Causes

In a healthy person, bone marrow makes new, immature blood cells that mature over time. Myelodysplastic syndromes occur when something disrupts this process so that the blood cells don't mature.

Instead of developing normally, the blood cells die in the bone marrow or just after entering the bloodstream. Over time, there are more immature, defective cells than healthy ones, leading to problems such as fatigue caused by too few healthy red blood cells (anemia), infections caused by too few healthy white blood cells (leukopenia) and bleeding caused by too few blood-clotting platelets (thrombocytopenia).

Most myelodysplastic syndromes have no known cause. Others are caused by exposure to cancer treatments, such as chemotherapy and radiation, or to toxic chemicals, such as benzene.

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:

• Myelodysplastic syndromes with single-lineage dysplasia. One blood cell type — white blood cells, red blood cells or platelets — is low in number and appears abnormal under the microscope.
• Myelodysplastic syndromes with multilineage dysplasia. In this subtype, two or three blood cell types are abnormal.
• Myelodysplastic syndromes with ring sideroblasts. This subtype involves a low number of one or more blood cell types. A characteristic feature is that existing red blood cells in the bone marrow contain rings of excess iron.
• Myelodysplastic syndromes with isolated del(5q) chromosome abnormality. People with this subtype have low numbers of red blood cells, and the cells have a specific mutation in their DNA.
• Myelodysplastic syndromes with excess blasts. In this subtype, any of the three types of blood cells — red blood cells, white blood cells or platelets — might be low and appear abnormal under a microscope. Very immature blood cells (blasts) are found in the blood and bone marrow.
• Myelodysplastic syndromes, unclassifiable. In this subtype, there are reduced numbers of one or more types of mature blood cells and the cells might look abnormal under the microscope. Sometimes the blood cells appear normal, but analysis might find that the cells have DNA changes that are associated with myelodysplastic syndromes.