Methods
We systematically searched MedLine (PubMed), Embase, the Health Management Information Consortium (HMIC), Medrxiv, the Cochrane Library, the Cochrane COVID-19 Study Register, NIHR Dissemination centre, Clinical Evidence, National Health Service Evidence and the National Institute of Clinical Excellence to identify the current published evidence which associates coronaviridae or similar RNA viruses with anosmia. The search strategy for Medline and Embase are demonstrated in Appendix 1. The final search was undertaken in 18thApril 2020. We included all years and all languages in the search.
The initial search identified 157 articles. 145 articles were excluded as they did not investigate a link between the current coronavirus outbreak and OD, were conference abstracts, isolated case reports or did not have an English version available. The literature search is presented in the Prisma flow diagram (image 1). One case series presented loss of smell and taste in combination, where patients were included if they had experienced either symptom. As such we were unable to isolate olfactory dysfunction in their population. Reporting related but independent symptoms in this way prevents formal analysis of their individual epidemiological factors and impact on patient outcomes.
We used the ROBINS-E (Risk of Bias in Non-randomised Studies – of Exposures) tool to assess the studies for bias. The articles were assessed across 7 parameters; confounding factors, selection of participants, classification of exposures, departures from intended exposures, missing data, measurement of outcomes and the reported result. All the studies were assessed at “serious” risk of bias due these common themes; lack of adjustment for confounding variables, differences in follow up and the start of exposure, variation in reporting, and numerous different subgroups reported.