Mast cell activation syndrome (MCAS) and systemic mastocytosis (SM) are two conditions that share many similarities. Both involve the abnormal activity and influence of mast cells, and both are activated during allergic reactions. Yet MCAS and SM differ in key ways.
Both are mast cell diseases
MCAS and SM are both mast cell diseases, a group of diseases caused when mast cells are abnormal or overly active.
Mast cells are a form of white blood cell that have a protective role in the human body. They are part of the immune system, and mediate inflammatory responses and allergic reactions. In both SM and MCAS, these reactions are outsized.
Both share similar symptoms
In MCAS, mast cell overactivity can cause symptoms such as flushing, dizziness, a raised heart rate, abdominal pain and hives. Generally, mast cell overactivity can lead to symptoms similar to an allergic reaction.
The same symptoms are often seen in SM. This is why the disorders can be challenging to differentiate without thorough testing.
The key difference
The key difference between MCAS and SM is in the number of mast cells and how they act.
In MCAS, the number of mast cells is normal, but for reasons researchers don’t fully understand the mast cells are overactive. They release too many chemicals (called mediators), and may respond to triggers that the body normally doesn’t see as harmful.
In contrast, SM is characterized by abnormally high levels of mast cells. This abnormal increase in mast cells is commonly caused by a genetic mutation. These excess mast cells need to go somewhere, which often leads to mast cell infiltration of the organs. This can cause an enlarged liver or spleen, a manifestation not seen in MCAS.
How the two disorders are diagnosed
Thorough testing is crucial to identifying when symptoms are caused by a mast cell disease, and to differentiate between MCAS and SM. Diagnosis may not be simple: an example is blood work, which typically shows elevated baseline tryptase levels in SM but not in MCAS. However, tryptase levels in the blood can spike in MCAS during an attack, complicating diagnosis.
Another useful test is a bone marrow biopsy, which is when a sample of the bone marrow is extracted for analysis. In SM, a bone marrow biopsy typically shows a cluster of mast cells. Genetic testing may show the presence of KIT D816V mutation in SM.
And what about MCAS? This disorder is mostly diagnosed via the presence of the symptoms. In contrast to SM, a bone marrow biopsy is typically normal, and there is typically no evidence of organ damage in MCAS.
Read more about SM testing and diagnosis
Are treatments different?
Therapies in both MCAS and SM can be similar; doctors often first prescribe antihistamines and mast cell stabilizers and see how patients respond.
In SM, due to the specific genetic mutation mentioned earlier, doctors can also use KIT inhibitors such as midostaurin or avapritinib.
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