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Loss of smell is a characteristic finding of COVID-19. It may outpersist the resolution of the disease, though recovery varies significantly, ranging from 15 to 180 days.1 Studies have purported central nervous system involvement in COVID-19 anosmia, mostly in the orbitofrontal cortex (OFC), the neural substrate for conscious olfactory perception.2 Given the high prevalence of COVID-19, an enormous number of patients worldwide are at risk of long-term loss of smell. Olfaction is essential for detecting environmental hazards and for enjoying food. Smell loss can cause mood disorders, even suicidal ideation.3 Treating olfactory dysfunction is therefore of paramount importance. Different interventions have been tried in order to alleviate COVID-19 hyposmia, but with limited efficacy.4
Non-invasive brain stimulation techniques can be deemed as a promising alternative to traditional neurorehabilitative approaches for several diseases, including smell disturbances.5 In this double-blind, sham-controlled study, we implemented a 2-week session of combined olfactory training (OT) and anodal transcranial direct current stimulation (A-tDCS) in seven patients with persistent COVID-19 hypo/anosmia, with the aim of investigating the effect of tDCS on olfactory function.
Materials and methods
Seven consecutive patients were enrolled in the study, according to the following inclusion criteria: (1) persistent (at least 6 months) hypo/anosmia due to COVID-19; (2) a score below 12 on the ‘Sniffin’ Sticks’ identification subtest. The exclusion criteria were: (1) severe mood disorder; (2) rhinological diseases; (3) epilepsy; (4) sensitive scalp. No medications for alleviating olfactory symptoms were allowed. Patients’ smell performances were first assessed immediately preceding stimulation (t0). A visual analogue scale smell …
Contributors LV and FB conceived the study and wrote the manuscript. LM and CT contributed to the design of the study. DB, RFC, CB and PC contributed to the acquisition of the data. DS and EC participated in the experiments. MP analysed the data.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.