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.
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.