Myelodysplasia (MDS) is often thought of as “pre-leukemia.” Which is kind of misleading, because only some cases of MDS go on to become leukemia; others stay the same and never progress.
In MDS, there is some sort of abnormality in a myeloid stem cell. If you look at the cells in the blood and bone marrow, you’ll see dysmyelopoiesis (the red cells, neutrophils and/or megakaryocytes look abnormal). The neutrophils in the blood often show dysgranulopoiesis (they lack specific granulation) or hypolobulation (there are only a couple segments to the nucleus – or sometimes no segments at all, just a single round nucleus). Check out the neutrophil in the photo above: the cytoplasm is almost clear (certainly not nice and fawn-pink like you’d expect for a segmented neutrophil), and the nucleus doesn’t have nice segments – just a couple poorly-defined lobes.
In addition to the dysmyelopoietic changes, most cases show a macrocytic anemia. This is an important diagnostic feature, and an easy one to remember. If you see an older patient with a macrocytic anemia, MDS is something you should keep in mind (in addition to the more common causes of a macrocytic anemia, such as B12/folate deficiency and ethanol ingestion).
There may be an increase in myeloblasts in the blood or bone marrow – but if so, it’s a small increase (and certainly not more than 20%, which is the cutoff for acute myeloid leukemia).
There are actually a bunch of different kinds of MDS, everything from refractory anemia (which is just what it sounds like: anemia that does not respond to therapy such as B12 or iron) to chronic myelomonocytic leukemia in transformation (which is a bad name, actually, because it’s a type of MDS, not technically a leukemia). You should not try to memorize these subtypes unless you plan to take your hematopathology boards soon (or unless you have a particularly sadistic pathology professor).
The treatment depends on the type of MDS. Lower-grade types (like refractory anemia) rarely transform into acute leukemia, so they are treated more conservatively. Higher-grade types (like chronic myelomonocytic leukemia in transformation) may be treated more aggressively, particularly if they occur in a younger person who can handle chemotherapy well.