Loss of Olfactory Function—Early Indicator for Covid-19, Other Viral Infections and Neurodegenerative Disorders

International audience; The loss of the senses of smell (anosmia) and taste (ageusia)are rather commondisorders, affecting up to 20% of the adult population. Yet,this condition has notreceived the attention it deserves, most probably because per se such a disorder isnot life threatening. However, loss of olfactory function significantly reduces the qualityof life of the affected patients, leading to dislike in food and insufficient, exaggeratedor unbalanced food intake, unintentional exposure to toxins such as household gas,social isolation, depression, and an overall insecurity. Not only is olfactory dysfunctionrather prevalent in the healthy population, it is, in many instances, also a correlate or anearly indicator of a panoply of diseases. Importantly, olfactory dysfunction is linked to thetwo most prominent neurodegenerative disorders, Parkinson’s disease and Alzheimer’sdisease. Anosmia and hyposmia (reduced sense of smell) affect a majority of patientsyears before the onset of cognitive or motor symptoms, establishing olfactory dysfunctionas early biomarker that can enable earlier diagnosis and preventative treatments. In thecurrent health crisis caused by SARS-CoV2, anosmia and dysgeusia as early-onsetsymptoms in virus-positive patients may prove to be highly relevant and crucial forpre-symptomatic Covid-19 detection from a public health perspective, preceding bydays the more classical respiratory tract symptoms such as cough, tightness of thechest or fever. Thus, the olfactory system seems to be at the frontline of pathologicassault, be it through pathogens or insults that can lead to or at least associatewith neurodegeneration. The aim of this review is to assemble current knowledgefrom different medical fields that all share a common denominator, olfactory/gustatorydysfunction, and to distill overarching etiologies and disease progression mechanisms.