Complete remission with histamine blocker in a patient with intractable hyperadrenergic postural orthostatic tachycardia syndrome secondary to long coronavirus disease syndrome

The COVID-19 pandemic caused by the novel severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2), has emerged as a global public health concern and its sequels have barely started to outcrop. A good percentage of patients who suffered from COVID-19 are prone to develop long-COVID or post-COVID condition (PCC), a multisystemic, heterogeneous, chronic disorder. Patients with PCC may experience diverse manifestations, of which cardiovascular and neurological symptoms are among the most frequently reported. Indeed, dysautonomia presented as orthostatic intolerance has gained room following recent reports linking postural orthostatic tachycardia syndrome (POTS) with PCC. Disturbances in heart rate (HR) and blood pressure (BP) during postural changes are the cornerstones of orthostatic intolerance seen in patients suffering from PCC. A subtype of POTS, hyperadrenergic POTS, has been widely studied because of its association with mast cell activation syndrome (MCAS). Although a causative relationship between PCC, hyperadrenergic POTS, and MCAS remains unrevealed, these syndromes can overlap. We want to propose here a correlation produced by a close-loop mechanism with positive feedback established after SARS-CoV-2 infection in a previously healthy young patient.

Learned cautions regarding antibody testing in mast cell activation syndrome

OBJECTIVES

To describe patterns observed in antibody titer trendlines in patients with mast cell activation syndrome (MCAS, a prevalent but underrecognized chronic multisystem inflammatory disorder of great clinical heterogeneity) and offer clinical lessons learned from such pattern recognition.

METHODS

The available records of 104 MCAS patients drawn from the authors' practices were reviewed, including all antibody tests therein.

RESULTS

All patients had positive/elevated antibodies of various sorts at various points, but for most of the antibodies which were found to be positive at least some points, the diseases classically associated with those antibodies were not present, marking such antibodies as clinically insignificant mimickers (likely consequent to inflammatory effects of MCAS on the immune system itself driving spurious/random antibody production) rather than "on-target" and pathogenic antibodies reflecting true disease warranting treatment. We also observed two distinct patterns in trendlines of the titers of the mimickers vs. the trendline pattern expected in a true case of an antibody-associated disease (AAD).

CONCLUSIONS

Our observations suggest most positive antibody tests in MCAS patients represent detection of clinically insignificant mimicking antibodies. As such, to reduce incorrect diagnoses of AADs and inappropriate treatment in MCAS patients, caution is warranted in interpreting positive antibody tests in these patients. Except in clinically urgent/emergent situations, patience in determining the trendline of a positive antibody in an MCAS patient, and more carefully assessing whether the AAD is truly present, is to be preferred.

Neuropsychiatric Manifestations of Mast Cell Activation Syndrome and Response to Mast-Cell-Directed Treatment: A Case Series

Mast cell activation syndrome (MCAS) is an immune disease with an estimated prevalence of 17%. Mast cell chemical mediators lead to heterogeneous multisystemic inflammatory and allergic manifestations. This syndrome is associated with various neurologic and psychiatric disorders, including headache, dysautonomia, depression, generalized anxiety disorder, and many others. Although MCAS is common, it is rarely recognized, and thus, patients can suffer for decades. The syndrome is caused by aberrant mast cell reactivity due to the mutation of the controller gene. A case series is presented herein including eight patients with significant neuropsychiatric disorders that were often refractory to standard medical therapeutics. Five patients had depression, five had generalized anxiety disorder, and four had panic disorder. Other psychiatric disorders included attention-deficit hyperactivity disorder, obsessive compulsive disorder, phobias, and bipolar disorder. All eight patients were subsequently diagnosed with mast cell activation syndrome; six had comorbid autonomic disorders, the most common being postural orthostatic tachycardia syndrome; and four had hypermobile Ehlers-Danlos syndrome. All patients experienced significant improvements regarding neuropsychiatric and multisystemic symptoms after mast-cell-directed therapy. In neuropsychiatric patients who have systemic symptoms and syndromes, it is important to consider the presence of an underlying or comorbid MCAS.

Gluten-free diet in postural orthostatic tachycardia syndrome (POTS)

OBJECTIVES

Postural orthostatic tachycardia syndrome (POTS) is a chronic disorder of the autonomic nervous system that is associated with orthostatic intolerance (OI) and gastrointestinal (GI) symptoms. We aimed to determine if a gluten-free diet (GFD) may be an effective treatment in patients with POTS.

METHODS

Patients with confirmed POTS and without celiac disease who followed GFD for  4 weeks retrospectively completed pre- and post- GFD COMPASS-31 questionnaires. Paired-samples t-tests were conducted to compare COMPASS-31 scores before and after adopting GFD.

RESULTS

All 20 patients (all females, age 16-62 years (mean age 33)), had POTS symptoms for 1-30 years (mean 10.8, SD  =  10.23 years). Eleven patients had co-morbid mast cell activation syndrome, and 8 had hypermobile Ehlers-Danlos syndrome. Pre-GFD COMPASS-31 scores (mean 57.8) were significantly higher than post-GFD scores (mean 38.2, t(19) = -8.92, p < 0.00001), with the largest improvement noted in the OI (p < 0.00001), vasomotor, (p  =  0.0034), and GI (p  =  0.0004) domains and with a mean reduction in total COMPASS-31 score by 33.9% after implementing GFD. All patients reported improved symptoms on GFD with mean self-assessed improvement of 50.5% (range 10-99%).

DISCUSSION

GFD may be effective in reducing the symptom burden in patients with POTS, particularly in the OI, vasomotor, and GI symptom domains. Large prospective studies are necessary to confirm whether GFD is an effective long-term treatment option for patients.

The Role of Mast Cells in the Induction and Maintenance of Inflammation in Selected Skin Diseases

Under physiological conditions, skin mast cells play an important role as guardians that quickly react to stimuli that disturb homeostasis. These cells efficiently support, fight infection, and heal the injured tissue. The substances secreted by mast cells allow for communication inside the body, including the immune, nervous, and blood systems. Pathologically non-cancerous mast cells participate in allergic processes but also may promote the development of autoinflammatory or neoplastic disease. In this article, we review the current literature regarding the role of mast cells in autoinflammatory, allergic, neoplastic skin disease, as well as the importance of these cells in systemic diseases with a pronounced course with skin symptoms.

