Chronic myelomonocytic leukaemia (CMML)
Chronic myelomonocytic leukaemia is a type of chronic leukaemia that develops slowly, often over many years. It affects a type of white blood cell called monocytes. There is a childhood form called juvenile myelomonocytic leukaemia (JMML), but this is extremely rare.
In CMML, patients have an elevated number of monocytes in the blood, with a monocyte count of at least 1,000 (per mm3). Often, the monocyte count is much higher, causing the total white blood cell count to become very high as well. Usually there are abnormal cells in the bone marrow, but the amount of blasts (very early/immature cells) is below 20%. Many patients also have enlarged spleens). About 15% to 30% of patients go on to develop acute myeloid leukaemia.
Because CMML patients have abnormal looking (dysplastic) cells in their bone marrow, for a long time CMML was considered to be a type of myelodysplastic syndrome (MDS), but it didn't fit in well with other diseases in that category. That is because the major problem in myelodysplastic syndrome is having too few blood cells. Patients with CMML may have shortages of some blood cells, but the main problem is too many of a certain type of white blood cell, too many monocytes.
The risk of CMML increases with age. This disease is rare in those younger than 40, with most cases found in people over 60. It is more common in men than women. Around 300 people are diagnosed with CMML in the UK every year.
Reviewed April 2013