Long-Term Olfactory Dysfunction in COVID-19 Patients: A Systematic Review

Olfactory dysfunction (OD) emerged early in the COVID-19 pandemic as a prevalent and often persistent symptom. While most individuals recover within weeks, a significant proportion continue to suffer from long-term impairments, including both quantitative and qualitative sensory deficits. Our review aimed to summarize current evidence on long-term post-COVID-19 OD with a duration of at least three months, including prevalence, recovery trajectory, and prognostic factors. The PubMed and Scopus databases were searched for relevant studies up to August 2024 following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Twenty-one studies were ultimately included, involving over 4,000 individuals. A remarkable proportion of patients continue to experience persistent dysfunction post-infection for a period ranging from several months to over two years. Qualitative disorders, such as parosmia and phantosmia, frequently appeared during recovery. Prognosis seemed to be related to age, initial severity, duration of OD, co-existing symptoms, and potentially sex. A consistent discrepancy between subjective reports and objective psychophysical test results was observed. Methodological heterogeneity limited comparability across studies. Olfactory dysfunction is a significant and often overlooked long-term complication of COVID-19. Standardized diagnostic criteria, validated outcome measures, and prospective longitudinal research are urgently needed to guide evidence-based management and improve patient outcomes.