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
where:
• 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
Exclusions:
(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).
