There are two non-advanced forms of systemic mastocytosis (SM): indolent SM and smoldering SM. Of the two, smoldering SM is the more severe.
What characterizes smoldering SM?
Smoldering SM is a slowly progressive subtype of SM. It is typically characterized by the buildup of mast cells in various organs.
Mast cell burden is a term used to describe an abnormally high proportion of mast cells in your body. Compared with indolent SM, smoldering SM has a higher mast cell burden. These excess mast cells can impact your health in many ways, such as leading to an enlarged spleen and skin lesions that may appear as red and itchy spots. However, unlike advanced SM subtypes, smoldering SM doesn’t usually mean organ damage.
There is no single test result that determines if you have smoldering SM. Instead, your symptoms and results from several tests must match at least two of those listed by the World Health Organization as markers of the disease — but at the same time, your results and symptoms can’t fall under categories that would indicate advanced SM.
In general, the results that indicate smoldering SM are things that accompany a high mast cell burden (such as an enlarged liver) but don’t take a dangerous toll (such as the liver’s ability to function being impacted).
Read more about SM testing and diagnosis
How does smoldering SM progress?
Studies have found that the risk of smoldering SM progressing is relatively mild; many patients are stable for years. One study analyzing disease progression in SM found that only 9% of patients with smoldering SM had developed a more advanced form of the disease during follow-up. Progression to a more severe disease form depends on a number of factors, including the extent to which the bone marrow is involved.
What is the prognosis of smoldering SM?
Similar to its disease burden, the prognosis of smoldering SM is typically worse than the prognosis for indolent SM, but better than the more aggressive subtypes. One study found that patients lived an average of 10 years after diagnosis, but this finding comes with a caveat: patients are typically diagnosed with smoldering SM at an older age, meaning they inherently have fewer years ahead of them than patients with subtypes diagnosed at younger ages. (The same study found the average age at diagnosis was 64.)
How is smoldering SM treated?
Treatment for smoldering SM depends on a patient’s specific symptoms. Medications such as antihistamines and mast cell stabilizers are often prescribed to help alleviate some of the more common symptoms associated with this disorder.
In addition, more potent medications such as midostaurin and cladribine may be prescribed to reduce the mast cell burden and further reduce symptoms. Midostaurin is a type of cancer drug known as a tyrosine kinase inhibitor and has been shown to alleviate symptoms and improve quality of life in patients with smoldering SM. Cladribine is a first-line drug in smoldering SM and is known for its efficacy in alleviating the mast cell burden and improving outcomes.
Key takeaways to remember
The smoldering subtype of SM is best described as being in the middle between the less-severe subtype indolent SM, and the more aggressive subtypes of SM. The disease course is often relatively mild and predictable, but it may develop into a more aggressive SM subtype. Following the treatments prescribed by your doctor, living a healthy lifestyle and seeking prompt care for new concerns can all help keep the disease stable.
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