Clinical Evidence of Type 2 Inflammation in Non-allergic Rhinitis with Eosinophilia Syndrome: a Systematic Review

Non-allergic rhinitis (NAR) includes different subtypes, among which NAR with eosinophilia syndrome (NARES) is the most important because of severity of symptoms and the high risk of comorbidities. Its pathophysiology is still object of debate, but a crucial role of chronic eosinophilic inflammation has been recognized. The aim of this review is to critically analyze the current evidence regarding the hypothesis that NARES may be considered a type 2 inflammatory disorder.The definition and diagnostic criteria for NARES are not universally shared and adopted, thus generating difficulties in reproducing the results. At present, there is extreme heterogeneity in sampling methods and disagreement in the cut-off of local eosinophilic count to determine a diagnosis of NARES. The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) standard was applied to identify English-language experimental and clinical articles regarding NARES. The search was performed in April 2021. Twenty-six articles were included. Our data suggest a particular heterogeneity regarding sampling and specific cut-offs adopted for diagnosis of NARES and consensus should be reached. We suggest that eosinophil count should be reported as an absolute value for at least 10 observed rich fields in order to increase the level of standardization. Consensus among authors on this topic should be reached with particular attention to the cut-off for diagnosis. In the future, this limitation may be overcome by the identification of repeatable biomarkers to refine diagnosis and prognosis of NARES. Furthermore, our data strongly suggest that NARES have numerous similarities with clinical features of the most common type 2 diseases such as eosinophilic asthma and chronic rhinosinusitis with nasal polyps (CRSwNP): late onset, association with type 2 comorbidities, selective eosinophilic tissue infiltration, remarkable response to oral and intranasal corticosteroids, and progression in a type 2 CRSwNP.