Contemporary classification of chronic rhinosinusitis beyond polyps vs no polyps: a review

Importance: Chronic rhinosinusitis (CRS) is a broadly defined process that has previously been used to describe many different sinonasal pathologic conditions from odontogenic sinusitis and allergic fungal sinusitis to the more contemporary definition of broad inflammatory airway conditions. Previous classification systems have dichotomized these conditions into CRS with nasal polyps and CRS without nasal polyps. However, clinicians are learning more about the inflammatory subtypes of CRS, which can lead to improved delivery and effectiveness of treatment. Observations: In clinical practice, treatment decisions are often based on observable findings, clinical history, presumed disease, and molecular pathophysiologic characteristics. A proposed classification system is simple and practical. It proposes that the functional anatomical compartments involved create the first level of separation into local and diffuse CRS, which are usually unilateral or bilateral in distribution. Diffuse does not imply "pansinusitis" but simply that the disease is not confined to a known functional anatomical unit. This classification takes into account whether local anatomical factors are associated with pathogenesis. Then the inflammatory endotype dominance is separated into a type 2 skewed inflammation, as this has both causal and treatment implications. The non-type 2 CRS encompasses everything else that is not yet known about inflammation and may change over time. The phenotypes or clinical examples are CRS entities that have been described and how they align with this system. Conclusions and Relevance: Although research continues to further define the subtypes of CRS into phenotypes and endotypes, the proposed classification system of primary CRS by anatomical distribution and endotype dominance allows for a pathway forward.