Acquired aplastic anemia is a rare, serious blood disorder, due to the failure of the bone marrow fails to produce blood cells. Bone marrow is the spongy substance found in the center of the bones of the body, in adults mainly the spine, pelvis, and large bones of the legs. The bone marrow contains hematopoietic stem cells. Stem cells can produce more stem cells (self-renewal) and also differentiate and proliferate, giving rise to red blood cells (erythrocytes), white blood cells (leukocytes), and platelets. In acquired aplastic anemia, an almost complete absence of hematopoietic stem cells results in low levels of red and white blood cells and platelets (pancytopenia). Symptoms of aplastic anemia are anemia, bleeding, and infection. Although bone marrow failure can occur secondary to other disorders, most aplastic anemia is due to the immune system mistakenly targeting the bone marrow (autoimmunity). Indeed, most patients can respond to therapy that suppresses the immune system, usually ATG and cyclosporine.
Aplastic anemia is classified as severe according to blood counts. Most of the discussion that follows relates to severe aplastic anemia. Patients with more moderately decreased blood counts; may not require treatment. Furthermore, some aplastic anemia that is genetically inherited may, first manifest in adulthood, sometimes without a family history of blood disease.
Causes
Most cases of acquired aplastic anemia occur unrelated to any identifiable causes, or for unknown reasons (idiopathic). Researchers believe that most are due to the immune system mistakenly targeting the bone marrow (autoimmunity). Autoimmune disorders are caused when the body’s natural defenses against “foreign” or invading organisms begin to attack healthy tissue for unknown reasons. Tests to confirm this in any individual case are not readily available, but there is much evidence to support this pathogenic mechanism.
The bone marrow contains hematopoietic stem cells. These stem cells can divide, differentiate and become red or white blood cells or platelets. In aplastic anemia, a precipitating event is hypothesized to trigger immune-mediated destruction of hematopoietic stem cells. It is believed that certain immune system cells (T-lymphocytes) target and destroy the most primitive cells capable of developing into blood cells, hematopoietic stem cells. Individuals with aplastic anemia do not have enough stem cells to produce mature blood cells. The bone marrow appears to be replaced by fat. Affected individuals eventually develop a deficiency of red and white blood cells and platelets (pancytopenia).
In the past, acquired aplastic anemia has been linked to a variety of environmental factors, especially benzene; benzene likely is directly harmful to bone marrow cells. The relationship between bone marrow failure to other chemicals, such as pesticides or insecticides, is less well established. The use of certain medical drugs also is rarely associated with aplastic anemia, as is nonviral hepatitis. Both may trigger the immune system response that mistakenly destroys hematopoietic stem cells. However, most cases of acquired aplastic anemia have no identifiable environmental trigger.
Diagnosis
A diagnosis of acquired aplastic anemia may be suspected when an otherwise healthy individual has low levels of all three blood cell types (pancytopenia). A diagnosis may be confirmed by a thorough clinical evaluation, a detailed patient history, and a variety of specialized tests, including a bone marrow biopsy. During this procedure, a small specimen of bone marrow tissue is surgically removed, usually from the hip or pelvis, and studied under a microscope. In acquired aplastic anemia this sample will show a dramatic reduction or complete lack of cells. Additional tests may be necessary to rule out other disorders such as leukemia and to determine if there is an inherited or genetic cause.
Treatment
Treatment of acquired aplastic anemia varies, depending upon the individual’s age, general health, and the severity of aplastic anemia. Treatment aims to correct the bone marrow failure, as well as to treat the patient’s immediate signs and symptoms. The two main forms of specific treatment are bone marrow transplantation and immunosuppressive therapies.
Initial treatment of acquired aplastic anemia may be directed toward improving the symptoms that may result from low blood counts. Such treatment consists of giving red blood cell transfusions to correct anemia, platelet transfusions to treat or prevent serious bleeding, and antibiotics to treat or prevent infections.
Bone marrow transplantation, specifically an allogeneic transplant, is the treatment of choice in children and younger adults. With an allogeneic bone marrow transplant, an affected individual’s abnormal bone marrow cells are eradicated or destroyed by chemotherapy and replaced with healthy marrow obtained from a donor. The donor marrow is transplanted by injecting the cells of the donor intravenously into the patient’s body, where it travels to the patient’s bone marrow and eventually begins producing new blood cells. The best match for a bone marrow transplant is an identical twin, sibling, or close relative who shares most of the same genetic makeup as the patient. However, in many cases, a search for an unrelated, matched donor is necessary, or more recently a partially matched family member is the donor.
Graft rejection and graft-versus-host disease are potential complications with any transplant procedures, including bone marrow transplants. Complications of graft-versus-host disease from a bone marrow transplant may range from mild to life-threatening. Drugs may be used to prevent or treat graft rejection or graft-versus-host disease. (For more information on this disorder, choose “graft versus host disease” as your search term in the Rare Disease Database.)
Individuals who are not candidates for a bone marrow transplant, either because of advanced age or lack of a suitable donor, are usually treated with immunosuppressive treatment. In this case, drugs are used to suppress the activity of the immune system. Since many cases of acquired aplastic anemia are believed to result from an individual’s immune system mistakenly attacking bone marrow, suppressing the activity of the immune system often allows the bone marrow to recover and eventually begin producing new blood cells. The two most commonly used immunosuppressive agents, given alone or in combination, are anti-thymocyte globulin (ATG) and cyclosporine. Horse ATG is more effective than rabbit ATG in the treatment of aplastic anemia.
Immunosuppressive therapy can restore an affected individual’s blood count to normal or near-normal levels for prolonged periods. However, the improvement may not be permanent and the treatment must be repeated if relapses of aplastic anemia occur. In addition, individuals who successfully respond to immunosuppressive therapy are still at risk of eventually developing PNH, myelodysplasia, or leukemia.
Approximately one-third of individuals treated with immunosuppressive drugs do not respond to therapy (refractory aplastic anemia). In these cases, treatment with hematopoietic stem cell transplantation may be considered. Immunosuppression can be repeated in refractory aplastic anemia and also for patients who have relapsed.
Hematopoietic growth factors such as erythropoietin and Neupogen are not effective in aplastic anemia, but surprisingly eltrombopag, a stimulator of platelet production, was effective in improving blood counts in refractory aplastic anemia. In 2014, Promacta was approved to treat patients with severe aplastic anemia who have had an insufficient response to immunosuppressive therapy and are not candidates for a hematopoietic stem cell transplant. When eltrombopag was combined with standard immunosuppression as first-line therapy, response and complete response rates were higher than with immunosuppression alone.
Investigational Therapies
Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. government funding, and some supported by private industry, are posted on this government website.
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