The Curious Case of Elevated Tryptase: Workup and Differential in Family of Four

Elevated basal serum tryptase (BST) levels are markers of both mast cell activation and overall mast cell burden. We present a family of four individuals with elevated tryptase levels greater than or equal to 20 mcg/L, all of whom exhibited signs and symptoms suggestive of mast cell activation. Differential diagnoses included hereditary alpha tryptasemia (HaT), systemic mastocytosis (SM), and mast cell activation syndrome (MCAS). In three individuals, SM was ruled out with normal morphology on bone marrow biopsy combined with negative genetic markers. Further workup would be required for the diagnosis of MCAS since serum tryptase levels were not obtained in our emergency department during acute episodes. Although genetic testing for HaT was not available upon initial workup, HaT remains the most likely explanation for this family's elevated BST.

Defining hereditary alpha-tryptasemia as a risk/modifying factor for anaphylaxis: are we there yet?

Summary

Hereditary α-tryptasemia (HαT) is a common autosomal dominant genetic trait with variable penetrance associated with increased serum baseline tryptase (SBT) levels. Clinical manifestations may range from an absence of symptoms to overtly severe and recurrent anaphylaxis. Symptoms have been claimed to result from excessive activation of EGF-like module-containing mucin-like hormone receptor-like 2 (EMR2) and protease-activated receptor 2 (PAR-2) receptors by α/β-tryptase heterotetramers. Herein, we aimed to review the evidence on whether HαT can be considered a hereditary risk factor or a modifying factor for anaphylaxis. Increased SBT levels have been linked to an increased risk of anaphylaxis. Likewise, recent studies have shown that HαT might be associated with a higher risk of developing anaphylaxis and more severe anaphylaxis. The same has also been shown for patients with clonal mast cell disorders, in whom the co-existence of HαT might lead to a greater propensity for severe, potentially life-threatening anaphylaxis. However, studies leading to such conclusions are generally limited in sample size, while other studies have shown opposing results. As such, further studies investigating the potential association of HαT with anaphylaxis caused by different triggers, and different severity grades, in both patients with clonal mast cell (MC) activation syndromes and the general population are still needed.

Mast cell evaluation in gastrointestinal biopsies: should we be counting? A critical review and practical guide for the surgical pathologist

Mast cells are residents of the tubular gastrointestinal (GI) tract, where they play an important role in host defence and other vital functions. Dysregulation of mast cells has been implicated in the pathogenesis of several neoplastic, inflammatory, and functional disorders, some of which may manifest with GI symptoms. Surgical pathologists must therefore confront when and how to evaluate GI biopsies for mast cells, and whether such decisions should be based on morphologic criteria, clinical context, or direct request from clinical colleagues. The pathologist's role in evaluation of mast cell infiltrates is best defined in the diagnosis of systemic mastocytosis, where the utility of morphologic assessment coupled with ancillary studies is well established. In contrast, in nonneoplastic mast cell disorders such as mast cell activation syndrome, irritable bowel syndrome, or so-called 'mastocytic enterocolitis', a role for histopathology, if any, is controversial. Despite this, pathologists have seen a sharp increase in requests for mast cell quantification in the latter setting, despite these requests not being supported by published evidence. Moreover, what constitutes a 'normal' number of mast cells in a luminal GI biopsy is not well established. As a result, there is considerable variation in how these requests are handled in practice. This review evaluates and summarizes the published evidence relating to mast cell evaluation in endoscopic GI biopsies in various clinical scenarios, with a goal of providing practical, evidence-based guidance for the surgical pathologist when approached with requests for mast cell quantification in GI biopsies.

What Is the Value of Counting Mast Cells in Gastrointestinal Mucosal Biopsies?

Neoplastic and nonneoplastic mast cell disorders can cause diarrhea, nausea, and abdominal pain that result from heightened release of mast cell mediators. Systemic mastocytosis is characterized by neoplastic mast cell aggregates in the bone marrow and other sites, particularly the skin and gastrointestinal tract. In this situation, extramedullary mast cell aggregates display atypical morphology, with aberrant immunostaining for CD25 in addition to staining for other mast cell markers, such as mast cell tryptase and CD117. Morphologically normal mast cells have also been implicated in nonneoplastic conditions. For example, increased mast cell numbers have been reported in the mucosal biopsy samples from patients with irritable bowel syndrome and hereditary alpha-tryptasemia. Patients with mast cell activation syndrome presumably experience symptoms related to the aberrant elaboration of histamine and other mediators from normal-appearing mast cells present in normal numbers. Unfortunately, similarities in terminology among these biologically distinct clinical conditions have caused considerable diagnostic confusion among clinical colleagues, resulting in frequent requests for pathologists to quantify and characterize mast cells in normal gastrointestinal biopsy samples from patients with diarrheal symptoms. The purpose of this review is to summarize the available data related to mast cell assessment in the gastrointestinal tract and provide pathologists with practical information so that they can help their clinical colleagues manage patients with presumed mast cell disorders.

Removal of Polymer Clips From the Gallbladder Fossa in a Patient With Ehlers-Danlos Syndrome (EDS) to Treat Mast Cell Activation Syndrome (MCAS): A Case Report

Ehlers-Danlos syndrome (EDS) is a group of hereditary disorders characterized by fragility of connective tissue. Clinical manifestations of the disorder involve the skin, joints, blood vessels, and other internal organs. We report the case of a 29-year-old female suffering from EDS and mast cell activation syndrome (MCAS). Her history includes multiple orthopedic surgeries leading to the worsening of her symptoms. This was determined to be due to medical implants placed during her multiple procedures predisposing her to severe immunological reactions. This case report emphasizes the importance of meticulous surgical intervention when managing patients with EDS.

Mast cell activation syndrome-anesthetic challenges in two different clinical scenarios

Mast cell activation syndrome (MCAS) includes a group of disorders that result in the inappropriate release of inflammatory mediators from mast cells. These mediators can affect multiple organ systems and lead to significant morbidity, and possible fatality. Although reactions, typically in response to various nonspecific stimuli, are usually mild, they may put those with MCAS at increased risk of anaphylaxis. In this case report, we present two clinical scenarios of MCAS, and identify possible factors triggering mast cell mediator release. We also define a preoperative preventive pathway, outline anesthetic considerations, and discuss the management of immediate hypersensitivity reactions in patients with MCAS. Meticulous preoperative preparation, avoidance of triggers, and development of a plan to treat possible adverse organ responses are paramount of good outcomes.

