What is MCAS?
Mast cells are immune cells that release chemical messengers called mediators, including histamine. These substances help the body respond to threats but can also produce allergic symptoms.
Mast cell activation syndrome (MCAS) involves recurrent episodes of excessive mediator release, typically causing symptoms in more than one body system at the same time. Symptoms may include hives, flushing, swelling, breathing difficulties, low blood pressure, abdominal cramping, nausea, and diarrhea.
How is it diagnosed?
Digestive symptoms or food reactions alone do not establish MCAS. Widely accepted diagnostic criteria require:
- Recurrent episodes consistent with systemic mast cell activation, usually involving at least two organ systems.
- Objective evidence of increased mast cell mediators during an episode, such as an appropriate rise in blood tryptase compared with the person’s baseline.
- Improvement with treatment that targets mast cells or their mediators.
An allergist or immunologist can assess symptoms, consider other explanations, and arrange appropriately timed tests. Improvement with an antihistamine alone is not enough to confirm MCAS.
MCAS, hEDS, and HSD
Some people have both MCAS and hEDS or HSD. However, current evidence does not justify saying that most people with hEDS have MCAS.
A 2025 systematic review found insufficient qualifying evidence to confirm the proposed relationship between mast cell activation disorders, EDS, and POTS. This does not prove that no relationship exists; further research using consistent diagnostic criteria is needed. Read the systematic review
Having hEDS or HSD should therefore not be used as a substitute for evaluating suspected MCAS.
How this relates to Diane
Diane lives with hEDS and has experienced significant digestive symptoms and reactions to some foods and medicines. Her episodes of nausea, cramping, and diarrhea overlap with symptoms that can occur in MCAS, but they can also have other causes.
She has also experienced a severe reaction after receiving a steroid medication, with throat swelling and difficulty breathing. That event is important to her allergy assessment, but it does not by itself establish MCAS.
We cannot conclude from her symptoms or improvement with dietary changes that MCAS is the explanation. We include this information as a possibility to discuss with her healthcare team, while distinguishing what we have observed from what has been medically confirmed.
Food reactions and treatment
If particular foods, drinks, or medicines appear to trigger symptoms, discuss the pattern with your healthcare team. Treatment may include avoiding identified triggers and using prescribed medicines.
Broad food restrictions can compromise nutrition, especially when eating is already difficult. A dietitian can help maintain adequate nourishment while suspected triggers are investigated. Discuss medication reactions with your prescriber rather than stopping or changing medicines independently.
For suspected anaphylaxis, use prescribed emergency epinephrine (adrenaline) immediately and call your local emergency number. Antihistamines do not replace epinephrine for anaphylaxis. NHS: anaphylaxis