Chronic myelomonocytic leukemia (CMML) is a rare type of blood cancer. Only about 1,100 people are diagnosed with CMML each year in the United States.

According to the Aplastic Anemia and MDS International Foundation, CMML is more common in men and older adults. About 90 percent of people who are diagnosed with CMML are at least 60 years old.

In this article, we’ll cover causes and risk factors for CMML, its symptoms, how it’s diagnosed, treatment options, and prognosis (outlook).

What Is Chronic Myelomonocytic Leukemia?

CMML is a type of leukemia that develops from monocytes. Monocytes are white blood cells that play a role in the immune system. Normal monocytes act as a lookout for germs and help coordinate other immune cells to fight infection.

People with CMML make high levels of monocytes that are immature. These abnormal monocytes don’t work correctly and crowd out other healthy blood cells.

CMML is not the same disease as chronic myeloid leukemia (CML). CMML and CML develop from different cells, are caused by different gene changes, and are treated with different therapies.

There are a couple of different types of CMML that come with different symptoms and lead to different outcomes.

Chronic Myelomonocytic Leukemia Subtypes

The World Health Organization (WHO) categorizes CMML into three different subtypes based on the number of blast cells (immature cells) in the blood and bone marrow:

• Type 0 CMML (CMML-0) — Less than 2 percent blasts in the blood and less than 5 percent blasts in the bone marrow
• Type 1 CMML (CMML-1) — 2 percent to 4 percent blasts in the blood and 5 percent to 9 percent blasts in the bone marrow
• Type 2 CMML (CMML-2) — 5 percent to 19 percent blasts in the blood and 10 percent to 19 percent blasts in the bone marrow

The way CMML is classified has changed in recent years. Some cases that weren’t included before may now be diagnosed as CMML. This includes oligomonocytic cases, which have lower monocyte levels. In some classification systems, these cases may now be included in a CMML diagnosis.

CMML can also be categorized in other ways. CMML is closely related to two other rare blood cancers:

• Myelodysplastic syndromes (MDS) — A group of conditions in which the body makes too many abnormal blood cells and too few healthy blood cells
• Myeloproliferative neoplasms (MPNs) — A group of disorders in which the body produces too many blood cells

CMML used to be considered a type of MDS. More recently, however, researchers have found that CMML cells are also similar to the cells seen in MPNs. Now, CMML has its own category that overlaps MDS and MPNs.

About half of the people with CMML have a dysplastic subtype that is similar to MDS, called MD-CMML. These people have low or normal white blood cell (WBC) levels when they are diagnosed, with a total count below 13 billion per liter.

The other half of people have a proliferative subtype that is more similar to an MPN, called MP-CMML. People with this subtype have high WBC levels, with a total count above 13 billion per liter.

Learning about your CMML subtype helps you know more about your prognosis and have better conversations with your doctor to determine which treatments may be most effective.

Causes of Chronic Myelomonocytic Leukemia

Blood cells are produced by special cells within bone marrow tissue. When these cells undergo multiple gene changes, they may turn cancerous. Cancer cells grow and divide more quickly than healthy cells.

Risk Factors for Developing Chronic Myelomonocytic Leukemia

Certain risk factors may increase a person’s chances of developing cancer-causing gene changes. These include:

• Older age, especially over 60
• Male sex
• Past exposure to certain chemicals or radiation
• Previous cancer treatment, such as chemotherapy or radiation therapy

CMML can result when bone marrow cells develop certain gene mutations (changes). Genes that are sometimes mutated in CMML include:

• TET2
• RAS
• ASXL1
• SRSF2
• RAS pathway

Genes are found on long pieces of DNA called chromosomes. About 20 percent to 30 percent of people with CMML also have changes that affect entire chromosomes. Part of a chromosome may be deleted, or extra copies of a chromosome may be made. These chromosomal abnormalities may play a role in the recommended treatment of the disease and its long-term outlook.

Chronic Myelomonocytic Leukemia Symptoms

Different subtypes of CMML may cause different symptoms. People with MD-CMML usually have anemia (low levels of red blood cells), thrombocytopenia (low levels of platelets), and leukopenia (low levels of white blood cells).

Not having enough healthy blood cells can lead to several symptoms for people with MD-CMML:

• Feelings of weakness or tiredness
• Petechiae (tiny sites of bleeding in the skin)
• Easy bruising
• Bleeding problems
• Frequent infections

People with MP-CMML often have high levels of blood cells, leading to a different set of symptoms:

• Splenomegaly (enlarged spleen)
• Enlarged liver
• A feeling of fullness after not eating very much
• Night sweats
• Bone pain
• Fatigue (extreme tiredness)
• Weight loss

In addition, both subtypes of CMML can affect the kidneys, causing complications ranging from urinary tract obstructions to kidney failure. Kidney issues can cause additional symptoms.

