What is Idiopathic Mast Cell Activation Syndrome?
Idiopathic Mast cell activation syndrome (MCAS) is one of several mast cell disorders. MCAS occurs when there are a normal number of mast cells in a person's body but they over-release mast cell mediators causing random allergic reactions in multiple systems of the body. MCAS is incredibly common being present in an estimated 17% of the population.
MCAS symptoms are incredibly varied and always occur in multiple systems of the body. Anaphylaxis is common.
[ID: A graphic labeled "Some common symptoms of Mast Cell Disease" A graphic of a person standing in the center with multiple organs visible is shown. Around the person are lines pointing to specific areas of the body labeled with the body system and symptoms. Clockwise these read "Neurological headache, brain fog, cognitive dysfunction, anxiety, depression Cutaneous (Skin) flushing of the face/neck/chest, hives, skin rashes, itching with or without rash Cardiovascular light-heartedness, syncope (fainting), rapid heart rate, chest pain, low blood pressure, high blood pressure at the start of a reaction, blood pressure instability Gynecological uterine cramps, bleeding Urinary bladder irritability, frequent voiding Systemic and/or organ specific Anaphylaxis angioedema (swelling) Skeletal bone/muscle pain, osteopenia, osteoporosis Gastrointestinal diarrhea, nausea, vomiting, abdominal pain, bloating, gastroesophageal reflux disease (GERD) Ear/Nose/Throat/Respiratory nasal itching and congestion, throat itching and swelling, wheezing, shortness of breath and more" In the bottom left corner "Symptoms can be sudden and unpredictable in onset learn more at tmsforacure.org"]
MCAS symptoms are specifically not allergies. the reactions may look like allergies but the two are not the same and MCAS is not a condition meaning "many allergies" While MCAS can have some consistent triggers one of the defining features of the disease is that reactions are random and happen unpredictably.
Anaphylactic shock is not a requirement for diagnosis.
MCAS is diagnosed by an immunologist. It is in part a diagnosis of exclusion and requires ruling out both allergies and systemic mastocytosis as well as other conditions such as certain types of tumors.
Diagnostic criteria for MCAS is debated. Some immunologists follow the symptom-based diagnosis approach in which case the diagnostic criteria are:
Recurring and severe anaphylactic-like episodes that involve more than one organ system
Positive response to mast cell stabilizing or mediator medications anaphylaxis-type symptoms
Others follow diagnostic criteria based on laboratory findings. In this case the diagnostic criteria are:
Episodic symptoms consistent with mast cell mediator release affecting two or more organ systems evidenced as follows:
Skin: urticaria, angioedema, flushing
Gastrointestinal: nausea, vomiting, diarrhea, abdominal cramping
Cardiovascular: hypotensive syncope or near syncope, tachycardia
Naso-ocular: conjunctival injection, pruritus, nasal stuffiness
A decrease in the frequency or severity; or resolution of symptoms with anti-mediator therapy: H1 and H2 histamine receptor antagonists, anti-leukotriene medications (cysLT receptor blockers or 5-LO inhibitor), or mast cell stabilizers (cromolyn sodium)
Evidence of an elevation in a validated urinary or serum marker of mast cell activation: Documentation of elevation of the marker above the patient’s baseline during a symptomatic period on at least two occasions; or if baseline tryptase levels are persistently >15ng, documentation of elevation of the tryptase above baseline on one occasion. Total serum tryptase is recommended as the markers of choice; less specific (also from basophils) 24 hour urine histamine metabolites, or 11-beta-prostaglandin F2.
Primary (clonal) and secondary disorders of mast cell activation ruled out.
These are not all proposed diagnostic criteria as the subject is heavily debated. Generally, a laboratory-confirmed MCAS diagnosis is considered more legitimate.
MCAS is a very treatable condition. Generally treatment follows a path from antihistamines -> mast cell mediators -> biologics.
Epipens are given to MCAS patients with a history of anaphylaxis.
Antihistamines are divided into 2 categories: H1 antagonists and H2 antagonists. These categories are determined based on the histamine receptor each one targets.
H1 antagonists mostly deal with systemic and cutaneous symptoms. H1 antagonists are also further divided into first and second generation antihistamines. first generation antihistamines include diphenhydramine (Benadryl) and Hydroxyzine. These tend to cause drowsiness. With second generation H1 antagonists cause fewer side effects and include drugs like loratadine (Claritin) and cetirizine (Zyrtec)
H2 antagonists primarily affect the gastrointestinal tract and include medications like famotidine (pepcid)
Typically when treating MCAS a person will be put on both a second generation H1 antagonist and an H2 antagonist.
When antihistamines do not treat symptoms well enough the next step is a mast cell mediator. The most common mast cell mediator is cromolyn sodium which is available by prescription only. (this is technically available OTC but it is at 1/50th the dose used for MCAS) Mast cell mediators work by preventing the degranulation of mast cells in the first place.
When both antihistamines and mast cell mediators are insufficient someone with MCAS might be prescribed a biologic such as Xolair to treat their remaining symptoms.
American Academy of Allergy, Asthma, and Immunology
Mast Cell Activation Syndrome: Proposed Diagnostic Criteria
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