Myeloproliferative Diseases Health Article

Definition

The myeloproliferative diseases are four conditions—essential thrombocythemia, polycythemia vera, chronic myelocytic leukemia, and agnogenic myeloid metaplasia—characterized by overproduction of normal-looking blood cells.

Because chronic myelocytic leukemia has its own individual entry, it is not covered in depth in this entry.

Description

The prefix "myelo—" refers to marrow. Bone marrow, a reddish substance in the middle of some bones, produces blood cells. In the myeloproliferative diseases, the body makes too many blood cells. Blood contains red blood cells to carry oxygen, white blood cells to fight infections, and platelets to begin blood clotting. Myeloproliferative diseases develop when a myeloid progenitor cell—a cell that makes red blood cells, platelets, and certain types of white blood cells—becomes overactive. The abnormal progenitor cell continues to make normal blood cells, but it makes too many of them. This excess of blood cells results in varying symptoms, depending on the progenitor cell involved.

Other problems develop when some of the abnormal myeloid progenitor cells travel to the spleen, liver, or lymph nodes and begin making blood cells there. Most often, they migrate to the spleen. An enlarged spleen can crowd other organs in the abdomen and cause discomfort or digestive troubles. It is also susceptible to painful damage from blocked arteries. Massively swollen spleens can use large amounts of energy and cause muscle wasting and weight loss.

In the later stages of myeloproliferative diseases, the bone marrow can become scarred. This may leave no space for progenitor cells. As a result, blood cell production can drop to dangerously low levels. The abnormal progenitor cells may also mutate and develop into leukemia. These two serious complications are rare in some myeloproliferative diseases but very common in others.

Types of myeloproliferative disease

The four myeloproliferative diseases include essential thrombocythemia, polycythemia vera, chronic myelocytic leukemia, and agnogenic myeloid metaplasia.

In essential thrombocythemia (primary thrombocythemia), the myeloid progenitor cell makes too many platelets. Blood containing too many platelets may either clot too easily or too slowly. Blood that clots too easily can lead to a variety of health problems, including strokes or heart attacks. Blood that clots too slowly can cause symptoms such as easy bruising, frequent nosebleeds, bleeding from the gums, or life-threatening hemorrhages. Excessive numbers of platelets can also cause headaches or erythromelalgia, an unusual condition characterized by warmth, redness and pain in the hands or feet. Typically, patients with this disease have long periods without symptoms, interspersed with clotting or bleeding episodes. Some patients may have no symptoms at all. Rarely, this disease ends in scarring of the bone marrow or leukemia. Patients with bone marrow scarring have symptoms identical to agnogenic myeloid metaplasia.

In polycythemia vera (primary polycythemia, Vaquez disease), the bone marrow makes too many red blood cells. Large numbers of red blood cells can make the blood too thick. Viscous blood flows sluggishly, pools in the veins, and delivers oxygen poorly. Patients may experience headaches, dizziness, fatigue, chest pains, or weakness and cramping in the calves while walking. The abnormal blood flow can also result in bleeding tendencies or blood clotting inside the veins. Many patients also have increased numbers of white blood cells or platelets, but most symptoms are caused by the sluggish blood flow. The spleen often enlarges. Polycythemia rarely leads to leukemia, but occasionally ends in bone marrow scarring.

In chronic myelocytic leukemia (chronic myelogenous leukemia), the myeloid progenitor cell makes a type of white blood cell called a granulocyte. With this condition, platelets can also increase. In the early stages of this disease, the white blood cells look outwardly normal. However, in 90-95% of patients, two chromosomes— number 9 and number 22— inside the progenitor cell have broken and exchanged parts. This chromosome rearrangement is known as the Philadelphia chromosome, and this genetic abnormality destabilizes these cells and inevitably they become cancerous.

Agnogenic myeloid metaplasia (idiopathic myelofibrosis, myelofibrosis with myeloid metaplasia) begins like other myeloproliferative diseases, with overproduction of blood cells. However, bone marrow scarring develops very quickly and causes most of the symptoms. Blood cell numbers drop, causing fatigue and weakness from anemia. Many of the cells found in the blood are also immature or oddly shaped. Although myeloid progenitor cells in the spleen and liver can partly compensate, the enlargement of these organs creates additional problems. Occasionally, this disease also ends in leukemia.

Demographics

Essential thrombocytopenia, polycythemia vera, and agnogenic myeloid metaplasia are usually diagnosed late in life, at an average (median) age of 60.

Essential thrombocythemia may be slightly more common in women and agnogenic myeloid metaplasia and polycythemia vera slightly more common in men; however, estimates vary. At one time, polycythemia vera was thought to develop more often in Jews. More recent statistics do not confirm this.