ABSTRACT

Objective

Multiple sclerosis (MS) is an inflammatory, demyelinating and neurodegenerative disease of the central nervous system. Although olfactory dysfunction in MS has been investigated for decades, systematic assessment remains limited. This study aimed to evaluate olfactory function in people with MS (PwMS) in relation to clinical parameters and magnetic resonance imaging (MRI) plaque distribution, using the Sniffin’ Sticks test.

Methods

Twenty-eight patients with MS, one participant with clinically isolated syndrome (22 women, 75.9%), and 27 healthy controls were included. Participants with primary olfactory disorders were excluded. Clinical disability was assessed using the Expanded Disability Status Scale (EDSS). Plaque locations on brain MRI scans were recorded. The Sniffin’ Sticks test was performed, and threshold, discrimination, identification, and total threshold + discrimination + identification (TDI) scores were obtained for each participant.

Results

Threshold, identification, and TDI scores were significantly lower in PwMS than in controls (p = 0.021, 0.004, and 0.005, respectively). Discrimination, identification, and TDI scores were significantly correlated with age (p = 0.001, 0.048, and 0.004, respectively) and with disease duration (p = 0.001, 0.028, and 0.001, respectively). Olfactory performance was unrelated to sex, smoking, clinical classification, EDSS, or MRI plaque distribution (all p > 0.05). A progressive disease course did not influence olfactory measures (p > 0.05).

Conclusion

Olfactory dysfunction can be observed across MS subtypes and becomes more pronounced with increasing age and longer disease duration. Threshold impairment may reflect inflammatory activity, whereas reductions in identification and TDI scores likely indicate cumulative neurodegeneration. Multidimensional olfactory testing is simple, accessible, and low-cost. Our findings support its use as a non-invasive complementary biomarker for monitoring long-term neurodegenerative processes in MS.

INTRODUCTION

Olfactory function is mediated by a neural network consisting of both peripheral and central pathways. Impaired olfactory function negatively affects daily functioning and quality of life (1, 2). Olfactory dysfunction is observed in several neurological diseases such as Parkinson’s disease, Alzheimer’s disease, Huntington disease, motor neuron disease, multiple sclerosis (MS) and migraine (3-7).

MS is a chronic, immune-mediated disease characterized by inflammation, demyelination and neuroaxonal degeneration of the central nervous system (8). Although optic nerve, brainstem, and spinal cord symptoms are well recognized, olfactory dysfunction in MS remains frequently underdiagnosed (6, 9). Histopathological evidence demonstrates demyelination and axonal degeneration in the olfactory bulb and tract in approximately 70% of MS brains (10).

The prevalence of olfactory dysfunction in MS ranges from 30% to 75% and has been associated with longer disease duration, higher Expanded Disability Status Scale (EDSS) scores and greater plaque load particularly within frontal and temporal regions (6, 7, 11, 12). Recent studies show that olfactory impairment is more pronounced in progressive MS and correlates with clinical worsening and disability progression (6, 7, 13).

Emerging evidence indicates that olfactory impairment reflects central network-level neurodegeneration, particularly involving orbitofrontal, limbic and associative cortices (14, 15). Longitudinal magnetic resonance imaging (MRI) studies have shown that a decline in olfactory performance parallels structural degeneration in these regions and that orbitofrontal cortex atrophy is strongly linked to impaired odor identification (16).

Various olfactory assessment tools have been used in MS, including the University of Pennsylvania smell identification test, the brief smell identification test, the Connecticut Chemosensory clinical research center test, and the Sniffin’ Sticks test (12, 17-19). The Sniffin’ Sticks test provides a three-domain evaluation (threshold, discrimination, identification) and has been validated in the Turkish population (20).

Furthermore, recent clinical work supports the Sniffin’ Sticks test as a practical clinical marker in MS monitoring (13, 16). This study aimed to evaluate olfactory function in MS using the Sniffin’ Sticks test and to determine relationships between olfactory function and disease duration, EDSS score, and MRI plaque distribution.

