Loss of Smell Worse for PPMS Patients, Study Finds

Olfactory dysfunction is worse for patients with primary progressive multiple sclerosis (PPMS) than for patients with the relapsing-remitting form of the disease, according to new research.

The study, conducted by scientists in the United States and Germany, is the first to draw a clear distinction between PPMS and relapsing-remitting MS (RRMS) when it comes to olfactory symptoms.

The authors, led by Felix A. Schmid, MD, of the Clinical and Experimental Multiple Sclerosis Research Center at the Charite-Medical University of Berlin, wrote they had successfully demonstrated a significantly worse olfactory function in PPMS versus RRMS patients while providing an adjusted model to “show that the differences are not merely reflective of general disease burden but seem to be particular to disease subtype.”

Schmid and colleagues enrolled a total of 96 patients in their study: one-third (32) had RRMS, another third had PPMS, and the final third were healthy control subjects. Patients with pre-existing olfactory problems unrelated to MS were excluded from the study.

Each patient was tested for olfactory threshold, odor discrimination, and odor identification. Each was also given a composite Threshold Discrimination Identification score (TDS).

The vast majority of PPMS patients (84%) were found to have olfactory dysfunction, compared to 31% of RRMS patients, and just 3% (1 patient) in the control group.

Odor discrimination, odor identification, and TDI scores remained lower for PPMS patients when researchers adjusted for age, sex, disease discrimination, and Expanded Disability Status Scale (EDSS) scores.

While the researchers have now shown differences between the rates of change in smell function based on disease type, they still don’t know the full story of why MS affects the olfactory system in the first place.

However, the authors said existing research offers some clues. Demyelination in the olfactory pathway has been found in autopsies of MS patients, and research has also noted a correlation between MS and decreased olfactory bulb volume.

Some patients have also reported olfactory dysfunction in the midst of acute relapses. The authors said it’s possible that PPMS affects the olfactory brain tissue more than RRMS, but they cautioned that more study is needed to fully elucidate the causes of olfactory dysfunction.

“Although the olfactory testing techniques used here were robust, we did not use MRI to measure the lesion load and atrophy of the olfactory pathway and compare it with other regions of the brain, which may have helped clarify the etiology of the differences we observed between the PPMS and RRMS groups,” the authors wrote.

They said their study was also limited because there aren’t yet fully validated methods to test lesion burden in smaller structures, such as the olfactory regions of the brain.

Still, the research offers another window into what patients can expect, depending on the type of MS they have.

The study, titled “Olfactory dysfunction in patients with primary progressive MS,” was published online in the July issue of Neurology: Neuroimmunology and Neuroinflammation.

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