Olfaction and neurocognition after COVID-19: a scoping review

Abstract

IntroductionCOVID-19 induces both acute and chronic neurological changes. Existing evidence suggests that chemosensory changes, particularly olfactory loss, may reflect central neurological dysfunction in neurodegenerative diseases and mark progression from mild cognitive impairment to Alzheimer’s. This scoping review summarizes the available literature to evaluate the relationship between neurocognition and olfaction in young to middle-aged adults with minimal comorbidities following COVID-19 infection.MethodsA literature search of PubMed, Ovid Embase, Web of Science, and Cochrane Library was conducted. Studies underwent title/abstract and full text screening by two reviewers, with a third reviewer resolving any conflicts. Remaining studies underwent data extraction.ResultsSeventeen studies were eligible for data extraction after the review process, where 12 studies found significantly poorer cognition in those suffering from olfactory dysfunction, four studies showed no association between cognition and olfaction, and one study reported lower anosmia prevalence among patients with cognitive impairment.ConclusionThe majority of studies in this review find that olfactory dysfunction is associated with poorer cognition. More rigorous studies are needed to further elucidate the relationship between olfaction and cognition after COVID-19.

Neuropsychological functioning and its correlates at 1 year follow‐up of severe COVID‐19

“…Although self-report measures of anosmia are widely used and are considered valid (Wu et al, 2023), the results of the studies using self-report measures are less consistent than the results of those studies using objective measures to predict cognitive impairment. 20 It is important to mention some of the limitations of our prospective study. The severity of the first wave of the COVID-19 pandemic crisis conditioned the selection of the population group and an incidental sample was recruited.…”

“…This contradicts previously described literature showing that anosmia is related to a more impaired cognitive performance. 19,20 However, in some studies anosmia is also related to a better general performance and lower mortality rate. 58,60 This inconsistence may be due to anosmia method of collection.…”

“…Although most studies of anosmia use subjective measures, the results of these studies are less consistent and it can increase the risk of bias, therefore more systematic measures of anosmia are recommended. 20 A number of strengths counterbalance the aforementioned limitations. An important strength is that our study included a comprehensive neuropsychological assessment to analyze how COVID-19 specifically affects each cognitive domain.…”

“…17 The presence of olfactory dysfunction has been related to an impaired cognitive performance after COVID-19 infection in most studies although more evidence is needed to better understand the relationship between anosmia and cognition after COVID-19. [18][19][20] Currently, there is increasing evidence regarding the causal link between SARS-CoV-2 infection and cognitive impairment in the first months after infection. 9,11 Nonetheless, evidence about the evolution of these symptoms at different time points is scarce.…”

Transcutaneous vagus nerve stimulation improves Long COVID symptoms in a female cohort: a pilot study

“…Neuroimaging studies have detected damages in various limbic and associative brain regions following COVID-19 infection, likely indicative of neuroinflammation and possible neurodegeneration ( 12 , 13 ). Notably, chronic loss of smell correlates with cognitive decline and is an early predictor of Alzheimer’s disease ( 14 ). Female sex, as well as early presence of olfactory and neurological symptoms, are well-known risk factors for dementia and Alzheimer’s disease ( 15 , 16 ).…”