Failing Sense of Smell Linked to Parkinson’s Duration and Progression

Loss of the sense of smell is associated with longer disease duration in Parkinson’s, suggesting this sense increasingly fails as the disease progresses.

Its decline may be a clinically useful biomarker of Parkinson’s progression and severity, the study’s researchers said.

These findings were in “Association between olfactory impairment and disease severity and duration in Parkinson’s disease,” published in Movement Disorders Clinical Practice.

Loss of smell is a common non-motor symptom of Parkinson’s, and one many patients start experiencing at early disease stages.

Because this symptom often occurs before motor symptoms are evident, testing for the sense of smell has gained interest as a possible way of screening for Parkinson’s early in its course.

But how it progresses as the diseases advances is less known.

Researchers in Japan measured sense of smell in three groups of people: 79 who were recently diagnosed with Parkinson’s (average disease duration, 7.9 months), 71 others with established Parkinson’s (average disease duration, 47.4 months or almost four years), and 128 people without Parkinson’s (controls). The three groups were similar in terms of age (late 70s, on average) and sex (slightly more females than males in all groups).

People with established Parkinson’s had significantly more severe disease, as indicated by higher scores on the Unified Parkinson’s Disease Rating Scale (10.9 vs. 6.3) and lower scores on the Mini-Mental State Exam (26.4 vs. 27.3).

Smell was evaluated using the odor-stick identification test for Japanese (OSIT-J). This test basically involves giving people samples of common smells, like garlic, curry, and menthol, then asking that they identify them. It is scored based on the number of correctly identified smells; the number of indistinguishable smells, meaning the person could smell something, but couldn’t tell what; or samples thought odorless, meaning the person couldn’t smell anything at all.

Compared with controls, those with early and established Parkinson’s had significantly fewer correct answers, as well as significantly more that they couldn’t distinguish among or found odorless.

Recently diagnosed patients had significantly higher average scores for correct smell identification than did those diagnosed years earlier (4.3 vs. 2.5). Newer patients also had significantly fewer smells that were odorless (1.2 vs. 3.8). These differences remained statistically significant after accounting for factors such as disease severity and treatment duration.

Although some previous research has suggested  sex-based differences in smell perceptions, no noteworthy differences in this study were found between men and woman in any group.

“Olfactory dysfunction (fewer correct answers and more ‘odorless’ responses) in both males and females was more severe in PD [Parkinson’s] patients with a previous diagnosis than in patients with recently diagnosed parkinsonism. Thus, olfactory dysfunction [decreased sense of smell] in PD may get worse over time,” the researchers concluded.

Since a longer disease duration was also linked with more severe disease, the researchers noted that a poorer sense of smell was also associated with greater disease severity.

“The assessment of olfactory function is a useful strategy to detect parkinsonism, particularly at the early stage of PD, and it should be more commonly utilized in clinical practice settings as a biomarker of disease progression and severity in PD,” the researchers wrote.