MCAS and histamine intolerance

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MCAS stands for mast cell activation syndrome. Some MCAS specialists believe that overactive mast cells are responsible for a wide range of health problems. While MCAS may be difficult to diagnose through medical testing, some patients are able to reduce their symptoms through a trial and error process of trying different foods and medications.

Some long haulers report that a low histamine diet and/or MCAS treatments improve their symptoms.

Low histamine diet

Histamines can cause reactions that mimic allergic reactions such as:

  • Itchiness
  • Redness with flushing
  • Rash/eczema
  • Hives / urticaria (red, itchy welts)
  • Dermatographia (you can 'draw' on your skin)
  • Rosacea (blushing or flushing and visible blood vessels in your face)
  • Psoriasis (red, itchy scaly patches)

Histamines can also cause symptoms that are common in long haulers: brain fog, insomnia, anxiety, fatigue, etc.

According to Tina Peers, histamine intolerance should be diagnosed if the patient has two or more symptoms and see improvement when eating a low-histamine diet for 4 weeks while taking antihistamines. Because the diet can be tricky to implement in the beginning, she suggests consulting a nutritionist or dietician familiar with the condition.

A low histamine food list can be found here: https://www.histaminintoleranz.ch/downloads/SIGHI-Leaflet_HistamineEliminationDiet.pdf

For those with MCAS, the same group (SIGHI) has compiled a more detailed (and complicated) food compatibility list: https://www.mastzellaktivierung.info/downloads/foodlist/21_FoodList_EN_alphabetic_withCateg.pdf

Medications

Antihistamine drugs do not lower histamine levels but they block the receptors for histamines. "Blocker" drugs can block histamines from attaching to H1 and H2 receptors. Both H1 and H2 can be blocked to reduce symptoms.

Over-the-counter antihistamines include:

  • Loratadine (Claritin), fexofenadine (Allegra), cetirizine (Zyrtec, Benadryl Allergy One a Day Relief 🇬🇧), levocetirizine (Xyzal) and acrivastine (Benadryl Allergy Relief 🇬🇧) are H1 blockers that are sold over-the-counter in most countries. Some of the drugs require a prescription in some countries, e.g. fexofenadine in the UK.
  • Famotidine (Pepcid) is a H2 blocker that is sold over-the-counter.

Some patients find that not all H1/H2 blockers improve their symptoms so trial and error is needed to find a blocker that they respond to.

Many other drugs (mostly prescription) are used to treat MCAS. This includes drugs such as:

  • Prescription H1 and H2 blockers
  • Mast cell stabilizers
  • Benzodiazepines. These are controlled substances that can lead to serious drug dependency/withdrawal problems if used for long periods of time. Your doctor may not necessarily provide you with informed consent regarding the difficulties some patients face trying to get off these drugs. Because these are controlled substances, refilling a prescription on short notice can be difficult so you should be diligent in refilling your prescriptions to avoid accidental withdrawal.
  • Low dose naltrexone (LDN)

MCAS specialists and doctors who educate themselves regarding MCAS (e.g. to treat long haul patients) would be able to guide you through such drugs and be able to provide you with a prescription.

MCAS specialists

The patient support organization The Mast Cell Disease Society has a list of medical centers treating MCAS at https://tmsforacure.org/resources/finding-a-physician/

MCAS specialists can also perform various medical tests to diagnose MCAS. However:

  • The tests often return false positives, e.g. when the patient isn't in the middle of a flare.
  • Medical tests do not predict whether or not a particular treatment will alleviate symptoms.

Guides and resources for HIT and MCAS

MCAS resources and introductory guide written by a MCAS sufferer at legalnomads.com.

Tina Peers has a histamine intolerance factsheet

Food lists:

DrBeen and other Youtubers have online discussions with MCAS specialists and/or discussions about MCAS in long haulers.

A MCAS diagnosis and treatment guideline by Lawrence Afrin and his colleagues can be found at https://doi.org/10.3109/07853890.2016.1161231

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