Olfactory and gustatory dysfunctions in SARS-CoV-2 infection: A systematic review

Abstract

Background Among Coronavirus Disease 2019 (COVID-19) manifestations, Olfactory (OD) and Gustatory (GD) Dysfunctions (OGD) have drawn considerable attention, becoming a sort of hallmark of the disease. Many have speculated on the pathogenesis and clinical characteristics of these disturbances; however, no definite answers have been produced on the topic. With this systematic review, we aimed to collect all the available evidence regarding the prevalence of OGD, the timing of their onset and their resolution, their rate of recovery and their role as diagnostic and prognostic tools for Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection. Methods A systematic review comprising all the observational studies that reported the prevalence and/or the longitudinal trajectories of OGD in COVID-19 patients, as self-reported by patients or measured through objective psychophysical tests. Results After the selection process, 155 studies were included, with a total of 70920 patients and 105291 not-infected individuals. Prevalence reports were extremely variable across studies, with wide ranges for OD (0% - 98%) and GD (0-89%) prevalence. OGD occurred early during the disease course and only rarely preceded other symptoms; out of 30 studies with a follow-up time of at least 20 days, only in 5 studies OGD fully resolved in more than 90% of patients. OGD had low sensitivity and high specificity for SARS-CoV-2 infection; accuracy of OD and GD for infection identification was higher than 80% in 10 out of 33 studies and in 8 out of 22 studies considered, respectively. 28 out of 30 studies that studied the association between OGD and disease severity found how OGD were associated with lower rates of severe pneumonia, hospitalization and mortality. Conclusions OGD seem to be highly prevalent in SARS-CoV-2 infection. They occur early, concomitantly with other symptoms and often persist after recovery, in some cases for months; whether a full recovery eventually occurs in all cases is not clear yet. OGD are good predictors of SARS-CoV-2 infection and are associated with a milder disease course.

The D614G Virus Mutation Enhances Anosmia in COVID-19 Patients: Evidence from a Systematic Review and Meta-analysis of Studies from South Asia

“… 1 , 83 − 86 Finally, objective methodology to assess olfactory dysfunction may be more sensitive than subjective recall, resulting in an increased prevalence, although studies are controversial. 7 Response bias may occur if early studies did not inquire about olfactory dysfunction as much as later studies.…”

“… 1 , 7 , 93 Some studies using objective tests showed an increase in the prevalence of olfactory dysfunction, 1 , 2 , 82 , 93 − 95 while other studies reported the opposite (reviewed in Boscutti et al, 2021). 7 In the studies on South Asian COVID-19 patients, objective tests were used in only three of the cohorts with G614 virus dominance. To avoid methodology as a confounding variable, we considered only studies with subjective questioning.…”

The D614G virus mutation enhances anosmia in COVID-19 patients: Evidence from a systematic review and meta-analysis of studies from South Asia

“…1,[83][84][85] Finally, objective methodology to assess olfactory dysfunction may be more sensitive than subjective recall, resulting in an increased prevalence, although studies are controversial. 7 Response bias may occur if early studies did not inquire about olfactory dysfunction as much as later studies.…”

“…1,7,93 Some studies using objective tests showed an increase in the prevalence of olfactory dysfunction, 1,2,82,[93][94][95] while other studies reported the opposite (reviewed in Boscutti et al, 2021). 7 In the studies on South Asian COVID-19 patients, objective tests were used in only three of the cohorts with G614 virus dominance. To avoid methodology as a confounding variable, we considered only studies with subjective questioning.…”

“…Some studies reported the prevalence of anosmia or hyposmia to be lower than 1%, while others reported a prevalence of over 70%. 1,2,[4][5][6][7] Such differences may be due to multiple factors, but two reasons are currently considered to be most relevant: differences at the level of the human host (variants in the virus entry proteins, angiotensin-converting enzyme 2, ACE2, and/or in the protease TMPRSS2), and differences at the level of the virus (mutations of the spike protein resulting in an altered efficiency of entry to the host cell, and therefore higher infectivity), or a combination of these host and virus factors. 1,2,4,5,[8][9][10][11][12][13] For historical reasons of how the pandemic unfolded, it has been difficult to define the relative contributions of host and virus factors for anosmia.…”