Myelodysplastic syndrome (MDS) is a rare disease of the blood. Most of the time it develops in older patients (over 60 years old), but it can occur at any age and affect children as well. However, children get different types of MDS than adults.

MDS develops in the bone marrow, the soft, spongy center of the long bones that produces the three major blood cells:

white blood cells to fight infection, red blood cells that carry oxygen, and platelets that help blood clot and stop bleeding.
In the normal bone marrow, growth and development of blood cells is carefully controlled to produce the correct number of each type of blood cell. All blood cells (while blood cells, red blood cells and platelets) are made in the bone marrow from a single type of a cell called a stem cell. Stem cells make up a very small portion of the cells in the bone marrow. The stem cells are stored in the bone marrow until a specific type of cell is needed, for example a red cell. When more red cells are needed, the bone marrow activates the stem cells to rapidly produce more red cells. At that time, the stem cell matures and develops more and more into a red cell.

In MDS, this process of maturation from a stem cell into a mature cell is disturbed. Red and white blood cells may mature, but not normally or in insufficient numbers. Sometimes the number of immature blood cells, called blasts, increases. As the disease progresses, these blasts continue to increase and invade the bone marrow, preventing it from working effectively. The blood cells lose their ability to mature and function properly.

In the past, MDS was called “smoldering leukemia” or “pre-leukemia,” but only about one-third of cases of MDS actually progress to leukemia, a cancer of the blood and bone marrow.