Eosinophilic myocarditis: comprehensive update on pathophysiology, diagnosis, prognosis and management

Hypereosinophilic syndromes are a group of disorders secondary to the accumulation of eosinophils leading to the injury of one or more organs. Among them, eosinophilic myocarditis (EM) is a rare form of inflammatory cardiomyopathy characterized by eosinophilic infiltration into myocardial tissue and subsequent release of substances with cell membrane damage and cell destruction. The degree of infiltration is thought to depend on the underlying condition, as well as the degree and duration of eosinophil exposure and ranges from mild localized disease to diffuse multifocal infiltrates associated with myocardial necrosis, thrombotic complications and endomyocardial fibrosis. The main causes of EM are hypersensitivity reactions, eosinophilic granulomatosis with polyangiitis, hypereosinophilic syndrome variants, infections and cancer. Clinical presentation can be variable, ranging from asymptomatic forms to life-threatening conditions, to chronic heart failure due to progression to chronic restrictive cardiomyopathy. Marked eosinophilia in peripheral blood, elevated serum eosinophilic cationic protein concentration and multimodality imaging may suggest the etiology of EM, but in most cases an endomyocardial biopsy must be performed to establish a definitive diagnosis. Systemic treatment varies greatly depending on the underlying cause, however the evidence of an eosinophilic infiltrate allows initiation of immunosuppressive therapy, which is the mainstay of treatment in idiopathic and in most forms of EM. Patients with helminthic infection benefit from anti-parasitic therapy, those with myeloid clone often need a tyrosine kinase inhibitor, while anticoagulant therapy should be undertaken in case of possible thrombotic complications.

Eosinophilic Lung Diseases

The eosinophilic lung diseases may manifest as chronic eosinophilic pneumonia, acute eosinophilic pneumonia, or as the Löffler syndrome (generally of parasitic etiology). The diagnosis of eosinophilic pneumonia is made when both characteristic clinical-imaging features and alveolar eosinophilia are present. Peripheral blood eosinophils are generally markedly elevated; however, eosinophilia may be absent at presentation. Lung biopsy is not indicated except in atypical cases after multidisciplinary discussion. The inquiry to possible causes (medications, toxic drugs, exposures, and infections especially parasitic) must be meticulous. Idiopathic acute eosinophilic pneumonia may be misdiagnosed as infectious pneumonia. Extrathoracic manifestations raise the suspicion of a systemic disease especially eosinophilic granulomatosis with polyangiitis. Airflow obstruction is frequent in allergic bronchopulmonary aspergillosis, idiopathic chronic eosinophilic pneumonia, eosinophilic granulomatosis with polyangiitis, and hypereosinophilic obliterative bronchiolitis. Corticosteroids are the cornerstone of therapy, but relapses are common. Therapies targeting interleukin 5/interleukin-5 are increasingly used in eosinophilic lung diseases.

Eosinophilic Lung Diseases

Eosinophilic lung diseases especially comprise eosinophilic pneumonia or as the more transient Löffler syndrome, which is most often due to parasitic infections. The diagnosis of eosinophilic pneumonia is based on characteristic clinical-imaging features and the demonstration of alveolar eosinophilia, defined as at least 25% eosinophils at BAL. Peripheral blood eosinophilia is common but may be absent at presentation in idiopathic acute eosinophilic pneumonia, which may be misdiagnosed as severe infectious pneumonia. All possible causes of eosinophilia, including drug, toxin, fungus related etiologies, must be thoroughly investigated. Extrathoracic manifestations should raise the suspicion of eosinophilic granulomatosis with polyangiitis.

Eosinophilic Pneumonia

Eosinophilic pneumonia may manifest as chronic or transient infiltrates with mild symptoms, chronic idiopathic eosinophilic pneumonia, or the frequently severe acute eosinophilic pneumonia that may be secondary to a variety of causes (drug intake, new onset of tobacco smoking, infection) and that may necessitate mechanical ventilation. When present, blood eosinophilia greater than 1 × 10⁹ eosinophils/L (and preferably greater than 1.5 × 10⁹/L) is of considerable help for suggesting the diagnosis, however it may be absent, as in the early phase of idiopathic acute eosinophilic pneumonia or when patients are already taking corticosteroids. On bronchoalveolar lavage, high eosinophilia (>25 %, and preferably >40 % of differential cell count) is considered diagnostic of eosinophilic pneumonia in a compatible setting, obviating the need of video-assisted thoracic surgical lung biopsy, which is now performed only on very rare occasions with inconsistency between clinical, biological, and imaging features. Inquiry as to drug intake must be meticulous (www.pneumotox.com) and any suspected drug should be withdrawn. Laboratory investigations for parasitic causes must take into account the travel history or residence and the epidemiology of parasites. In patients with associated extrathoracic manifestations, the diagnosis of eosinophilic granulomatosis with polyangiitis or of the hypereosinophilic syndromes should be raised. Presence of airflow obstruction can be found in hypereosinophilic asthma, allergic bronchopulmonary aspergillosis, idiopathic chronic eosinophilic pneumonia, eosinophilic granulomatosis with polyangiitis, or in the recently identified syndrome of hyperosinophilic obliterative bronchiolitis. Corticosteroids remain the cornerstone of symptomatic treatment for eosinophilic pneumonias, with a generally dramatic response. Relapses are common when tapering the doses or after stopping treatment especially in idiopathic chronic eosinophilic pneumonia. Cyclophosphamide is necessary only in patients with eosinophilic granulomatosis with polyangiitis and poor-prognostic factors. Imatinib is very effective in the treatment of the myeloproliferative variant of hypereosinophilic syndromes. Anti-interleukin-5 monoclonal antibodies are promising in the spectrum of eosinophilic disorders.

Granuloma in ANCA-associated vasculitides: another reason to distinguish between syndromes?

The 2012 renewed Chapel Hill Consensus Conference (CHCC) officially named three clinicopathological entities, i.e. granulomatosis with polyangiitis (GPA), eosinophilic granulomatosis with polyangiitis (EGPA), and microscopic polyangiitis (MPA), as major variants of anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitides (AAV). Recent genetic and cohort studies revealed the need for further differentiation between the entities, for example regarding differences in outcome. As well as ANCA reactivity, upper and lower airway disease were found to be differentiating factors for AAV variants, improving prognostic ability regarding relapse prediction and associated clinical features. Extravascular granulomatosis, or "granuloma", which describes both clinically relevant granulomatous manifestations and histopathologically documented granulomatous inflammation, is characteristic of localized and systemic GPA, but not MPA. This review summarizes new knowledge regarding granuloma in the head and neck region of AAV, its histomorphological equivalents in the upper and lower respiratory tract, and evidence for a granulomatous phenotype of a persistent localized GPA variant. This comprises the development of disease activity and damage scores for extravascular lesions in the ear, nose, and throat (ENT) regions, and imaging techniques. In addition, findings linking extravascular manifestations to granulomatous inflammation are described. We hypothesize that, as for ANCA, necrotizing granulomatous inflammation and its clinical manifestations are discriminators, assisting subclassification of AAV and/or GPA subphenotypes which will be useful both for designing clinical trials and for treating patients successfully.