Objective: We tested the hypothesis that olfactory function is more impaired in patients with primary progressive MS (PPMS) than that in relapsing-remitting MS (RRMS).
Methods: Standardized olfactory testing was performed in 32 patients with PPMS, 32 patients with RRMS, and 32 healthy controls (HCs). Patients with olfactory dysfunction due to an alternative primary etiology were excluded. The validated olfactory testing method yielded individual scores for olfactory threshold, odor discrimination, and odor identification, along with a composite Threshold Discrimination Identification (TDI) score.
Results: Olfactory dysfunction was identified in 27 (84%) patients with PPMS, 10 (31%) patients with RRMS, and 1 (3%) HC. While age and sex were similar between PPMS and HCs, the TDI score and all olfactory subscores were significantly worse in patients with PPMS compared with HCs (all p < 0.001). After adjustment for differences in age, sex, Expanded Disability Status Scale (EDSS), and disease duration, odor discrimination, odor identification, and the composite TDI score were worse in patients with PPMS vs RRMS (p = 0.03, 0.04, and 0.02, respectively). Neither age, sex, EDSS, nor disease duration was significantly associated with the composite TDI score.
Conclusions: Olfactory dysfunction was more frequent and severe in PPMS compared with RRMS, independent of disease duration and overall disability status. Further research on cellular level differences in olfactory neural pathways may lead to new insights about disease pathogenesis in MS.