MATERIALS AND METHODS

Patient Selection

This study included 29 patients aged 18-65 years, comprising 28 patients with MS and one participant with clinically isolated syndrome (CIS), classified according to the 2017 revised McDonald criteria (21) together with 27 healthy controls. Patients were recruited from the MS outpatient clinic of the Department of Neurology, and healthy controls from the Department of Otorhinolaryngology at patients were recruited from the MS outpatient clinic of the Department of Neurology, and healthy controls from the outpatient clinic of the Department of Otorhinolaryngology at Sakarya University Training and Research Hospital. The control group consisted of individuals without neurological disease or known olfactory dysfunction. The study was approved by the Institutional Ethics Committee of Sakarya University, Faculty of Medicine (ethical approval number: E-16214662-050.01.04-28539-97, date: 21.04.2021), and written informed consent was obtained from all participants.

Exclusion criteria included subjective olfactory complaints, nasal obstruction, history of nasal surgery, nasal polyps, rhinitis, paranasal sinusitis, recent upper respiratory tract infection, head trauma, toxic exposures, smoking smoking >20 cigarettes/day cigarettes/day, pregnancy, hormone replacement therapy, diabetes mellitus, hypertension, major neurological or psychiatric disorders, severe cognitive impairment, Alzheimer’s disease, and Parkinson’s disease. Patients who had experienced an MS relapse or received corticosteroid treatment within 30 days of olfactory testing were also excluded.

Demographic and clinical data, including age, sex, smoking status, relapse history, and medication use, were recorded. Disease-related variables included disease duration and clinical classification: CIS, relapsing-remitting MS (RRMS), primary progressive MS (PPMS), or secondary progressive MS (SPMS). Neurological examinations were performed by the same neurologist. Disability level was assessed using the EDSS.

Olfactory Assessment

Olfactory function was assessed using the Sniffin’ Sticks test (Burghart Medical Technology, Wedel, Germany), a validated psychophysical battery that evaluates three domains: Odor threshold, odor discrimination and odor identification. The three subscores were summed to obtain the TDI composite score.

The test was performed in a well-ventilated, odor-free room, following standardized procedures. Participants were instructed to refrain from consuming food or beverages (other than water), smoking, or using scented products for at least 1 hour before testing. Odorants were presented in felt-tip dispensers. The pen cap was removed for a few seconds, and the tip was held approximately 1-2 cm in front of both nostrils.

Odor threshold was assessed using n-butanol in a 3-alternative forced-choice staircase paradigm. For each trial, one pen contained n-butanol at ascending dilutions (1:2 dilution steps), and two pens contained only solvent. Participants identified the pen with the odorant, and the threshold score was calculated after seven reversals based on the lowest correctly recognized concentration. Odor discrimination was evaluated using 16 triplets of pens, each consisting of two pens with identical odors and one pen with a different odor. Participants were asked to identify the pen with a different odor. Odor identification was assessed using 16 common odorants, each presented with four response options. A minimum 30-second interval was maintained between odor presentations to prevent olfactory desensitization. Each subtest had a maximum score of 16 points, and the total TDI score reflected global olfactory performance (range: 0-48), with higher scores indicating better function (Table 1).

Based on established normative criteria, TDI scores <16.5 were classified as anosmia, scores of 16.5-30.5 as hyposmia, and scores >30.5 as normosmia. Threshold, discrimination, and identification scores of <10, <12, and <11, respectively, were considered abnormal (Table 2).

MRI

MRI was performed using a 1.5 T scanner. MRI abnormalities were evaluated by an experienced neurologist blinded to the olfactory test results. T2-weighted and FLAIR sequences were used to assess hyperintense demyelinating plaques, while T1-weighted sequences were used to assess hypointense lesions. Post-contrast gadolinium-enhanced T1 images were reviewed to determine the presence of active lesions. Plaque localization was recorded across the following regions: Frontal, temporal, parietal, and occipital; brainstem; cerebellum; cervical spinal cord; and periventricular and/or juxtacortical areas.

