Medically reviewed by Marla Anderson, MD
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- Indolent systemic mastocytosis (ISM) is the most common form of systemic mastocytosis.
- Though symptoms can significantly affect daily life, most people with ISM have a normal life expectancy.
- Only a small number of people with ISM see the disease progress to a more serious, life-threatening form.
Getting an indolent systemic mastocytosis (ISM) diagnosis can bring up a lot of fear—especially about what it means for your future. Though ISM is a very rare disease, treatments have improved significantly in recent years, and most people with the condition have a positive prognosis.
Does Indolent Systemic Mastocytosis Shorten Your Lifespan?
The usual prognosis for ISM is a reassuring one—most research suggests that people usually have positive outcomes and normal lifespans.
ISM is the most common and least aggressive form of systemic mastocytosis (SM). SM is the umbrella term for a number of conditions characterized by an overproduction of immune cells called mast cells, which build up in organs and bone marrow (tissue inside the bones). The vast majority of SM cases, including ISM, are caused by a genetic mutation.
ISM is "indolent," or slow-moving, so progression to a more serious, advanced form of the disease is possible, but very uncommon.
When Can ISM Turn Into Something More Serious?
For most people with ISM, the focus is on feeling better day-to-day, rather than fighting a fast-moving disease. However, ISM can sometimes progress to advanced (SM), which is linked with organ dysfunction and a shorter lifespan. In one study, people who had their condition progress to advanced SM had an average life expectancy of 5 years.
However, the large majority of people with ISM do not develop advanced SM. In a large study, about 4.9% of people with ISM saw their disease progress to some degree, and about 2.9% advanced to a more aggressive form of SM. Other research has placed the risk of progression as under 3%.
Certain specific genetic changes and immune system markers may be linked to a higher risk of ISM progression. Your hematologist—a doctor who specializes in blood conditions—and the rest of your care team may help you assess whether you might be at a higher risk.
How Does ISM Affect Your Body?
While most people with ISM have normal life expectancies, living with the disease means managing ongoing symptoms that can affect quality of life.
Symptoms affect a number of different systems across the body and commonly include:
- Skin flushing
- Skin itching
- Stomach pain
- Diarrhea
- Fatigue
- Congestion
- Brain fog
ISM also carries the risk of more serious symptoms too, such as:
- Anaphylaxis: People with ISM have too many mast cells—these immune system cells, when triggered, release histamines and other chemicals that cause anaphylaxis, or allergic reactions. Anaphylaxis can sometimes be severe and life-threatening, particularly when swelling closes a person's airway or makes breathing more difficult. Researchers estimate 22–49% of people with SM develop anaphylaxis.
- Bone pain: Bone marrow (the tissue inside your bones) is most commonly affected by ISM. That can lead to frequent bone pain or bone loss, called osteoporosis. This may raise the risk of bone fractures.
Many people with ISM report that their symptoms make it hard to keep up with daily activities or maintain regular work.
How Is ISM Treated and Managed?
To manage ISM, the goal is to stop the body's mast cells from releasing high amounts of symptom-causing chemicals. That usually involves avoiding any triggers that cause you to have an allergic reaction, in addition to using certain medications, such as:
- Antihistamines: These widely-available drugs are usually the first step for ISM management. People with the condition may be encouraged to take H1 antihistamines—such as loratadine (Claritin) and fexofenadine (Allegra)—alongside H2 antihistamines like famotidine (Pepcid).
- Epinephrine auto-injector (EpiPen): Because people with ISM are at a high risk of anaphylaxis, they should carry one (or maybe even two) EpiPens with them in case they experience an allergic reaction.
- Targeted treatments: New treatments are being approved that directly target the underlying gene change that drives many ISM symptoms. Medications like avapritinib (Ayvakit) and midostaurin (Rydapt) have been shown to reduce symptom severity for people with ISM.
Because ISM can affect so many parts of the body, care often involves a team of specialists—including blood doctors, allergy specialists, and digestive health doctors—working together.
Next Steps
Your healthcare provider is the best source of information about your individual ISM prognosis. If you've been recently diagnosed, have honest conversations with your care team so you can better understand what to expect in the long term with ISM.
Avoiding triggers, monitoring bone health, carrying an EpiPen, and taking any prescribed medications are crucial ways to manage ISM. It's also important to get support from others—the Mast Cell Disease Society and similar websites offer ways to connect with others who have ISM, as well as written resources for patients and loved ones.
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