Mast Cell Activation Syndrome (MCAS)
Irene Wilson, Hon. Chair of the UK Mastocytosis Support Group

A disorder or syndrome in which there is evidence of the systemic, inappropriate release of mast cell mediators. While people with MCAS sometimes have a near-normal tryptase level and a bone marrow biopsy that contains a normal number of mast cells, they experience most of the same symptoms as does someone with mastocytosis.

Patients with MCAS present with recurrent symptoms in a similar way to a patient with Mastocytosis although patients with MCAS lack Urticaria Pigmentosa.

Patients can present with recurrent episodes suggestive of allergic reactions but no consistent allergic cause has been identified. Symptoms can be triggered by multiple factors including food, exertion, environmental factors, stress, insect stings, heat and alcohol. Symptoms can include Urticaria, Angioedema, Flushing, Nausea, Vomiting, Diarrhoea, Abdominal Cramps, Tachycardia, Wheezing, Itching and Anaphylaxis.

Due to the range of symptoms patients can sometimes be diagnosed with Idiopathic Anaphylaxis and this has to be ruled out before a diagnosis of MCAS can be made.

There is no cure for MCAS and an individual’s treatment should target specific symptoms. The avoidance of triggers that have been identified and the use of H1 and H2 Histamine Receptor Antagonists, Leukotriene Inhibitors and Mast Cell Stabilisers can help.

It is a very complex syndrome that can present with unusual symptoms in multiple organ systems

Diagnosis and Treatment for MCAS is available under the NHS and from private specialists. Please contact our UK Mastocytosis Support Group for a list of Consultants able to diagnose and treat MCAS.

Mast Cell Activation Syndrome (MCAS)
Dr. Matthew Helbert Consultant Immunologist, Manchester Royal Infirmary

Mast cell activation syndrome (MCAS) is a recently defined condition which causes severe allergic-like symptoms. The symptoms of MCAS range from severe itching and swellings to full blown allergic reactions (anaphylaxis) which may be life threatening. Patients with MCAS can either develop symptoms spontaneously or after exposure to substances to which they are allergic (for example wasp stings).
Systemic Mastocytosis is a related condition which causes identical allergic symptoms and leads to similar test results as MCAS. However, Systemic Mastocytosis may behave as a type of malignancy and so must be distinguished from MCAS.

Mast cells are involved in allergy as well as our defences against cancer and infection. In MCAS, mast cells are either increased in number, over respond to stimulation or both. Little is known about why this happens in MCAS. For example we do not know 1) how it is related to Systemic Mastocytosis 2) what simple non invasive tests can be routinely used to diagnose MCAS and distinguish it from Systemic Mastocytosis and 3) which chemicals cause symptoms in MCAS and thus, how best is it treated.

Mast cell activation syndrome patients are routinely referred to adult allergy investigation services by GPs and hospital practitioners for investigation of symptoms thought to have an allergic basis. After investigating, about 1:500 of such patients are found to have symptoms suggestive of MCAS or Systemic Mastocytosis. Bone marrow (or less frequently other tissue) sampling is then recommended. Results of these will confirm or rule out MCAS. These patients are monitored under ongoing review (typically seen twice a year) because of the unpredictable nature of the syndrome and the possibility of the need for further investigation or treatment