Statistical Analysis

Statistical analyses were performed using the SPSS 24.0 software package (SPSS Inc., Chicago, IL, USA). Categorical variables were expressed as frequencies and percentages, and continuous variables as means and standard deviations or medians and interquartile ranges, as appropriate. Normality of the distribution was assessed using the Kolmogorov-Smirnov test. Group comparisons were performed using the chi-squared test for categorical variables, and the independent t-test, Mann-Whitney U test, One-Way ANOVA, or Kruskal-Wallis test for continuous variables, depending on distributional assumptions. Correlations between olfactory measures and clinical variables were examined using Pearson’s correlation analysis. Values of p < 0.05 were considered statistically significant.

RESULTS

The patient group comprised 28 patients with MS and one patient with CIS; the control group included 27 healthy participants. There were no significant differences between the groups regarding age, sex distribution, or smoking status (all p > 0.05). Demographic and clinical characteristics are presented in Table 3.

Based on the TDI score, one participant in the patient group (3.4%) was classified as anosmic, and 17 (58.6%) were classified as hyposmic. Abnormal threshold, discrimination, and identification scores were observed in 16 (55.2%), 20 (69.0%), and 17 (58.6%) participants, respectively. In the control group, 5 (18.5%) participants were classified as hyposmic based on the TDI score, while abnormal threshold, discrimination, and identification scores were observed in 4 (14.8%), 19 (70.4%), and 8 (29.6%) participants, respectively. Compared with healthy controls, the patient group showed significantly lower odor threshold, identification, and total TDI scores (p = 0.021, 0.004, 0.005, respectively) (Table 3).

According to clinical classification, 13 patients (44.8%) had RRMS, 1 patient (3.4%) had CIS, 1 patient (3.4%) had PPMS, and 14 patients (48.3%) had SPMS. 15 patients (51.7%) had a progressive disease course. There were no significant differences in olfactory threshold, discrimination, identification, or total TDI scores across the clinical classifications (all p > 0.05). The presence of a progressive disease course did not influence olfactory performance (all p > 0.05).

MRI showed periventricular plaques in all 29 participants in the patient group (100%), followed by juxtacortical plaques in 18 (62.1%), frontal plaques in 16 (55.2%), cervical spinal cord plaques in 16 (55.2%), temporal plaques in 12 (41.4%), brainstem plaques in 12 (41.4%), and cerebellar plaques in 5 (17.2%) (Table 4). No significant associations were found between plaque distribution in any region and olfactory scores (all p > 0.05).

Correlation analysis showed that, in the patient group, odor discrimination, identification, and total TDI scores were significantly correlated with age (r = -0.600, -0.370 and -0.514; p = 0.001, 0.048 and 0.004, respectively) and disease duration (r = -0.592, -0.409 and -0.579; p = 0.001, 0.028 and 0.001, respectively) (Table 5). No significant correlation was observed between EDSS scores and any olfactory measure (all p > 0.05).

DISCUSSION

Olfactory function is increasingly recognized as a clinically relevant neurobiological marker in MS (6, 13). In this study, we demonstrated impaired global olfactory performance in the patient group compared with healthy controls, consistent with previous studies reporting olfactory dysfunction in 30% to 75% of patients with MS (6, 22). In our cohort, 58.6% of patients were classified as hyposmic and 3.4% as anosmic based on TDI scores, corresponding to an overall olfactory dysfunction frequency of 62.1%. Odor threshold, identification, and total TDI scores were significantly lower than those of the control group. Odor discrimination did not differ significantly between the patient and control groups, supporting the notion that individual olfactory subdomains may reflect distinct underlying mechanisms.

Consistent with earlier findings, we observed that odor identification, discrimination, and TDI scores were lower with increasing age and longer disease duration (7). This observation supports the hypothesis that olfactory performance reflects cumulative neurodegenerative burden rather than acute inflammatory activity. In contrast, odor threshold impairment has been associated with short-term inflammatory processes, particularly during relapsing-remitting phases (23). Other studies suggest that threshold abnormalities may show sensitivity to inflammatory activity, whereas identification and discrimination deficits more strongly reflect neurodegenerative mechanisms (23-25). Our results align with this view, as abnormal threshold scores were observed in more than half of the patients, suggesting that threshold impairment may indicate ongoing inflammatory activity.