Diagnosis of Chronic Myelomonocytic Leukemia

Doctors diagnose CMML by studying cells in the blood and bone marrow.

Bone marrow samples are obtained through a bone marrow aspiration and biopsy. During this procedure, a doctor will remove a small sample of cells and fluid from the bone.

Doctors use blood and bone marrow samples to measure levels of monocytes, blasts, and other blood cells. They may also run tests to look for certain gene changes that help rule out other similar types of leukemia.

The criteria to be diagnosed with CMML include:

• Persistent blood monocytosis, with an absolute monocyte count of at least 0.5 billion per liter and a relative monocyte level of at least 10 percent
• No sign of the Philadelphia chromosome, a gene change found in CML, or other gene rearrangements linked to myeloid/lymphoid neoplasms with eosinophilia, including PDGFRA, PDGFRB, FGFR1, or JAK2
• Abnormalities in immature blood cells
• Fewer than 20 percent blast cells in the blood and bone marrow, with more than 20 percent possibly meaning acute leukemia

Having abnormal blood cell levels on one single test is not enough to be diagnosed with CMML. You need to have unusual test results multiple times. This is because other diseases can cause high or low blood cell levels.

For example, low vitamin levels can cause reduced blood cell counts, and infection can cause elevated white blood cell counts. Seeing unusual blood cell levels over a longer period of time helps rule out these other conditions.

Your doctor may also use your blood or bone marrow samples for cytogenetic or molecular tests. These tests can identify changes in chromosomes or genes. Genetic tests help doctors identify the type of leukemia and recommend effective treatments.

Chronic Myelomonocytic Leukemia Treatments

When recommending a treatment plan, your doctor will consider many different factors. These can include your health, which symptoms you’re experiencing, and how quickly your CMML is progressing.

Bone Marrow Transplant

A bone marrow transplant from a donor is the only possible cure for CMML. It’s the only treatment that may make leukemia go away and keep it from returning.

People who choose this treatment option will receive a high dose of chemotherapy, radiation, or both, which kills off leukemic cells as well as normal blood cells. After this treatment, people receive an infusion of blood stem cells from a donor. The donated cells will travel to the bone marrow and make new, healthy blood cells.

Only a portion of people with CMML will undergo a bone marrow transplant. This treatment has many serious or even life-threatening risks. Bone marrow transplantation is generally only an option for people who:

• Are younger
• Are in good health
• Have more advanced leukemia
• Have a matched donor
• Have leukemia that isn’t responding to other treatments

Chemotherapy

Sometimes, chemotherapy drugs used to treat other types of blood cancers are given to people who have CMML. Chemotherapy drugs don’t cure CMML. However, they may normalize blood cell counts, which may help reduce symptoms.

Treatments for the Spleen

CMML may make the spleen grow very large. People who have symptoms from an enlarged spleen may be able to shrink it through radiation therapy.

Alternatively, the spleen may be surgically removed. These treatments are not common for people with CMML.

Supportive Care

The goal of supportive care is not to cure leukemia but rather to reduce symptoms and improve quality of life. Supportive care is an important part of CMML treatment.

Many people with CMML have low blood counts. One possible treatment for this is a blood transfusion, in which a person with CMML receives blood cells from a donor.

Another supportive care option for people with low blood cell counts is growth factors. These molecules encourage the body to produce new blood cells.

Clinical Trials

CMML treatment is evolving. If you’re interested in learning about clinical trials for new ways to treat CMML or help manage symptoms, talk to your hematologist/oncologist.

Chronic Myelomonocytic Leukemia Prognosis

Survival after a CMML diagnosis can differ from person to person. People in the highest-risk group may live about 16 months, while people in the lowest-risk group may live about 8 years. This is why general survival rates are not always very helpful.

Your individual outlook for CMML depends on many factors, including:

• Your age
• Whether you have other health conditions
• Your blood cell counts
• Which genetic changes your CMML cells have
• The percentage of blasts in the blood and bone marrow
• Blood levels of certain chemicals

Your CMML subtype also plays a big role in your outlook. People with dysplastic CMML (MD-CMML) generally have a more positive outlook than people with the proliferative subtype (MP-CMML). About 15 percent to 30 percent of people with CMML later develop acute myeloid leukemia (AML).

Your blood cancer specialist can help you understand your prognosis based on your overall health and individual risk factors.

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