Blast Crisis in Chronic Myeloid Leukemia

During the course of CML with or without therapy, regular monitoring of the differential smear is particularly important, since over periods of varying duration the relative proportions of blasts and promyelocytes increases noticeably. When the blast and promyelocyte fractions together make up 30%, and at the same time Hb has decreased to less than 10g/dl and the thrombocyte count is less than 100 000/^1, an incipient acute blast crisis must be assumed. this blast crisis is often accompanied or preceded by a markedly increased basophil count. Further blast expansion—usually largely recalcitrant to treatment—leads to a clinical picture not always clearly distinguishable from acute leukemia. If in the chronic phase the disease was "latent" and medical treatment was not sought, enlargement of the spleen, slight eosinophilia and basophilia, and the occasional presence of normoblasts, together with the overwhelming myeloblast fraction, are all signs indicating CML as the cause of the blast crisis.

As in AML, in two-thirds of cases cytological and immunological tests are able to identify the blasts as myeloid. In the remaining one-third of cases, the cells carry the same markers as cells in ALL. This is a sign of dedifferentiation. A final megakaryoblastic or a final erythremic crisis is extremely rare.

Bone marrow cytology is particularly indicated when clinical symptoms such as fatigue, fever, and painful bones suggest an acceleration of CML which is not yet manifest in the CBC. In such a case, bone marrow analysis will frequently show a much more marked shift to blasts and promyelocytes than the CBC. A proportion of more than 20% immature cell fractions is sufficient to diagnose a blast crisis.

The prominence of other cell series (erythropoiesis, thrombopoiesis) is reduced. The basophil count may be elevated.

A bone marrow aspiration may turn out to be empty (sicca) or scarcely yield any material. This suggests fibrosis of the bone marrow, which is frequently a complicating symptom of long-standing disease. Staining of the fibers will demonstrate this condition in the bone marrow histology.

In the course of chronic myeloid leukemia, an acute crisis may develop in which blasts predominate

Acute Leukemia With Blast Crisis

Fig. 41 Acute blast crisis in CML. a Myeloblasts (1) with somewhat atypical nuclear lobes. Basophilic granulocyte (2) and band granulocyte (3). Thrombocytopenia. The proliferation of basophilic granulocytes often precedes the blast crisis. b Myeloblasts in an acute CML blast crisis. Typical sand-like chromatin structure with nucleoli. A lymphocyte. c Bone marrow cytology in acute CML blast crisis: blasts of variable sizes around a hyperlobulated megakaryocyte (in this case during a lymphatic blast crisis).

Fig. 41 Acute blast crisis in CML. a Myeloblasts (1) with somewhat atypical nuclear lobes. Basophilic granulocyte (2) and band granulocyte (3). Thrombocytopenia. The proliferation of basophilic granulocytes often precedes the blast crisis. b Myeloblasts in an acute CML blast crisis. Typical sand-like chromatin structure with nucleoli. A lymphocyte. c Bone marrow cytology in acute CML blast crisis: blasts of variable sizes around a hyperlobulated megakaryocyte (in this case during a lymphatic blast crisis).

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    How to treat Blast Crisis in AML?
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