Myelodysplasia is an acquired hematopoietic stem cell disorder of dogs characterized by clonal proliferation of erythroid myeloblasts from bone marrow.
The disease, which presents clinically as a normocytic, normochromic non-regenerative anemia, may be a primary disorder, or occur as a secondary phenomenon due to other underlying disease(s) processes.
In humans, karyotyping and molecular genetic analysis are used to diagnose this condition but in veterinary medicine, such techniques are rarely available, therefore a less accurate method is employed.
Specifically, dogs with less than 30% of cells present on histopathological examination of bone marrow samples are myeloblasts, this is termed myelodysplasia. In cases where > 30% of bone marrow cells are myeloblasts, this is usually categorized as leukemia.
There are three loose categories of myelodysplasia recognized in dogs:
- Myelodysplastic syndrome with refractory cytopenia (<6% myeloblasts) - anemia, no pancytopenia, usually secondary to other diseases
- Myelodysplastic syndrome with excess myeloblasts (6 - 30% myeloblasts) - anemia, leukopenia, thrombocytopenia, usually primary
- Myelodysplastic syndrome with sideroblastic differentiation (6 - 30% myeloblasts)
When myeloblasts and monoblasts together constitute more than 30% of all nucleated cells in the bone marrow, and differentiated granulocytes and monocytes each comprise more than 20% of all nucleated cells, the definition is acute myelomonocytic leukemia.
Cell morphology alone cannot be used to distinguish primary and secondary myelodysplastic syndromes in dogs.
Secondary conditions associated with myelodysplasia include:
- Immune-mediated thrombocytopenia
- Immune-mediated hemolytic anemia
- Multiple myeloma following melphalan administration
- Pyometra following estrogen administration
- Polycythemia vera
Dogs with refractory cytopenias tend to be clinically stable when first evaluated, have a better response to erythropoietin, blood transfusions, prednisolone and vitamin supplementation therapy and longer survival times, compared with dogs with primary myelodysplasia with excess myeloblasts which have short survival and do not respond to standard treatments.
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