Long-term self-reported symptoms and psychophysical tests in COVID-19 subjects experiencing persistent olfactory dysfunction: a 4-year follow-up study

Background: Since the onset of the COVID-19 pandemic, chemosensory dysfunction (CD), including olfactory and taste quantitative dysfunction (OD/TD), has emerged as a prevalent and early symptom in SARS-CoV-2-infected subjects. This study explores the prevalence, duration, and recovery trajectory of COVID-19-related olfactory dysfunction (C19OD), with a specific focus on the four-year follow-up. Methods: Using a combination of psychophysical tests (Sniffin' sticks) and patient-reported outcome measures (sVAS and tVAS), 83 participants were prospectively evaluated for OD and parosmia. Factors influencing long-term olfactory recovery were analysed. Results: Baseline assessments revealed OD in 56.6% of patients, with progressive improvement observed over 4 years. At the four-year follow-up, 92.3% of patients recovered their olfaction while the remaining still reported hyposmia. Younger age and olfactory training were found to be favourable prognostic factors. Conclusion: Our findings show that, despite most individuals with C19OD recover olfaction within the first year, a subset of them continue to experience prolonged CD, demonstrating a slow, constant and meaningful improvement over years. This prolonged recovery period highlights the complexity of SARS-CoV-2's impact on olfactory function and highlights the need of further research on CD pathophysiology with the aim to improve therapeutic approaches to C19OD.