Olfactory deficits — a diminished sense of smell — affected 95% of Parkinson’s disease patients in a small study, but such problems did not appear to be linked to cognitive impairment or disease-related dementia.

The study, “Association between olfactory loss and cognitive deficits in Parkinson’s disease,” was published at the journal Clinical Neurology and Neurosurgery.

Previous research has suggested that a poor sense of smell is a non-motor symptom of Parkinson’s disease experienced by many patients — about 90% — during its initial and moderate stages.

Researchers have also found possible associations between olfactory dysfunction and a patient’s risk of developing dementia, with recent studies indicating a three times higher risk of cognitive deficits appearing in 10 years in these patients.

A research team in Brazil compared the cognitive and olfactory capacity of 42 Parkinson’s patients and 38 age- and sex-matched healthy volunteers.

Patients had lower cognitive scores compared to the control group, as determined by the Scales for Outcomes in Parkinson’s Disease-Cognition (SCOPA-Cog) and Mini-Mental State Examination (MMSE) tests. Thirty-four, or 81%, of them were diagnosed with dementia based on the International Parkinson and Movement Disorder Society criteria.

This high number of Parkinson’s dementia patients “can be partly explained by the fact that the study was carried out in a reference center for Parkinson’s disease treatment, where patients had longer disease duration and more complications,” the researchers said.

Patients’ sense of smell was assessed using the Sniffin’ Sticks Screening 12 Test, in which the participants are asked to identify each stick’s respective scent from among four options.

Based on the number of correct answers, the prevalence of olfactory dysfunction in Parkinson’s patients was 95.24% — 40 out of 42 patients were classified has having an impaired sense of smell.

Further analysis found no association between olfactory and cognitive impairment among patients, regardless of their age (although older patients had worse results — an expected finding as sense of smell is known to diminish with age, especially in men, the study noted). The only exception was in attention, the one cognitive domain that correlated with a loss or diminishment of olfactory abilities.

Among volunteers, cognitive and olfactory scores strongly correlated, both in the SCOPA-Cog and MMSE results.

A possible explanation for these results, the researchers said, could be a difficulty recognizing odors due to poor cognitive ability.

This mean a study limitation might be “the choice of the instrument to measure olfaction, because although olfaction can be evaluated in several ways (…) the Sniffin ‘Sticks Screening Test only evaluates odor identification,” the researchers wrote. “Because odor identification requires more cognitive processing and memory use than the other two ways of assessing smell (odor threshold and discrimination), it would be possible that cognitive impairment at the time olfaction was assessed affected the olfactory scores.”

Collectively, this study revealed that “patients with advanced Parkinson’s disease have cognitive and olfactory deficits more commonly than other elderly patients,” the researchers wrote.

But a link between olfactory deficit and the onset of Parkinson’s dementia could not be established. Further studies are necessary to assess sense of smell in a larger Parkinson’s population, and to explore a potential link with dementia and frontal lobe dysfunction, they concluded.