Epidemiology

​Mast cell activation syndrome (MCAS) is a rare condition but there are no reliable data available to estimate the incidence or prevalence among either adults or children in the general population. The highest prevalence of MCAS appears in patients who have both an IgE-dependent allergy and mastocytosis.[5]

The most common clinical presentation of MCAS is recurrent episodes of anaphylaxis. A working group of the American College of Allergy, Asthma and Immunology estimated a lifetime prevalence of anaphylaxis of between 0.05% and 2.00%, citing studies from North America, Europe, and Australia.[6]​ UK data have shown a substantial rise in hospital admissions for anaphylaxis over recent years.[7]​ Data from 10 years of a nationwide notification system for anaphylaxis in Portugal found that anaphylaxis recurred at least once in 41% of cases and 21% had ≥3 anaphylactic episodes.[8]​ One US study of 19,039 patients who had an initial episode of anaphylaxis found that 11% had a recurrent episode within the following 12 months.[9]

The prevalence of mastocytosis (an underlying cause of primary MCAS) has been estimated at around 1 per 10,000 in three population-based studies from Europe and the US.[10][11][12]​ There are scarce data available to estimate the proportion of people with mastocytosis who will also meet the three criteria for a concomitant diagnosis of MCAS. Among patients with systemic mastocytosis, the lifetime incidence of anaphylaxis has been reported to be as high as 50%, but not all of this group will experience recurrent episodes.

​​There is a large group of patients who experience less severe and/or more localised symptoms related to mast cell activation (e.g., pruritus, headache, flushing) that are clinically relevant but do not meet the diagnostic criterion for MCAS; in some cases, this is because they are already taking a prophylactic anti-mast cell mediator drug, such as an antihistamine. In addition, there is a wide range of other conditions with some overlapping symptoms that have been confused with MCAS by both clinicians and patients.[1]​ These two factors have resulted in erroneous overestimates that up to 17% of the general population have MCAS, but these studies have been criticised for including excessively broad and non-specific criteria to make a diagnosis of the condition.[13]

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