COVID-19 related olfactory impairment – diagnostics, significance and treatment

Abstract

Aim: Smell disorders are frequent symptoms of COVID-19 disease. This systematic review covers the pathophysiology, diagnostics, prognostic significance, and treatment of COVID-19 related smell disorders. Methods: The articles were selected in accordance with the PRISMA guidelines. The inclusion criteria were: 1. peer-reviewed publications;2. studies on human subjects;3. studies published in English or Czech;and 4. used filters: meta-analysis/systematic review/randomized controlled trial. The exclusion criteria were: 1. abstracts from conferences;2. commentaries;and 3. inclusion of subjects younger than 18 years. The databases PubMed and Web of Science were searched. Using the search term “anosmia OR smell loss OR smell disorders OR dysosmia OR parosmia AND COVID-19”, 157 papers were selected for analysis, with 109 being ruled out based on the exclusion criteria. The full texts and their references were obtained and studied, and the references meeting the inclusion criteria were also included in this review, leading to a total of 68 papers selected for the review. Results: Approximately 60% of patients with COVID-19 disease present with smell disorders. Most studies agree anosmia appears more frequently in the mild course of the disease. The inflammation and demyelination of the olfactory nerve fibres probably play a key role in the pathogenesis. Damage to the supporting cells of the olfactory epithelium may also play an important role. Olfactory disorder may persist over a varying period of time after the resolution of the acute phase of COVID-19. Olfactory training seems to be beneficial in the treatment. The data regarding the efficacy of topical corticosteroids are inconsistent. No other drugs have yet demonstrated a clinical effect. Conclusion: Loss of smell is a common accompanying symptom in mostly mild forms of COVID-19. Olfactory training seems to be effective in treating this dysfunction. Pharmacotherapy (including local and systemic corticosteroids) has not yet proven effective.