Mast Cell Activation Syndrome (MCAS) was first recognized in 1991 and was given a name in 2007. It is a complex condition, caused by the systemic and inappropriate release of mast cell mediators, with symptoms involving the skin, gastrointestinal, cardiovascular, respiratory and sometimes neurological systems.

Patients present with symptoms of mast cell degranulation as seen in Mastocytosis - this can include but is not limited to, urticaria, angioedema, flushing, nausea, vomiting, diarrhea, abdominal cramps, tachycardia, wheezing, itching and anaphylaxis - however they do not meet the WHO diagnostic criteria for systemic mastocytosis.

Symptoms can be triggered by multiple factors including food, exertion, environmental factors, stress, viruses, insect stings, temperature changes and alcohol.

MCAS patients may have an increased baseline serum tryptase level, but some have a normal or near normal tryptase. Tryptase may increase during symptomatic episodes but must be measured within 2 to 3 hours of an incident, and these rises do not occur in all patients. It may be possible to document increases in other mast cell mediators, such as prostaglandin D2 and metabolites of histamine in the urine.

The avoidance of known triggers plus the use of H1, H2 receptor antagonists, leukotrine inhibitors and mast cell stabilisers can help patients manage their condition, though not all patients tolerate these medications. Anaphylaxis is an emergency situation and should be treated with epinephrine.

Recent research has identified a group of people with symptoms of inappropriate mast cell activation concurrently with postural orthostatic tachycardia syndrome (POTS) and hypermobility consistent with a diagnosis of Ehler’s Danlos Syndrome (EDS). Some patients find that treating one of the three improves symptoms of the other conditions.

Research into MCAS is ongoing. There is more than one view on the optimal diagnostic criteria, testing, and treatments and the prevalence of the condition.

This presents a challenging environment to patients waiting for diagnosis and treatment, especially within the NHS. As of 2016, there are a limited but growing number of doctors with an interest in MCAS working both in the NHS and privately.