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

Abstract

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

Acute Loss of Smell or Taste Without Nasal Blockage Should Raise Suspicion for COVID-19 Infection

“…Loss of smell and/or taste are common clinical manifestations that have long been associated with a spectrum of medical conditions, including neurodegenerative diseases such as Alzheimer's, in which olfaction is affected. 1 The evidence associating these clinical symptoms to COVID-19 has led to their inclusion on the list of COVID-19 symptoms by the Unites States Centers for Disease Control in April 2020 and brought these symptoms to the spotlight. The olfactory disturbances (anosmia) typically observed in upper respiratory infections are associated with inflammation and irritation of nasal mucosa or nasal blockage (rhinitis).…”

Olfactory dysfunction in SARS-CoV-2 infection: Focus on odorant specificity and chronic persistence

“…Following an accumulation of publications in 2020, it has become widely accepted that complete (anosmia) or partial (hyposmia or microsmia) loss of olfaction as well as the loss of taste (ageusia) are common symptoms induced by SARS-CoV-2 ( [1] , [2] , [3] , [4] , [5] , [6] , [7] , [8] , [9] , [10] ). Due to its early onset during the disease course, it was proposed that olfactory testing be implemented as an early screening measure ( [11] , [12] , [13] , [14] , [15] ).…”

The Association between Olfactory Function and Cognitive Impairment in Older Persons with Cognitive Impairments: A Cross-Sectional Study

“…Several previous studies have demonstrated that cognitive impairment in older persons is related to general characteristics such as age, sex, and education [ 2 ], lifestyle factors such as drinking and smoking [ 3 ], and disease factors such as diabetes [ 4 ] and depression [ 5 ]. In recent years, many researchers have become increasingly interested in studies on the olfactory impairment observed in the early stages of neurodegenerative diseases, such as Alzheimer’s disease (AD), and Parkinson’s disease (PD), and cognitive impairment [ 6 , 7 , 8 , 9 , 10 , 11 ].…”