Mast cell activation syndrome: A new outlook

Abstract

Mast Cell Activation Syndrome (MCAS) is a severe relapsing disease requiring inpatient treatment, with clinical pattern including the features of anaphylaxis. The article presents diagnostic criteria aimed for differentiation of MCAS from similar severe conditions as well as discusses local forms of mast cell activation. The consensus group has established distinct criteria for diagnosing MCAS. The agreed criteria include episodic (recurrent) occurrence of typical systemic symptoms caused by release of mast cell mediators and involve, at least, two organs; an increase in serum tryptase level by, at least, 20% over individual baseline tryptase plus 2 ng/mL tryptase during 3-4 hours of the pathological reaction; a positive response to drugs that either target mast cells mediators, or their effects. In principle, the classification of MCAS is based on its etiology being subdivided into primary (clonal) MCAS, secondary MCAS, and idiopathic MCAS. The primary MCAS is determined by clonal expansion of mast cells and is considered systemic mastocytosis. In secondary MCAS, normal mast cells are activated by the known triggers, e.g., IgE. If neither clonal expansion nor a trigger for mast cells activation are identified, the condition is defined as idiopathic MCAS.The new COVID-19 infection has attracted particular interest in MCAS, since the severe course of COVID-19 was thought to develop due to latent MCAS, but the criteria for MCAS in these patients were not reproduced. In the presence of local symptoms, such as urticaria, or in cases of single-organ involvement, e.g., isolated gastrointestinal symptoms, and suspected mast cell activation being basic to pathogenesis, the term mast cell activation disorder was introduced. Moreover, the article discusses several different mediators that are proposed as markers in the diagnosis of MCAS.However, over-diagnosis of MCAS entails the risk of missing the underlying pathology, which is not associated with MCAS, and requires differential diagnosis with a number of diseases. In the absence of severe attacks (with hypotension and shock), the likelihood of MCAS is generally very low. Of course, the patients with mastocytosis and/or confirmed IgE-dependent allergy are at higher risk of developing MCAS, but a key diagnostic marker is an event-related increase in mast cells tryptase from baseline determined over the asymptomatic period. The diagnosis of MCAS is highly likely if the tryptase level rises above a certain threshold (20% of baseline plus 2 ng/mL).

Keywords

• mast cell activation syndrome
• mast cell activation disorder
• mast cells
• tryptase
• systemic mastocytosis
• chronic urticaria