Myeloproliferative Neoplasms (MPNs)

Myeloproliferative Neoplasms (MPNs) are a group of rare, chronic blood cancers. These diseases begin within the bone marrow which is found at the centre of major bones in your body and produces stem cells which grow to form various types of blood cells. MPNs involve abnormal stem cells within the bone marrow (myelo) which divide (proliferate) to produce excess numbers 
of one or more types of  blood cells (white cells, red cells and/or platelets). 

These abnormal cells change the thickness of the blood, and sometimes function abnormally. MPNs can cause serious health problems unless properly treated and controlled. They are chronic diseases that, in most cases, remain stable for many years and progress gradually over time.

MPNs are usually described according to the type of blood cell which is most affected. It is not uncommon for people to have features of more than one MPN when they are first diagnosed, or during the course of their illness. In some cases, one disorder may transform over time to another MPN or to a type of leukaemia called acute myeloid leukaemia.

Types of MPNs

MPNs are typically named according to the type of cell which is most affected. There are three main types of MPNs: 

  • Polycythaemia vera (PV) - overproduction of red cells 
  • Essential thrombocythemia (ET) - overproduction of platelets
  • Primary myelofibrosis (MF) - production of fibrous scar-like material in the marrow

Less common types of myeloproliferative neoplasms include:

  • Chronic neutrophilic leukaemia (CNL) – too many neutrophils (a type of white cell) in blood and bone marrow.
  • Chronic eosinophilic leukaemia (CEL) / hypereosinophilic    syndrome – too many eosinophils (a type of white cell) in blood and bone marrow
  • Chronic myelomonocytic leukaemia (CMML) – too many monocytes (a type of white cell)
  • Myeloproliferative neoplasm (unclassifiable) – an abnormal, overactive bone marrow that doesn’t fit within other subcategories

Polycythaemia vera – (PV)

Polycythaemia vera (PV) is a disorder in which too many red cells are produced in the bone marrow. These cells accumulate in the bone marrow and in the blood where they cause the blood to become thicker than normal. 

Many people with PV also have too many platelets and white cells. Although it can occur at any age, PV usually affects older people, with most patients diagnosed over the age of 50. It occurs more commonly in males than in females. 

Essential thrombocythaemia – (ET)

Essential thrombocythaemia (ET) is a disorder in which too many platelets are produced in the bone marrow. Platelets are normally needed in the body to control bleeding. However, excess numbers of platelets can lead to abnormal blood clotting, which can block the flow of blood in the blood vessels. 

There are a number of other conditions that can cause a rise in 
the number of platelets in the circulating blood (thrombocytosis) including bleeding, infection and some types of cancer. In ET however, the blood platelet count is persistently elevated as a result of increased bone marrow production of platelets. Like polycythaemia vera, ET is a rare chronic disease and although it can occur at any age, even (rarely) in children, it usually affects older people, with most patients diagnosed between the ages of 50 and 70. It occurs more frequently in females than males.

Primary Myelofibrosis

Primary myelofibrosis is a disorder in which the proliferation of platelet producing cells triggers the production of fibrous scar-like material in the bone marrow. Over time, this destroys the normal bone marrow environment, preventing the production of adequate numbers of red cells, white cells and platelets. This results in anaemia, low platelet counts and the production of blood cells in areas outside the bone marrow. The production occurs in organs such as the spleen and liver which become enlarged as a result.

Myelofibrosis is a rare chronic disorder and it can occur at any age but is usually diagnosed later in life, between the ages of 60 and 70 years. Around one third of people with myelofibrosis have been previously diagnosed with polycythaemia (post-polycythaemic myelofibrosis) or essential thrombocythaemia (post-thrombocythaemic myelofibrosis).

Myeloproliferative Neoplasm – Unclassifiable (MPN-U)

The MPN sub-category of MPN-U includes MPN-like neoplasms that cannot be clearly classified as one of the other subcategories of MPNs. Patients with MPN-U might present with otherwise unexplained thrombosis (clot formation), especially splanchnic vein thrombosis, which is associated with a normal blood count.

The reported incidence of MPN-U varies significantly in different studies with most studies showing an incidence of 10–15%. Patients with MPN-U are treated based on their clinical abnormalities, blood count and bone marrow findings, and molecular abnormalities. In some cases, a diagnosis of MPN-U may reflect a very early stage of the disease, and sub- classification may become possible as the disease progresses.

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