Mast cell activation syndrome
Mast cell activation syndrome (MCAS) is a type of mast cell activation disorder (MCAD), and is an immunological condition in which mast cells inappropriately and excessively release chemical mediators, resulting in a range of chronic symptoms, sometimes including anaphylaxis or near-anaphylaxis attacks. Primary symptoms include cardiovascular, dermatological, gastrointestinal, neurological and respiratory problems.Multiple diagnostic schemes for MCAS have been proposed, and the condition is increasingly over-diagnosed or misdiagnosed.

Signs and symptoms
MCAS is a condition that affects multiple systems, generally in an inflammatory manner. Symptoms typically wax and wane over time, varying in severity and duration. Many signs and symptoms are the same as those for mastocytosis, because both conditions result in too many mediators released by mast cells. It has many overlapping characteristics with recurrent idiopathic anaphylaxis, although there are distinguishing symptoms, specifically hives and angioedema.Common symptoms include:
Dermatological
flushing
hives
easy bruising
either a reddish or a pale complexion
itchiness
burning feeling
dermatographism
Cardiovascular
lightheadedness, dizziness, presyncope, syncope, arrhythmia, tachycardia
Gastrointestinal
diarrhea and/or constipation, cramping, intestinal discomfort
nausea, vomiting, acid reflux
swallowing difficulty, throat tightness
Respiratory
congestion, coughing, wheezing
Anaphylaxis If too many mediators are released into a patients system, they may also experience anaphylaxis, which primarily includes: difficulty breathing, itchy hives, flushing or pale skin, feeling of warmth, weak and rapid pulse, nausea, vomiting, diarrhea, dizziness and fainting.

Causes
There are no known causes, but the condition appears to be inherited in some patients. Symptoms of MCAS are caused by excessive chemical mediators inappropriately released by mast cells. Mediators include leukotrienes, histamines, prostaglandin, and tryptase.  The condition may be mild until exacerbated by stressful life events, or symptoms may develop and slowly trend worse with time.

Pathophysiology
Mast cell activation can be localized or systemic. MCAS can present with a wide range of symptoms in multiple body systems, these symptoms may range from digestive discomfort to chronic pain, mental issues as well as an anaphylactic reaction. Some examples of  tissue specific consequences of mast cell activation include urticaria, allergic rhinitis, and wheezing. Systemic mast cell activation presents with symptoms involving two or more organ systems (skin: urticaria, angioedema, and flushing; gastrointestinal: nausea, vomiting, diarrhea, and abdominal cramping; cardiovascular: hypotensive syncope or near syncope and tachycardia; respiratory: wheezing; naso-ocular: conjunctival injection, pruritus, and nasal stuffiness). This can result from the release of mediators from a specific site, such as the skin or mucosal tissue, or activation of mast cells around the vasculature.

Diagnosis
MCAS is often difficult to identify due to the heterogeneity of symptoms and the "lack of flagrant acute presentation". Many of the numerous symptoms are non-specific in nature.   Mast cell activation was assigned an ICD-10 code (D89.40, along with subtype codes D89.41-43 and D89.49) in October 2016.
Although different diagnostic criteria are published, a commonly used strategy to diagnose patients is to use all three of the following:

Symptoms consistent with chronic/recurrent mast cell release:  Recurrent abdominal pain, diarrhea, flushing, itching, nasal congestion, coughing, chest tightness, wheezing, lightheadedness (usually a combination of some of these symptoms is present)
Laboratory evidence of mast cell mediator (elevated serum tryptase, N-methyl histamine, prostaglandin D2 or 11-beta- prostaglandin F2 alpha, leukotriene E4 and others)
Improvement in symptoms with the use of medications that block or treat elevations in these mediators"The World Health Organization has not published diagnostic criteria.

Treatment
Common pharmacological treatments include:

Mast cell stabilizers, including cromolyn sodium and natural stabilizers such as quercetin
H1-antihistamines, such as cetirizine or ketotifen or fexofenadine or loratadine
H2-antihistamines, such as ranitidine or famotidine
Antileukotrienes, such as montelukast or zileuton as well as natural products (e.g., curcumin or St. Johns wort extracts)
Nonsteroidal anti-inflammatory drugs, including aspirin can be very helpful in reducing inflammation in some patients, while others can have dangerous reactions

Prognosis
The prognosis of MCAS is uncertain because of lack of studies.

History
The condition was hypothesized by the pharmacologists John Oates and Jack Roberts of Vanderbilt University in 1991, and following a build-up of evidence featured in papers by Sonneck et al. and Akin et al., named in 2007.Diagnostic criteria were proposed in 2010 and revised in 2019.

See also
FcεRI
Histamine intolerance
Immunoglobulin E

References


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