Juvenile myelomonocytic leukemia (JMML) is classified by the WHO as a mixed myelodysplasia and myeloproliferative disorder. It can be defined based on hematologic and cytologic findings.


Age of onset: Most cases occur in children < 3 years of age but can occur into early adolescence


Common clinical findings:

(1) hepatosplenomegaly

(2) lymphadenopathy

(3) pallor

(4) fever

(5) skin rash


General features:

(1) persistent monocytosis > 1,000 per µL in the peripheral blood

(2) < 20% blasts in blood or bone marrow

(3) 2 or more of the following:

(3a) hemoglobin F increased for age

(3b) immature granulocytes in the peripheral blood

(3c) clonal cytogenetic abnormality in the bone marrow

(3d) WBC count > 10,000 per µL

(3e) in vitro GM-CSF hypersensitivity of myeloid progenitors



• Blasts include myeloblasts, monoblasts and promonocytes.


Additional features:

(1) boys are affected about twice as frequently as girls

(2) about 10% of cases have clinical neurofibromatosis



(1) Philadelphia chromosome

(2) BCR/ABL fusion genes

(3) >= 20% blasts in blood or bone marrow (indicates AML)


NOTE: The combination of no circulating blasts, low marrow blast count, absolute monocytosis, leukocytosis and immature granulocytes in the blood could be seen in an infection. I would exclude other causes before accepting the diagnosis of JMML with this pattern of findings, perhaps also requiring persistence of findings for at least 3 months. Both of these are used in the diagnosis of CMML (above).


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