Importantly, we found no significant relationships between olfactory test performance and clinical MS subtype, EDSS score, or MRI plaque distribution. Previous studies have reported mixed results, with some indicating correlations between olfactory dysfunction, higher EDSS scores, and progressive disease course (13, 26, 27), while others found no such associations (6, 17, 28, 29). Our findings suggest that EDSS scores and plaque counts obtained by conventional MRI may not adequately reflect the functional impact of MS on the olfactory network. Standard imaging techniques often lack the sensitivity to detect subtle microstructural and network-level changes that likely contribute to olfactory impairment.

The network-based model of olfactory dysfunction provides an integrative framework for understanding the multisystem nature of olfactory impairment in MS. Consequently, plaque burden detected on conventional MRI may not adequately represent the physiologically relevant network-level damage underlying olfactory dysfunction. Advanced imaging studies have shown significant correlations between olfactory deficits and reduced olfactory bulb volume, frontotemporal cortical thinning, and reduced connectivity across olfactory network pathways (6, 7, 30-33). In our cohort, plaques were predominantly located in periventricular and juxtacortical regions; however, no significant associations were found between plaque topography and olfactory scores. This finding may be attributed to the limited spatial resolution of standard MRI rather than the absence of true structural involvement.

Taken together, these findings emphasize that olfactory impairment in MS arises from distributed inflammatory and neurodegenerative processes rather than from isolated focal lesions. This highlights the importance of comprehensive olfactory assessment as a clinically feasible and non-invasive approach for detecting subtle network-level alterations that may occur before measurable functional decline.

Among the olfactory subcomponents, threshold impairment appears to reflect ongoing inflammatory activity, whereas identification and discrimination deficits may correspond to cumulative neurodegenerative burden. This pattern has been observed in several recent studies demonstrating that odor threshold fluctuates with relapse-related inflammation, while identification and discrimination scores show progressive reduction with longer disease duration and greater cortical atrophy (13,23-25,34). Evaluating these subcomponents separately, rather than relying solely on the composite TDI score, may provide deeper insights into the transition from inflammation to neurodegeneration in MS and may support the development of olfactory testing as a complementary clinical biomarker.

From a clinical perspective, olfactory assessment represents a simple, inexpensive, and non-invasive tool that can be easily incorporated into routine neurological evaluations. By assessing threshold, discrimination, and identification separately, multidimensional olfactory testing may help distinguish inflammatory activity from cumulative neurodegeneration in people with MS. Integrating olfactory evaluation with neuroimaging and cognitive assessment may enhance early detection of disease progression and identify patients at higher risk for neurodegenerative decline (13). As recent studies have underscored the value of sensory biomarkers in neuroinflammatory disorders, olfactory measures could serve as complementary indicators reflecting both functional and structural aspects of central nervous system involvement (13, 25, 34). Further longitudinal studies are warranted to determine whether specific olfactory parameters can reliably predict clinical or radiological disease activity, thereby refining their role as complementary biomarkers in MS monitoring.

Study Limitations

This study has several limitations. First, the relatively small sample size and cross-sectional design limit the generalizability of our findings and preclude causal inferences. The small number of patients within individual clinical categories also reduced the statistical power of subgroup comparisons. Therefore, the absence of significant subtype-related differences should be interpreted cautiously. Second, patients experiencing acute relapses or receiving corticosteroid treatment were excluded, limiting our ability to evaluate the direct effects of acute inflammation on olfactory function. Third, only conventional MRI techniques were used, which may not have been sufficiently sensitive to detect subtle microstructural or network-level alterations associated with olfactory impairment. Future longitudinal studies using advanced neuroimaging methods in larger and more diverse MS populations are needed to confirm and extend these findings.

CONCLUSION

Olfactory dysfunction was frequent in patients with MS. Lower discrimination, identification, and total TDI scores were associated with older age and longer disease duration. Threshold impairment may be related to inflammatory processes, whereas lower identification and discrimination scores may reflect cumulative neurodegenerative changes. Comprehensive olfactory assessment is a practical, non-invasive, and inexpensive approach that may complement clinical evaluation in MS. Longitudinal studies are needed to determine whether olfactory measures can be used to monitor neurodegenerative changes or disease progression.