The MastAttack 107: The Layperson’s Guide to Understanding Mast Cell Diseases, Part 40

49. What is the relationship between FPIES and MCAS?

FPIES is food protein induced enterocolitis syndrome, a severe type of food allergy. It causes continuous vomiting and diarrhea upon ingestion of a trigger. FPIES reactions can cause dehydration and dangerous drop in blood pressure. I cannot emphasize enough that FPIES can be extremely serious and that the reactions can be life threatening if they are not managed properly.

FPIES almost exclusively affects children starting in infancy and resolves around the age of 5. The reasons for this are unknown. FPIES is a diagnosis of exclusion. There are no tests to identify FPIES.

An important point is that trigger avoidance is generally sufficient for management in children with FPIES. When the child is not being exposed to a trigger, they should not have lingering symptoms.

If a child with FPIES continues to have symptoms, the conventional thinking is often that there must be a trigger that has not yet been eliminated from their diet. In children with continuing symptoms, they frequently have more traditional allergy type symptoms than the profuse GI issues seen with FPIES exposures. This is where FPIES starts to overlap with MCAS. MCAS can cause the same reactions to foods seen in FPIES. MCAS can also cause daily symptoms even if food triggers are avoided. Increasingly, children who were initially diagnosed with FPIES are later diagnosed with MCAS.

There are a few possible scenarios here. Firstly, it is possible that the child has FPIES and has MCAS secondarily to the FPIES. It is also possible that the child was misdiagnosed with FPIES and had MCAS all along. It may also be that FPIES is some form of MCAS. They have a lot in common.

Because there is no test for FPIES, and it is very difficult to accurately perform mediator testing to look for mast cell disease in infants, it is hard to be definitive at that age anyway. In some cases, investigation of MCAS as a possible diagnosis for these children only occurs when they fail to “grow out of” FPIES around age 5. Having anaphylaxis also provides a clue towards MCAS as a potential diagnosis.

For more detailed reading, please visit these posts:

Food allergy series: FPIES (Part 1)

Food allergy series: FPIES (Part 2)

Food allergy series: Mast cell food reactions and the low histamine diet

The Provider Primers Series: Mast cell activation syndrome (MCAS)