Myelodysplastic syndromes occur when something happens to disrupt the orderly and controlled production of blood cells. People with myelodysplastic syndromes have blood cells that are immature and defective, and instead of developing normally, they die in the bone marrow or just after entering the bloodstream. Over time, the number of immature, defective cells begins to surpass that of healthy blood cells, leading to problems such as anemia, infections and excess bleeding.

Doctors divide myelodysplastic syndromes into two categories based on their cause:

  • Myelodysplastic syndromes with no known cause. Called de novo myelodysplastic syndromes, doctors don't know what causes these. De novo myelodysplastic syndromes are often more easily treated than are myelodysplastic syndromes with a known cause.
  • Myelodysplastic syndromes caused by chemicals and radiation. Myelodysplastic syndromes that occur in response to cancer treatments, such as chemotherapy and radiation, or in response to chemical exposure are called secondary myelodysplastic syndromes. Secondary myelodysplastic syndromes are often more difficult to treat.

Types of myelodysplastic syndromes

The World Health Organization divides myelodysplastic syndromes into subtypes based on the type of blood cells — red cells, white cells and platelets — involved. Myelodysplastic syndrome subtypes include:

  • Refractory cytopenia with unilineage dysplasia. In this type, one blood cell type is low in number. This type of blood cell appears abnormal under the microscope.
  • Refractory anemia with ringed sideroblasts. This type involves a low number of red blood cells. The existing red blood cells contain excess amounts of iron (ringed sideroblasts).
  • Refractory cytopenia with multilineage dysplasia. In this myelodysplastic syndrome, two of the three types of blood cells are abnormal, and less than 1 percent of the cells in the bloodstream are immature cells (blasts).
  • Refractory anemia with excess blasts — types 1 and 2. In both these syndromes, any of the three types of blood cells — red blood cells, white blood cells or platelets — may be low in number and appear abnormal under a microscope. Very immature blood cells (blasts) are found in the blood.
  • Myelodysplastic syndrome, unclassified. In this uncommon syndrome, there are reduced numbers of one of the three types of mature blood cells, and either the white blood cells or platelets look abnormal under a microscope.
  • Myelodysplastic syndrome associated with isolated del(5q) chromosome abnormality. People with this syndrome have low numbers of red blood cells, and the cells have a specific defect in their DNA.
Nov. 04, 2011

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