Thrombocytopenia is a deficiency of platelets (thrombocytes).

The blood usually contains about 140,000 to 440,000 platelets per microliter. Bleeding can occur with relatively minor trauma when the platelet count falls below about 50,000 platelets per microliter of blood. The most serious risk of bleeding, however, generally does not occur until the platelet count falls below 10,000 to 20,000 platelets per microliter. At these very low levels, bleeding may occur without any injury.

Causes

Many diseases can cause thrombocytopenia. Thrombocytopenia can occur when the bone marrow does not produce enough platelets, as happens in leukemia and some anemias. Infection with the human immunodeficiency virus (HIV), the virus that causes AIDS, often results in thrombocytopenia. Platelets can become entrapped in an enlarged spleen, as happens in myelofibrosis and Gaucher's disease, reducing the number of platelets in the bloodstream. Massive blood transfusions can dilute the concentration of platelets in the blood. Finally, the body may use or destroy too many platelets, as occurs in many disorders, three of the most notable being idiopathic thrombocytopenic purpura, thrombotic thrombocytopenic purpura, and hemolytic-uremic syndrome.

Causes of Thrombocytopenia Bone marrow does not produce enough platelets Leukemia Lymphoma Aplastic anemia Heavy alcohol consumption Megaloblastic anemias, including vitamin B12 and folic acid deficiency anemias Some bone marrow disorders Platelets become entrapped in an enlarged spleen Cirrhosis with congestive splenomegaly Myelofibrosis Gaucher's disease Platelets become diluted Massive blood replacement or exchange transfusion with stored blood containing too few platelets Cardiopulmonary bypass surgery Use or destruction of platelets increases Idiopathic thrombocytopenic purpura HIV infection Drugs such as heparin, quinidine, quinine, sulfa-containing antibiotics, some oral drugs for diabetes, gold salts, and rifampin Conditions involving disseminated intravascular coagulation within blood vessels, such as can occur with obstetric complications, cancer, blood poisoning (septicemia) from gram-negative bacteria, and traumatic brain damage Thrombotic thrombocytopenic purpura Hemolytic-uremic syndrome Paroxysmal nocturnal hemoglobinuria

Idiopathic thrombocytopenic purpura (ITP): ITP is a disease in which antibodies form and destroy the body's platelets. Why the antibodies form is not known. Although the bone marrow increases platelet production to compensate for the destruction, the supply cannot keep up with the demand.

Thrombotic thrombocytopenic purpura (TTP): TTP is a rare disease in which small blood clots form suddenly throughout the body. The blood clots mean that an abnormally high number of platelets are being used, which leads to a sharp decrease in the number of platelets in the bloodstream.

Hemolytic-uremic syndrome (HUS): HUS is a disorder related to TTP in which the number of platelets suddenly decreases, red blood cells are destroyed, and the kidneys stop functioning. HUS is rare but can occur with certain bacterial infections (particularly intestinal infections with Escherichia coli O157:H7 or some strains of Shigella dysenteriae) and with the use of some drugs (including quinine, cyclosporine, and mitomycin C). The syndrome is most common in infants, young children, and women who are pregnant or have just given birth, although it can occur in older children, adults, and women who are not pregnant.

Symptoms and Complications

Bleeding in the skin may be the first sign of a low platelet count. Many tiny red dots (petechiae) often appear in the skin on the lower legs, and minor injuries may cause small scattered bruises. The gums may bleed, and blood may appear in the stool or urine. Menstrual periods may be unusually heavy. Bleeding may be hard to stop.

Bleeding worsens as the number of platelets decreases. People who have very few platelets may lose large amounts of blood into the digestive tract or may develop life-threatening bleeding in the brain even though they have not been injured.

The rate at which symptoms develop can vary depending on the cause of thrombocytopenia. For example, in TTP and HUS, symptoms develop suddenly. In ITP, symptoms may develop suddenly or gradually and subtly.

Symptoms in TTP and HUS are quite distinct from symptoms of most other forms of thrombocytopenia. In TTP, the small blood clots that develop (using up platelets) cause a wide range of symptoms and complications, some of which can be life threatening. Symptoms that result from clots in the brain may include headache, confusion, seizures, and coma. Symptoms that result from clots elsewhere in the body include abnormal heart rhythms, blood in the urine that accompanies kidney damage, and abdominal pain. The predominant symptoms and complications of HUS are related to blood clots that develop in the kidneys, causing damage that is usually severe and may progress to kidney failure.

Diagnosis

Doctors suspect thrombocytopenia in people who have abnormal bruising and bleeding. They often check the number of platelets routinely in people who have disorders that cause thrombocytopenia. Sometimes they discover thrombocytopenia when blood tests are performed for other reasons in people who have no bruising or bleeding.

Determining the cause of thrombocytopenia is critical to treating the condition. Certain symptoms may help determine the cause. For example, people usually have a fever when thrombocytopenia results from an infection. In contrast, they usually do not have a fever when the cause is ITP, TTP, or HUS. An enlarged spleen, which a doctor may be able to feel during a physical examination, suggests that the spleen is trapping platelets and that thrombocytopenia results from a disorder that causes the spleen to enlarge. HUS is diagnosed when poor kidney function is identified by blood tests that show high levels of urea nitrogen and creatinine.

A sample of blood may be examined under a microscope, or the platelet count may be measured with an electronic counter to determine the severity of thrombocytopenia and provide clues to its cause. A sample of bone marrow removed and examined under a microscope (bone marrow aspiration and biopsy (see Symptoms and Diagnosis of Blood Disorders: Bone Marrow Examination) may be needed to provide information about platelet production.

Treatment

People who have a very low platelet count are often treated in a hospital or advised to stay in bed to avoid accidental injury. When bleeding is severe, platelets may be transfused.

Addressing the underlying cause can often treat the thrombocytopenia. Thrombocytopenia caused by a drug usually is corrected by discontinuing the drug. The effects produced by antibodies that destroy platelets in ITP can be blocked temporarily with a corticosteroid (for example, prednisone) or intravenous immune globulin, allowing the number of platelets to increase. Danazol may have similar effects as prednisone. Drugs that suppress the immune system, including cyclophosphamide and sometimes azathioprine, may reduce the formation of antibodies. Most adults (but not children) with ITP eventually require surgical removal of the spleen (splenectomy) to increase the number of platelets. People with TTP are often treated with plasma transfusions along with plasmapheresis (see Testing Donated Blood for Infections). The two procedures together are called plasma exchange.

Complications that require long-term treatment can result from some causes of thrombocytopenia. For example, the number of platelets usually increases as people recover from the hemolytic-uremic syndromeHUS; however, lifelong dialysis or kidney transplantation may be needed if the kidney failure persists.

Last full review/revision May 2006 by Joel L. Moake, MD