Olfactory dysfunction in patients with multiple sclerosis; A systematic review and meta-analysis

Abstract

Background The importance and prevalence of olfactory dysfunction is recently gaining attention in patients with multiple sclerosis (MS) as a result of their chronic inflammatory disease, yet different prevalence rates are reported for it. Therefore, we have designed this systematic review to estimate the pooled prevalence of olfactory dysfunction in patients with MS. To our knowledge, this is the first systematic review and meta-analysis on the prevalence of olfactory dysfunction in MS patients. Methods We searched PubMed, Scopus, EMBASE, Web of Science, ProQuest, and gray literature including references from the identified studies, review studies, and conference abstracts which were published up to January 2021. Articles that were relevant to our topic and could provide information regarding the prevalence of olfactory dysfunction, or the scores of smell threshold, discrimination, or identification (TDI) among MS patients and healthy individuals were included. The pooled prevalence was calculated using a random-effects model and a funnel plot and Egger’s regression test were used to see publication bias. Results The literature search found 1630 articles. After eliminating duplicates, 897 articles remained. Two conference abstracts were included for final analysis. A total of 1099 MS cases and 299 MS patients with olfactory dysfunction were included in the analysis. The pooled prevalence of olfactory dysfunction in the included studies was 27.2%. Also, the overall TDI score in MS patients was lower than that in the control group, and the level of Threshold, Discrimination, and Identification per se were lower in MS compared with control respectively. Conclusion The results of this systematic review show that the prevalence of olfactory dysfunction in MS patients is high and more attention needs to be drawn to this aspect of MS.

Smell as a Disease Marker in Multiple Sclerosis

“…A TDI < 30.75 (defining hyposmia) was detected in 30.8% of the pwMS (26.5% of the patients with RRMS, 37.5% of those with PPMS, and 43.75% of those with SPMS). Our findings are corroborated by a recently published systematic review and meta-analysis of 1099 MS cases which found that the pooled prevalence of olfactory dysfunction was 27.2% (95% CI: 19.7%, 35.4%) [ 29 ]. Compared to age- and sex-stratified updated Sniffin’ Sticks normative data, 18.8% of the patients were found to be hyposmic (scores lower than the 10th percentile [ 16 ].…”

“…Existing data suggest that pwMS are at an elevated risk for experiencing olfactory impairment [ 2 , 29 ]. It is crucial to evaluate olfactory dysfunction as it affects the quality of life [ 2 , 30 ]; smell is related to a person’s physical, behavioral, emotional, and cognitive state, and also because there is growing evidence that the hyposmia in pwMS can be used as a potential disease marker [ 2 , 29 ].…”

“…Compared to our age- and sex-matched controls, the patients’ OD, and TDI scores were significantly lower. Mirmosayyeb et al reported lower TDI, OD, OI, and OT patients’ scores compared to controls [ 29 ]. In accordance with our findings, Bsteh et al published data suggesting that the odor threshold was markedly impaired in patients with relapse activity within 12 months, and recovered in the absence of relapse [ 2 ].…”

“…Greater disability and longer disease duration correlated with worse olfactory ability (total score) and odor discrimination. A recent meta-analysis found that the pooled prevalence of olfactory dysfunction was higher in studies with a mean EDSS higher than three compared to those with EDSS lower than three (32.6% vs. 15.9%, p = 0.048) [ 29 ]. In a prospective 3-year longitudinal study on 151 pwMS, odor discrimination strongly associated with EDSS progression [ 2 ].…”

Olfactory threshold as a biomarker of disease activity

“…Date received: 9 July 2022; revised: 14 July 2022; accepted: 14 July 2022 Dear Editor, Olfactory dysfunction has been studied in multiple sclerosis (MS). 1 We have recently read the paper by Bsteh et al, 2 stating that olfactory threshold (OT) predicts treatment response in relapsing multiple sclerosis (RMS). Absolute OT score (hazard ratio (HR) = 0.44, p < 0.001) and OT score improvement (HR = 0.12, p < 0.001) at 3 months after disease modifying treatment (DMT) initiation were associated with reduced relapse risk in the following 5 years.…”