Background/Purpose: Mast Cell Activation Syndromes (MCAS) are a group of disorders presenting with episodic multisystem symptoms, predominantly anaphylaxis, as the result of mast cell mediator release. Primary MCAS, that are those with a clonal origin such as systemic mastocytosis (SM), present a high prevalence of fragility fractures (FF), which varied from 5 to 37% of patients [1]. On the contrary, the idiopathic (non-clonal) forms of MCAS, do not appear to be associated with this risk [2]. The main objective of our study is to describe the presence of osteoporosis (OP), FF and the principal risk factors associated with FF in patients with MCAS.

Methods: The study population included all the patients with primary MCAS and idiopathic MCAS evaluated by the Rheumatology and Allergology departments at Hospital General Universitario Gregorio Marañón (Madrid, Spain), a tertiary-level hospital, until January 2023. The diagnosis was made according to the diagnostic and classification criteria updated in the consensus of 2021 [3]. Demographic, clinical, and analytical characteristics were collected. Bone involvement was defined as densitometric OP (T score ≤ -2.5) and/or the presence of FF on thoracic and lumbar spine X-rays. Fracture Risk Assessment Tool (FRAX) was used for estimating the risk of osteoporotic fracture with a 10-year probability. The differences between groups were tested using t-student test for continuous variables and chi-square test for categorical variables. A value of p < 0.05 was considered statistically significant.

Results: A total of 48 patients were included: 36 with primary MCAS (33 with mastocytosis, 5 cutaneous and 28 non-advanced systemic forms, and 3 clonal MCAS) and 12 patients with idiopathic MCAS. In the primary MCAS group, 25% of patients developed at least one FF, of which 44.4% were men with a mean age of 42 years (p=0.02); these patients also have a lower bone mineral density at the femoral neck (T-score -1.07; p=0.04) and a higher FRAX score compared to those who did not present fractures. In the group of patients with idiopathic MCAS, only one presented FF. Table 1 shows a comparison between the primary and idiopathic MCAS patients. Patients with primary MCAS have a higher prevalence OP and FF, higher tryptase levels (p=0.003) and a lower body mass index (BMI) (p=0.004). However, the risk of fracture calculated by FRAX, both for major and hip fractures, is increased in both groups.

Conclusion: The prevalence of OP and FF is higher in patients with primary MCAS compared to patients with idiopathic MCAS. A higher BMI and lower tryptase levels could explain the lower risk in the idiopathic MCAS group, although due to the small sample size the results must be handled with caution.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/clinical-characteristics-and-risk-factors-associated-with-fragility-fractures-in-patients-with-primary-and-idiopathic-mast-cell-activation-syndromes/