Limited observed efficacy of olfactory training in severe long-term post-COVID-19 syndrome: a prospective cohort study.

PURPOSE: Persistent chemosensory dysfunction is a debilitating symptom of Post-COVID-19 Syndrome (PCS). While olfactory training (OT) is effective in early post-viral smell loss, its efficacy in chronic, multisystemic PCS remains unclear. This study aimed to characterize a severely affected cohort and to evaluate changes in olfactory function during a structured OT intervention. METHODS: In this prospective study, 29 patients with persistent PCS lasting > 12 weeks (mean duration approximately 30 months) were recruited from a specialized post-COVID outpatient clinic. All patients underwent structured OT (standardized twice-daily exposure protocol) for 12 weeks. Participants underwent psychophysical testing (Sniffin' Sticks TDI, Taste Strips) and completed questionnaires (self-MOQ, VAS, EQ-5D-5 L) before and after a 12-week OT period. RESULTS: Of 25 patients included in the final analysis, 9 presented with psychophysical anosmia based on the TDI score. The cohort exhibited a high symptom burden, with health-related quality of life below population norms. Following OT, no statistically significant changes were observed in the objective TDI scores (19.0 ± 8.2 vs. 20.1 ± 10.5; p = 0.52). Similarly, subjective ratings (VAS) and questionnaires assessing olfactory quality of life showed no significant differences between baseline and follow-up. While individual transient improvements were reported, a persistant discrepancy between objective data and subjective perception was observed. CONCLUSION: Structured olfactory training was not associated with measurable improvement within the 12-week observation period in this cohort of chronic PCS patients. The lack of response may indicate that, in this severely affected subgroup, the effectiveness of olfactory training may be limited by factors beyond peripheral olfactory dysfunction, including reduced cognitive and functional capacity.