According to study results reported in Multiple Sclerosis Journal, odor identification and discrimination are olfactory qualities associated with neurodegeneration in patients with multiple sclerosis (MS).

A total of 260 patients with MS were recruited from an outpatient clinic of the Medical University of Innsbruck in Austria. The 3-stage Sniffin’ Sticks test was used to evaluate olfactory function, particularly with regard to olfactory threshold, discrimination, and identification. In the test, participants were asked to identify the stick with an odorant and distinguish between different sticks with the same odorant.

In addition, the investigators used spectral-domain optical coherence tomography to measure peripapillary retinal nerve fiber layer (pRNFL) thickness. Correlations were made between olfactory testing results and age, sex, disease duration, relapses, Expanded Disability Status Scale, cognitive function, depression, smoking, and pRNFL thickness via multivariable linear regression models.

The study investigators found a significant correlation between the olfactory threshold and the number of relapses in the year before assessment. Specifically, the threshold was impaired in patients with a relapse in the year before vs patients without a relapse in the same period (4.75 vs 7.00, respectively; P <.0001). In addition, a significant association was found between olfactory threshold and shorter disease duration (ρ= 0.295; P <.001).

Discrimination and identification were also correlated with longer disease duration (discrimination: ρ= −0.376, P =.023; identification: ρ= −0.253, P <.001), higher Expanded Disability Status Scale (discrimination: ρ= −0.334, P <.001; ρ= −0.512, P <.001), and reduced cognitive function (cognitive function: ρ= 0.269, P <.001; ρ= 0.417, P <.001). Odor identification and discrimination were the only qualities associated with pRNFL thickness (identification: ρ= 0.384, P <.001; discrimination: ρ= 0.205, P <.001).

The lack of magnetic resonance imaging or body fluid biomarker data available for analyzing olfactory function represents a possible study limitation.

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“Assessment of olfactory function provides an opportunity to stratify MS patients with regard to inflammation and neurodegeneration,” the researchers wrote. “Future research should focus on their potential role as predictors of future disease course and treatment response.”

Reference

Bsteh G, Berek K, Hegen H, et al. Smelling multiple sclerosis: Different qualities of olfactory function reflect either inflammatory activity or neurodegeneration [published online November 22, 2018]. Mult Scler. doi:10.1177/1352458518814113