Mast cell activation syndrome

What is mast cell activation syndrome?

Mast cells are immune system cells found in the skin, the digestive tract, the airways and around blood vessels. They play a role in the body’s defences and in allergic reactions, by releasing “mediators” such as histamine or tryptase.

In mast cell activation syndrome (MCAS), these cells release their mediators excessively or inappropriately, in repeated episodes, with symptoms in at least two body systems (skin, digestion, heart and blood vessels, breathing).

It should not be confused with mastocytosis, a rare disease in which too many mast cells are produced. MCAS can be linked to mastocytosis or an allergy, or have no identified cause.

Three criteria to meet

According to the international consensus criteria, mast cell activation syndrome is diagnosed when three conditions are met:

1. repeated episodes affecting at least two body systems;
2. a rise in a mediator measured during a flare-up, especially blood tryptase taken in the following hours, compared with its usual level: a rise is considered significant when it exceeds 20% of the usual level + 2 ng/mL;
3. an improvement with treatment targeting mast cells or their mediators.

The doctor also looks for mastocytosis or another cause that could explain the symptoms.

EDS, dysautonomia and mast cell activation

A link between hypermobile EDS, postural orthostatic tachycardia syndrome (POTS) and mast cell activation is often described by patients and by some doctors.

Studies on this subject remain limited and their results are debated: this link has not been proven to date. As the symptoms can be similar (fainting, digestive problems, fatigue), a thorough assessment helps tell them apart.