People with multiple sclerosis (MS) can indeed have a poorer-than-usual sense of smell, with problems possibly starting at early diseases stages, a small Turkish study reports.
This work supports previous research noting olfactory problems in MS patients. It also argues that longer disease duration and more relapses are associated with greater difficulties, reflecting “more extensive involvement of the central nervous system and olfactory network in MS disease,” the researchers said.
Such problems are a point of dispute among scientists, because lesions are rarely evident on the olfactory nerve.
A diminished ability to smell odors or an altered sense of how smells are perceived are among nonmotor symptoms of MS, but a disease manifestation that is often under-diagnosed.
Olfaction, the sense of smell, is regulated by a network that includes the olfactory nerve and diverse brain areas — namely, the olfactory bulb and tract, the orbitofrontal region and cortex, the insula, and the cerebellum.
As the olfactory nerve largely lacks myelin — the protective coat on neurons that is destroyed in MS — researchers have thought that olfaction was spared damage. But recent evidence points to greater involvement of the olfactory network in MS than previously assumed.
Research has shown that olfaction is damaged in the early stages of neurodegenerative diseases, such as Parkinson’s or Alzheimer’s disease. In fact, studies suggest that olfactory impairment may be used as a biomarker for diagnosing neurodegeneration.
Clinical studies addressing smell function in MS, however, have contradictory results. Data from some indicate a 20% to 40% olfactory impairment in MS, mostly in patients with relapsing-remitting MS but also among those with primary progressive MS. Other studies report no olfactory problems associated with MS.
Addressing this conflict, researchers in Turkey used a validated test to evaluate the link between olfactory dysfunction and MS duration and progression, as well as with patients’ cognition. Sense of smell was assessed using the Connecticut Chemosensory Clinical Research Center olfactory test, while thinking abilities were evaluated using the Montreal Cognitive Assessment.
The team analyzed a total of 31 adult patients, with a minimum follow-up period of five years, and 24 healthy controls at an MS outpatient clinic in Istanbul. Patients with other conditions that might cause olfactory problems were excluded.
Everyday items were used in the smell identification test: peanut butter, soap, chocolate, coffee, cinnamon, mothballs, and baby powder.
Results revealed that MS patients, particularly those with longer disease duration and more frequent relapses, performed worse than controls on the olfactory test. Better cognitive status was also linked to superior olfactory test scores.
“Our results may have important clinical implications: olfactory testing could be a useful surrogate for cognitive dysfunction in MS patients, apart from their physical disability status,” the investigators wrote.
No correlation was found between patients’ level of disability and their olfactory perceptions. This result may be due to the small sample size and low level of disability in the MS group analyzed, the team said.
Of note, the poorer sense of smell found in MS patients compared to controls was more pronounced than in prior studies, even though these patients were still in early disease stages.
“This study supports the presence of a possible relationship between MS and olfactory system involvement, and the correlation of cognitive impairment with olfactory dysfunction, even in the early stages of the disease in young and less physically disabled patients,” the researchers concluded.
“The assessment of olfaction may be helpful as a surrogate method for tracking disease progression in patients over time,” they added.