Mast Cell's Role in Cytokine Release Syndrome and Related Manifestations of COVID-19 Disease

In this narrative review, firstly, we describe the characteristics of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the pathogenesis of its infection in humans. Later, the importance of mast cells in SARS-CoV-2 infection and their role in Coronavirus Disease 2019 (COVID-19) will be discussed. SARS-CoV-2 is a transmissible agent frequently detected in some mammalian species and also in humans. Literature data published in PubMed that covered mast cells' role in cytokine release syndrome and related manifestations of COVID-19 disease were reviewed by the authors independently and collectively. Recommendations for the management of cytokine release syndrome and related manifestations were made by the authors. Mast cells are concentrated in environments where they encounter viruses, bacteria, and toxins, especially in the skin, nasal mucosa, lungs, airways, gastrointestinal tract, and meninges, to prevent their entry into the human body. Once SARS-CoV-2 enters the host, it stimulates one of the mast cells, together with pre-existing innate immune cells that form a defensive barrier in the submucosa of the respiratory tract and nasal cavities against pathogenic microorganisms. The roles of mast cells in SARS-CoV-2-induced hyperinflammation and cytokine storms have recently been one of the hot topics in the literature. Physicians should keep in mind the mast cells' role in cytokine release syndrome and related manifestations of COVID-19 disease. Mast cell-targeting therapies (e.g., H1 and H2 receptor antagonists) can reduce the severity and course of the disease when used after complications associated with COVID-19 are suspected or seen.

Idiopathic mast cell activation syndrome is more often suspected than diagnosed-A prospective real-life study

BACKGROUND

Idiopathic mast cell activation syndrome (MCAS) is characterized by three diagnostic criteria: (1) episodic mast cell (MC)-driven signs/symptoms of at least two organ systems in the absence of clonal MC expansion and definite triggers, (2) episodic increase in tryptase, and (3) response to MC-targeted treatment. Many patients believe they have MCAS, but how often this is the case remains unknown.

METHODS

We prospectively investigated patients with suspected MCAS (n = 100) for the diagnostic criteria including baseline tryptase, KIT D816V mutation, and patient-reported outcome measures (PROMs) over the course of 12 weeks. Comorbid depression and anxiety were explored with the Hospital Anxiety and Depression Scale (HADS).

RESULTS

In 53% of our patients (80% females), suspicion of MCAS was based on self-evaluation. In total, patients reported 87 different symptoms, mostly fatigue (n = 57), musculoskeletal pain/weakness (n = 49), and abdominal pain (n = 43), with overall high disease activity and impact. Two of 79 patients had increased tryptase (by >20% +2 ng/ml) following an episode. Only 5%, with any of the PROMs used, showed complete response to MC-targeted treatment. Depression and anxiety disorders were frequent comorbidities (n = 23 each), and 65 patients had pathological HADS values, which were linked to high disease impact and poor symptom control.

CONCLUSION

Mast cell activation syndrome was confirmed in only 2% of patients, which implies that it is not MC activation that drives signs and symptoms in most patients with suspected MCAS. There is a high need for comprehensive research efforts aimed at the identification of the true underlying pathomechanism(s) in patients with suspected MCAS.

Systemic Mastocytosis and Other Entities Involving Mast Cells: A Practical Review and Update

Evidence in the recent literature suggests that the presentation spectrum of mast cell neoplasms is broad. In this article, we elaborate on recent data pertaining to minor diagnostic criteria of systemic mastocytosis (SM), including sensitive testing methods for detection of activating mutations in the KIT gene or its variants, and adjusted serum tryptase levels in cases with hereditary α-tryptasemia. We also summarize entities that require differential diagnosis, such as the recently reclassified SM subtype named bone marrow mastocytosis, mast cell leukemia (an SM subtype that can be acute or chronic); the rare morphological variant of all SM subtypes known as well-differentiated systemic mastocytosis; the extremely rare myelomastocytic leukemia and its differentiating features from mast cell leukemia; and mast cell activation syndrome. In addition, we provide a concise clinical update of the latest adjusted risk stratification model incorporating genomic data to define prognosis in SM and new treatments that were approved for advanced SM (midostaurin, avapritinib).

Evaluation and Classification of Mast Cell Disorders: A Difficult to Manage Pathology in Clinical Practice

Mast cells are granulocytic immunomodulatory cells with an important role in physiologic and pathogenic processes due to their location at the junction between the internal and external environment and to their capacity to release a broad range of active mediators. Mast cells mediators have both pro-inflammatory and anti-inflammatory activities and are implicated in various and complex pathology. Mast cells disorders (MCDs) represent a heterogeneous pathology, with frequently difficult and challenging evaluation and diagnostic workup. MCDs can be primary, secondary to other diseases, or idiopathic. Increased research interest in this field was noted during the last decade and various classification criteria, as well as diagnostic and treatment recommendations, were proposed. The aim of this paper is to review the most recent published data on the classification and evaluation of mast cells disorders and to point out the main difficulties in diagnosing and managing these complex diseases in medical practice.

Post-HPV-Vaccination Mast Cell Activation Syndrome: Possible Vaccine-Triggered Escalation of Undiagnosed Pre-Existing Mast Cell Disease?

For nearly a decade, case reports and series have emerged regarding dysautonomias-particularly postural orthostatic tachycardia syndrome (POTS)-presenting soon after vaccination against human papilloma virus (HPV). We too have observed a number of such cases (all following vaccination with the Gardasil product), and have found several to have detectable mast cell activation syndrome (MCAS) as well as histories suggesting that MCAS was likely present long before vaccination. We detail 11 such cases here, posing a hypothesis that HPV vaccination (at least with the Gardasil product) may have triggered or exacerbated MCAS in teenagers previously not recognized to have it. Only recently recognized, MCAS is being increasingly appreciated as a prevalent and chronic multisystem disorder, often emerging early in life and presenting with inflammatory ± allergic phenomena following from known mast cell (MC) mediator effects. There is rising recognition, too, of associations of MCAS with central and peripheral neuropathic disorders, including autonomic disorders such as POTS. Given the recognized potential for many antigens to trigger a major and permanent escalation of baseline MC misbehavior in a given MCAS patient, we hypothesize that in our patients described herein, vaccination with Gardasil may have caused pre-existing (but not yet clinically recognized) MCAS to worsen to a clinically significantly degree, with the emergence of POTS and other issues. The recognition and management of MCAS prior to vaccinations in general may be a strategy worth investigating for reducing adverse events following HPV vaccinations and perhaps even other types of vaccinations.

