Impaired olfaction post-coronavirus disease 2019: a systematic review of smell recovery predictive factors

Abstract

Background The devastating coronavirus disease (COVID-19) pandemic seemed not yet to cease. Numerous studies regarding its typical sign and symptoms have been done, presenting one of the most promising predictors of the infection: olfactory dysfunction. Although not life-threatening, the symptom could decrease one’s quality of life, especially if persistent throughout their entire life. Among the countless literature regarding loss of smell, only limited studies denote predictors of smell recovery. This systematic review aimed to bridge the knowledge gap of olfactory impairment prevalence and recovery predictors in people with COVID-19. Methods This review was carried out through journal databases, including PubMed, Science Direct, Google Scholar, and medRxiv. Literature published from 2020 to 2022 that complied with the inclusion and exclusion criteria was retrieved, scanned for duplicates with Zotero, and reported using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocols 2020 (PRISMA) guidelines. Results Of the 2243 studies, seven were assessed with the Prediction model Risk Of Bias ASsessment Tool (PROBAST) to analyze the risk of bias, which five cohort studies deemed relevant. Olfactory dysfunction, olfactory recovery, and its predictive factors are noted. This review is registered in PROSPERO (Registration Number CRD42022318412). Conclusions No clinical markers predicted the recovery of olfactory dysfunction, but patients who are more likely to recover are associated with younger age, female sex, and having COVID-19-related symptoms such as nasal congestion and trigeminal sensation. Modifiable factors are still dubious in predicting the olfaction recovery.

Olfactory Dysfunction in the COVID-19 Era: An Umbrella Review Focused on Neuroimaging, Management, and Follow-up

“…In most reviews, Age, gender, and medical comorbidities were predictors of post-COVID-19 olfactory recovery. It is hypothesized that individuals aged<40 years have more tissue injury resistance than older individuals, and the age of >70 years is a predictor of poor long-term recovery from COVID-19-related OD [28], On the other hand, another study found that young adults are at an increased risk of olfactory dysfunction [34]. In contrast to two studies demonstrating a higher recovery rate in women [26, 28], another review concluded that women were more likely to continue the olfactory dysfunction [30].…”

“…It is hypothesized that individuals aged<40 years have more tissue injury resistance than older individuals, and the age of >70 years is a predictor of poor long-term recovery from COVID-19-related OD [28], On the other hand, another study found that young adults are at an increased risk of olfactory dysfunction [34]. In contrast to two studies demonstrating a higher recovery rate in women [26, 28], another review concluded that women were more likely to continue the olfactory dysfunction [30]. A less severe form of COVID-19 disease in non-hospitalized patients is associated with a greater prevalence of anosmia, and longer symptoms [26, 31, 33].…”

“…A less severe form of COVID-19 disease in non-hospitalized patients is associated with a greater prevalence of anosmia, and longer symptoms [26, 31, 33]. Two other studies have suggested that highly severe cases have a higher risk of developing long-lasting anosmia and experiences a persistent deficit [28, 32].…”

“…Ten studies examined the long-term course and recovery rate from OD [19,[26][27][28][29][30][31][32][33][34]. According to two studies, the majority of OD cases remained chronic, and most patients recovered incompletely within the first weeks after the elimination of other symptoms.…”

“…Theophylline is proven to be effective in neural generation by influencing cell massages. One study on hyposmia showed approximately half of the patients benefited from theophylline [28]. Moreover, another study showed greater doses of theophylline could make more improvement in OD [58].…”