Definition and Overview
Mast cell activation syndrome (MCAS) is a condition characterized by episodic, excessive release of mast cell mediators causing multi-organ symptoms. Unlike systemic mastocytosis (clonal mast cell proliferation), MCAS involves inappropriate activation of morphologically normal mast cells [1].
MCAS prevalence is estimated at 1-17% depending on diagnostic criteria used. It frequently co-occurs with hypermobile Ehlers-Danlos syndrome (hEDS) and POTS, forming the "triad" increasingly recognized in clinical practice [2].
Pathophysiology
Mast cells are tissue-resident immune cells containing granules filled with preformed mediators (histamine, tryptase, heparin) and synthesize new mediators (prostaglandins, leukotrienes, cytokines) upon activation [1].
In MCAS, mast cells degranulate in response to triggers including physical stimuli (heat, pressure, vibration), emotional stress, foods, medications, infections, and chemical exposures. The underlying cause of hyper-reactivity may involve genetic variants in KIT, altered IgE signaling, or connective tissue abnormalities [3].
Symptoms
MCAS causes episodic multi-system symptoms that wax and wane:
- Skin: flushing, urticaria, angioedema, pruritus
- Gastrointestinal: abdominal cramping, diarrhea, nausea, reflux
- Cardiovascular: tachycardia, hypotension, presyncope
- Respiratory: nasal congestion, wheezing, throat tightness
- Neurological: headache, brain fog, dizziness
- Constitutional: fatigue, malaise
- Anaphylaxis: severe, life-threatening reactions (rare but possible)
The EDS-POTS-MCAS Triad
The co-occurrence of hEDS, POTS, and MCAS is increasingly recognized. Hypotheses include abnormal connective tissue affecting mast cell anchoring, autonomic dysregulation triggering mast cell activation, and shared genetic susceptibility [2].
Diagnosis
Consensus diagnostic criteria require all three [4]:
- Episodic symptoms involving two or more organ systems consistent with mast cell mediator release
- Elevated mast cell mediators: serum tryptase increase >20% + 2 ng/mL above baseline (measured within 1-4 hours of symptoms), or elevated 24-hour urine N-methylhistamine, prostaglandin D2, or leukotriene E4
- Response to mast cell-targeted therapy
Treatment
First-line: H1 antihistamines (cetirizine, fexofenadine) and H2 antihistamines (famotidine) taken regularly, not just as needed. Mast cell stabilizers (cromolyn sodium, ketotifen) prevent degranulation [5].
Second-line: leukotriene receptor antagonists (montelukast), aspirin (if tolerated, blocks prostaglandin D2), and omalizumab (anti-IgE) for refractory cases.
Trigger avoidance is essential. Patients should carry epinephrine auto-injectors for anaphylaxis risk. Low-histamine diet may reduce symptom burden.