Some cases of hypermobile Ehlers-Danlos syndrome may be rooted in mast cell activation syndrome

Hypermobile Ehlers-Danlos syndrome (hEDS) is the most common type of EDS, yet has remained steadfastly inscrutable vis-à-vis efforts to identify its cellular, molecular, and pathophysiologic roots. Once thought to principally affect just connective tissues, hEDS is now appreciated to be a multisystem disease of great heterogeneity with many symptoms and findings difficult to attribute solely to disordered connective tissue development. In the last decade, there has been growth in the appreciation of the existence of a wide range of disorders of chronic inappropriate mast cell (MC) activation (a large heterogeneous pool of MC activation syndromes [MCAS]) distinguishable from other MC disorders such as rare neoplastic mastocytosis. Via chronic aberrant release of the MC's vast repertoire of potent mediators, MCAS can drive extraordinary arrays of pathologies, most commonly of inflammatory, allergic, and dystrophic natures. Although hEDS is seen in only a minority of MCAS cases, limited studies have identified an association between hEDS and MCAS, fueling speculation that certain variants of MCAS may drive hEDS. No laboratory studies probing cellular or molecular linkages between hEDS and MCAS have been conducted yet, and research efforts to identify the genetic roots of hEDS should also consider those of MCAS.

Anesthetic Management of a Patient With Mast Cell Activation Syndrome: A Case Study

Mast cell activation syndrome (MCAS) is a relatively new diagnosis for a constellation of symptoms with sometimes devastating results for patients. A 40-year old woman with MCAS underwent arthroscopic repair of her right shoulder, with successful anesthetic management. This case report discusses the basic immunologic physiology surrounding this syndrome, myriad medications often used by this patient population, and the anesthetic management of this patient. With additional knowledge of this disorder, exposure to its clinical presentation in the perioperative setting, and anesthetic considerations specific to MCAS, the Certified Registered Nurse Anesthetist will be better equipped to effectively manage the complex requirements of this patient population.

A case of omalizumab as a successful treatment for telangiectasia macularis eruptiva perstans

Treatment for telangiectasia macularis eruptiva perstans (TMEP) is often challenging due to lack of an established first-line therapy and as such is primarily focused on symptomatic relief. Omalizumab shows promise as a potential therapy for mast cell disorders; however, its efficacy in TMEP is yet to be established. This case describes a 72-year-old woman with chronic refractory TMEP achieving symptomatic remission within 4 months of commencing omalizumab therapy.

PATHOGENIC AND DIAGNOSTIC RELEVANCE OF KIT IN PRIMARY MAST CELL ACTIVATION DISORDERS

OBJECTIVE

Mast cell (MC) activation (MCA) defines the mechanism by which certain patients suffer from symptoms due to the effect of a wide range of mediators released from MC upon their activation triggered by different stimuli. When these symptoms are severe and recurrent, the diagnosis of MCA syndrome (MCAS) might be considered. Here we review the relevant aspects related to the pathogenesis of MCAS, with special emphasis on the prevalence and diagnostic relevance of KIT mutations.

DATA SOURCES

PubMed was searched between 1980 and 2021 using the following terms: Mast cell activation syndromes, mast cell activation, anaphylaxis, KIT mutations, KIT D816V, indolent systemic mastocytosis, bone marrow mastocytosis, cutaneous mastocytosis, IgE anaphylaxis and idiopathic anaphylaxis.

STUDY SELECTIONS

Only articles published in English were selected based on their relevance to MCAS and/or severe and recurrent anaphylaxis.

RESULTS

MCAS can be classified in clonal MCAS and non-clonal MCAS depending on the presence vs. absence of an underlying KIT mutation (mostly KIT D816V), respectively. In contrast to clonal MCAS in which MCA is associated with a primary MC disorder (i.e. primary MCAS) such as mastocytosis or monoclonal MCAS, non-clonal MCAS can be secondary to known or unidentified triggers (i.e. secondary and idiopathic MCAS, respectively).

CONCLUSION

The clinical heterogeneity and complexity of the molecular assays needed for the study of MCAS patients might lead to misdiagnosis, particularly when patients are evaluated at non-specialized centers. Thus, referral of patients suffering from clinical manifestations suggestive of MCAS to Reference Centers on mastocytosis and MC diseases is strongly recommended.

Diagnosis of mast cell activation syndrome: a global "consensus-2"

The concept that disease rooted principally in chronic aberrant constitutive and reactive activation of mast cells (MCs), without the gross MC neoplasia in mastocytosis, first emerged in the 1980s, but only in the last decade has recognition of "mast cell activation syndrome" (MCAS) grown significantly. Two principal proposals for diagnostic criteria have emerged. One, originally published in 2012, is labeled by its authors as a "consensus" (re-termed here as "consensus-1"). Another sizable contingent of investigators and practitioners favor a different approach (originally published in 2011, newly termed here as "consensus-2"), resembling "consensus-1" in some respects but differing in others, leading to substantial differences between these proposals in the numbers of patients qualifying for diagnosis (and thus treatment). Overdiagnosis by "consensus-2" criteria has potential to be problematic, but underdiagnosis by "consensus-1" criteria seems the far larger problem given (1) increasing appreciation that MCAS is prevalent (up to 17% of the general population), and (2) most MCAS patients, regardless of illness duration prior to diagnosis, can eventually identify treatment yielding sustained improvement. We analyze these proposals (and others) and suggest that, until careful research provides more definitive answers, diagnosis by either proposal is valid, reasonable, and helpful.

Depression, psychosocial correlates, and psychosocial resources in individuals with mast cell activation syndrome

Individuals with Mast Cell Activation Syndrome (MCAS), a rare chronic disease, experience unpredictable physical symptoms and diagnostic challenges resulting in poor emotional states. The prevalence and correlates of depressive symptoms were examined among 125 participants who completed the CES-D and relevant instruments. The majority reported a clinically-significant level of depression which was especially common among younger participants and those who reported greater loneliness or more disease-specific stressors. Greater magnitude of depressive symptoms was associated with greater illness intrusiveness, less social support, and lower optimism. Results highlight the value of interventions targeting loneliness and stressors unique to this population.

Prevalence of mastocytosis and hymenoptera venom allergy in the United States

BACKGROUND

Mastocytosis is a risk factor for hymenoptera venom anaphylaxis (HVA). Current guidelines recommend measuring tryptase in patients with HVA and that those with mastocytosis pursue lifelong venom immunotherapy (VIT). Available data on HVA and mastocytosis largely derive from European single-center studies, and the prevalence of HVA with and without mastocytosis in the United States is unknown.

