen to enlarge is treated Sometimes the spleen must be removed surgically Symptoms tend to be mild Often, an enlarged spleen also traps platelets and white blood cells, thus reducing their number in the bloodstream Mechanical damage to red blood cells Abnormalities in blood vessels (such as an aneurysm), an artificial or damaged heart valve, or extremely high blood pressure can break normal red blood cells apart The cause of the damage is identified and corrected The kidneys eventually filter the damaged red blood cells out of the blood but may be damaged by them The spleen also filters the damaged red cells out of the blood Paroxysmal nocturnal hemoglobinuria The immune system destroys red blood cells Hemoglobin from these damaged cells is concentrated in urine during the night, resulting in dark, reddish urine in the morning Corticosteroids and a new antibody drug, eculizumab, help relieve symptoms, but the only cure is allogeneic bone marrow transplant People with blood clots may need to take an anticoagulant Bone marrow transplantation may be needed People may have severe stomach cramps and clotting in the large veins of the abdomen and legs Symptoms often occur in episodes (paroxysmally) Hereditary spherocytosis Red blood cells become misshapen and rigid, getting trapped and destroyed in the spleen Treatment is usually not needed, but severe anemia may require removal of the spleen This hereditary disorder can also cause bone abnormalities, such as a tower-shaped skull Hereditary elliptocytosis Red blood cells are oval or elliptical in shape rather than the normal disk shape Severe anemia may require removal of the spleen The anemia is usually mild and requires no treatment Glucose-6-phosphate dehydrogenase (G6PD) deficiency The G6PD enzyme is missing from red blood cell membranes Without this enzyme, red blood cells are more likely to break apart when the person is stressed by triggers such as fever, infection, diabetic crisis, or certain drugs such as aspirin and sulfa drugs Anemia can be prevented by preventing or avoiding things that trigger it This hereditary disorder almost always affects males About 10% of black males and a smaller percentage of white people of Mediterranean origin have the disorder

Symptoms and Diagnosis

Symptoms vary depending on the severity of the anemia and how rapidly it develops. Some people with mild anemia, particularly when it develops slowly, have no symptoms at all. Other people may experience symptoms only with physical exertion. More severe anemia may produce symptoms even when people are resting. Symptoms are more severe when mild or severe anemia develops rapidly, such as with bleeding that occurs when a blood vessel ruptures.

Mild anemia often causes fatigue, weakness, and paleness. In addition to these symptoms, more severe anemia may produce faintness, dizziness, increased thirst, sweating, a weak and rapid pulse, and rapid breathing. Severe anemia may produce painful lower leg cramps during exercise, shortness of breath, and chest pain, especially if people already have impaired blood circulation in the legs or certain types of lung or heart disease.

Sometimes anemia is detected before people notice symptoms, when routine blood tests are done.

Low levels of hemoglobin and a low hematocrit (the percentage of red blood cells in the total blood volume) found in a blood sample confirm the anemia. Other tests, such as examining a blood sample under a microscope and less often examining a sample taken from the bone marrow, help determine the cause of the anemia.

Spotlight on Aging Many disorders that cause anemia, especially cancer, tend to be more common among older people. Thus, many older people develop anemia. Iron deficiency anemia, usually due to abnormal bleeding, is the most common anemia among older people. Symptoms of anemia are basically the same in older people as in younger people. However, older people may not look pale. Also, older people with anemia, even when mild, are more likely to become confused, depressed, agitated, or listless than younger people. They may also become unsteady and have difficulty walking. These problems can interfere with being able to live independently. However, some older people with mild anemia have no symptoms at all, particularly when anemia develops gradually, as it often does in older people. In older people, anemia caused by vitamin B12 deficiency may be mistaken for dementia because this type of anemia may affect the nerves and mental function. Having anemia may shorten the life expectancy of older people. Thus, identifying the cause and correcting it is particularly important.

Last full review/revision June 2008 by Alan E. Lichtin, MD