Acquired aplastic anemia is a rare disorder caused by profound, almost complete bone marrow failure. 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 produces specialized cells (hematopoietic stem cells) that grow and eventually develop into red blood cells (erythrocytes), white blood cells (leukocytes), and platelets. In acquired aplastic anemia, an almost complete absence of hematopoietic stem cells eventually results in low levels of red and white blood cells and platelets (pancytopenia). Specific symptoms associated with acquired aplastic anemia may vary, but include fatigue, chronic infections, dizziness, weakness, headaches, and episodes of bleeding, usually in the skin and mucous membranes. Although some cases of acquired aplastic anemia occurs secondary to other disorders, researchers now believe that most cases result from a disorder of the patient's immune system, which mistakenly targets the bone marrow (autoimmunity). This is based on the response of the majority of patients to immunotherapy, whether it is ATG and cyclosporine, high-dose corticosteroids or cyclophosphamide.
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, without a family history of blood disease.
The symptoms of acquired aplastic anemia occur as a consequence of the bone marrow failing to produce enough functioning blood cells. The specific symptoms and progression of the disorder vary from case to case. Some individuals may have mild symptoms that remain stable for many years; others may have serious symptoms that can progress to life-threatening complications.
Red and white blood cells and platelets are formed in the bone marrow. The cells are released into the bloodstream to travel throughout the body performing their specific functions. Red blood cells deliver oxygen to the body, white blood cells help in fighting off infections and platelets allow the body to form clots to stop bleeding. A low level of circulating red blood cells is known as anemia. A low level of white blood cells is known as leukopenia. A low level of platelets is known as thrombocytopenia.
Individuals with anemia may experience tiredness, increased need for sleep, weakness, lightheadedness, dizziness, irritability, headaches, pale skin color, difficulty breathing (dyspnea), and cardiac symptoms. Individuals with leukopenia have an increase in their risk of contracting bacterial and fungal infections. Individuals with thrombocytopenia are more susceptible to excessive bruising following minimal injury and to spontaneous bleeding from the mucous membranes, especially those of the gums and nose. Women may develop increased menstrual blood loss (menorrhagia). Symptoms are dependent on the severity of the anemia, leukopenia, and thrombocytopenia.
Some individuals with acquired aplastic anemia also have another disorder at the same time, called paroxysmal nocturnal hemoglobinuria (PNH). Acquired aplastic and PNH have an extremely close relationship that is not fully understood by researchers. It is believed that PNH arises in the setting of autoimmune acquired aplastic anemia and bone marrow failure. Individuals affected with acquired aplastic anemia are also at risk that it will evolve into another similar disorder known as myelodysplasia. In rare cases, acquired aplastic anemia may eventually evolve into leukemia. PNH is caused by an acquired genetic defect affecting the PIGA gene, limited to the stem cells. The PIGA gene mutations cause blood cells to become sensitive to increased destruction by complement, a blood immunity protein. Twenty percent or more of patients with aplastic anemia have evidence of PNH at presentation, as detected by flow cytometry. Furthermore, patients who respond following immunosuppressive therapy frequently recover with clonal hematopiesis and PNH. There are a minority of MDS patients with hypoplastic or low cellularity bone marrow, as seen in acquired aplastic anemia. These conditions are often mistaken for each other, so whether one is transformed to another is uncertain. (For more information on these disorders, see the Related Disorders section of this report.)
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 very readily available, but there is much evidence to support this pathogenic mechanism.
The bone marrow contains specialized cells called hematopoietic stem cells. These stem cells eventually 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 believe 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. In some cases, the bone marrow, with no blood cell production, becomes replaced by large quantities of fat. Affected individuals eventually develop a deficiency of red and white blood cells and platelets (pancytopenia). It is interesting that hypoplastic MDS may respond to immunotherapy, similar to AA, and that benzene also can damage genes (genotoxic) and cause the condition to be converted to leukemia (leukemic transformation), so the overlap between these syndromes is becoming more apparent.
In the past, acquired aplastic anemia has been linked to a variety of environmental factors including exposure to toxic environmental chemicals such as benzene, pesticides or insecticides; the use of certain drugs; and certain viral infections, especially hepatitis. These environmental factors are believed to trigger the immune system response that mistakenly destroys hematopoietic stem cells. However, most cases of acquired aplastic anemia have no identifiable environmental trigger.
Acquired aplastic anemia affects males and females in about equal numbers. Most cases affect older children, teen-agers or young adults. The incidence of aplastic anemia in Europe and Israel is 2 new cases among 1 million people per year. The incidence rate is two or three times greater in Asia. The exact incidence rates exist for the United States is unknown although some sources say that approximately 500-1,000 new cases of aplastic anemia are diagnosed each year.
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.
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 allogeneic bone marrow transplant, an affected individual's abnormal bone marrow cells are eradicated or destroyed by chemotherapy and/or radiation, 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.
A similar procedure called peripheral hematopoietic stem cell transplantation may also be used to treat individuals with acquired aplastic anemia. In this procedure, healthy stem cells are taken from a donor by collecting them from the donor's bloodstream rather than the bone marrow. These are then injected intravenously into the patient's bloodstream. However, marrow is preferred to blood as a source of stem cells in aplastic anemia.
Graft rejection and graft-versus-host disease are potential complications with any transplant procedures, including bone marrow transplant. 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 to begin producing new blood cells. The two most commonly used immunosuppressive agents, given alone or in combination, are antithymocyte 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 is often not 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. Treatment with substances that stimulate blood cell production, called growth factors, may be also be beneficial in refractory aplastic anemia. Growth factors are substances normally found in the body that assist in the production of blood cells. They include granulocyte-colony stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF) and erythropoietin (EPO). These growth factors stimulate the production of neutrophils, neutrophils and macrophages (both types of white blood cells that fight infection), and red blood cells, respectively. In some cases, treatment of refractory aplastic anemia with growth factors had led to clinically beneficial improvement in blood cell counts.
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Contact for additional information about acquired aplastic anemia:
Neal S Young, MD
Chief, Hematology Branch
National Heart, Lung, and Blood Institute (NHLBI)
National Institutes of Health
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