Olfactory threshold is impaired in early, active multiple sclerosis

Background: Olfactory dysfunction has been reported in multiple sclerosis (MS). However, to date no data are available on different qualities of olfactory function, namely odour identification, odour discrimination and odour perception threshold. Objective: To assess different qualities of olfactory function in patients with MS and correlate these with demographic data, clinical data, depression, quality of life and cognitive functions. Methods: In this cross-sectional study, 50 patients with MS or clinically isolated syndrome and 30 healthy controls were included. Olfactory function was measured using the Sniffin' Sticks test. Results: The scores for odour identification (p = 0.001), odour perception threshold (p = 0.037) and the combined score of odour identification, discrimination and perception threshold (TDI, p = 0.002) were significantly lower in MS. Hyposmia for identification (p = 0.0017), threshold (p = 0.017) and TDI score (p = 0.0014) was more frequent in MS. Olfactory threshold was impaired in patients who were clinically active in the previous year (p = 0.026) and in patients with a disease duration less than 2 years (p = 0.0093). Identification score was negatively correlated with disease duration (p = 0.0017). Olfactory function was not associated with disability, depression or quality of life. Conclusions: We report evidence for qualitatively distinct hyposmia in MS, with increased smell threshold in the early inflammatory phases of the disease and impaired identification with a more widespread chronic disease.