ABSTRACT
Introduction Olfactory dysfunction is mostly caused by post upper respiratory tract infections (URTI). This is currently becoming apparent due to the Covid-19 pandemic, with olfactory dysfunction being a common early symptom that affects a significant fraction of people infected by Covid-19. Objectives The objective of this systematic review is to determine evidence on interventions to improve olfactory performance among patients with post-infectious smell disorders and identify implications for those affected by Covid-19 induced olfactory dysfunctions. Methods A systematic search of relevant literature published between January 2010 and November 2020 was conducted in the databases Pubmed, CINAHL, Embase and Cochrane Central Register of Controlled Trials. Further studies were assessed by reference tracking. Results Based on 13 studies included in this review, two types of interventions to improve olfactory function in postinfectious patients were found. Eight studies describing olfactory training interventions showed a significant improvement of TDI Score and/or its subscores odour threshold (T), odour discrimination (D) and odour identification (I). Five studies assessing the effect of olfactory therapy or olfactory therapy combined with olfactory training revealed diverging results with only significant effects on small scale. A negative correlation of disease duration and improvement of olfactory function was found in four trials. Conclusion Results of this review show a promising but limited effect to improve olfactory function in patients with postinfectious olfactory disorder with either olfactory training or olfactory therapy. The work presented encourages to continue researching in this field and raise awareness of this highly topical issue.