OBJECTIVE

We sought to determine the prevalence of HVA and mastocytosis in the United States using an insurance claims database and evaluate the impact of mastocytosis on VIT in patients with HVA in a US cohort.

METHODS

The IBM Watson Database, consisting of insurance claims from approximately 27 million US patients in 2018, was queried to identify patients with HVA and/or mastocytosis. Furthermore, a retrospective study of 161 patients undergoing VIT between 2015 and 2018 at the University of Michigan was conducted.

RESULTS

In the IBM Watson Database, the prevalence of HVA was 167 per 100,000 (0.167%) and the prevalence of mastocytosis 10 per 100,000 (0.010%) overall and 97 per 100,000 (0.097%) among those with HVA. Mastocytosis showed a 9.7-fold increase among patients with HVA versus the general population. In the U-M cohort, 2.6% of patients with VIT had mastocytosis. Tryptase level did not correlate with venom reaction severity but was higher in patients with systemic VIT reactions.

CONCLUSIONS

We observed a lower US HVA prevalence than previously reported. Mastocytosis was more common in US patients with HVA, though at lower rates than previously reported. In patients with VIT there was no correlation between tryptase level and reaction severity.

Metabolome and lipidome derangements during a severe mast cell activation event in a patient with indolent systemic mastocytosis

BACKGROUND

The number of mast cells in various organs is elevated manifold in individuals with systemic mastocytosis. Degranulation can lead to life-threatening symptomatology. No data about the alterations of the metabolome and lipidome during an attack have been published.

OBJECTIVE

Our aim was to analyze changes in metabolomics and lipidomics during the acute phase of a severe mast cell activation event.

METHODS

A total of 43 metabolites and 11 lipid classes comprising 200 subvariants from multiple plasma samples in duplicate, covering 72 hours of a severe mast cell activation attack with nausea and vomiting, were compared with 2 baseline samples by using quantitative liquid chromatography-mass spectrometry.

RESULTS

A strong enterocyte dysfunction reflected in an almost 20-fold reduction in the functional small bowel length was extrapolated from strongly reduced ornithine and citrulline concentrations and was very likely secondary to severe endothelial cell dysfunction with hypoperfusion and extensive vascular leakage. Highly increased histamine and lactate concentrations accompanied the peak in clinical symptoms. Elevated asymmetric and symmetric dimethylarginine levels combined with reduced arginine levels compromised endothelial nitric oxide synthase activity and nitric oxide signaling. Specific and extensive depletion of many lysophosphatidylcholine variants indicates localized autotaxin activation and lysophosphatidic acid release. A strong correlation of clinical parameters with histamine concentrations and symptom reduction after 100-fold elevated plasma diamine oxidase concentrations implies that histamine is the key driver of the acute phase.

CONCLUSIONS

Rapid elimination of elevated histamine concentrations through use of recombinant human diamine oxidase, supplementation of lysophosphatidylcholine for immunomodulation, inhibition of autotaxin activity, and/or blockade of lysophosphatidic acid receptors might represent new treatment options for life-threatening mast cell activation events.

Persistent Antiphospholipid Antibodies, Mast Cell Activation Syndrome, Postural Orthostatic Tachycardia Syndrome and Post-COVID Syndrome: 1 Year On

This is the first case report of a patient with post-COVID-19 postural orthostatic tachycardia syndrome (POTS) with multiple persistent antiphospholipid antibody (aPL)-positivity more than a year after illness onset who also meets Global Consensus-2 criteria for mast cell activation syndrome (MCAS), suggesting pathological activation of the acquired and innate immune systems by SARS-CoV-2. While the patient continues to meet criteria for POTS 1 year on, her functional ability has improved significantly with therapy directed at MCAS, POTS and aPL-positivity.

LEARNING POINTS

A subset of long-haul COVID-19 patients have postural orthostatic tachycardia syndrome (POTS), which can be diagnosed by a 10-minute in-office stand test.Antiphospholipid antibodies may be associated with POTS in patients with long-haul COVID-19 and have important clinical implications.Mast cell activation syndrome (MCAS) may be associated with long-haul COVID-19 (with or without POTS) and can often be easily treated, including with over-the-counter medications, supplements and dietary changes.

Proceedings from the Inaugural American Initiative in Mast Cell Diseases (AIM) Investigator Conference

The American Initiative in Mast Cell Diseases (AIM) held its inaugural investigator conference at Stanford University School of Medicine in May 2019. The overarching goal of this meeting was to establish a Pan-American organization of physicians and scientists with multidisciplinary expertise in mast cell disease. To serve this unmet need, AIM envisions a network where basic, translational, and clinical researchers could establish collaborations with both academia and biopharma to support the development of new diagnostic methods, enhanced understanding of the biology of mast cells in human health and disease, and the testing of novel therapies. In these AIM proceedings, we highlight selected topics relevant to mast cell biology and provide updates regarding the recently described hereditary alpha-tryptasemia. In addition, we discuss the evaluation and treatment of mast cell activation (syndromes), allergy and anaphylaxis in mast cell disorders, and the clinical and biologic heterogeneity of the more indolent forms of mastocytosis. Because mast cell disorders are relatively rare, AIM hopes to achieve a coordination of scientific efforts not only in the Americas but also in Europe by collaborating with the well-established European Competence Network on Mastocytosis.

Systemic mastocytosis with flushing and hypotension: A case report and literature review

Systemic mastocytosis (SM) is a heterogeneous disease of the bone marrow, which is characterized by the abnormal proliferation and infiltration of mast cells in one or more organs, such as the skin, bone marrow, digestive tract, liver and spleen. Urticaria pigmentosa is a typical but infrequent manifestation of SM. Other clinical presentations are non-specific, varying from pruritus and hypotension to multiple organ dysfunction, which may be lethal when hemodynamic changes occur, such as the sharp decline in blood pressure observed in the present case. In patients who lack skin lesions, the diagnosis of SM is frequently challenging. The present study reported on a 58-year-old male who presented with episodic flushing and syncope. The patient demonstrated marked neutrophilia and reduced blood potassium concentrations soon after the onset of each episode, which was able to last several hours, ranging from once to four times a year. SM without skin lesions was suspected and confirmed after multifocal bone marrow aspiration, which revealed dense infiltrates of mast cells (≥15 mast cells), with positive toluidine blue and CD117 staining. The present case illustrates the significance of taking SM or mast cell activation syndrome into consideration when unexplained recurrent hypotension or even syncope are observed, care should be taken to exclude differential diagnoses, as some of them may have much poorer prognoses and require alternative treatments.

