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Mast cell activation syndrome
Other Resources UpToDate PubMed

Mast cell activation syndrome

Contributors: Siddhanth Hegde, Susan Burgin MD
Other Resources UpToDate PubMed


Mast cell activation syndrome (MCAS) is a group of rare disorders that presents with symptoms of excessive chronic mast cell activation. MCAS can be primary, secondary, or idiopathic. In primary MCAS, a clonal population of mast cells is present. Patients may fulfill the diagnostic criteria for systemic mastocytosis or, in cases of a low mast cell burden, they may not (monoclonal mast cell activation syndrome). Secondary MCAS is most commonly caused by immunoglobulin E (IgE)-mediated allergic disease but is sometimes caused by non-IgE-mediated causes such as drugs, venoms, or other inflammatory disorders.

The underlying pathophysiology of MCAS involves excessive systemic activation of mast cells, leading to degranulation and release of vasoactive mediators such as histamine and tryptase. These mediators cause an array of symptoms and signs including flushing, pruritus, urticaria and angioedema, nasal congestion and pruritus, bronchoconstriction and throat swelling, headache, and diarrhea. A severe complication of MCAS is systemic anaphylaxis. The disease is often episodic in nature, with symptoms waxing and waning over time. MCAS is most commonly found in White females. Symptoms often start in childhood and continue into adulthood if left untreated.


D89.40 – Mast cell activation, unspecified

16697871000119100 – Mast cell activation syndrome

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Diagnostic Pearls

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Differential Diagnosis & Pitfalls

Differential diagnosis of flushing (the most common causes of flushing):
  • Benign cutaneous flushing – Caused by emotion, temperature, food, or beverages
  • Fever
  • Alcohol – May cause flushing directly or via its metabolite acetaldehyde. More common among patients of Asian descent that may have aldehyde dehydrogenase-2 deficiency. Certain drugs combined with alcohol may cause flushing.
  • Menopause – Can be provoked by warmth, hot drinks, alcohol, and mental stress.
  • Rosacea
  • Cluster headache
Uncommon, serious, and malignant causes:
Neurologic causes:
Other causes:

Best Tests

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Management Pearls

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Drug Reaction Data

Below is a list of drugs with literature evidence indicating an adverse association with this diagnosis. The list is continually updated through ongoing research and new medication approvals. Click on Citations to sort by number of citations or click on Medication to sort the medications alphabetically.

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Last Reviewed:01/31/2022
Last Updated:02/06/2022
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Mast cell activation syndrome
A medical illustration showing key findings of Mast cell activation syndrome : Abdominal pain, Diarrhea, Flushing, Nasal congestion, Dyspnea, Urticaria, Wheezing, Abdominal bloating
Copyright © 2023 VisualDx®. All rights reserved.