Olfactory function in patients with multiple sclerosis: A diffusion tensor imaging study

Abstract Background Previous research has shown that multiple sclerosis (MS) patients can develop olfactory disturbances. Objective The purpose of our study was to investigate how olfactory function in MS patients correlates with cerebral magnetic resonance imaging (MRI) including diffusion tensor imaging (DTI). Methods Olfactory performance was tested in 30 MS patients and 30 controls to determine odour threshold (T), odour discrimination (D), and odour identification (I) summarised in the TDI score. The lesion load (number and total volume of lesions) was measured on proton-density (PD)- and T2-weighted images of the olfactory brain and the total brain. Fractional anisotropy (FA) of the lesions and the surrounding normal-appearing brain tissue (NABT) was quantified using DTI. Results The median FA of white matter lesions was 0.29 and was on average 11.1% lower than in the surrounding NABT. The normalised TDI score and the normalised I subscore were significantly poorer in the MS group compared to controls (p < 0.0001), while the T and D subscores were similar in both groups. The median FA of lesions in the olfactory brain correlated inversely with the decreased I subscore (p = 0.001). There was also a strong correlation between the TDI score and the Expanded Disability Status Scale (EDSS) (p = 0.001). Conclusion A strong inverse relationship between decreased odour identification ability of MS patients and FA values in the olfactory brain indicates that the reduction in I is more strongly affected by lesions in areas with high FA values, i.e., with an increased amount of affected white matter tracts.