Clinical Impact of Inherited and Acquired Genetic Variants in Mastocytosis

Mastocytosis is a rare and complex disease characterized by expansion of clonal mast cells (MC) in skin and/or various internal organ systems. Involvement of internal organs leads to the diagnosis of systemic mastocytosis (SM). The WHO classification divides SM into indolent SM, smoldering SM and advanced SM variants, including SM with an associated hematologic neoplasm, aggressive SM, and MC leukemia. Historically, genetic analysis of individuals with pure cutaneous mastocytosis (CM) and SM have focused primarily on cohort studies of inherited single nucleotide variants and acquired pathogenic variants. The most prevalent pathogenic variant (mutation) in patients with SM is KIT p.D816V, which is detectable in most adult patients. Other somatic mutations have also been identified-especially in advanced SM-in TET2, SRSF2, ASXL1, RUNX1, CBL and JAK2, and shown to impact clinical and cellular phenotypes. Although only small patient cohorts have been analyzed, disease associations have also been identified in several germline variants within genes encoding certain cytokines or their receptors (IL13, IL6, IL6R, IL31, IL4R) and toll-like receptors. More recently, an increased prevalence of hereditary alpha-tryptasemia (HαT) caused by increased TPSAB1 copy number encoding alpha-tryptase has been described in patients with SM. Whereas HαT is found in 3-6% of general Western populations, it is identified in up to 17% of patients with SM. In the current manuscript we review the prevalence, functional role and clinical impact of various germline and somatic genetic variants in patients with mastocytosis.

Carbamazepine-induced Stevens-Johnson syndrome in a patient with history of methotrexate-induced mast cell activation syndrome

Stevens-Johnson syndrome (SJS) is serious conditions that happen as a result of infection, side effects to medications, or unknown etiology. Carbamazepine is one of the common medications that can cause SJS. Good history taking is crucial if treatment with carbamazepine is clinically indicated. We would like to alert all physicians that carbamazepine should be avoided in any patient with a previous history of drug reaction such as mast cell activation syndrome.

Case Report: Hereditary Alpha Tryptasemia in Children: A Pediatric Case Series and a Brief Overview of Literature

Hereditary alpha tryptasemia (HαT) is a recently described autosomal dominant genetic trait caused by an increased copy number of the TPSAB1 gene. It commonly leads to elevated basal serum tryptase levels, and it is associated with heterogeneous clinical manifestations. Some individuals report few to no symptoms, while others present with a spectrum of debilitating features. Most symptoms related to HαT may be explained by mast cell activation and mediator release, namely multiple allergies, anaphylaxis, and skin rash. However, the genotype-phenotype correlation has not yet been clearly understood. In particular, the characterization of the clinical spectrum lacks in children, where differential diagnosis could be challenging. Systemic mastocytosis, HαT, and mast cell activation syndrome are all associated with overlapping pathophysiology and symptoms, making the distinction between these conditions a difficult task. We herein describe two pediatric cases of HαT and their respective families at our tertiary care teaching hospital, highlighting the diagnostic workup and differential diagnosis. We also provide a brief review of the literature to underline the peculiar features of this condition in children.

The Relationship between Autism and Ehlers-Danlos Syndromes/Hypermobility Spectrum Disorders

Considerable interest has arisen concerning the relationship between hereditary connective tissue disorders such as the Ehlers-Danlos syndromes (EDS)/hypermobility spectrum disorders (HSD) and autism, both in terms of their comorbidity as well as co-occurrence within the same families. This paper reviews our current state of knowledge, as well as highlighting unanswered questions concerning this remarkable patient group, which we hope will attract further scientific interest in coming years. In particular, patients themselves are demanding more research into this growing area of interest, although science has been slow to answer that call. Here, we address the overlap between these two spectrum conditions, including neurobehavioral, psychiatric, and neurological commonalities, shared peripheral neuropathies and neuropathologies, and similar autonomic and immune dysregulation. Together, these data highlight the potential relatedness of these two conditions and suggest that EDS/HSD may represent a subtype of autism.

Risk and management of patients with mastocytosis and MCAS in the SARS-CoV-2 (COVID-19) pandemic: Expert opinions

The coronavirus disease 2019 (COVID-19) (caused by severe acute respiratory syndrome coronavirus 2) pandemic has massively distorted our health care systems and caused catastrophic consequences in our affected communities. The number of victims continues to increase, and patients at risk can only be protected to a degree, because the virulent state may be asymptomatic. Risk factors concerning COVID-19-induced morbidity and mortality include advanced age, an impaired immune system, cardiovascular or pulmonary diseases, obesity, diabetes mellitus, and cancer treated with chemotherapy. Here, we discuss the risk and impact of COVID-19 in patients with mastocytosis and mast cell activation syndromes. Because no published data are yet available, expert opinions are, by necessity, based on case experience and reports from patients. Although the overall risk to acquire the severe acute respiratory syndrome coronavirus 2 may not be elevated in mast cell disease, certain conditions may increase the risk of infected patients to develop severe COVID-19. These factors include certain comorbidities, mast cell activation-related events affecting the cardiovascular or bronchopulmonary system, and chemotherapy or immunosuppressive drugs. Therefore, such treatments should be carefully evaluated on a case-by-case basis during a COVID-19 infection. In contrast, other therapies, such as anti-mediator-type drugs, venom immunotherapy, or vitamin D, should be continued. Overall, patients with mast cell disorders should follow the general and local guidelines in the COVID-19 pandemic and advice from their medical provider.

Patient characteristics of suspected mast-cell activation syndrome with sinonasal obstruction: a single institution experience

BACKGROUND

Mast-cell activation syndrome (MCAS) is increasingly recognized. Sinonasal obstruction is common among these patients. There is a paucity of literature describing the characteristics of MCAS and treatment outcomes.

METHODS

Retrospective review of 192 patients with nasal congestion July 2017 to May 2019 among 3 providers (1 allergist, 2 rhinologists) was conducted. Suspected MCAS criteria were as follows: (1) at least 2 recurrent severe symptoms in addition to nasal congestion: flushing, pruritus, urticaria, angioedema, wheezing, throat swelling, headache, hypotension, diarrhea; (2) clinical response to medications that target mast cell mediators. Quality of life (QOL) outcomes were quantified using the 22-item Sino-Nasal Outcome Test (SNOT-22).

