Anosmia in COVID-19 and post-COVID syndrome

Anosmia is one of the most frequent complaints in COVID-19 and post-COVID syndrome. The exact mechanism through which the virus leads to this symptom is still unclear. A proposed mechanism is that in "immune-balanced” individuals, the coronavirus damages olfactory epithelium and induce acute local inflammation with edema and the development of anosmia, which is recovered after acute phase. However, in subjects with genetical predisposition for developing autoimmunity, the immune response to the virus could create a parallel autoimmune response creating autoantibodies targeting the olfactory receptors and similar peptides (G-protein-coupled receptors), permitting an inflammatory process in the central nervous system through the olfactory bulb, where induces inflammation with the development of neurological symptoms, including persistent anosmia, fatigue, brain fog, etc. © 2023 Elsevier Inc. All rights reserved.

Insights into the molecular triggers of parosmia based on gas chromatography olfactometry.

Background: Parosmia is a debilitating condition in which familiar smells become distorted and disgusting, with consequences for diet and mental health. It is a feature of post-infectious olfactory loss, particularly resulting from COVID-19. There is currently little understanding of its pathophysiology, and the prevailing hypothesis for the underlying mechanism is aberrant growth of regenerating olfactory sensory neurons after damage. Methods: We use gas-chromatograph olfactometry to individually present components of a complex olfactory mixture as a rapid screening tool for assessment of both quantitative and qualitative olfactory dysfunction in those with and without parosmia. This allows them to report the associated sensory effects and to identify those molecules which are altered or parosmic in nature. Results: Here we show 15 different molecular triggers of this symptom. These trigger molecules are common to many in the parosmic volunteer group and share certain characteristics such as extremely low olfactory threshold and common molecular structure. Conclusions: We posit that specific highly odour-active molecules are the cause of the parosmic symptom in most cases and initiate the sense of disgust, suggesting that parosmia is, at least in part, a receptor-level phenomenon.

COVID-19-Related Anosmia: The Olfactory Pathway Hypothesis and Early Intervention.

Anosmia is a well-described symptom of Corona Virus Disease 2019 (COVID-19). Several respiratory viruses are able to cause post-viral olfactory dysfunction, suggesting a sensorineural damage. Since the olfactory bulb is considered an immunological organ contributing to prevent the invasion of viruses, it could have a role in host defense. The inflammatory products locally released in COVID-19, leading to a local damage and causing olfactory loss, simultaneously may interfere with the viral spread into the central nervous system. In this context, olfactory receptors could play a role as an alternative way of SARS-CoV-2 entry into cells locally, in the central nervous system, and systemically. Differences in olfactory bulb due to sex and age may contribute to clarify the different susceptibility to infection and understand the role of age in transmission and disease severity. Finally, evaluation of the degree of functional impairment (grading), central/peripheral anosmia (localization), and the temporal course (evolution) may be useful tools to counteract COVID-19.