SM Basics

Systemic mastocytosis (SM) is a rare hematological disease characterized by mast cells—specialized cells of the immune system that mediate inflammatory responses and allergic reactions—that are overactive and accumulate in different parts of the body such as the bone marrow, liver, spleen, gastrointestinal tract and lymph nodes.

The disease can affect people of any age but is more common among adults, with the average age at diagnosis being 60.  

History of SM

The first report of a mast cell disorder was a two-year-old patient with urticaria pigmentosa, a skin condition characterized by discolored lesions and itchy skin.

The association between urticaria pigmentosa and mast cells was identified in 1887 by Unna, following their discovery by Paul Ehrlich in 1879.

The first reported case of SM was in 1949 following an autopsy in a one-year-old patient who died due to mast cell infiltration.

How common is SM?

SM is a rare disease that is thought to affect one in every 10,000 to 20,000 people worldwide.

Types of SM

SM can be classified into five different subtypes. These are indolent SM, smoldering SM, aggressive SM, SM with an associated hematologic neoplasm and mast cell leukemia.

The most common type of SM is indolent SM, seen in around 90% of adults with the disease. Patients with this subtype usually have no other illness and have a normal life expectancy. The risk of having their disease progress to more severe types is also low, at around 3% to 5%. 

Patients with smoldering SM have reduced life expectancy compared to those with the indolent form of the disease. This means the risk of their disease progressing to more severe types is higher compared to patients with indolent SM.

Patients with aggressive SM may have enlarged liver and lymph nodes, anemia and low platelet counts due to mast cell infiltration. This type of disease is often associated with a loss of bone tissue. leading to a greater likelihood of bone fractures. 

SM with an associated hematologic neoplasm affects around one in five of all patients. Patients usually experience other problems due to associated blood malignancy, such as myeloproliferative and myelodysplastic disorders. 

The rarest subtype of SM is mast cell leukemia. Affected people have a shortened life expectancy, due to organ damage.

Complications of SM

SM can lead to many complications affecting different organs and systems in the body. 

For example, it can lead to anaphylactic reactions that can cause rapid heartbeat, fainting, loss of consciousness and shock.

It can also cause blood disorders, such as anemia and reduced blood clotting. In severe cases, SM may lead to chronic leukemia.

Another complication caused by SM is peptic ulcer disease, which is characterized by chronic stomach irritation that can lead to ulcers and bleeding in the digestive tract.

Because SM can affect the bones and bone marrow, it can cause reduced bone density and increase the risk of bone problems like osteoporosis.

Finally, SM can lead to organ damage and organ failure due to the accumulation of mast cells.