RESULTS

Thirty-two patients with nasal congestion were suspected of MCAS. The median age was 47 years; 24 of 32 were female; 13 of 32 had prior history of sinonasal surgery and 11 of 32 allergen immunotherapy. Out of 32, 19 had history of asthma, 10 drug allergy, 11 food allergy, and 10 anaphylaxis. The median number of medications targeting mast cell activation was 4 (range, 2-7). Eleven patients were offered surgery by a rhinologist after adequate medical management. Three of 32 patients showed elevation of serum tryptase. Fourteen completed pretreatment and posttreatment SNOT-22 (4/14 surgery, 10/14 medical management). Pretreatment score was 59.8 ± 6.2 (mean ± standard error [SEM]) and posttreatment score was 42.8 ± 6.7; the difference was statistically and clinically significant (p = 0.0015). Both groups showed a mean 17-point reduction.

CONCLUSION

A multidisciplinary approach to the treatment of sinonasal symptoms using both escalation of medical therapy and surgical approaches may improve QOL of patients with suspected MCAS. Consensus criteria for MCAS, which includes elevation in tryptase over baseline during an episode, may exclude the full spectrum of individuals with MCAS from potentially beneficial treatment.

Restless legs syndrome is associated with mast cell activation syndrome

STUDY OBJECTIVES

Mast cell activation syndrome (MCAS) is an inflammatory and allergic disorder. We determined the prevalence of restless legs syndrome (RLS) in MCAS because each common syndrome may be inflammatory in nature and associated with dysautonomia.

METHODS

Individuals with MCAS were evaluated for RLS by two standard questionnaires. Prevalence comparisons included spouse control patients and two prevalence publications. MCAS diagnosis required mast cell (MC) symptoms in ≥ 2 organs plus ≥ 1 elevated MC mediators, improvement with MC therapy, and/or increased intestinal MC density. Clinical variables were studied.

RESULTS

There were 174 patients with MCAS (146 female, 28 male, mean age 44.8 years) and 85 spouse control patients (12 female, 73 male, mean age 50.9 years). Patients with MCAS as a whole had a higher prevalence of RLS (40.8%) than spouse control (12.9%) (P < .0001) Male patients with MCAS had a higher prevalence of RLS (32.1%) than male controls (12.3%, odds ratio [OR] 3.4, confidence interval [CI] 1.2-9.7, P = .025), American men (8.4%, OR 5.2, CI 2.2-12.0, P < .001), and French men (5.8%, OR 7.7, CI 3.4-17.1, P < .001). Female patients with MCAS also had a higher prevalence of RLS (42.5%) than female controls (16.7%) but this did not reach statistical significance perhaps because of the sample size of the female controls. However, female patients with MCAS had a statistically higher prevalence of RLS than American women (10.0%, OR 6.7, CI 4.5-9.7, P < .0001) and French women (10.8%, OR 6.1, CI 4.4-8.6, P < .0001).

CONCLUSIONS

RLS appears to be associated with MCAS. Effects of mast cell mediators, inflammation, immune mechanisms, dysautonomia, or hypoxia may theoretically activate RLS in MCAS.

Successful treatment of idiopathic mast cell activation syndrome with low-dose Omalizumab

Objectives

Idiopathic mast cell disorders, a recently defined and recognised syndrome in clinical practice, are similar to the previously termed non-clonal mast cell disorder. Patients with idiopathic mast cell activation syndrome (MCAS) suffer all the classical signs of mast cell activation but do not have evidence of mast cell clonality. Furthermore, treatment of these patients can be limited and burdensome in those with refractory symptoms.

Methods

Here, we describe treatment of a patient with idiopathic MCAS utilising a single monthly subcutaneous injection of omalizumab and review the current classification and therapeutic options for clonal and non-clonal MCAS.

Results

Low-dose omalizumab treatment has successfully led to a 5-year, sustained clinical response, controlled debilitating symptoms of mast cell activation and allowed for reintroduction and long-term maintenance of bee venom subcutaneous immunotherapy.

Conclusion

Low-dose omalizumab of 150 mg monthly should be considered for maintenance management of patients with idiopathic MCAS for its cost and quality-of-life benefits.

Mast cell activation syndromes

Mast cell activation is common and possibly necessary for maintenance of survival. Disordered mast cell activation occurs when mast cells are pathologically overproduced or if their activation is out of proportion to the perceived threat to homeostasis. Mast cell activation syndrome refers to a group of disorders with diverse causes presenting with episodic multisystem symptoms as the result of mast cell mediator release. Despite introduction of diagnostic criteria and some advances in treatment in the last decade, many areas of mast cell activation syndrome are in need of research. This article reviews our current knowledge about the various types of mast cell activation disorders, their treatment, and areas of uncertainty in need of future investigation.

Insights in Anaphylaxis and Clonal Mast Cell Disorders

The prevalence of anaphylaxis among patients with clonal mast cell disorders (MCD) is clearly higher comparing to the general population. Due to a lower frequency of symptoms outside of acute episodes, clonal MCD in the absence of skin lesions might sometimes be difficult to identify which may lead to underdiagnosis, and anaphylaxis is commonly the presenting symptom in these patients. Although the release of mast cell (MC) mediators upon MC activation might present with a wide variety of symptoms, particular clinical features typically characterize MC mediator release episodes in patients with clonal MCD without skin involvement. Final diagnosis requires a bone marrow study, and it is recommended that this should be done in reference centers. In this article, we address the main triggers for anaphylaxis, risk factors, clinical presentation, diagnosis, and management of patients with MC activation syndromes (MCASs), with special emphasis on clonal MCAS [systemic mastocytosis and mono(clonal) MC activations syndromes].

