Sense of Smell Declines With REM Sleep Disorder, But Patients Unaware

Objective olfactory test best, as RBD can be early Parkinson's symptom

Lindsey Shapiro, PhD avatar

by Lindsey Shapiro, PhD |

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People with REM sleep behavior disorder (RBD) — an early nonmotor symptom of Parkinson’s disease — were significantly more likely than others to wrongly think their sense of smell was normal when olfactory deficits were evident, a small study found.

Parkinson’s patients with RBD also had a diminished sense of smell relative to healthy people in objective evaluations.

Since perceptions of olfactory, or smelling, ability is one strategy clinicians use to predict the likelihood that a person with RBD will develop Parkinson’s, study findings have important implications for the disease’s diagnostic process, its researchers noted.

The adoption of routine objective, rather than subjective, smell testing with RBD could be “a cost-effective and accessible method to inform prognosis and potentially improve healthcare outcomes for RBD patients,” they wrote.

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The study, “Inaccurate self-report of olfactory dysfunction in REM Sleep Behavior Disorder and implications for prognosis,” was published in Clinical Parkinsonism & Related Disorders.

RBD is a condition marked by the acting out of behaviors while dreaming, which is normally suppressed during healthy sleep. It’s a common nonmotor symptom of Parkinson’s, and often emerges years before a formal diagnosis.

Doctors use the presence of RBD as a factor in predicting whether a person might develop alpha-synucleinopathies, a group of diseases marked by the buildup of toxic clumps of the alpha-synuclein protein in the brain. Related diseases include Parkinson’s, dementia with Lewy bodies (DLB), and multiple system atrophy (MSA).

A diminished sense of smell (hyposmia) — another early sign of Parkinson’s — in combination with RBD also indicates an increased likelihood of Parkinson’s or DLB but not MSA, where the sense of smell is relatively well preserved.

While “self-report of olfactory dysfunction is unreliable and declines with age,” the researchers wrote, “the majority of clinicians rely on patient self-reports and do not perform further olfaction assessments” of RBD patients.

A team of researchers in the U.K. compared self-reported and clinically assessed smell function among 16 people with RBD but not Parkinson’s, 17 people with early-stage Parkinson’s, and 19 healthy people.

Patients were enrolled at a North Bristol hospital, while healthy controls “were recruited from the local population, including partners or friends of patient group participants,” the team wrote.

Of the Parkinson’s patients, six were considered likely to have RBD but lacked a confirmed diagnosis.

Most participants (90.4%) were white and of British ethnicity. There were no significant group differences in terms of sex and age, with each group consisting mostly of men, and with a mean age of 64.6 to 69.6 years.

Participants were asked to self-report their sense of smell and taste.

Olfactory function was also objectively quantified using the Sniffin’ Sticks test. Here, patients are given a particular scent to smell for two to three seconds, then asked to identify the correct scent out of four options. The test contains 16 different smells, with a score of 16 reflecting perfect accuracy.

Based on their performance, people were categorized as normosmic (normal sense of smell), hyposmic, or anosmic (complete lack of smell).

Self-reported smell and taste abilities did not significantly differ among the three groups. However, the Parkinson’s group was the only one in which more than half of participants — nine of 17 — self-reported hyposmia or a diminished sense of smell.

Poorer smell test scores for Parkinson’s and RBD patients

In the Sniffin’ Sticks test, RBD and Parkinson’s patients scored significantly lower than healthy controls, reflecting marked olfactory dysfunction. Specifically, the mean score for controls was 11.68, compared with 7.47 among Parkinson’s patients and 7.63 in the RBD group.

Most healthy participants (89.5%) were considered normosmic, while 23.5% of those with Parkinson’s and 31.3% of RBD patients were normosmic. Conversely, significantly more Parkinson’s (47.1%) and RBD (50%) patients were considered anosmic relative to controls, none of whom had a complete lack of smell.

Notably, there were no significant differences in the Sniffin’ Sticks scores between participants with and without nasal problems, or between non-smokers and ex-smokers. No current smokers were among participants.

Perceived sense of smell correctly aligned with Sniffin’ Sticks performance for a majority of participants in all three groups. Overall, about 73.7% of healthy people, as well as 56.3% of RBD patients and 64.7% of Parkinson’s patients correctly perceived their sense of smell.

However, significantly more people in the RBD (37.5%) and Parkinson’s (29.4%) groups reported having a normal sense of smell when Sniffin’ Sticks test results classified them as hyposmic or anosmic, compared with those in the control group (5.3%).

“Our results indicate that RBD patients are less likely to be aware of any olfactory dysfunction than Control and [Parkinson’s disease] individuals, calling into question the reliance of clinicians on olfactory self-report for RBD prognosis, where olfactory [dysfunction] arising in RBD increases the likelihood of” Parkinson’s or DLB, the team wrote.

If RBD patients cannot accurately detect smell loss, this reporting of its status becomes less reliable, the researchers noted.

“To the best of our knowledge, this is the first paper to consider the fallibility of olfactory function self-report in the context of RBD prognosis,” the researchers wrote, adding that their work supports the use of “routine objective testing of olfactory function in the clinic upon initial RBD diagnosis.”

A larger and more diverse study would help to support these findings, they added.

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