The nose: gatekeeper and trigger of bronchial disease.

The nose is strategically placed at the entrance of the airway. Nose breathing takes place under physiologic circumstances and protects the lower airways from exposure to unconditioned air and exogenous particles. Alternatively, nasal disease may have a negative impact on lower airway biology, being involved in aggravation of bronchial disease. The interaction between upper and lower airway disease has been recognized for centuries. Due to the increase in prevalence of allergic diseases during the last decades, new interest has been gained in understanding the mechanisms underlying the interaction between rhinitis and asthma. Nowadays, allergic rhinitis and asthma are considered part of a global airway disease, with both diagnostic and therapeutic consequences for every day clinical practice. Besides allergy, other inflammatory conditions of the upper airways are associated with lower airway disease via unknown mechanisms. Viral rhinitis often coincides with exacerbations of bronchial disease, chronic sinus disease with or without nasal polyps frequently relates to bronchial dysfunction and occupational rhinitis and asthma are often present in the same individuals. In spite of the clinical relevance of considering the airway as one organ with major involvement of disease in upper, lower or both parts, many clues to understand the pathology still remain to be explored. This manuscript aims at providing a comprehensive overview of the current knowledge on the interaction between nasal disease and lower airway biology and stresses the importance of further research on this important matter.