Olfactory dysfunction in multiple sclerosis: Relation to plaque load in inferior frontal and temporal lobes

The question of whether and to what degree multiple sclerosis (MS) influences the ability to smell is controversial. We administered the University of Pennsylvania Smell Identification Test (UPSIT) to 26 MS patients and concurrently employed high-resolution magnetic resonance imaging (MRI) to quantify the number of demyelinating plaques within central brain structures. 38.5% of the patients demonstrated olfactory loss, with 7.7% exhibiting severe microsmia, 19.2% moderate microsmia, and 11.5% mild microsmia. None was anosmic, and no consistent left:right asymmetry in olfactory function or in hemispheric plaque numbers was observed. A strong negative correlation was found (Spearman r = -0.94) between UPSIT scores and the number of plaques within the inferior frontal and temporal lobes, but not within the rest of the brain. This study unequivocally demonstrates that a sizable proportion of MS patients suffer from olfactory loss commensurate with plaque activity within olfactory-related central brain regions, and is the first to explicate a physical basis for the olfactory dysfunction of any common neurologic disease.