Key Terms: Asymptomatic, Congenital, Gamma globulin, Microliter, Neonatal, Stent.
Description
Thrombocytopenia (thrombocythemia) is a blood disorder characterized by an abnormally low number of circulating platelets (thrombocytes) in the bloodstream. Because platelets play an important role in the process of coagulation (blood clotting) and in the plugging of damaged blood vessels, persons with decreased platelets bruise easily and can have episodes of excessive bleeding (hemorrhage). Thrombocytopenia is usually an acquired disorder, but it can also be congenital, as in neonatal rubella (German measles).
Platelets are irregular, disc-shaped fragments of large cells called megakaryocytes, which are found in the spongy center of long bones (bone marrow). They are the smallest cell-like structures in the blood. When a blood vessel is punctured or damaged, normal mature platelets have a tendency to aggregate (group) together at the site, forming a plug that stops the bleeding. The lifespan of platelets in the blood is relatively short (five to ten days), so the bone marrow of healthy individuals is continually producing new platelets to replace the old ones.
Doctors usually use a combination of the physical examination, the medical history, and laboratory testing to diagnose this disorder. The platelet count, which is part of a complete blood count (CBC), is a key diagnostic tool. It measures the number of platelets in a volume of blood. The blood normally contains between 150,000 and 400,000 platelets per microliter (cubic millimeter or mm3) of blood. (A million microliters is equal to one liter, or about 1.1 quarts.) In adults, a platelet count of less than 100,000/microliter is considered low, but might occur without symptoms. Abnormal bleeding often occurs when the platelet count is below 30,000/microliter. If the count falls below 10,000/microliter, abnormal external bleeding is usually evident, and serious internal bleeding can be life threatening.
Causes
Thrombocytopenia occurs when any of the following abnormal conditions exist:
- decreased production of platelets by the bone marrow
- increased destruction of circulating platelets
- increased trapping of platelets by the spleen
- platelet loss from hemorrhage
The most common cause of thrombocytopenia is a decrease in the production of platelets by the bone marrow. When abnormalities develop in the bone marrow, the megakaryocytes (platelet precursors) can lose their ability to produce platelets in sufficient amounts. This is a common side effect of blood cancers such as leukemia, which causes an abnormal growth of white blood cells in the bone marrow. These abnormal cells crowd out the normal bone marrow cells, including the platelets. Other diseases that cause this condition are tumors that spread (metasta-size) to the bone, aplastic anemia, and viral infections such as rubella. Radiation and drugs used in cancer chemotherapy and in the treatment of other serious diseases can also cause the bone marrow to malfunction in this way, especially if they are used together. Some drugs, such as aspirin or heparin, do not actually cause a decrease in the number of platelets, but they destroy the functional ability of the platelets to aggregate.
Platelets can break down in unusually high amounts in persons with abnormalities in their blood vessel walls, with blood clots, or with man-made replacement heart valves. Devices (stents) placed inside blood vessels to keep them from closing (because of weakened walls or fat build-up) can also cause an increased destruction of platelets. In addition, severe microbial infections, infection with the human immunodeficiency virus (HIV), the virus that causes AIDS, and other changes in the immune system can speed up the removal of platelets from the circulation.
Normally, the spleen holds about one-third of the body's platelets as part of this organ's function to recycle certain aging or damaged blood cells. When liver disease or cancer of the spleen is present, the spleen can become enlarged (a condition called splenomegaly) and trap many more platelets than normal. Because a greater number of platelets remain in the enlarged organ, fewer platelets are circulating in the bloodstream.
Treatments
Sometimes this disorder is asymptomatic and does not require any treatment. This is often the case when thrombocytopenia occurs in children following a viral infection. Even when the disorder is a side effect of both radiation therapy and chemotherapy, if the thrombocytopenia is not severe, it is often reversible on its own once the therapies end.
Treatments, when necessary, vary with the severity of the disorder, the abnormal condition that caused the disorder, and any underlying or secondary cause. When possible, the best form of treatment is to eliminate whatever is causing the condition. For example, if a drug is causing the thrombocytopenia, eliminating that drug would be the ideal solution. However, when the disorder is a side effect of chemotherapy, the patient might need to continue the drug therapy. In such cases, the doctor must decide whether it is in the best interest of the patient to continue with the same dosage, to lower the dosage, to try an alternative drug, or to give the patient a platelet transfusion. For diseases other than blood cancers, doctors can sometimes continue the chemotherapy at full dosage by also giving the patient a platelet growth factor called Oprelvekin (marketed as Neumega) to boost the production of normal platelets in the bone marrow.
If a dysfunctional immune system is destroying the patient's platelets, the doctor might use a corticosteroid (such as prednisone) or gamma globulin to suppress the patient's immune response and to help maintain adequate platelet levels. Corticosteroids can also have unwanted side effects, so doctors usually do not use this treatment for very long.
If an enlarged spleen is the underlying cause of the thrombocytopenia, the doctor might want to try corticosteroids or epinephrine to release platelets from the spleen. If these methods fail, surgical removal of the spleen (splenectomy) can help to raise the platelet level since the spleen is no longer there to capture the platelets. However, the disease that caused the enlarged spleen, such as lymphoma or cancer that spread to the spleen from another area of the body, should be treated as well.
If the patient is having severe external or internal bleeding as the result of injury or disease, a platelet transfusion might be necessary for immediate results. This is especially true if laboratory tests show a decreased production of platelets in the bone marrow.
Alternative and Complementary Therapies
A natural substance called thrombopoietin shows promise as a regulator of platelet production.
Many over-the-counter medicines, herbal supplements (such as garlic, ginger, feverfew, and ginko biloba) and vitamins can affect the ability of platelets to function properly. To determine the best treatment for a patient and to avoid drug interactions, the doctor needs to know every drug and remedy a patient is taking.
Resources
Books
Altman, Roberta, and Michael J. Sarg. The Cancer Dictionary, Rev. ed. New York: Checkmark Books, 2000.
Komaroff, Anthony L. Harvard Medical School Family Health Guide. New York: Simon & Schuster, 1999.
Periodicals
Henderson, C.W. "Study Results Suggest Hope for Previously Untreatable Patients." Blood Weekly 16 November 2000: 20.
Other
"Side Effect Management Series: Low Blood Platelets (Thrombocytopenia)." Oncology.com. 2001. [cited June 28, 2001]. .
—Beverly Miller, MT(ASCP); Dominic De Bellis
