Indolent systemic mastocytosis (SM) is the least aggressive form of the five SM subtypes recognized by the World Health Organization. The majority of patients with SM — about 90% — have indolent SMIndolent SM A subtype of nonadvanced SM caused by the abnormal accumulation of mast cells in the bone marrow and other organs. Indolent SM accounts for around 90% of SM cases..
What characterizes indolent SM?
If you have been diagnosed with indolent SM, you have a usually benign (i.e., non-aggressive) form of SM caused by the abnormal accumulation of mast cells in the bone marrow and other organs. Mast cells are immune cells that are released to fight allergic reactions and inflammation in the body.
Physicians now largely determine your SM subtype via specific criteria set by the World Health Organization. These criteria are highly technical and depend on specific laboratory findings — but in general, an indolent SM diagnosis means milder signs and symptoms of the disease. For example, one of the criteria for the diagnosis of indolent SM is the presence of clusters of abnormal mast cells in the bone marrow, but without evidence of more severe findings that would suggest a more aggressive course of SM.
Read more about SM testing and diagnosis
What is the clinical course of indolent SM?
Though it is the least aggressive form of SM, patients with indolent SM may still experience significant symptoms. One study found that more than two-thirds (70%) of patients reported some form of functional limitation. In addition, the most common symptoms of indolent SM — such as skin disease, pruritus (severe itching), food allergies and muscle pain — can have a significant psychological effect, as patients lose some of their ability to navigate their world with confidence. For this and many other reasons, potential triggers of SM symptoms should be carefully avoided.
What is the prognosis?
Though patients may struggle with symptoms, people with indolent SM generally have a life expectancy similar to that of individuals without the disease. A study in 2019 of 1,006 patients with indolent SM found that the median survival after a diagnosis was approximately 28.4 years. Indolent SM is a subtype that has a relatively slow and stable disease course; only 3% to 5% of cases progress to a more advanced form of the disease.
How is indolent SM treated?
The focus of indolent SM treatment is to ensure that the disease is under control as much as possible, to ensure that quality of life is not too heavily impacted.
In 2023, the tyrosine kinase inhibitorTyrosine kinase inhibitor Targeted therapies designed to block the KIT D816V mutation that drives most cases of SM. Includes avapritinib, midostaurin and imatinib. avapritinib became the first treatment approved by the U.S. Food and Drug Administration specifically for indolent SM.
Treatment for indolent SM largely depends on the symptoms present, but many treatments aim to minimize the risk of anaphylaxisAnaphylaxis A severe allergic reaction that can turn fatal without treatment. Patients with SM are at a higher risk of developing anaphylaxis.. Your doctor may prescribe you antihistaminesAntihistamines Medications that block the effects of histamine, the chemical found in mast cells that is responsible for many of the symptoms of SM, as well as many allergic reactions. to counter the effects of mast cellMast cell A type of white blood cell produced in the bone marrow. They help defend against infections and play a key role in allergic reactions. In SM, mast cells become overactive and build up throughout the body. histamineHistamine A chemical released by mast cells when they are activated. Histamine causes many of the allergy-type symptoms in mastocytosis, including flushing, itching and hives. release. Most patients are advised to carry an epinephrineEpinephrine Also called adrenaline, this hormone is used in emergency situations to reverse symptoms during a severe allergic reaction (anaphylaxis). auto-injector for on-demand use. Venom immunotherapy may be recommended to patients with severe reactions to bee and wasp stings.
Because indolent SM is typically mild with no presence of organ damage, patients don’t typically require the drugs prescribed with more aggressive forms of SM to treat organ damage.
Key takeaways to remember
In summary, indolent SM is generally considered the mildest subtype of SM. In some patients, symptoms may hardly be bothersome, if they indeed appear at all. However, it is important to comply with the therapies prescribed to ensure that the disease burden is kept to a minimum and that the impact on quality of life is low.
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