Initially, around the beginning of 1990 and until recently, Mast Cell Activation Syndrome (MCAS) was the medical terminology for a new expression of mast cell disease.  It has never been an official diagnosis such as Cutaneous Mastocytosis or Systemic Mastocytosis¹.  This syndrome was proposed when experts and clinicians were faced with patients experiencing symptoms similar to Mastocytosis but without the mast cell proliferation in one or more organs (WHO criteria of Mastocytosis).

In 2012, a consensus proposal was written and published by a large group of specialists and researchers from Europe and United States.  Some goals of this consensus were to guide toward a clearer identification and diagnosis of patients with MCAS and to avoid misdiagnoses or overinterpretation of clinical symptoms². 

In this international consensus, MCAS is defined by three criteria.  First, symptoms of MCAS are linked to mast cell mediators released and are affecting two or more organ systems.  MCAS patient would evoke typical symptoms such as flushing or angioedema, gastro-intestinal distress, cardiovascular symptoms, respiratory distress or naso-ocular symptoms².  Two other criteria are proposed such as an elevated serum marker (tryptase) or urine marker of mast cell activation and a reduction of symptoms with treatment^2,3.

Many aspects of MCAS are still not fully understood and there is no common diagnostic criteria for MCAS.  It is a journey for the patient to be diagnosed with one of these rare diseases, when it is actually a newly recognized syndrome in some cases, such as idiopathic mast cell activation syndrome or idiopathic anaphylaxis.

References

1. Soderberg ML (2015) The Mast Cell Activation Syndrome: A mini review.  MOJ Immunol 2(1): 00032. http://medcraveonline.com/MOJI/MOJI-02-00032.pdf
2. Valent P, Akin C, Arrack M, Brockow K, Butterfield P, et al. (2012) Definitions, criteria and global classification of mast cell activation syndrome: a consensus proposal. Int Arch Allergy Immunol  157(3): 215-225  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3224511/
3. Akin C, Valent P, Metcalfe DD (2010) Mast cell activation syndrome : Proposed diagnostic criteria. J Allergy Clin Immunol 126(6): 1099-1104  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3753019/