Olfactory dysfunction (OD), defined as the reduced or distorted ability to smell during sniffing (orthonasal olfaction)*** or eating (retronasal olfaction)***, is often reported in mild or even asymptomatic cases.
OD could act as a marker for disease, particularly among individuals who are otherwise minimally symptomatic or asymptomatic, prompted organizations, such as the American Academy of Otolaryngology–Head and Neck Surgery and ENT UK, to recommend inclusion of sudden-onset loss of smell and/or taste as part of the diagnostic criteria for COVID-19 disease, as has now been done by the CDC.
Coronaviruses are one of many pathogens known to cause postinfectious OD***, and nasal epithelial cells show relatively high expression of the angiotensin-converting enzyme 2 receptor, which is required for SARS-CoV-2 entry.
Disruption of cells in the olfactory neuroepithelium may result in inflammatory changes that impair olfactory receptor neuron function, cause subsequent olfactory receptor neuron damage, and/or impair subsequent neurogenesis.