Olfactory Dysfunction in Patients With Relapsing-Remitting Multiple Sclerosis Treated With Disease-Modifying Therapies

Background: Olfactory dysfunction evaluated with time-consuming tests was more common in patients with multiple sclerosis (MS) than in controls and correlated with neurological deficit. The aim of the present study was to compare olfactory function between patients with relapsing-remitting MS (RRMS) and controls with short and simple screening tool—the Sniffin’ Sticks Identification Test (SSIT)—and search for its association with clinical and radiological features of the disease. Methods: The study included 30 controls and 30 patients with RRMS treated with disease-modifying therapies—injectables (interferon β or glatiramer acetate, N = 18) and oral drugs (dimethyl fumarate or fingolimod, N = 12). Hyposmia was defined as a score of 6 points or fewer in the SSIT olfactory test. The data concerning number of previous relapses, disability in Expanded Disability Status Scale (EDSS), and recent brain magnetic resonance imaging (MRI) scan were collected. Moreover, thalamic volume and third ventricle width were recorded in every patient. Additionally, cognition and fatigue in patients were evaluated 24 months after olfactory assessment with the Symbol Digit Modalities Test (SDMT) and Fatigue Scale for Motor and Cognitive Functions (FSMC), respectively. Results: Patients with RRMS had a higher risk of hyposmia than controls (66.7% vs 36.7%, OR = 1.82, 95% CI, 1.10-3.67, P = .02). Neither inflammatory (number of previous relapses or new brain MRI lesions) nor neurodegenerative (EDSS, SDMT, and FSMC scores; thalamic volume; third ventricle width) MS features did not correlate with SSIT score (P > .05). In patients treated with oral drugs, olfactory dysfunction correlated with FSMC cognitive subscale (r = −0.90, P = .006). Conclusions: Olfactory dysfunction is nearly twice as common in RRMS as in controls and correlates with fatigue level in patients treated with dimethyl fumarate or fingolimod.