María Amparo Callejon-Leblic

Background: Olfactory disorders (ODs) are reported to be an early non-motor sign before the onset of deterioration in neurodegenerative diseases (NDs) such as Alzheimer’s and Parkinson’s. This systematic revision aims to review the current literature and the value of subjective olfactometry (SO) in the early diagnosis of cognitive decline and NDs. Methods: A systematic literature review was conducted following the PRISMA framework. Four different authors reviewed six different databases. The main variables analyzed were olfactory function and cognitive status. The quality of results was evaluated using the Oxford Centre of Evidence-based Medicine Levels. Results: Twenty-one cross-sectional and cohort studies and six meta-analyses were included. Most of them found an association between ODs and NDs. A prevalence of ODs greater than 80% was shown in Parkinson’s disease, proportional to the severity of symptoms. In Alzheimer’s, ODs were associated with early diagnosis and prognosis. All SO tests employed in the literature showed enough predictive value to correlate with early stages of cognitive decline. Conclusions: SO should be considered a pivotal tool when diagnosing NDs due to their association with early symptoms and prognosis. However, in the current literature, no firm consensus exists on the optimal SO tests and protocols that should be applied to the study of NDs, which prevents the interpretability and comparability of results among studies.