Mastocytosis: the puzzling clinical spectrum and challenging diagnostic aspects of an enigmatic disease

Mastocytosis is a complex disorder characterized by the accumulation of abnormal mast cells (MC) in the skin, bone marrow and/or other visceral organs. The clinical manifestations result from MC-derived mediators and, less frequently, from destructive infiltration of MCs. Patients suffer from a variety of symptoms including pruritus, flushing and life-threatening anaphylaxis. Whilst mastocytosis is likely to be suspected in a patient with typical skin lesions [i.e. urticaria pigmentosa (UP)], the absence of cutaneous signs does not rule out the diagnosis of this disease. Mastocytosis should be suspected in cases of recurrent, unexplained or severe insect-induced anaphylaxis or symptoms of MC degranulation without true allergy. In rare cases, unexplained osteoporosis or unexplained haematological abnormalities can be underlying feature of mastocytosis, particularly when these conditions are associated with elevated baseline serum tryptase levels. The diagnosis is based on the World Health Organization criteria, in which the tryptase level, histopathological and immunophenotypic evaluation of MCs and molecular analysis are crucial. A somatic KIT mutation, the most common of which is D816V, is usually detectable in MCs and their progenitors. Once a diagnosis of systemic mastocytosis (SM) is made, it is mandatory to assess the burden of the disease, its activity, subtype and prognosis, and the appropriate therapy. Mastocytosis comprises seven different categories that range from indolent forms, such as cutaneous and indolent SM, to progressive forms, such as aggressive SM and MC leukaemia. Although prognosis is good in patients with indolent forms of the disease, patients with advanced categories have a poor prognosis.

Often seen, rarely recognized: mast cell activation disease--a guide to diagnosis and therapeutic options

Mast cell (MC) disease has long been thought to be just the rare disease of mastocytosis (in various forms, principally cutaneous and systemic), with aberrant MC mediator release at symptomatic levels due to neoplastic MC proliferation. Recent discoveries now show a new view is in order, with mastocytosis capping a metaphorical iceberg now called "MC activation disease" (MCAD, i.e. disease principally manifesting inappropriate MC activation), with the bulk of the iceberg being the recently recognized "MC activation syndrome" (MCAS), featuring inappropriate MC activation to symptomatic levels with little to no inappropriate MC proliferation. Given increasing appreciation of a great menagerie of mutations in MC regulatory elements in mastocytosis and MCAS, the great heterogeneity of MCAD's clinical presentation is unsurprising. Most MCAD patients present with decades of chronic multisystem polymorbidity generally of an inflammatory ± allergic theme. Preliminary epidemiologic investigation suggests MCAD, while often misrecognized, may be substantially prevalent, making it increasingly important that practitioners of all stripes learn how to recognize its more common forms such as MCAS. We review the diagnostically challenging presentation of MCAD (with an emphasis on MCAS) and current thoughts regarding its biology, epidemiology, natural history, diagnostic evaluation, and treatment.

Successful treatment of mast cell activation syndrome with sunitinib

Mast cell (MC) activation syndrome (MCAS) is a recently recognized, likely prevalent collection of heterogeneous illnesses of inappropriate MC activation with little to no MC neoplasia likely driven by heterogeneous patterns of constitutively activating mutations in MC regulatory elements including various tyrosine kinases (TKs, dominantly KIT). MCAS typically presents as chronic multisystem polymorbidity of generally inflammatory ± allergic theme. As with indolent systemic mastocytosis (SM), treatment of MCAS focuses more against MC mediators than MC neoplasia, but some cases prove refractory even to the TK inhibitor (TKI) imatinib reported useful both in uncommon SM cases not bearing SM's usual imatinib-resistant KIT-D816V mutation and in some cases of MCAS (which rarely bears KIT-D816V). Most allergy is principally a MC activation phenomenon and sunitinib is a multitargeted TKI shown helpful in controlling a murine model of oral allergy syndrome. We present the first report of use of sunitinib in life-threatening MCAS refractory to multiple agents including imatinib achieving immediate, complete, sustained, non-toxic remission suggesting a new option for treatment of aggressive MC disease.

Mast Cell Activation Disease and Microbiotic Interactions

PURPOSE

This article reviews the diagnostically challenging presentation of mast cell activation disease (MCAD) and current thoughts regarding interactions between microbiota and MCs.

METHODS

A search for all studies on interactions between mast cells, mast cell activation disease, and microbiota published on pubmed.gov and scholar.google.com between 1960 and 2015 was conducted using the search terms mast cell, mastocyte, mastocytosis, mast cell activation, mast cell activation disease, mast cell activation syndrome, microbiome, microbiota. A manual review of the references from identified studies was also conducted. Studies were excluded if they were not accessible electronically or by interlibrary loan.

FINDINGS

Research increasingly is revealing essential involvement of MCs in normal human biology and in human disease. Via many methods, normal MCs-present sparsely in every tissue-sense their environment and reactively exert influences that, directly and indirectly, locally and remotely, improve health. The dysfunctional MCs of the "iceberg" of MCAD, on the other hand, sense abnormally, react abnormally, activate constitutively, and sometimes (in mastocytosis, the "tip" of the MCAD iceberg) even proliferate neoplastically. MCAD causes chronic multisystem illness generally, but not necessarily, of an inflammatory ± allergic theme and with great variability in behavior among patients and within any patient over time. Furthermore, the range of signals to which MCs respond and react include signals from the body's microbiota, and regardless of whether an MCAD patient has clonal mastocytosis or the bulk of the iceberg now known as MC activation syndrome (also suspected to be clonal but without significant MC proliferation), dysfunctional MCs interact as dysfunctionally with those microbiota as they interact with other human tissues, potentially leading to many adverse consequences.

IMPLICATIONS

Interactions between microbiota and MCs are complex at baseline. The potential for both pathology and benefit may be amplified when compositionally variant microbiota interact with aberrant MCs in various types of MCAD. More research is needed to better understand and leverage these interactions.

Utility of hydroxyurea in mast cell activation syndrome

Mast cell activation syndrome (MCAS) is a relatively recently recognized cause of chronic multisystem polymorbidity of a generally inflammatory theme. Patients with MCAS often report migratory soft tissue and/or bone pain which frequently responds poorly to typical (narcotic and non-narcotic) analgesics as well as atypical analgesics such as antidepressants and anticonvulsants. Hydroxyurea (HU) is an oral ribonucleotide reductase inhibitor commonly used in the treatment of chronic myeloproliferative neoplasms and sickle cell anemia. HU has been used to treat systemic mastocytosis, sometimes effecting improvement in MC activation symptoms but not tumor burden, suggesting potential utility of the drug in MCAS, too. Reported here are five cases of successful use of relatively low-dose HU in MCAS to reduce symptoms including previously refractory soft tissue and/or bone pain. HU may be useful in treating mediator symptoms in MCAS, but further study is needed to define optimal dosing strategies and patient subpopulations most likely to benefit.