Objectives: Mast cell activation syndrome (MCAS) is a multi-systemic inflammatory and allergic disorder caused by uncontrolled, inappropriate mast cell (MC) activity. Intestinal symptoms in MCAS are common and may mimic irritable bowel syndrome. The latter can be associated with small intestinal bacterial overgrowth (SIBO). The aim was to determine the prevalence of SIBO in MCAS patients with gastrointestinal symptoms.

Methods: Patients presenting with refractory intestinal-type symptoms were prospectively evaluated for MCAS and SIBO. MCAS was diagnosed with symptoms of MC activation in ≥2 organ systems plus ≥1 of increased MC mediators, clinical improvement with MC-therapy, and/or increased intestinal MC density. SIBO was defined as a rise of hydrogen rise ≥20 ppm from baseline in a 90 min lactulose breath test and were compared to healthy controls. Impact of co-morbid syndromes and medications were analyzed.

Results: Study included 139 patients with MCAS (83.4% female, 46.6 ±16.9 years) and 30 completely healthy controls (63.3% female, 44.0 ±14.0 years). Intestinal symptoms included mid to lower abdominal pain (87.1%), bloating (74.8%), constipation (66.9%), and diarrhea (63.3%). SIBO was present in 30.9% MCAS subjects vs.10.0% controls (p=0.023). Postural orthostatic tachycardia syndrome, hypermobile Ehlers-Danlos syndrome, MC density, MC mediators, and medications were not associated with breath test changes. Bloating was not associated with breath test results.

Discussion/Conclusion: SIBO is common in MCAS but does not explain intestinal symptoms in all patients. Additional disturbances from MC mediators in the gut and the effect of mediators in paraneuronal tissue may play roles.