Previous studies reviewing the presence of olfactory dysfunction (OD) in Multiple Sclerosis (MS) have focused on the prevalence and types of dysfunction, but have not focused on longitudinal changes.
In this review, the existing literature investigating MS and the changes of olfaction over time was examined. Olfactory dysfunctions role in the diagnosis and prognostication of MS was also explored.
A scoping review of the literature was performed to identify longitudinal studies of MS and OD. Systematic searches of PubMed, Google Scholar, AgeLine, CINAHL, PSYCH info, Web of Science, Embase, and Ovid MEDLINE were performed using terms that encompassed both MS and olfaction. The aim of our review was to build upon the existing literature by summarizing findings that can only be demonstrated longitudinally. Of 6938 articles that were identified from the search, 9 studies met inclusion criteria and underwent review. Each study observed olfaction in relapsing-remitting MS and/or progressive MS in a longitudinal manner. Olfaction was measured and scored using various testing arrays; these scores were then correlated against a multitude of clinical markers.
Across all studies, patients with MS demonstrated increased OD. Two main patterns were identified: (1) clinical markers of inflammation correlated most with an increased odor threshold in MS. (2) Clinical markers of neurodegeneration, or progression of disease, correlated most with a decreased ability to discriminate and identify odors.
These studies suggest that olfaction is a dynamic, dependent variable of inflammation and neurodegeneration, correlating with disease activity and progression. This opens the door for future exploration of olfactions relationship with MS diagnosis, prognostication, and monitoring of disease.