Serum neurofilament light levels correlate with change of olfactory function in multiple sclerosis

Background: Serum neurofilament light chain levels (sNfL) and impairment of olfactory function emerge as biomarkers in multiple sclerosis (MS). However, the relation between sNfL and olfactory function in MS has not been investigated yet. Objective: We aimed to determine whether sNfL levels correlate with olfactory function in relapsing-remitting (RR) MS. Methods: We annually measured sNfL and olfactory function (Sniffin' Sticks test: Threshold (T) and combined discrimination-identification (DI) score) in 80 RRMS patients and compared sNfL to T and DI scores. Results: T scores significantly correlated with sNfL levels at simultaneous measurement (-1.5 points, 95% CI: -2.6-0.5 per 10 pg/ml sNfL increase; p<0.001 per 10 pg/ml sNfL increase), but not at temporally distant measurement. Patients with >= 2 sNfL measures above the 75th percentile displayed significantly larger DI decrease (median 3.0 points, IQR 2.0-4.5) compared to patients with no or only one sNfL measure above the 75th percentile (0.0, IQR -0.5-0.5, p<0.001 and 1.0, IQR 0.0-3.30, p = 0.008, respectively). 13-18% of the variance in T and 22% in DI decrease could be predicted from sNfL levels. Conclusions: sNfL correlates with different qualities of olfactory function in patients with RRMS further strengthening the value of olfactory function as a biomarker of inflammation and axonal damage in MS.