Myelodysplastic syndromes (MDSs) are a group of rare cancers that affect blood-producing cells in your blood marrow. They lead to the overproduction of abnormal, immature blood cells that overcrowd healthy red blood cells, white blood cells, or platelets.

MDSs sometimes develop into acute myeloid leukemia. MDSs used to be known as pre-leukemia, but this term isn’t frequently used anymore because many cases won’t transform into leukemia.

The World Health Organization (WHO) recognizes six primary types of MDSs. These categories are:

  • MDS with multi-lineage dysplasia (MDS-MLD)
  • MDS with single lineage dysplasia (MDS-SLD)
  • MDS with excess blasts (MDS-EB)
  • MDS with ring sideroblasts (MDS-RS)
  • MDS with isolated del(5q)
  • MDS, unclassifiable (MDS-U)

Understanding which type you have can help you and your doctor predict how quickly your MDS will progress and the best treatment options. Read on to learn more about each of these MDS types.

MDSs are classified based on how your blood and bone marrow cells look under a microscope and whether there are genetic changes to these cells. Specific factors doctors consider include:

  • your white blood cell, red blood cell, and platelet counts
  • your percentage of blood cells that look abnormal under a microscope
  • certain changes to the chromosomes in your bone marrow cells
  • the portion of cells in your blood and bone marrow that are abnormal and immature
  • the presence and proportion of abnormal red blood cell precursors called ring sideroblasts

MDS-MLD is the most common type of MDS. Its features include:

  • at least 10 percent of 2 or 3 types of immature blood cells in your bone marrow are abnormal
  • a low blood cell count of at least one type of blood cell
  • less than 5 percent of the cells in your bone marrow are abnormal immature cells called blasts (there are less than 1 percent or no blasts in your blood)

MDS-SLD is uncommon. Features of MDS-SLD include:

  • at least 10 percent of at least one type of immature blood cell in your bone marrow is abnormal
  • a low number of one or two types of blood cells (but not three)
  • abnormal immature cells make up less than 5 percent of cells in your bone marrow and very few in your blood

MDS-EB is classified by a higher-than-normal number of abnormal, immature cells called blasts. It makes up about one-quarter of cases of MDS. It’s further classified into:

  • MDS-EB1. This is when 5 to 9 percent of cells in the bone marrow or 2 to 4 percent of cells in the blood are abnormal and immature.
  • MDS-EB2. This is when 10 to 19 percent of cells in the bone marrow or 5 to 19 percent of cells in the blood are abnormal and immature.

MDS-EB is one of the more likely types of MDS to turn into AML.

MDS-RS characterized by many abnormal red blood cells called ring sideroblasts. Ring sideroblasts are immature red blood cells that contain extra iron around their nucleus.

Doctors diagnose an MDS as MDS-RS when at least 15 percent of immature red blood cells are sideroblasts, or at least 5 percent if they also have a mutation in the SF3B1 gene.

MDS-RS is further subdivided into two types:

  • MDS-RS with single lineage dysplasia (MDS-RS-SLD). This is when only one type of blood cell develops abnormally.
  • MDS-RS with multilineage dysplasia MDS-RS-MLD). This is when more than one type of blood cell develops abnormally. This type is more common.

Cells in the bone marrow of people with MDS 5q are missing part of chromosome 5. Cells may also show other genetic abnormalities that don’t include a partial or full loss of chromosome 7.

Other characterizations of MDS 5q include:

  • a low number of one or two types of blood cells, with red blood cells most commonly affected
  • an elevated number of at least one type of abnormal blood cell.

According to the American Cancer Society (ACS), this type is rare and most often occurs in older women. Prognosis is usually good, and MDS 5q rarely develops into AML.

An MDS is classified as MDS-U when it doesn’t fit into any other categories. MDS-U is rare, and its outlook still isn’t well understood.

MDS-U can be further subdivided into subtypes depending on its features:

  • MDS-U with 1 percent blood blasts (MDS-U BL)
  • MDS-U with SLD and pancytopenia (MDS-U Pan)
  • MDS-U based on a defining cytogenic abnormality (MDS-U CG)

The subtype MDS-U BL tends to have a poor prognosis while MDS-I Pan and CG tend to progress slowly.

MDSs are classified as primary MDSs when there’s no obvious cause, which is most common. If a specific cause is suspected, it’s referred to as secondary MDS.

According to the ACS, secondary MDS, although rarer, is much less likely to respond to treatment. Some of the potential causes of a secondary MDS include:

  • prior treatment with chemotherapy (treatment-related MDS)
  • smoking
  • exposure to high doses of radiation, like in people who survive atomic bomb explosions or nuclear reactor accidents
  • long-term exposure to the chemical benzene and other chemicals

The Revised International Prognostic Scoring System (R-IPSS) is a commonly used diagnostic tool for myelodysplastic syndromes. It helps doctors predict a person’s overall survival and risk of transformation to acute leukemia.

This scoring system projects a score of 1-5 based on the following:

  • a person’s genetic changes
  • the number of blasts in the bone marrow
  • the degree of cytopenias

The R-IPSS scoring system also plays a role in determining treatment options for those with myelodysplastic syndrome.

Every person’s situation is different based on overall health, age, MDS subtype, and response to treatment. It’s always best to discuss your individual outlook with your doctor or oncology team.

MDSs are a group of blood cancers characterized by the overproduction of abnormal, immature blood cells and a low count of healthy blood cells. MDSs are subdivided depending on the way bone marrow and blood cells appear under a microscope.

Understanding which type of MDS you have can help your doctor predict how quickly your condition will progress. Some types, like MDS 5q, usually have a good outlook and rarely turn into leukemia, while others like MDS-EB are more likely to turn into leukemia and have a more serious outlook.