Problems with Sense of Smell Are Worse in Primary Progressive MS Than Relapsing-Remitting Form, Study Reports

Problems with sense of smell are more frequent and severe in patients with primary progressive multiple sclerosis (PPMS) than in those with relapsing-remitting multiple sclerosis (RRMS), a study reports.

The research, “Olfactory dysfunction in patients with primary progressive MS,” was published in the journal Neurology: Neuroimmunology and Neuroinflammation.

Multiple sclerosis is a chronic inflammatory disease of the central nervous system whose hallmark is destruction of the protective myelin sheath surrounding nerve cells, or neurons.

A distinguishing feature of RRMS, the most common form of the disease, is attacks of new or increasing neurologic symptoms, such as movement disorders, and then recovery periods.

About 15 percent of patients have the primary progressive form, or PPMS. Its main feature is gradually increasing neurologic disability without recovery periods.

Some scientists believe PPMS is a less inflammatory course of MS. The differences in the processes that underlie each form are not well understood, however.

Several researchers think that studying differences in the two groups’ ability to smell — or olfactory dysfunction — could shed light on these underlying processes. Autopsies of MS patients in one study showed that 71 percent had experienced demyelination, or loss of neurons, in the brain’s olfactory pathway. The processes that led to this dysfunction were unclear, however.

Researchers decided to test the hypothesis that olfactory impairment is more pronounced in patients with PPMS than RRMS. The team examined 32 patients with PPMS, 32 with RRMS, and 32 healthy controls.

The yardstick they used to evaluate sense of smell was the olfactory Threshold Discrimination Identification (TDI) Test. It involves patients smelling 48 sniffin’ sticks.

In addition to an overall TDI, there were subscores on olfactory threshold, odor identification and odor discrimination. Olfactory threshold refers to the lowest concentration of an odor that a person can detect.

Researchers found olfactory dysfunction in 27, or 84 percent, of the PPMS patients; 10, or 31 percent, of the RRMS patients; and 1, or 3 percent, of the healthy controls.

The TDI score and all subscores were significantly worse in patients with PPMS than in the controls, when considering patients of similar age and the same sex. The TDI score was also worse in PPMS patients than in the RRMS group, after adjusting for age, sex, MS disability level, the length of time patients had the disease, and patients’ ability to identify and discriminate among odors.

Researchers acknowledged limitations to the study. One was the small size of the groups in the research. Another was not using magnetic resonance imaging, or MRI, to measure olfactory pathway deterioration.

Comparing the brain’s olfactory pathway region with other brain regions in both the RRMS and PPMS groups could have shed light on the processes underlying the olfactory dysfunction differences between the two, researchers said.

“The findings suggest that olfactory dysfunction might be a surrogate of neurodegeneration in these patients,” the researchers wrote. “Studies correlating olfactory function with radiologic and clinical markers of disease progression would be of interest.”